Milky Way

Milky Way

Overview
The Milky Way is the galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 that contains the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 for "milky road", Via Lactea, in turn derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 kyklos galaktikos or "milky circle", "milk" also being the root for the Greek word for galaxy, γαλαξίας (galaxias).

The galaxy has this appearance because of the Earth's position within the galactic plane around two thirds of the way out from the center, on the inner edge of the Orion–Cygnus Arm, with the majority of the galaxy being seen edge on.
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Encyclopedia
The Milky Way is the galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 that contains the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 for "milky road", Via Lactea, in turn derived from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 kyklos galaktikos or "milky circle", "milk" also being the root for the Greek word for galaxy, γαλαξίας (galaxias).

The galaxy has this appearance because of the Earth's position within the galactic plane around two thirds of the way out from the center, on the inner edge of the Orion–Cygnus Arm, with the majority of the galaxy being seen edge on. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 used his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

 showed that the Milky Way was just one of around 200 billion galaxies in the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

.

The Milky Way is a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...

 100,000-120,000 light years in diameter containing 200–600 billion stars. The galaxy is estimated to contain 50 billion planets, 500 million of which could be located in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of their parent star. Depending on its structure the entire galaxy has a rotational rate of 1 per 15 to 50 million years. The galaxy is also moving at a rate of 552 to 630 km per second depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

. The Milky Way is part of the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...

 of galaxies.

Appearance



All the stars that can be seen in the night sky are part of the Milky Way galaxy, but for the purposes of the astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 activity of observing the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 the term "Milky Way" is limited to the hazy band of white light arching around the entire night sky. The light originates from un-resolved
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

 stars and other material that lie within the galactic plane. Dark regions within the band, such as the Great Rift
Great Rift (astronomy)
In astronomy, the Great Rift is a series of overlapping, non-luminous, molecular dust clouds that are located between the Solar System and the Sagittarius Arm of the Milky Way Galaxy at a distance of about 100 parsecs or about 300 light years from Earth...

 and the Coalsack
Coalsack Nebula
The Coalsack Dark Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a dark patch silhouetted against the southern Milky Way. It was known pre-historically in the Southern Hemisphere and was observed by Vicente Yáñez Pinzón in 1499...

, correspond to areas where light from distant stars is blocked by dark nebulae. The Milky Way has a relatively low surface brightness
Surface brightness
The overall brightness of an extended astronomical object such as a galaxy, star cluster, or nebula, can be measured by its total magnitude, integrated magnitude or integrated visual magnitude; a related concept is surface brightness, which specifies the brightness of a standard-sized piece of an...

 with an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of around +4.5 to +5. Its visibility is greatly affected by how bright the night sky is (limiting magnitude
Limiting magnitude
In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument....

) due to light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...

, stray light from the moon, etc. It becomes visible at limiting magnitudes of +5.1 or higher, showing a great deal of detail above +6.1. This makes the Milky Way difficult to see from any urban
Urban area
An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...

 or suburban location due to light pollution, but very prominent when viewed from a rural area.

The center of the galaxy
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

 lies in the direction of Sagittarius
Sagittarius (constellation)
Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...

, and it is here that the Milky Way looks brightest. From Sagittarius, the hazy band of white light appears to pass westward through the constellations of Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...

, Ara
Ara (constellation)
Ara is a southern constellation situated between Scorpius and Triangulum Australe. Its name is Latin for "altar". Ara was one of the 48 Greek constellations described by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations defined by the International Astronomical...

, Norma
Norma (constellation)
Norma is a small and inconspicuous constellation in the southern hemisphere between Scorpius and Centaurus. Its name is Latin for normal, referring to a right angle, and is variously considered to represent a rule, a carpenter's square, a set square or a level....

, Triangulum Australe
Triangulum Australe
Triangulum Australe is a small constellation in the southern sky, created in the sixteenth century. Its name is Latin for 'the southern triangle', which distinguishes it from Triangulum in the northern sky...

, Circinus
Circinus
The constellation Circinus is a small constellation in the southern sky, first defined in the 18th century. Its name is Latin for compass, referring to the drafting tool used for drawing circles; it should not be confused with Pyxis, a constellation that represents a mariner's...

, Centaurus
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...

, Musca
Musca
Musca is one of the minor southern constellations. The constellation was one of twelve constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman and it first appeared on a 35-cm diameter celestial globe published in 1597 in Amsterdam by...

, Crux
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

, Carina
Carina (constellation)
Carina is a constellation in the southern sky. Its name is Latin for the keel of a ship, and it was formerly part of the larger constellation of Argo Navis until that constellation was divided in three.-Stars:...

