The
Milky Way, or simply
the Galaxy, is the
galaxyA 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 name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
in which the
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
is located. It is a
barred spiral galaxyA barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately half of all spiral galaxies...
that is part of the
Local GroupThe Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises about 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
of galaxies. It is one of billions of galaxies in the
observable universeIn 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...
.
Its name is a translation of the
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
Via Lactea, in turn translated from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
Γαλαξίας (
Galaxias), referring to the pale band of light formed by the galactic plane as seen from Earth (see etymology of
galaxy). Some sources hold that, strictly speaking, the term
Milky Way should refer exclusively to the band of light that the galaxy forms in the
night skyNight sky is a commonly used term most often employed to refer to the sky as it is seen at night. The term is usually associated with astronomy, with reference to views of heavenly bodies such as stars, the Moon and planets that become visible on a clear night after the Sun has set.The night sky...
, while the galaxy should receive the full name
Milky Way Galaxy, or alternatively
the Galaxy. However, it is unclear how widespread this convention is, and the term
Milky Way is routinely used in either context.
Appearance from Earth
The Milky Way Galaxy, as viewed from
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, itself situated on a spur off one of the spiral arms of the galaxy (see Sun's location and neighborhood), appears as a hazy band of white light in the night sky arching across the entire
celestial sphereIn 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...
and originating from
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
s and other material that lie within the galactic plane. The plane of the Milky Way is inclined by about 60° to the
eclipticThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
(the plane of the Earth's orbit), with the North Galactic Pole situated at
right ascensionRight 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,
declinationIn astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator...
+27.4° (B1950) near
beta Comae BerenicesBeta Coma Berenices is a main sequence dwarf star in the constellation of Coma Berenices. The Greek letter beta usually indicates that the star has the second highest visual magnitude in the constellation...
. The South Galactic Pole is near
alpha SculptorisAlpha 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...
.
The center of the galaxy is in the direction of
SagittariusSagittarius is one of the constellations of the zodiac. 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 the Milky Way then "passes" (going westward) through
ScorpiusScorpius is one of the constellations of the zodiac; the associated astrological sign is called Scorpio. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
,
AraAra is a southern constellation situated between Scorpius and Triangulum Australe. Its name is Latin for altar. Ara was one of Ptolemy's original 48 Greek constellations, and it is now one of the 88 constellations defined by the International Astronomical Union.-Stars:Ara's brightest star, β Arae,...
,
NormaNorma 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 AustraleTriangulum 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...
,
CircinusThe 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...
,
CentaurusCentaurus is a bright constellation in the southern sky. One of the largest constellations in the sky, Centaurus was included among the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations.-Stars:...
,
MuscaMusca 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...
,
CruxCrux 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 because since the time of Christ it has been easily visible from the southern...
,
CarinaCarina 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.-Notable features:...
,
VelaVela 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:...
,
PuppisPuppis is a constellation in the southern sky. Its name is Latin for the poop deck 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 Vela; Puppis is the largest of the three.-Named...
,
Canis MajorCanis Major is a constellation, included in the 2nd century astronomer Ptolemy's 48 constellations, and still included among the 88 modern constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...
,
MonocerosMonoceros is a faint constellation on the celestial equator. Its name is Greek for unicorn. Its creation 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...
,
OrionOrion, 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 largest, most conspicuous, and most recognizable in the night sky...
&
GeminiGemini is one of the constellations of the zodiac. Its name is Latin for "twins", and it is associated with the twins Castor and Pollux in Greek mythology. Its symbol is . It lies between Taurus to the west and the dim Cancer to the east, with Auriga and Lynx to the north and Monoceros and Canis...
,
TaurusTaurus is one of the constellations of the zodiac. Its name is Latin for bull, and its symbol is , a stylized bull's head. Taurus is a large and prominent constellation in the northern hemisphere's winter sky, between Aries to the west and Gemini to the east; to the north lie Perseus and Auriga,...
,
AurigaAuriga 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 is included among the 88 modern...
,
PerseusPerseus 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...
,
AndromedaAndromeda is a constellation in the northern sky. It is named after Andromeda, the princess of a mythological kingdom Ethiopia in Greek mythology. From the Earth's perspective, the constellation appears to contain the Andromeda Galaxy...
,
CassiopeiaCassiopeia is a constellation in the northern sky. In Greek mythology it was considered to represent the vain queen Cassiopeia, who boasted about her unrivaled beauty. It was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern...
