Andromeda Galaxy

Andromeda Galaxy

Overview
The Andromeda Galaxy is a 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...

 approximately 2.5 million light-years from 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...

 in the constellation Andromeda. It is also known as Messier
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...

 31
, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

in older texts. Andromeda is the nearest spiral galaxy to the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, but not the closest 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....

 overall. It gets its name from the area of the sky in which it appears, the Andromeda constellation, which was named after the mythological princess Andromeda
Andromeda (mythology)
Andromeda is a princess from Greek mythology who, as divine punishment for her mother's bragging, the Boast of Cassiopeia, was chained to a rock as a sacrifice to a sea monster. She was saved from death by Perseus, her future husband. Her name is the Latinized form of the Greek Ἀνδρομέδη...

.
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Unanswered Questions
Encyclopedia
The Andromeda Galaxy is a 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...

 approximately 2.5 million light-years from 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...

 in the constellation Andromeda. It is also known as Messier
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...

 31
, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

in older texts. Andromeda is the nearest spiral galaxy to the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, but not the closest 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....

 overall. It gets its name from the area of the sky in which it appears, the Andromeda constellation, which was named after the mythological princess Andromeda
Andromeda (mythology)
Andromeda is a princess from Greek mythology who, as divine punishment for her mother's bragging, the Boast of Cassiopeia, was chained to a rock as a sacrifice to a sea monster. She was saved from death by Perseus, her future husband. Her name is the Latinized form of the Greek Ἀνδρομέδη...

. Andromeda is the largest 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...

 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...

, which consists of the Andromeda Galaxy, the Milky Way, the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...

, and about 30 other smaller galaxies. Although the largest, Andromeda may not be the most massive, as recent findings suggest that the Milky Way contains more 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...

 and may be the most massive in the grouping. The 2006 observations 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...

 revealed that M31 contains one trillion (1012) stars,: at least twice more than the number of stars in our own galaxy, which is estimated to be c. 200–400 billion.

Andromeda is estimated to be 7.1 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. In comparison a 2009 study estimated that the Milky Way and Andromeda are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of Andromeda. The Andromeda Galaxy and the Milky Way are expected to collide in perhaps 4.5 billion years.

At 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 3.4, the Andromeda Galaxy is notable for being one of the brightest 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...

s, making it visible to the naked eye on moonless nights even when viewed from areas with moderate 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:...

. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

, only the brighter central region is visible to the naked eye or when viewed using binoculars
Binoculars
Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

 or a small telescope.

Observation history


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 Abd al-Rahman al-Sufi wrote a tantalizing line about the chained constellation in his Book of Fixed Stars
Book of Fixed Stars
The Book of Fixed Stars is an astronomical text written by Abd al-Rahman al-Sufi around 964. The book was written in Arabic, although the author himself was Persian...

around 964, describing it as a "small cloud". Star chart
Star chart
A star chart is a map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate astronomical objects such as stars, constellations and galaxies. They have been used for human navigation since time immemorial...

s of that period have it labeled as the Little Cloud. The first description of the object based on telescopic observation was given by German astronomer Simon Marius
Simon Marius
Simon Marius was a German astronomer. He was born in Gunzenhausen, near Nuremberg, but he spent most of his life in the city of Ansbach....

 in 1612. Charles Messier
Charles Messier
Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 "Messier objects"...

 catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. In 1785, the astronomer 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...

 noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and based on the colour and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...

.

William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...

 in 1864 observed the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

 of M31 and noted that it differed from a gaseous nebula. The spectra of M31 displayed a continuum of frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

, superimposed with dark absorption lines that help identify the chemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, and from this it was deduced that M31 had a stellar nature. In 1885, a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 (known as "S Andromedae
S Andromedae
|- style="background-color: #A0B0FF;" colspan="3"| Database References|- bgcolor="#FFFAFA"| Simbad || |- bgcolor="#FFFAFA"| ||...

") was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a 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...