, Vela
Vela (constellation)
Vela is a constellation in the southern sky. Its name is Latin for the sails of a ship, and it was originally part of a larger constellation, the ship Argo Navis, which was later divided into three parts, the others being Carina and Puppis.-Stars:...

, Puppis
Puppis
Puppis is a constellation in the southern sky. Its name is the Latin word for the poop deck of a ship, and Puppis represents the deck of the ship and its deckhouses...

, Canis Major
Canis Major
Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

, Monoceros
Monoceros
Monoceros is a faint constellation on the celestial equator. Its name is Greek for unicorn. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south and Hydra to the east...

, Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

 and Gemini
Gemini (constellation)
Gemini is one of the constellations of the zodiac. It was one of the 48 constellations described by the 2nd century astronomer Ptolemy and it remains one of the 88 modern constellations today. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology...

, Taurus
Taurus (constellation)
Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...

, Auriga
Auriga (constellation)
Auriga is a constellation in the northern sky. Its name is Latin for 'charioteer' and its stars form a shape that has been associated with the pointed helmet of a charioteer. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains among the 88 modern...

, Perseus
Perseus (constellation)
Perseus is a constellation in the northern sky, named after the Greek hero Perseus. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations defined by the International Astronomical Union...

, Andromeda
Andromeda (constellation)
Andromeda is a constellation in the northern sky. It is named after Andromeda, the princess in the Greek legend of Perseus who was chained to a rock to be eaten by the sea monster Cetus...

, Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...

, Cepheus
Cepheus (constellation)
Cepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:...

 and Lacerta
Lacerta
Lacerta is one of the 88 modern constellations defined by the International Astronomical Union. Its name is Latin for lizard. A small, faint constellation, it was created in 1687 by the astronomer Johannes Hevelius. Its brightest stars form a "W" shape similar to that of Cassiopeia, and it is thus...

, Cygnus
Cygnus (constellation)
Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...

, Vulpecula
Vulpecula
Vulpecula is a faint constellation in the northern sky. Its name is Latin for "little fox", although it is commonly known simply as the fox. It was identified in the seventeenth century, and is located in the middle of the Summer Triangle .-Stars:There are no stars brighter than 4th magnitude in...

, Sagitta
Sagitta
Sagitta is a constellation. Its name is Latin for "arrow", and it should not be confused with the larger constellation Sagittarius, the archer. Although ancient, it is insignificant, for it has no star brighter than the 4th magnitude and is the third smallest of all constellations...

, Aquila
Aquila (constellation)
Aquila is a stellar constellation. Its name is Latin for 'eagle' and it is commonly represented as such. In mythology, Aquila was owned by the Roman god Jupiter and performed many tasks for him....

, Ophiuchus
Ophiuchus
Ophiuchus is a large constellation located around the celestial equator. Its name is from the Greek "serpent-bearer", and it is commonly represented as a man grasping the snake that is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century...

, Scutum
Scutum
Scutum is a small constellation introduced in the seventeenth century. Its name is Latin for shield.-History:Scutum is the only constellation that owes its name to a non-classical historical figure...

, and back to Sagittarius
Sagittarius (constellation)
Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...

. The fact that the band divides the night sky into two roughly equal hemispheres
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 indicates that the Solar System lies close to the galactic plane
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...

.

The galactic plane is inclined by about 60 degrees to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 (the plane of the Earth's orbit). Relative to the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

, it passes as far north as the constellation of Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...

 and as far south as the constellation of Crux
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

, indicating the high inclination of Earth's equatorial plane
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 and the plane of the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 relative to the galactic plane. The north galactic pole is situated at right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

 12h 49m, declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

 +27.4° (B1950) near beta Comae Berenices
Beta Comae Berenices
Beta Comae Berenices is a main sequence dwarf star in the constellation of Coma Berenices. It is located at a distance of about 30 light years. The Greek letter beta usually indicates that the star has the second highest visual magnitude in the constellation...

, and the south galactic pole is near alpha Sculptoris
Alpha Sculptoris
Alpha Sculptoris is a star in the constellation Sculptor.Alpha Sculptoris is a blue-white B-type giant with an apparent magnitude of +4.30. It is approximately 680 light years from Earth...

.