,
CepheusCepheus is a constellation in the northern sky. It is named after Cepheus, King of Aethiopia in Greek mythology, and is considered to represent a king. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.-Stars:γ Cephei...
&
LacertaLacerta 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...
,
CygnusCygnus is a northern constellation. 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...
,
VulpeculaVulpecula 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 created in the seventeenth century, and is located in the middle of the Summer Triangle .-Stars:There are no stars brighter than 4th magnitude in this...
,
SagittaSagitta 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...
,
AquilaAquila is a constellation. Its name is Latin for 'eagle' and it is commonly represented as such. It lies roughly at the celestial equator. The alpha star, Altair, is a vertex of the Summer Triangle asterism.-History:...
,
OphiuchusOphiuchus is a large constellation located around the celestial equator. Its name is Greek for 'snake-holder', 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...
,
ScutumScutum 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
SagittariusSagittarius is one of the constellations of the zodiac. 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 Milky Way looks brightest in the direction of the
constellationIn modern astronomy, a constellation is an area of the celestial sphere, defined by exact boundaries.The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.-Definitions:...
of
SagittariusSagittarius is one of the constellations of the zodiac. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
, toward the
galactic centerThe Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 7.62±0.32 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
. Relative to the
celestial equatorThe 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
CassiopeiaCassiopeia is a constellation in the northern sky. In Greek mythology it was considered to represent the vain queen Cassiopeia, who boasted about her unrivaled beauty. It was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern...
and as far south as the constellation of
CruxCrux 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 because since the time of Christ it has been easily visible from the southern...
, indicating the high inclination of Earth's
equatorial planeThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
and the plane of the
eclipticThe ecliptic is the apparent path that the Sun traces out in the sky during the year, appearing to move eastwards on an imaginary spherical surface, the celestial sphere, relative to the fixed stars. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is...
relative to the galactic plane. The fact that the Milky Way divides the night sky into two roughly equal
hemispheresIn 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. The Milky Way has a relatively low
surface brightnessSurface brightness is a concept used in astronomy when describing extended astronomical objects such as galaxies and nebulae.-General description:...
, making it difficult to see from any
urbanAn 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 suffering from
light pollutionLight pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light. The International Dark-Sky Association , "The Light Pollution Authority," defines light pollution as: It obscures the stars in the night sky for city dwellers, interferes with...
.
Size
The stellar disk of the Milky Way Galaxy is approximately in diameter, and is considered to be, on average, about 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 stars, which is highly uncertain. This can be compared to the one trillion (10
12) stars of the neighbouring
Andromeda GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
.
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 —twice the previously accepted value. As a guide to the relative physical
scaleThe scale ratio of some sort of model which represents an original proportionally is the ratio of a linear dimension of the model to the same dimension of the original. Examples include a 3-dimensional scale model of a building or the scale drawings of the elevations or plans of a building. In such...
of the Milky Way, if it were reduced to 10m in diameter, the Solar System, including the
Oort cloudThe Oort Cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
, would be no more than 0.1mm in width (0.001%).
The
Galactic HaloThe 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 CloudsThe two Magellanic Clouds are irregular dwarf galaxies, which are members of our Local Group of galaxies. Once they were thought to be orbiting our Milky Way galaxy. However, new research seems to indicate that this is not the case...
, whose
perigalacticonIn celestial mechanics, an apsis, plural apsides is the point of greatest or least distance of the elliptical orbit of an object from its center of attraction, which is generally the center of mass of the system....
is at ~. 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.
Recent measurements by the
Very Long Baseline ArrayThe 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 revealed that the Milky Way is much heavier than some previously thought. The mass of our home galaxy is now considered to be roughly similar to that of our largest local neighbour, the
Andromeda GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
. By using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts. The newer and more accurate estimate of the galaxy's rotational speed (and in turn the amount of dark matter contained by the galaxy) puts the figure at about 254 km/s, significantly higher than the widely accepted value of 220 km/s. This in turn implies that the Milky Way has a total mass equivalent to around 3 trillion Suns, about 50% more massive than some previously thought.
Age
It is extremely difficult to define the age of the Milky Way but the age of the oldest star in the galaxy yet discovered,
HE 1523-0901HE 1523-0901 is the designation given to a red giant star located in the Milky Way galaxy approximately 7500 light years away. 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 Hamburg/ESO Survey...
, is estimated to be about 13.2 billion years, nearly as
old as the UniverseThe age of the universe is the time elapsed between the Big Bang and the present day. Current theory and observations suggest that the universe is between 13.5 and 14 billion years old. The uncertainty range has been obtained by the agreement of a number of scientific research projects...
itself.