, and was named accordingly "Nova 1885".

The first photographs of M31 were taken in 1887 by Isaac Roberts
Isaac Roberts
Isaac Roberts was a Welsh engineer and business man best known for his work as an amateur astronomer, pioneering the field of astrophotography of nebulae. He was a member of the Liverpool Astronomical Society in England and was a fellow of the Royal Geological Society...

 from his private observatory in Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time. However, at the time this object was still commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. The radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

 of this object with respect to our 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...

 was measured in 1912 by Vesto Slipher
Vesto Slipher
Vesto Melvin Slipher was an American astronomer. His brother Earl C. Slipher was also an astronomer and a director at the Lowell Observatory....

 at the Lowell Observatory
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....

, using spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

. The result was the largest velocity recorded at that time, at 300 km/s, moving in the direction of the Sun.

Island universe


In 1917, American astronomer Heber Curtis Olson
Heber Doust Curtis
Heber Doust Curtis was an American astronomer.He studied at the University of Michigan and at the University of Virginia, where he got a degree in astronomy....

 observed a nova within M31. Searching the photographic record, 11 more novae were discovered. 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 elsewhere in the sky. As a result he was able to come up with a distance estimate of 500000 ly. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae
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...

 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...

 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 Curtis took place, concerning the nature of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, spiral nebulae, and the dimensions of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

. To support his claim that the Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift. In 1922 Ernst Öpik
Ernst Öpik
Ernst Julius Öpik was a noted Estonian astronomer and astrophysicist who spent the second half of his career at the Armagh Observatory in Northern Ireland.-Education:...

 presented a very elegant and simple astrophysical method to estimate the distance of M31. His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

, which is about 1,500,000 ly. 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...

 settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5 metre (100 in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own.
Andromeda plays an important role in galactic studies, since it is the nearest spiral galaxy (although not the nearest galaxy). In 1943, Walter Baade
Walter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-Biography:He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the...

 was the first person to resolve stars in the central region of the Andromeda Galaxy. Based on his observations of this galaxy, he was able to discern two distinct populations of stars based on their metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

, naming the young, high velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort
Jan Oort
Jan Hendrik Oort was a Dutch astronomer. He was a pioneer in the field of radio astronomy. The Oort cloud of comets bears his name....

.) Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe.

Radio emission from the Andromeda Galaxy was first detected by Hanbury Brown
Robert Hanbury Brown
Robert Hanbury Brown, AC FRS was a British astronomer and physicist born in Aruvankadu, India. He made notable contributions to the development of radar and he later conducted pioneering work in the field of radio astronomy...

 and Cyril Hazard at Jodrell Bank Observatory using the 218-ft Transit Telescope, which were announced in 1950 (earlier observations were made by radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

 pioneer Grote Reber
Grote Reber
Grote Reber was an amateur astronomer and pioneer of radio astronomy. He was instrumental in investigating and extending Karl Jansky's pioneering work, and conducted the first sky survey in the radio frequencies...

 in 1940, but were inconclusive, and were later shown to be an order of magnitude too high). The first radio maps
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

 of the galaxy were made in the 1950s by John Baldwin
John E. Baldwin
John Evan Baldwin FRS has worked at the Cavendish Astrophysics Group since 1954. He played a pivotal role in the development of interferometry in Radio Astronomy, and later astronomical optical interferometry and lucky imaging...

 and collaborators at the Cambridge Radio Astronomy Group
Cavendish Astrophysics Group
The Cavendish Astrophysics Group is based at the Cavendish Laboratory at the University of Cambridge. The group operates all of the telescopes at the Mullard Radio Astronomy Observatory except for the 32m MERLIN telescope, which is operated by Jodrell Bank.The group is the second largest of three...

. The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This candidate was detected using a technique called microlensing, which is caused by the deflection of light by a massive object.

General


The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, measurements of both standard red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass 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...

s as well as 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 from the Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

satellite measurements were used to calibrate the Cepheid distances.