Size


The stellar disk of the Milky Way galaxy is approximately 100,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s (30 kiloparsecs, 9×1017 km) in diameter, and is considered to be, on average, about 1000 ly thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass, or dwarf star
Dwarf star
The term dwarf star refers to a variety of distinct classes of stars.* Dwarf star alone generally refers to any main sequence star, a star of luminosity class V.** Red dwarfs are low-mass main sequence stars....

s, which are hard to detect, especially more than 300 ly from the Sun, and so current estimates of the total number remain highly uncertain, though often speculated to be around 250 billion. This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

.
The stellar disc does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the galaxy. Beyond a radius of roughly 40000 ly, the number of stars drops much faster with radius, for reasons that are not understood. Recent estimates give the galaxy a population of at least 50 billion planets, 500 million of which could be located in the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 of their parent star. New data suggests there may be up to twice as many free-floating planets
Rogue Planet
- Literature :* "Rogue Planet" , a Dan Dare story that ran in the original Eagle comic from Volume 6, Issue 48 to Volume 8, Issue 7* Rogue Planet , a 2000 novel set in the Star Wars galaxy- Other :...

 in the Milky Way as there are stars.

Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12000 ly—twice the previously accepted value.

As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 were reduced to the size of a US quarter
Quarter (United States coin)
A quarter dollar, commonly shortened to quarter, is a coin worth ¼ of a United States dollar, or 25 cents. The quarter has been produced since 1796. The choice of 25¢ as a denomination, as opposed to 20¢ which is more common in other parts of the world, originated with the practice of dividing...

 (approximately 2.5 cm in diameter) the Milky Way would cover an area of 3.286 million square kilometers, an area equivalent to about a third of the United States (3.276 million square km), the total area of India (3.287 million square km), or the combined areas of Great Britain, France, Spain, Germany, Italy, Poland, Sweden, Finland and Norway (3.378 million square km).

The Galactic Halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...

 extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...

, whose perigalacticon
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 is at about 180000 ly. At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.

Age



The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

, then comparing the results to estimates of their original abundance. These yield values of about for CS 31082-001 and for BD+17° 3248. Once a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

 star is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as . Globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s are among the oldest objects in the Milky Way galaxy, which thus set a lower limit on the galaxy age. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 Ga, and a 95% confidence upper limit of 16 Ga.

In 2007, a star in the Galactic halo, HE 1523-0901
HE 1523-0901
HE 1523-0901 is the designation given to a red giant star located in the Milky Way galaxy approximately seven and a half light-millennia from Earth. It is thought to be a second generation Population II, or metal-poor, star . The star was found in the sample of bright metal-poor halo stars from the...

, was estimated to be about 13.2 billion years old, nearly as old as the Universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way. This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

 to measure
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 the relative strengths of spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s caused by the presence of Thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

 and other elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 created by the R-process
R-process
The r-process is a nucleosynthesis process, likely occurring in core-collapse supernovae responsible for the creation of approximately half of the neutron-rich atomic nuclei that are heavier than iron. The process entails a succession of rapid neutron captures on seed nuclei, typically Ni-56,...

. The line strengths yield abundances of different elemental isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s, from which an estimate of the age of the star can be derived using nucleocosmochronology
Nucleocosmochronology
Nucleocosmochronology, also known as cosmochronology, is a relatively new technique used to determine timescales for astrophysical objects and events...

.

The age of stars in the Galactic thin disk can be estimated in the same way as HE 1523-0901. Measurements of thin disk stars yield an estimate that the thin disk formed between 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the Galactic halo and the thin disk.

Composition and structure



The galaxy consists of a bar-shaped core region surrounded by a disc of gas, dust
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 and stars forming four distinct arm structures spiraling outward in a logarithmic spiral
Logarithmic spiral
A logarithmic spiral, equiangular spiral or growth spiral is a special kind of spiral curve which often appears in nature. The logarithmic spiral was first described by Descartes and later extensively investigated by Jacob Bernoulli, who called it Spira mirabilis, "the marvelous...

 shape (see Spiral arms). The mass distribution within the galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely wound arms. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...

, rather than an ordinary spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...

, in the 1990s. Their suspicions were confirmed by the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 observations in 2005 that showed the galaxy's central bar to be larger than previously suspected.

Estimates for the mass of the Milky Way vary, depending upon the method and data used. Recent estimates at the low end have placed the mass of the Milky Way at 5.8 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es (M), somewhat smaller than the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

. Other measurements by the Very Long Baseline Array
Very Long Baseline Array
The Very Long Baseline Array is a system of ten radio telescopes controlled remotely from the Array Operations Center in Socorro, New Mexico by the National Radio Astronomy Observatory. The array works together as the world's largest dedicated, full-time astronomical instrument using the...