This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the
Very Large TelescopeThe Very Large Telescope is a very large telescope made up of four separate optical telescopes organized in an array formation, built and operated by the European Southern Observatory at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain...
to
measureIn science, measurement is the process of obtaining the magnitude of a quantity, such as length or mass, relative to a unit of measurement, such as a meter or a kilogram...
, for the first time, the
berylliumBeryllium is the chemical element with the symbol Be and atomic number 4.A bivalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include Beryl and Chrysoberyl...
content of two stars in
globular clusterA 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...
NGC 6397NGC 6397 is a globular cluster in the Ara constellation. It is located about 7,200 light-years from Earth, making it one of the two nearest globular clusters to Earth . NGC 6397 is one of the at least 20 globulars of our Milky Way Galaxy which have undergone a core collapse, meaning that its core...
. From this research, the elapsed time between the rise of the first generation of stars in the entire galaxy and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.
Composition and structure
The galaxy consists of a bar-shaped core region surrounded by a disk of
gas, dustIn astronomy, the interstellar medium is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
and stars forming four distinct arm structures spiralling outward in a
logarithmic spiralA 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 Jakob 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 galaxyA barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately half of all spiral galaxies...
in the 1990s rather than an ordinary
spiral galaxyA spiral galaxy is a galaxy belonging to one of the three main classes 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...
. Their suspicions were confirmed by the
Spitzer Space TelescopeThe Spitzer Space Telescope is an infrared space observatory launched in 2003...
observations in 2005 which showed the galaxy's central bar to be larger than previously suspected.
The Milky Way's
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
is thought to be about 5.8
solar massThe solar mass , , is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth or 1,048 times the mass of Jupiter.The solar mass can be...
es (M
☉) comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the galaxy is thought to be
dark matterIn astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...
, forming a
dark matter haloThe dark matter halo is the hypothetical gravitational core of a galaxy, consisting of dark matter.-Rotation curves as evidence of a dark matter halo:...
of an estimated 600–3000 billion M
☉ which is spread out relatively uniformly.
Galactic Center
The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years. The distance from the Sun to the galactic center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge.
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*Sagittarius A* is a bright and very compact astronomical radio source at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location...
, thought to mark the center of the Milky Way, is newly confirmed to be a
supermassive black holeA supermassive black hole is a black hole with the highest classification of mass, on the order of hundreds of thousands to billions of solar masses...
. For a photo see
Chandra X-ray Observatory; Jan. 6, 2003. Most galaxies are believed to have a supermassive black hole at their center.
The galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient (see
red dwarfAccording to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type...
,
red giantA red giant is a luminous giant star of low or intermediate mass that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
). The bar is 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 formationStar 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 GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
, it would be the brightest feature of our own galaxy.
Spiral arms
Each spiral arm describes a
logarithmic spiralA 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 Jakob Bernoulli, who called it Spira mirabilis, "the marvelous...
(as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are 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 The parsec is a unit of length, equal to just under 31 million million kilometres , or about 3.26 light-years. The parsec measurement unit is used in astronomy and is defined as the length of the adjacent side of an imaginary right triangle in space...
and Perseus ArmThe Perseus Arm is a major spiral arm of the Milky Way galaxy.The Milky Way is a barred spiral galaxy with four major arms and at least two minor arms or spurs. The Perseus Spiral Arm, with a radius of approximately 10,700 parsecs, is located between the Cygnus Arm and the Sagittarius Arm...
|
| purple |
Norma and Outer arm (Along with a newly discovered extension) |
| green |
Scutum-Crux ArmThe Scutum-Crux arm is a secondary spiral arm of the Milky Way. This arm is located between the Sagittarius Arm and the Norma Arm.The arm starts near the core as the Scutum arm, then gradually turns into the Crux arm....
|
| pink |
Carina and Sagittarius Arm The Sagittarius Arm is generally thought to be one of the spiral arms of our home galaxy, the Milky Way. Each spiral arm is a long, diffuse curving streamer of stars that radiates out from the galactic center...
|
| There are at least two smaller arms or spurs, including: |
| orange |
Orion-Cygnus armThe Orion 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 and Earth are within the Orion Arm. It is also referred to as the Local Arm, the Local Spur or the Orion Spur.The Orion Arm is named for its...
(which contains the SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
and Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
) |
Outside of the major spiral arms is the Outer Ring or
Monoceros RingThe Monoceros Ring is a proposed ring of stars around the Milky Way which consists 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...
, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and
Heidi Jo NewbergHeidi Jo Newberg is an American astrophysicist known for her work in understanding the structure of our Milky Way galaxy. Among her team's findings is the first-ever evidence that the Milky Way is "cannibalizing" stars from smaller galaxies...
, which consists 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 speedThe 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 periodThe 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 matterIn astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...
. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that 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 arms form as a result of a matter-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. Thus this results in several spiral arms where there are a lot of stars and gas. 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.
Observations presented in 2008 by Robert Benjamin of the
University of Wisconsin-WhitewaterThe University of Wisconsin–Whitewater is part of the University of Wisconsin System, located in Whitewater, Wisconsin. It became Wisconsin's second public college on April 21, 1868 when it opened its doors to 39 students taught by nine faculty members...
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
Halo
The galactic disk is surrounded by a spheroid halo of old stars and
globular clusterA 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 100,000 light-years, 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. While the disk contains gas and dust which obscure the view in some wavelengths, the spheroid component does not. Active
star formationStar 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 clusterAn open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters have been found only in spiral and irregular...
s also occur primarily in the disk.
Recent discoveries have added dimension to the knowledge of the Milky Way's structure. With the discovery that the disc of the
Andromeda GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
(M31) extends much further than previously thought, the possibility of the disk of our own galaxy extending further is apparent, and this is supported by evidence of the newly discovered Outer Arm extension of the
Cygnus ArmThe Milky Way Galaxy in which the Sun dwells, in common with many other galaxies of similar type, consists of a large mass of stars drawn by gravitational forces into the form of a relatively flat disc. The disc is rotating, with the dense central body of stars moving at greater speeds than those...
. With the discovery of the
Sagittarius Dwarf Elliptical GalaxyThe Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. The main cluster which, in 1994, was the first to be discovered, is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth and travelling in a polar...
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 GalaxyThe 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 JurićMario Jurić is a Croatian astronomer.He is a prolific discoverer of asteroids, and has also discovered one comet, 183P/Korlević-Jurić, with Korado Korlević. He took part in discovery of the Sloan Great Wall, the largest known structure in the Universe....
and others of
Princeton UniversityPrinceton University a private university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League and is considered one of the Colonial Colleges....
announced that the
Sloan Digital Sky SurveyThe 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. 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 galaxyA 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. The Large Magellanic Cloud, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy...
is merging with the Milky Way. This galaxy is tentatively named the
Virgo Stellar StreamThe Virgo Stellar Stream 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
VirgoVirgo 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 30,000 light-years away.
Sun's location and neighborhood
The
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
(and therefore the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and the
Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
) may be found close to the inner rim of the galaxy's
Orion ArmThe Orion 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 and Earth are within the Orion Arm. It is also referred to as the Local Arm, the Local Spur or the Orion Spur.The Orion Arm is named for its...
, in the Local Fluff inside the
Local BubbleThe 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 BeltThe Gould Belt is a partial ring of stars about 3000 light years across, tilted toward the galactic plane by about 16 to 20 degrees. It contains many O- and B-type stars, and may represent the local spiral arm of which the Sun is a member — about 325 light years from its center. It is thought to be...
, at a distance of 7.62±0.32 kpc (~25,000±1,000
lyAs defined by the International Astronomical Union , a light-year is the distance that light travels in a vacuum in one Julian year.The speed of light is 299,792,458 metres per second...
) from the
Galactic CenterThe Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 7.62±0.32 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...
.
The Sun is currently 5–30 parsecs from the central plane of the galactic disc. The distance between the local arm and the next arm out, the
Perseus ArmThe Perseus Arm is a major spiral arm of the Milky Way galaxy.The Milky Way is a barred spiral galaxy with four major arms and at least two minor arms or spurs. The Perseus Spiral Arm, with a radius of approximately 10,700 parsecs, is located between the Cygnus Arm and the Sagittarius Arm...
, is about 6,500 light-years. The Sun, and thus the Solar System, is found in the galactic habitable zone.
There are about 208 stars brighter than
absolute magnitudeIn astronomy, absolute magnitude measures a celestial object's intrinsic brightness. To derive the absolute magnitude from the observed apparent magnitude of a celestial object its value is corrected for distance to the observer...
8.5 within 15
parsecThe parsec is a unit of length, equal to just under 31 million million kilometres , or about 3.26 light-years. The parsec measurement unit is used in astronomy and is defined as the length of the adjacent side of an imaginary right triangle in space...
s of the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 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 dwarfBrown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth...
s) within 5 parsecs 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 apexThe solar apex, or the Apex of the Sun's Way, refers to the direction that the Sun travels with respect to the Local Standard of Rest. This is not to be confused with the Sun's apparent motion through the constellations of the zodiac, which is illusory – this supposed motion is actually caused by...