Origins


According to a team of astronomers reporting in 2010, Andromeda was formed out of the collision of two smaller galaxies between 5 and 9 billion years ago.

Recent distance estimate


At least four distinct techniques have been used to measure distances to the Andromeda Galaxy.

In 2003, using the infrared surface brightness fluctuation
Surface brightness fluctuation
Surface brightness fluctuation is a secondary distance indicator used to estimate distances to galaxies. The technique uses the fact that galaxies are made up of a finite number of stars. The number of stars in any small patch of the galaxy will vary from point to point, creating a noise-like...

s (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 Mly was derived.
Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.

In 2005, a group of astronomers consisting of Ignasi Ribas (CSIC, Institute for Space Studies of Catalonia
Catalonia
Catalonia is an autonomous community in northeastern Spain, with the official status of a "nationality" of Spain. Catalonia comprises four provinces: Barcelona, Girona, Lleida, and Tarragona. Its capital and largest city is Barcelona. Catalonia covers an area of 32,114 km² and has an...

 (IEEC)) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236, has two luminous and hot blue stars of types
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the 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...

 of the stars. When the visual
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...

 and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 Mly and the whole Andromeda Galaxy at about 2.5 Mly. This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Andromeda is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 Mly.

Averaged together, all these distance measurements give a combined distance estimate of 2.54 Mly. Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 kly. Applying trigonometry
Trigonometry
Trigonometry is a branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides. Trigonometry defines the trigonometric functions, which describe those relationships and have applicability to cyclical phenomena, such as waves...

 (arctangent), that figures to extending at an apparent 3.18°
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

 angle in the sky.

Mass and luminosity estimates


Mass estimates for the Andromeda halo (including 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...

) give a value of approximately 1.23 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

 (or 1.2 trillion 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) compared to 1.9 M for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. Even so, the masses of the Milky Way and M31 are comparable, and M31's spheroid actually has a higher stellar density than that of the Milky Way.

In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...

 of M31, ~2.6 L, is about 25% higher than that of our own galaxy. However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e in the Milky Way is also double that of M31. This suggests that M31 once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity in the Milky Way may eventually overtake that of M31.


Image:Local_Group.JPG|frame|center|Local group (clickable map)

circle 167 27 20 Sextans B
Sextans B
Sextans B is an irregular galaxy that may be part of the Local Group, or lie just beyond it. Sextans B is 4.44 million light-years away from Earth and thus is one of most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A...


circle 120 36 23 Sextans A
Sextans A
Sextans A , is a tiny dwarf irregular galaxy. It spans about 5000 light-years across, and is located within the Local Group of galaxies, which includes our Milky Way galaxy. At 4.3 million light-years away from Earth, Sextans A is one of most distant members of the Local Group, and is notable for...


circle 318 239 20 Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...


circle 289 197 16 Leo I (dwarf galaxy)
Leo I (dwarf galaxy)
Leo I is a dwarf spheroidal galaxy in the constellation Leo. At about 820,000 light-years distant, it is a member of the Local Group of galaxies and is thought to be one of the most distant satellites of the Milky Way galaxy...


circle 334 201 15 Canes Dwarf
rect 303 185 318 215 Leo II (dwarf galaxy)
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....


circle 357 289 28 NGC 6822
NGC 6822
NGC 6822 is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1881 , with a six-inch refractor telescope. It is one of the closer galaxies to the Milky Way...


circle 288 323 24 Phoenix Dwarf
Phoenix Dwarf
The Phoenix Dwarf is a dwarf galaxy and an irregular galaxy that was discovered in 1976 by Hans-Emil Schuster and Richard Martin West and mistaken for a globular cluster. It is currently 1.44 Mly away from Earth...