 (VLBA) have found velocities as large as 254 km/s for stars at the edge of the Milky Way, higher than the previously accepted value of 220 km/s. As the orbital velocity depends on the mass enclosed, this implies that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7 M within 50 kpc of its center. A recent measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7 M. Most of the mass of the galaxy is thought to be dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

, which forms a dark matter halo
Dark matter halo
A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

 that is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic Center. Modelling of the Milky Way suggests that the overall mass of the entire galaxy lies in the range 1-1.5 M

This mass in baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...

ic matter is estimated to include 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9.

Galactic Center




The galactic disc, which bulges outward at the Galactic Center, has a diameter of 70000 ly. The exact distance from the Sun to the Galactic Center is actively debated. The latest estimates from geometric-based methods and standard candles
Standard Candles
Standard Candles is a compilation of short stories by American science fiction author Jack McDevitt. The sixteen stories in the anthology were originally published in various magazines from 1982 to 1996...

 yield distances to the Galactic Center of 8 kpc. The fact that the estimates span nearly 1 kpc only underscores the true uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...

 associated with the distance to the Galactic Center.

The Galactic Center harbors a compact object of very large mass as determined by the motion of material around the center. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is confirmed to be a supermassive black hole
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...

.For a photo see Chandra X-ray Observatory; January 6, 2003 Most galaxies are believed to have supermassive black holes at their centers.

The nature of the galaxy's bar is also actively debated, with estimates for its half-length and orientation spanning from 1 kpc (short or a long bar) and 10–50 degrees. Certain authors advocate that the galaxy features two distinct bars, one nestled within the other. The bar is delineated by red clump
Red clump
The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants...

 stars, however, RR Lyr variables do not trace a prominent Galactic bar. The bar may be surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the galaxy, as well as most of the Milky Way's star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

 activity. Viewed from the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

, it would be the brightest feature of our own galaxy.

Notes

Spiral arms


Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences. Some 150 years after Alexander (1852) first suggested that the Milky Way was a spiral, there is currently no consensus on the number or nature of the Galaxy's spiral arms. Perfect grand design logarithmic spiral patterns ineptly describe features near the Sun, namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are likely not unique, and exist elsewhere in the galaxy.

Each spiral arm describes a logarithmic spiral
Logarithmic spiral
A logarithmic spiral, equiangular spiral or growth spiral is a special kind of spiral curve which often appears in nature. The logarithmic spiral was first described by Descartes and later extensively investigated by Jacob Bernoulli, who called it Spira mirabilis, "the marvelous...

 (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. Until recently, there were believed to be four major spiral arms which all start near the galaxy's center. These are named as follows, according to the image at right:

Color Arm(s)
cyan 3-kpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

 and Perseus Arm
Perseus Arm
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called Scutum–Centaurus Arm. Perseus Arm begins from the distal end of the long Milky Way....

purple Norma and Outer arm (Along with a newly discovered extension)
green Scutum–Centaurus Arm
pink Carina–Sagittarius Arm
There are at least two smaller arms or spurs, including:
orange Orion–Cygnus Arm (which contains the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 and Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

)



Observations presented in 2008 by Robert Benjamin of the University of Wisconsin–Whitewater suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum-Centaurus arm. The rest of the arms are minor or adjunct arms.See also
This would mean that the Milky Way is similar in appearance to NGC 1365
NGC 1365
NGC 1365, also known as the Great Barred Spiral Galaxy, is a barred spiral galaxy about 56 million light-years away in the constellation Fornax.The core is an oval shape with an apparent size of about 50″ × 40″....

.

Outside of the major spiral arms is the Monoceros Ring
Monoceros Ring
The Monoceros Ring is a ring of stars around the Milky Way which is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy as it merges with the Milky Way over the course of billions of years...

 (or Outer Ring), proposed by astronomers Brian Yanny and Heidi Jo Newberg
Heidi Jo Newberg
Heidi Jo Newberg is an American astrophysicist known for her work in understanding the structure of our Milky Way galaxy...

, a ring of gas and stars torn from other galaxies billions of years ago.

As is typical for many galaxies, the distribution of mass in the Milky Way galaxy is such that the orbital speed
Orbital speed
The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...

 of most stars in the galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. Hence the orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

 of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits are expected to have significantly different velocities associated with them. This difference is one of the major pieces of evidence for the existence of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.

Halo


The galactic disk is surrounded by a spheroid halo of old stars and globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s, of which 90% lie within 100000 ly, suggesting a stellar halo diameter of 200,000 light-years. However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of these clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation. The globular clusters can follow rosette orbits about the galaxy, in contrast to the elliptical orbit of a planet.