, 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
VegaVega 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
HerculesHercules is a constellation. It is 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 CenterThe Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 7.62±0.32 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 often coincide with mass extinction periods on Earth; presumably the higher density of stars close to the galactic plane leads to more impact events.
It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a
galactic yearThe 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 speedThe 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. 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 unitAn astronomical unit is a unit of length roughly equal to the mean distance between the Earth and the Sun. It is approximately 150 million kilometres ....
).
Environment
The Milky Way and the
Andromeda GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
are a
binary systemA 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 GroupThe Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises about 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
, itself being part of the
Virgo SuperclusterThe Virgo Supercluster or Local Supercluster is the irregular supercluster that contains 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 galaxiesA 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. The Large Magellanic Cloud, containing over 30 billion stars, is sometimes classified as a dwarf galaxy while others consider it a full-fledged galaxy...
in the Local Group
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
the Milky Way. The largest of these is the
Large Magellanic CloudThe Large Magellanic Cloud is a nearby irregular galaxy, once thought to be a satellite of our own. 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...
with a diameter of 20,000 light-years. It has a close companion, the
Small Magellanic CloudThe Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years. and contains several hundred million stars....
. The
Magellanic StreamHierarchical Clustering tells us that galaxies are built up over time from collisions of smaller galaxies. These collisions are still going on today, with the Milky Way still cannibalizing its smaller neighbours. The best known and most studied example of these mergers, is the Magellanic Stream...
is a peculiar streamer of neutral
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
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 are Canis Major Dwarf (the closest),
Sagittarius Dwarf Elliptical GalaxyThe Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. The main cluster which, in 1994, was the first to be discovered, is roughly 10,000 light-years in diameter, and is currently about 70,000 light-years from Earth and travelling in a polar...
,
Ursa Minor DwarfThe 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 DwarfThe 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 the...
, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include
Carina DwarfThe Carina Dwarf Spheroidal is a dwarf galaxy in the Carina constellation. It was discovered in 1977 with the UK Schmidt Telescope. 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 E206-G220 or PGC 19441...
,
Draco DwarfThe 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 DwarfLeo 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 cannibalized by the Milky Way, such as
Omega CentauriOmega Centauri or NGC 5139 is a globular clusterseen in 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.The English...
. Observations through the
zone of avoidanceThe 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 matterIn astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...
, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twenty-fold 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 GalaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
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 galaxyAn 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 flattened 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
EinsteinAlbert Einstein was a theoretical physicist. His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first post-Newtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of...
's
special theory of relativitySpecial relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies"...
, which declares that there is no "preferred"
inertial frame of referenceIn physics, an inertial frame of reference is a member of the subset of reference frames with the property that every physical law takes the same form in each such frame. In contrast, in the set of non-inertial frames the laws of physics are frame-dependent, and the usual physical forces must be...
in space with which to compare the Galaxy's motion. (Motion must always be specified with respect to another object.)
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
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
. The Milky Way is thought to be moving in the direction of the
Great AttractorThe 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 GroupThe Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises about 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 galaxyThe Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
) is part of a
superclusterSuperclusters are large groups of smaller galaxy groups and clusters and are among the largest structures of the cosmos.-Existence:The existence of superclusters indicates that the galaxies in our Universe are not uniformly distributed; most of them are drawn together in groups and clusters, with...
called the Local Supercluster, centered near the
Virgo ClusterThe Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 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 ascensionRight 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°
declinationIn astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator...
(J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the
Wilkinson Microwave Anisotropy ProbeThe 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-shiftedThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, 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 an observer...
in the direction of the motion and
red-shiftedIn physics and astronomy, redshift occurs when electromagnetic radiation—usually visible light—emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum due to the Doppler effect or other gravitationally-induced effects...
in the opposite direction.
The galaxy rotates about its center according to its
galaxy rotation curveThe 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 matterIn astronomy and cosmology, dark matter is hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...
.