circle 248 391 35 Tucana Dwarf
circle 363 416 20 Wolf-Lundmark-Melotte
Wolf-Lundmark-Melotte
The Wolf-Lundmark-Melotte galaxy is an irregular galaxy discovered in 1909 by Max Wolf, and is located on the outer edges of the local group. The discovery of the nature of the galaxy was accredited to Knut Lundmark and Philibert Jacques Melotte in 1926. It is in the constellation Cetus.-Star...


circle 363 383 17 Cetus Dwarf
Cetus Dwarf
Cetus Dwarf is a dwarf spheroidal galaxy. It lies approximately 2.46 Million light-years from Earth. It is an isolated galaxy of the Local Group, which also contains the Milky Way...


circle 369 346 11 IC 1613
IC 1613
IC 1613 is an irregular dwarf galaxy in the constellation Cetus near the star 26 Ceti. It was discovered in 1906 by Max Wolf, and is approaching Earth at 234 km/s....


rect 381 335 393 357 SagDIG
rect 393 335 406 356 Aquarius Dwarf
Aquarius Dwarf
The Aquarius Dwarf galaxy is a dwarf galaxy and an irregular galaxy, that was first catalogued in 1959 by the DDO survey. Its most distinctive characteristic is that it is one of the few galaxies known to display a blueshift, as it is traveling towards the Milky Way at 137 km/s. Aquarius...


circle 417 304 17 Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...


circle 417 254 15 NGC 185
NGC 185
NGC 185 is a dwarf spheroidal galaxy about 2.08 million light-years away in the constellation Cassiopeia. It is a member of the Local group, and is a satellite galaxy of the Andromeda Galaxy . NGC 185 was discovered by William Herschel on November 30, 1787, and he cataloged it "H II.707"...


rect 432 237 447 260 NGC 147
NGC 147
NGC 147 is a dwarf spheroidal galaxy about 2.58 Mly away in the constellation Cassiopeia. NGC 147 is a member of the Local group of galaxies and a satellite galaxy of the Andromeda Galaxy . It forms a physical pair with the nearby galaxy NGC 185,another remote satellite of M31. It was...


circle 461 229 17 IC 10
IC 10
IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by Lewis Swift in 1887. Nicholas U. Mayall was the first to suggest that the object is extragalactic in 1935. Edwin Hubble suspected it might belong to the Local Group of galaxies, but its status remained uncertain for...


poly 440 282 455 260 511 259 493 285 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...


poly 450 264 434 265 431 280 442 280 Messier 110
Messier 110
Messier 110 is a dwarf spheroidal galaxy that is a satellite of the Andromeda Galaxy. M110 contains some dust and hints of recent star formation, which is unusual for dwarf elliptical galaxies in general.-History:...


circle 295 110 20 Leo A
Leo A
Leo A is an irregular galaxy that is part of the Local Group. It lies 2.25 million light-years from Earth. This galaxy was discovered by Fritz Zwicky in 1942. The estimated mass of this galaxy is solar masses, with at least 80% consisting of an unknown dark matter...


circle 84 128 20 NGC 3109
NGC 3109
NGC 3109 is a small spiral or irregular galaxy around 4.2 Mly away in the direction of the constellation of Hydra. It is the most prominent member of a Local Group subgroup. NGC 3109 is believed to be tidally interacting with the dwarf elliptical galaxy, Antlia Dwarf...


circle 109 149 14 Antlia Dwarf
Antlia Dwarf
The Antlia Dwarf is a dwarf spheroidal/irregular galaxy. It lies about 1.3 Mpc from Earth in the constellation Antlia. It is the fourth and faintest member of the nearby Antlia Group of galaxies. The galaxy contains stars of all ages, contains significant amounts of gas, and has experienced...


circle 412 332 12 LGS 3
circle 460 361 21 Pegasus Dwarf
Pegasus Dwarf Spheroidal Galaxy
The Pegasus Dwarf Spheroidal is a dwarf spheroidal galaxy about 2.7 million light-years away in the constellation Pegasus...