While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

 takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

s also occur primarily in the disk.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disk of the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

 (M31) extends much further than previously thought, the possibility of the disk of the Milky Way galaxy extending further is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm
Cygnus Arm
The Norma Arm is a minor spiral arm of the Milky Way galaxy extending from and around the central hub region of the Milky Way Galaxy. The inner portion of the Arm is called Norma Arm in narrow meaning...

. With the discovery of the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...

 came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy
Canis Major Dwarf Galaxy
The Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation Canis Major. The galaxy contains a relatively high percentage of red giant stars, and is thought to contain an estimated one billion stars in all....

, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the galactic disk.

On January 9, 2006, Mario Jurić
Mario Juric
Mario Jurić is a Croatian astronomer.Jurić was born in Zagreb. He graduated from the University of Zagreb Faculty of Science, and received a doctorate at Princeton University in 2007....

 and others of Princeton University
Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....

 announced that the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

 of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the galaxy. The proposed likely interpretation is that a dwarf galaxy
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...

 is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream
Virgo Stellar Stream
The Virgo Stellar Stream, also known as Virgo Overdensity, is the proposed name for a stellar stream in the constellation of Virgo which was discovered in 2005. The stream is thought to be the remains of a dwarf spheroidal galaxy that is in the process of merging with the Milky Way...

 and is found in the direction of Virgo
Virgo (constellation)
Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...

 about 30000 ly away.

Gamma-ray bubbles



On November 9, 2010, Doug Finkbeiner of the Harvard–Smithsonian Center for Astrophysics announced that he had detected two gigantic spherical bubbles of energy erupting to the north and the south from the center of the Milky Way, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25000 ly; they stretch up to Grus
Grus (constellation)
Grus is a constellation in the southern sky. Its name is Latin for the crane, a species of bird. It was introduced in the late sixteenth century.-History:The stars that form Grus were originally considered part of Piscis Austrinus...

 and to Virgo
Virgo (constellation)
Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...

 on the night-sky of the southern hemisphere. Their origin remains unclear, so far.

Sun's location and neighborhood



The Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 (and therefore the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

) may be found close to the inner rim of the galaxy's Orion Arm
Orion Arm
The Orion–Cygnus Arm is a minor spiral arm of the Milky Way galaxy some 3,500 light years across and approximately 10,000 light years in length. The Solar System is within the Orion–Cygnus Arm...

, in the Local Fluff inside the Local Bubble
Local Bubble
The Local Bubble is a cavity in the interstellar medium of the Orion Arm of the Milky Way. It is at least 300 light years across and has a neutral hydrogen density of about 0.05 atoms per cubic centimetre, or approximately one tenth of the average for the ISM in the Milky Way , and half that for...

, and in the Gould Belt
Gould Belt
The Gould Belt is a partial ring of stars in the Milky Way galaxy, about 3000 light years across, tilted toward the galactic plane by about 16 to 20 degrees. It contains many spectral class O- and B-type stars, and may represent the local spiral arm to which the Sun belongs—currently the Sun is...

, at a distance of 8.33 ± from the Galactic Center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

. The Sun is currently 5 pc from the central plane of the galactic disc. The distance between the local arm and the next arm out, the Perseus Arm
Perseus Arm
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called Scutum–Centaurus Arm. Perseus Arm begins from the distal end of the long Milky Way....

, is about 6500 ly. The Sun, and thus the Solar System, is found in the galactic habitable zone.

There are about 208 stars brighter than absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

 8.5 within 15 pc of the Sun, giving a density of 0.0147 such stars per cubic parsec, or per cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s) within 5 pc of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5.

The Apex of the Sun's Way, or the solar apex
Solar apex
The solar apex is the direction that the Sun travels with respect to the Local Standard of Rest. In lay terms, it's the "target" within the Milky Way that the Sun appears to be "chasing" as it orbits the galaxy...

, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...

 near the constellation of Hercules
Hercules (constellation)
Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...

, at an angle of roughly 60 sky degrees to the direction of the Galactic Center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

. The Sun's orbit around the galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass extinction periods on Earth.
However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find these correlations.

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year
Galactic year
The galactic year, also known as a cosmic year, is the duration of time required for the Solar System to orbit once around the center of the Milky Way Galaxy. Estimates of the length of one orbit range from 225 to 250 million "terrestrial" years....

), so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed
Orbital speed
The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body...

 of the Solar System about the center of the galaxy is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

).