Etymology and beliefs
There are many creation myths around the world which explain the origin of the Milky Way and give it its name. The
EnglishEnglish is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
phrase is a translation from
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
Γαλαξίας,
Galaxias, which is derived from the word for milk (γάλα,
gala). This is also the origin of the word
galaxyA 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 name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
. In
Greek mythGreek 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
HeraIn the Olympian pantheon of classical Greek Mythology, Hera or Here was the wife and older sister of Zeus. Her chief function was as goddess of women and marriage. In Roman mythology, Juno was the equivalent mythical character. The cow, and later, the peacock were sacred to her...
when suckling
HeraclesIn Greek mythology, Heracles or Herakles , Alcides or Alcaeus , was a divine hero, the son of Zeus and Alcmene, foster son of Amphitryon and great-grandson of Perseus...
.
The term
Milky Way first appeared in
English literatureEnglish literature is the literature written in the English language, including literature composed in English by writers not necessarily from England; Joseph Conrad was born in Poland, Robert Burns was Scottish, James Joyce was Irish, Dylan Thomas was Welsh, Edgar Allan Poe was American, V.S....
in a poem by
ChaucerGeoffrey Chaucer was an English author, poet, philosopher, bureaucrat, courtier and diplomat. Although he wrote many works, he is best remembered for his unfinished frame narrative The Canterbury Tales...
.
In
SanskritSanskrit is a historical Indo-Aryan language, one of the liturgical languages of Hinduism and Buddhism, and one of the 22 official languages of India. It is also declared as a classical language by the government of India....
and several other
Indo-Aryan languagesThe Indo-Aryan languages are a branch of the Indo-European language family.SIL International in a 2005 estimate counted a total of 209 varieties, the largest in terms of native speakers being Hindustani , Bengali , Punjabi , Marathi ,...
, 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
PuranasThe Puranas are a group of important Hindu religious texts, notably consisting of narratives of the history of the Universe from creation to destruction, genealogies of the kings, heroes, sages, and demigods, and descriptions of Hindu cosmology, philosophy, and geography...
, 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.
In a large area from
Central AsiaAsia is a region of Asia from the Caspian Sea in the west to central China in the east, and from southern Russia in the north to northern India in the south. It is also sometimes known as Middle Asia or Inner Asia, and is within the scope of the wider Eurasian continent.Various definitions of its...
to
AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
, the name for the Milky Way is related to the word for "
strawStraw is an agricultural by-product, the dry stalk of a cereal plant, after the grain or seed has been removed. Straw makes up about half of the yield of cereal crops such as barley, oats, rice, rye and wheat...
". This may have originated in ancient
Armenian mythologyVery little is known about pre-Christian Armenian mythology, the oldest source being the legends of Xorenatsi's History of Armenia.Armenian mythology was strongly influenced by Zoroastrianism, with deities such as Aramazd, Mihr or Anahit, as well as Assyrian traditions, such as Barsamin, but there...
, (Յարդ զողի Ճանապարհ
hard goghi chanaparh, or "Trail of the Straw Thief"), and been carried abroad by
ArabArab people or Arabs are an ethnic group whose members identify along linguistic, cultural or genealogical grounds...
s. In several
UralicThe Uralic languages constitute a language family of 39 languages spoken by approximately 25 million people. The healthiest Uralic languages in terms of the number of native speakers are Hungarian, Finnish, Estonian, Mari and Udmurt...
,
Turkic languagesThe Turkic languages constitute a language family of some thirty languages, spoken by Turkic peoples across a vast area from Eastern Europe and the Mediterranean to Siberia and Western China, and are considered to be part of the proposed Altaic language family.Turkic languages are spoken by some...
, Fenno-Ugric languages and in the
Baltic languagesThe Baltic languages are a group of related languages belonging to the Indo-European language family and spoken mainly in areas extending east and southeast of the Baltic Sea in Northern Europe...
the Milky Way is called the "Birds' Path" (
Linnunrata in
FinnishFinnish is the language spoken by the majority of the population in Finland and by ethnic Finns outside of Finland. It is one of the official languages of Finland and an official minority language in Sweden. In Sweden, both standard Finnish and Meänkieli, a Finnish dialect, are spoken...
), since the route of the migratory birds appear to follow the Milky Way. (The
Qi XiQixi Festival , also known as Magpie Festival, falls on the seventh day of the seventh lunar month on the Chinese calendar; thus its name. It also inspired Tanabata in Japan, Chilseok in Korea, and That Tich in Vietnam...
legend celebrated in many
Asian culturesThe culture of Asia is the artificial aggregate of the cultural heritage of many nationalities, societies, religions, and ethnic groups in the region, traditionally called a continent from a Western-centric perspective, of Asia...
references a seasonal bridge
across the Milky Way formed by birds, usually magpies or crows.) The
ChineseChinese or the Sinitic language is a language family consisting of languages mutually unintelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the two branches of Sino-Tibetan family of languages...
name "Silver River" (銀河) is used throughout
East AsiaEast Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms. Geographically and geo-politically, it covers about , or about 28 percent of the Asian continent, about 15 percent bigger than the area of Europe, though some categorize Tibet, Xinjiang,...