circle 394 272 14 Andromeda II
Andromeda II
Andromeda II is a dwarf spheroidal galaxy about 2.22 Mly away in the constellation Andromeda. It is part of the Local group of galaxies and is a satellite galaxy of the Andromeda Galaxy but it's also situated closely to the Triangulum Galaxy , it is not quite clear if it is a satellite of the...


rect 427 279 438 294 Andromeda III
Andromeda III
Andromeda III is a dwarf spheroidal galaxy about 2.44 million light-years away in the constellation Andromeda. And III is part of the Local group of galaxies and is a satellite galaxy of the Andromeda Galaxy . And III was discovered by Sidney van den Bergh on photographic plates taken in 1970 and...


rect 438 282 450 294 Andromeda I
Andromeda I
Andromeda I is a dwarf spheroidal galaxy about 2.40 million light-years away in the constellation Andromeda. Andromeda I is part of the Local group of galaxies and a satellite galaxy of the Andromeda Galaxy . It is roughly 3.5 degrees south and slightly east of M31...



desc bottom-left

Structure




Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs-Sandage extended classification system of spiral galaxies. However, data from the 2MASS
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...

 survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually 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...

 like the Milky Way, with the Andromeda Galaxy's bar viewed almost directly along its long axis.

In 2005, astronomers used the Keck telescopes
Keck telescopes
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of near the summit of Mauna Kea in Hawai'i. The primary mirrors of each of the two telescopes are in diameter, making them the second largest optical telescopes in the world, slightly behind the Gran Telescopio...

 to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220000 ly in diameter. Previously, estimates of Andromeda's size ranged from 70000 ly across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy M33
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...

 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of M31
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....

 at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 km/s at a radius of 1300 ly, then descends to a minimum at 7000 ly where the rotation velocity may be as low as 50 km/s. Thereafter the velocity steadily climbs again out to a radius of 33000 ly, where it reaches a peak of 250 km/s. The velocities slowly decline beyond that distance, dropping to around 200 km/s at 80000 ly. These velocity measurements imply a concentrated mass of about 6 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

 in the nucleus. The total mass of the galaxy increases linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...

ly out to 45000 ly, then more slowly beyond that radius.

The spiral arms of Andromeda are outlined by a series of H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...

s that Baade described as resembling "beads on a string". They appear to be tightly wound, although they are more widely spaced than in our galaxy. Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13000 ly. These can be followed outward from a distance of roughly 1600 ly from the core. The most likely cause of the spiral pattern is thought to be interaction with M32
Messier 32
Messier 32 is a dwarf elliptical galaxy about 2.65 million light-years away in the constellation Andromeda. M32 is a satellite galaxy of the famous Andromeda Galaxy and was discovered by Le Gentil in 1749. M32 measures only 6.5 ± 0.2 kly in diameter at the widest point...

. This can be seen by the displacement of the neutral hydrogen clouds
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...

 from the stars.

In 1998, images from the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

's Infrared Space Observatory
Infrared Space Observatory
The Infrared Space Observatory was a space telescope for infrared light designed and operated by the European Space Agency , in cooperation with ISAS and NASA...

 demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy
Ring galaxy
A ring galaxy is a galaxy with a ring-like appearance. The ring consists of massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through...

. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32000 ly from the core. This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.

Close examination of the inner region of Andromeda showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of Andromeda along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.

Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...

", and increasingly so with greater distance. This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years. The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.

Nucleus


M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.
In 1991 Tod R. Lauer
Tod R. Lauer
Tod R. Lauer is an astronomer on the research staff of the National Optical Astronomy Observatory. He was a member of the Hubble Space Telescope Wide Field and Planetary Camera team, and is a founding member of the Nuker Team...

 used WFPC
Wide Field and Planetary Camera
The Wide Field/Planetary Camera was a camera installed on the Hubble Space Telescope until December 1993. It was one of the instruments on Hubble at launch, but its functionality was severely impaired by the defects of the main mirror optics which afflicted the telescope...