Environment




The Milky Way and the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

 are a binary system
Binary system (astronomy)
A binary system is an astronomical term referring to two objects in space which are so close that their gravitational interaction causes them to orbit about a common center of mass. Some definitions A binary system is an astronomical term referring to two objects in space (usually stars, but also...

 of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...

, itself being part of the Virgo Supercluster
Virgo Supercluster
The Virgo Supercluster or Local Supercluster is the irregular supercluster that contains the Virgo Cluster in addition to the Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs...

.

Two smaller galaxies and a number of dwarf galaxies
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...

 in the Local Group orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 the Milky Way. The largest of these is the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

 with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....

. The Magellanic Stream
Magellanic Stream
The Magellanic Stream is a high-velocity cloud of gas connecting the Large and the Small Magellanic Clouds. It came into existence by a near-collision of both galaxies some 2.5 billion years ago.-Discovery and early observations:...

 is a peculiar streamer of neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way
Milky Way's satellite galaxies
The Milky Way Galaxy has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup. This subgroup is part of the local galaxy cluster, the Local Group.The Milky Way's satellite galaxies include the following:-Clickable map:...

 are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...

, Ursa Minor Dwarf
Ursa Minor Dwarf
The Ursa Minor Dwarf dwarf elliptical galaxy was discovered by A.G. Wilson of the Lowell Observatory in 1954. It is part of the Ursa Minor constellation, and a satellite galaxy to the Milky Way...

, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf
Fornax Dwarf
The Fornax Dwarf Spheroidal is an elliptical dwarf galaxy in the constellation Fornax that was discovered in 1938 by Harlow Shapley. He discovered it while he was in South Africa on photographic plates taken by a 24 inch reflecting telescope at Boyden Observatory, shortly after he discovered...

, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf
Carina Dwarf
The Carina Dwarf Spheroidal is a dwarf galaxy in the Carina constellation. It was discovered in 1977 with the UK Schmidt Telescope by Cannon et al. The Carina Dwarf is receding from the Milky Way at 230 km/s and is a satellite galaxy to the Milky Way. The galaxy may also be referred to as...

, Draco Dwarf
Draco Dwarf
The Draco Dwarf is a spheroidal galaxy which was discovered by Albert George Wilson of Lowell Observatory in 1954 on photographic plates of the National Geographic Society's Palomar Observatory Sky Survey . It is part of the local group and a satellite galaxy of the Milky Way galaxy...

, and Leo II Dwarf
Leo II (dwarf galaxy)
Leo II is an dwarf spheroidal galaxy about 690,000 light-years away in the constellation Leo. As of October 2008 it is one of 24 known satellite galaxies of the Milky Way....

. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri
Omega Centauri
Omega Centauri or NGC 5139 is a globular clusterin the constellation of Centaurus, discovered by Edmond Halley in 1677 who listed it as a nebula. Omega Centauri had been listed in Ptolemy's catalog 2000 years ago as a star. Lacaille included it in his catalog as number I.5...

. Observations through the Zone of Avoidance
Zone of Avoidance
The Zone of Avoidance is the area of the night sky that is obscured by our own galaxy, the Milky Way.-Term:The ZOA was originally called the "Zone of Few Nebulae" in an 1878 paper by English astronomer Richard Proctor that referred to the distribution of "nebulae" in Sir John Herschel's General...

 are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the galaxy, causing vibrations at certain frequencies when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twentyfold increase in mass for the galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. In this model, the dark matter is spreading out from the galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the galaxy.

Current measurements suggest the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

 is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

 over the course of about a billion years.

Velocity


In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's special theory of relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, which declares that there is no "preferred" inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

 in space with which to compare the object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the galaxy's motion.

Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow. If the galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

. The Milky Way is thought to be moving in the direction of the Great Attractor
Great Attractor
The Great Attractor is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster that reveals the existence of a localised concentration of mass equivalent to tens of thousands of Milky Ways, observable by its effect on the motion of galaxies and their associated...

. The Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...

 (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

) is part of a supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...

 called the Local Supercluster, centered near the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

: although they are moving away from each other at 967 km/s as part of the Hubble flow, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.

Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s with respect to the photons of the CMB, toward 10.5 right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

, −24° declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

 (J2000 epoch, near the center of Hydra
Hydra (constellation)
Hydra is the largest of the 88 modern constellations, measuring 1303 square degrees. It has a long history, having been included among the 48 constellations listed by the 2nd century astronomer Ptolemy. It is commonly represented as a water snake...

). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...

 in the direction of the motion and red-shifted
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

 in the opposite direction.