, including
KoreaKorea is a civilization and formerly unified nation currently divided into two states. Located on the Korean Peninsula, it borders China to the northwest, Russia to the northeast, and is separated from Japan to the east by the Korea Strait....
and
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly
Han RiverThe Han River is a left tributary of the Yangtze River with a length of 1532 km. Historically it was referred to as Hànshuǐ and the name is still occasionally used today....
"(天汉). In
Japaneseis a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family. There are a number of proposed relationships with other languages, but none have gained general acceptance...
, "Silver River" (銀河
ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系
gingakei) or the "River of Heaven" (天の川
Amanokawa or Amanogawa). In
SwedishSwedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along the coast and on the Åland islands. It is to a considerable extent mutually intelligible with Norwegian and to a lesser extent with Danish...
, it is called
Vintergatan, or "Winter Avenue", because the stars in the belt were used to predict time of the approaching winter. In some of the
Iberian languagesIberian languages is a generic term for the languages currently or formerly spoken in the Iberian peninsula.- Pre-Roman languages :The following languages were spoken in the Iberian peninsula before the Roman occupation.* Aquitanian* Celtiberian...
, the name refers to "Road of Saint James"
Discovery
As
AristotleAristotle was a Greek philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology.Together with Plato and Socrates , Aristotle is one of...
(384-322 BC) informs us in
Meteorologica (DK 59 A80), the
Greek philosophersGreek philosophy focused on the role of reason and inquiry. Many philosophers today concede that Greek philosophy has shaped the entire Western thought since its inception...
AnaxagorasAnaxagoras 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
DemocritusDemocritus was an Ancient Greek philosopher born in Abdera in the north of Greece. He was the most prolific, and ultimately the most influential, of the pre-Socratic philosophers; his atomic theory may be regarded as the culmination of early Greek thought.His exact contributions are difficult to...
(450–370 BC) proposed that the Milky Way might consist of distant
starA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
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
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
, in the
region of the world which is continuous with the heavenly motionsThe sublunary sphere is a concept derived from Greek astronomy. It is the region of the cosmos from the Earth to the Moon, consisting of the four classical elements: earth, water, air, and fire. Beginning with the Moon, up to the limits of the universe, everything is made of aether...
." The
Arabian astronomerIn the history of astronomy, Islamic astronomy or Arabic astronomy refers to the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age , and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia,...
, Alhazen (965-1037 AD), refuted this by making the first attempt at observing and measuring the Milky Way's
parallaxParallax is an apparent displacement or difference of orientation 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...
, and he thus "determined that because the Milky Way had no parallax, it was very remote from the
earthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and did not belong to the atmosphere."
The
Persian astronomerIn the history of astronomy, Islamic astronomy or Arabic astronomy refers to the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age , and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia,...
, Abū Rayhān al-Bīrūnī (973-1048), proposed the Milky Way
galaxyA 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 name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...
to be a collection of countless
nebulousA nebula is an interstellar cloud of dust, hydrogen gas, helium gas and plasma...
stars.
AvempaceAbū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian-Arab Muslim polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, 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
refractionRefraction is the change in direction of a wave due to a change in its velocity. This is most commonly observed when a wave passes from one medium to another...
in the
Earth's atmosphereThe Earth's atmosphere 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...
. 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 that these stars are larger than
planetA 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.
Actual proof of the Milky Way consisting of many stars came in 1610 when
Galileo GalileiGalileo 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
telescopeAn 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 KantImmanuel Kant was an 18th-century German philosopher from the Prussian city of Königsberg...
, drawing on earlier work by
Thomas WrightThomas 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.-Life and Works:...
, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by
gravitational forcesGravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the...
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
nebulaA nebula is an interstellar cloud of dust, hydrogen gas, helium gas and plasma...
e visible in the night sky might be separate "galaxies" themselves, similar to our own.
The first attempt to describe the shape of the Milky Way and the position of the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
within it was carried out by
William HerschelSir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel, was a German-born English astronomer, technical expert and composer who became famous for discovering Uranus...
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 RosseWilliam Parsons, 3rd Earl of Rosse KP built several telescopes including the world's largest telescope in 1845 and it remained the world's largest for the rest of the century. Using this telescope he saw and cataloged a large number of galaxies...