, then on board the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

, to image Andromeda's inner nucleus. The nucleus consists of two concentrations separated by 1.5 pc. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

 measured at 3-5 × 107 M
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

 in 1993, and at 1.1–2.3 × 108 M in 2005.

Scott Tremaine
Scott Tremaine
Scott Duncan Tremaine is a Canadian-born astrophysicist. He is a fellow of the Royal Society of London, the Royal Society of Canada and the National Academy of Sciences. Tremaine is widely regarded as one of the world's leading astrophysicists for his contributions to the theory of solar system...

 has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 around the central black hole. The eccentricity is such that stars linger at the orbital apocenter
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...

, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy "cannibalized" by Andromeda, this is no longer considered to be a viable explanation. The primary reason is that such a nucleus would have an exceedingly short lifetime due to tidal disruption
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

 by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.

Discrete sources


Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy. A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from M31.

Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the ESA's
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 XMM-Newton
XMM-Newton
The XMM-Newton is an orbiting X-ray observatory launched by ESA in December 1999 on a Ariane 5 rocket...

 orbiting observatory. Robin Barnard et al. hypothesized that these are candidate black holes or neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.

There are approximately 460 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 associated with the Andromeda galaxy. The most massive of these clusters, identified as Mayall II
Mayall II
Mayall II also known as NGC-224-G1, SKHB 1, GSC 2788:2139, HBK 0-1, M31GC J003247+393440 or Andromeda's Cluster is a globular cluster orbiting M31, the Andromeda Galaxy....

, nicknamed Globular One, has a greater luminosity than any other known globular cluster in 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. It contains several million stars, and is about twice as luminous 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...

, the brightest known globular cluster in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of 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...

 that was consumed by M31 in the distant past. The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.

In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger–several hundred light-years across–and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.

Satellites


Like the Milky Way, the Andromeda Galaxy has satellite galaxies
Satellite galaxy
A satellite galaxy orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a large number of objects which are not connected to each other, it has a center of mass, which represents a weighted average of the positions of each component object...

, consisting of 14 known 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...

. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of 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...

 in the core region, which lasted until the relatively recent past.

M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies. M110 does contain a dusty lane, which may indicate recent or ongoing star formation.

In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.

Future collision of the Milky Way with Andromeda


The Andromeda Galaxy is approaching the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 at about 100 km/s (400 lightyears every million years), making it one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two. A likely outcome of the collision is that the galaxies will merge to form a giant 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...

. Such events are frequent among the galaxies in galaxy groups. The fate of 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...

 in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.

See also


  • Galaxies in fiction
    Galaxies in fiction
    Galaxies other than the Milky Way are popular settings for creators of science fiction, particularly those working with broad-scale space opera settings. Among the most common settings are the Andromeda Galaxy, the Magellanic Clouds, and the Triangulum Galaxy, all part of the Local Group close to...

  • List of Messier objects
  • List of galaxies
  • Mayall II
    Mayall II
    Mayall II also known as NGC-224-G1, SKHB 1, GSC 2788:2139, HBK 0-1, M31GC J003247+393440 or Andromeda's Cluster is a globular cluster orbiting M31, the Andromeda Galaxy....

     – the largest 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...

     in the Andromeda Galaxy and in the Local Group
  • New General Catalogue
    New General Catalogue
    The New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...

  • NGC 206
    NGC 206
    NGC 206 is a bright star cloud in the Andromeda Galaxy. It is notable for being the brightest star cloud in Andromeda as viewed from Earth.NGC 206 is the richest and most conspicuous star cloud in M31. It contains more than 300 stars brighter than Mb=-3.6....

     – the brightest star cloud
    Star cloud
    A star cloud is a group of stars that appear to be in the same position in the sky. These are not true star clusters, but rather dependent on the point of view of the observer. The stars that make up the cloud do not actually reside close to each other, they are just in the same line-of-sight.-See...

     in the Andromeda Galaxy

External links