The galaxy rotates about its center according to its galaxy rotation curve
Galaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....

 as shown in the figure. The discrepancy between the observed curve (relatively flat) and the curve based upon the known mass of the stars and gas in the Milky Way (decaying curve) is attributed to dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

.

Etymology


There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 phrase is a translation from Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 Γαλαξίας, Galaxias, which is derived from the word for milk (γάλα, gala). This is also the origin of the word galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

. In Greek myth
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

, the Milky Way was caused by milk spilt by Hera
Hera
Hera was the wife and one of three sisters of Zeus in the Olympian pantheon of Greek mythology and religion. Her chief function was as the goddess of women and marriage. Her counterpart in the religion of ancient Rome was Juno. The cow and the peacock were sacred to her...

 when suckling Heracles
Heracles
Heracles ,born Alcaeus or Alcides , was a divine hero in Greek mythology, the son of Zeus and Alcmene, foster son of Amphitryon and great-grandson of Perseus...

.

In Sanskrit
Sanskrit
Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 and several other Indo-Aryan languages
Indo-Aryan languages
The Indo-Aryan languages constitutes a branch of the Indo-Iranian languages, itself a branch of the Indo-European language family...

, the Milky Way is called Akash Ganga (आकाशगंगा, Ganges of the heavens). The milky way is held to be sacred in the Hindu scriptures known as the Puranas
Puranas
The Puranas are a genre of important Hindu, Jain and Buddhist religious texts, notably consisting of narratives of the history of the universe from creation to destruction, genealogies of kings, heroes, sages, and demigods, and descriptions of Hindu cosmology, philosophy, and geography.Puranas...

, and the Ganges and the Milky Way are considered to be terrestrial-celestial analogs of each other. However, the term Kshira (क्षीर, milk) is also used as an alternative name for the milky way in Hindu texts.

Discovery



As Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 (384–322 BC) informs us in Meteorologica (DK 59 A80), the Greek philosophers
Greek philosophy
Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...

 Anaxagoras
Anaxagoras
Anaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae in Asia Minor, Anaxagoras was the first philosopher to bring philosophy from Ionia to Athens. He attempted to give a scientific account of eclipses, meteors, rainbows, and the sun, which he described as a fiery mass larger than...

 (ca. 500–428 BC) and Democritus
Democritus
Democritus was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher and pupil of Leucippus, who formulated an atomic theory for the cosmos....

 (450–370 BC) proposed the Milky Way might consist of distant star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

, in the region of the world which is continuous with the heavenly motions." The Neoplatonist
Neoplatonism
Neoplatonism , is the modern term for a school of religious and mystical philosophy that took shape in the 3rd century AD, based on the teachings of Plato and earlier Platonists, with its earliest contributor believed to be Plotinus, and his teacher Ammonius Saccas...

 philosopher Olympiodorus the Younger
Olympiodorus the Younger
Olympiodorus the Younger was a Neoplatonist philosopher, astrologer and teacher who lived in the early years of the Byzantine Empire, after Justinian's Decree of 529 A.D. which closed Plato's Academy in Athens and other pagan schools...

 (c. 495–570 A.D.) criticized this view, arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world
Muslim world
The term Muslim world has several meanings. In a religious sense, it refers to those who adhere to the teachings of Islam, referred to as Muslims. In a cultural sense, it refers to Islamic civilization, inclusive of non-Muslims living in that civilization...

.

The Arabian astronomer, Alhazen (965–1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...

, and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 and did not belong to the atmosphere."

The Persian
Persian people
The Persian people are part of the Iranian peoples who speak the modern Persian language and closely akin Iranian dialects and languages. The origin of the ethnic Iranian/Persian peoples are traced to the Ancient Iranian peoples, who were part of the ancient Indo-Iranians and themselves part of...

 astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 to be a collection of countless nebulous
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 stars. The Andalusian
Al-Andalus
Al-Andalus was the Arabic name given to a nation and territorial region also commonly referred to as Moorish Iberia. The name describes parts of the Iberian Peninsula and Septimania governed by Muslims , at various times in the period between 711 and 1492, although the territorial boundaries...

 astronomer Avempace
Ibn Bajjah
Abū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, botanist, poet and scientist. He was known in the West by his Latinized name, Avempace...

 (d. 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

 in the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s.

The Persian astronomer Naṣīr al-Dīn al-Ṭūsī
Nasir al-Din al-Tusi
Khawaja Muḥammad ibn Muḥammad ibn Ḥasan Ṭūsī , better known as Naṣīr al-Dīn al-Ṭūsī , was a Persian polymath and prolific writer: an astronomer, biologist, chemist, mathematician, philosopher, physician, physicist, scientist, theologian and Marja Taqleed...