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|- style="background-color: #A0B0FF;" colspan="3"| Database References|- bgcolor="#FFFAFA"| Simbad || |- bgcolor="#FFFAFA"| ||...
within the "Great
AndromedaAndromeda is a constellation in the northern sky. It is named after Andromeda, the princess of a mythological kingdom Ethiopia in Greek mythology. From the Earth's perspective, the constellation appears to contain the Andromeda Galaxy...
Nebula" (
Messier objectThe Messier objects are a set of astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses et des Amas d'Étoiles" included in the Connaissance des Temps for 1774...
M31The Andromeda Galaxy is a spiral galaxy approximately away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way...
). Searching the photographic record, he found 11 more
novaA nova is a cataclysmic nuclear explosion caused by the accretion of hydrogen onto the surface of a white dwarf star. Novae are not to be confused with supernovae or Luminous Red Novae.-Occurrence rate, and astrophysical significance:...
e. Curtis noticed that these novae were, on average, 10
magnitudesIn astronomy, magnitude refers to the logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths....
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 DebateThis is about the famous discussion of astronomy. For the Dream Theater song about stem cell research, see 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...
took place between
Harlow ShapleyHarlow 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 shiftThe Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842, 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 an observer...
.
The matter was conclusively settled by
Edwin HubbleEdwin Powell Hubble was an American astronomer. He profoundly changed our understanding of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the...
in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some
Cepheid variableA Cepheid is a member of a class of pulsating variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale.Typical classical Cepheids pulsate with...
s, thus allowing him to estimate the distance to the nebulae: 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 sequenceThe 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
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
The dark matter halo is the hypothetical gravitational core of a galaxy, consisting of dark matter.-Rotation curves as evidence of a dark matter halo:...
- 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...
- Oort Constants
The Oort Constants, named after Jan Oort, are empirically measurable constants relating to the sun's distance from and motion relative to the galactic centre.A = 14.82 ± 0.84 km/B = −12.37 ± 0.64 km/...
- The Great Rift
In astronomy, the Great Rift, sometimes called the Dark Rift, is a series of overlapping non-luminous molecular dust clouds 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.
Further reading
- Thorsten Dambeck in Sky and Telescope, "Gaia's Mission to the Milky Way", March 2008, p. 36–39.
External links
- The Milky Way Galaxy from An Atlas of the Universe
- A 3D map of the Milky Way Galaxy
- Milky Way – IRAS (infrared) survey wikisky.org
- Milky Way – H-Alpha survey wikisky.org
- Running Rings Around the Galaxy Spitzer Space Telescope News
- The Milky Way Galaxy, SEDS Messier pages
- MultiWavelength Milky Way, NASA site with images and VRML
VRML is a standard file format for representing 3-dimensional interactive vector graphics, designed particularly with the World Wide Web in mind...
models
- Milky Way Explorer, detailed images in infrared with radio, microwave and hydrogen-alpha as well
- Face-on Milky Way maps, within about 10 thousand parsecs
- The Milky Way at the Astro-Photography Site Of Mister T. Yoshida.
- Widefield Image of the Summer Milky Way
- Proposed Ring around the Milky Way
- Milky Way spiral gets an extra arm, New Scientist.com
- Possible New Milky Way Spiral Arm, Sky and Telescope.com
- The Milky Way spiral arms and a possible climate connection
- Galactic center mosaic via sun-orbiting Spitzer infrared telescope
- Milky Way Plan Views, The University of Calgary Radio Astronomy Laboratory
- Our Growing, Breathing Galaxy, Scientific American Magazine (January 2004 Issue)
- Deriving The Shape Of The Galactic Stellar Disc, SkyNightly (March 17, 2006)
- Digital Sky LLC, Digital Sky's Milky Way Panorama and other images
- A new view of the Milky Way galaxy obtained by the Diffuse Infrared Background Experiment (DIRBE) on NASA's Cosmic Background Explorer satellite (COBE).
- Image of Milky Way galaxy arms, Chandra X-ray Observatory Center
- The 1920 Shapley – Curtis Debate on the size of the Milky Way
- Milky Way Voyage – India's First & Largest Star Party
- Astronomy Picture of the Day:
- Moving Milkyway seen from Teneriffe without any lightpollution
- Multi-Gigapixel Infrared Milky Way A zoomable, annotated version of the Spitzer Space Telescope GLIMPSE survey.
- Animated tour of the Milky Way, University of Glamorgan