 (1201,1274) in his Tadhkira writes:
"The Milky Way, i.e. the galaxy, is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. because of this, it was likened to milk in color."

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

 used a telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

 to study the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

, drawing on earlier work by Thomas Wright
Thomas Wright (astronomer)
Thomas Wright was an English astronomer, mathematician, instrument maker, architect and garden designer. He was the first to describe the shape of the Milky Way and speculate that faint nebulae were distant galaxies....

, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

 akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

e visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both our galaxy and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.

The first attempt to describe the shape of the Milky Way and the position of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 within it was carried out by William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

 in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center.

In 1845, Lord Rosse
William Parsons, 3rd Earl of Rosse
William Parsons, 3rd Earl of Rosse, Knight of the Order of St Patrick was an Irish astronomer who had several telescopes built. His 72-inch telescope "Leviathan", built 1845, was the world's largest telescope until the early 20th century.-Life:He was born in Yorkshire, England, in the city of...

 constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

In 1917, Heber Curtis had observed the nova S Andromedae
S Andromedae
|- style="background-color: #A0B0FF;" colspan="3"| Database References|- bgcolor="#FFFAFA"| Simbad || |- bgcolor="#FFFAFA"| ||...

 within the "Great Andromeda
Andromeda (constellation)
Andromeda is a constellation in the northern sky. It is named after Andromeda, the princess in the Greek legend of Perseus who was chained to a rock to be eaten by the sea monster Cetus...

 Nebula" (Messier object
Messier object
The Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in 1771. The original motivation of the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets...

 M31
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

). Searching the photographic record, he found 11 more nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...

e. Curtis noticed that these novae were, on average, 10 magnitudes
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...

 fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies. In 1920 the Great Debate
The Great Debate
In astronomy, the Great Debate, also called the Shapley–Curtis Debate, was an influential debate between the astronomers Harlow Shapley and Heber Curtis which concerned the nature of spiral nebulae and the size of the universe...

 took place between Harlow Shapley
Harlow Shapley
Harlow Shapley was an American astronomer.-Career:He was born on a farm in Nashville, Missouri, and dropped out of school with only the equivalent of a fifth-grade education...

 and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...

.

The matter was conclusively settled by Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

 in the early 1920s using the Mount Wilson observatory 100 inch (2.5 m) Hooker telescope. With the light-gathering power of this new telescope he was able to produce astronomical photographs
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

 that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variable
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...

s that he could use as a benchmark
Benchmark
-Geology:*Benchmark , a point of reference for a measurement**Benchmarking , an activity involving finding benchmarks*Benchmark , used in pricing crude oil-Technology:...

 to estimate the distance to the nebulae: proving they were far too distant to be part of the Milky Way. In 1936, Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence
Hubble sequence
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the Hubble tuning-fork diagram because of the shape in which it is traditionally represented....

.

See also


  • Galactic coordinate system
    Galactic coordinate system
    The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...

  • Dark matter halo
    Dark matter halo
    A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...

  • Smith's Cloud
    Smith's Cloud
    Smith's Cloud is a high-velocity cloud of hydrogen gas located in the constellation Aquila at Galactic coordinates l = 39°, b = −13°. The cloud was discovered in 1963 by Gail Bieger, née Smith, who was an astronomy student at Leiden University in the Netherlands...

  • Baade's Window
    Baade's Window
    Baade's Window is an area of the sky with relatively low amounts of interstellar "dust" along the line of sight from the Earth. This area is considered an observational "window" as the normally obscured Galactic Center of the Milky Way is visible in this direction. It is named for astronomer Walter...

  • Oort Constants
  • The Great Rift
    Great Rift (astronomy)
    In astronomy, the Great Rift is a series of overlapping, non-luminous, molecular dust clouds that are located between the Solar System and the Sagittarius Arm of the Milky Way Galaxy at a distance of about 100 parsecs or about 300 light years from Earth...

    , a molecular dust cloud located between the solar system and the Sagittarius Arm of the Milky Way which appears to split the Milky Way into two lanes over a third of its length
  • MilkyWay@Home
    MilkyWay@Home
    MilkyWay@home is a volunteer distributed computing project in astrophysics running on the Berkeley Open Infrastructure for Network Computing platform...

    , a distributed computing project that attempts to generate highly accurate three-dimensional dynamic models of stellar streams in the immediate vicinity of our Milky Way galaxy.

Further reading


External links