Andromeda

The **Andromeda Galaxy** (), also known as **Messier 31**, **M31**, or **NGC 224**, is a approximately 780 (2.5 million ) from . It is the nearest major to the and was often referred to as the **Great Andromeda Nebula** in older texts. It received its name from the area of the sky in which it appears, the , which was named after the mythological princess .

Andromeda is approximately 220,000 light years across, and it is the largest galaxy of the , which also contains the Milky Way, the , and other smaller galaxies. Despite earlier findings that suggested that the Milky Way contains more and could be the largest in the grouping, the 2006 observations by the revealed that Andromeda contains stars: at least twice the number of stars in the Milky Way, which is estimated to be 200–400 billion. The mass of the Andromeda Galaxy is estimated to be 1.5×10 , while the Milky Way is estimated to be 8.5×10 solar masses.

The Milky Way and Andromeda galaxies are in 3.75 billion years, eventually merging to form a giant or perhaps a large . The of the Andromeda Galaxy, at 3.4, is among the brightest of the , making it visible to the on moonless nights, even when viewed from areas with moderate .

## Observation history

In the year 964, the astronomer described the Andromeda Galaxy, in his as a "nebulous smear". of that period labeled it as the *Little Cloud*. In 1612, the German astronomer gave an early description of the Andromeda Galaxy based on telescopic observations. In 1764, catalogued Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", and based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of . In 1850, , saw and made the first drawing of Andromeda's .

In 1864, noted that the of Andromeda differs from a gaseous nebula. The spectra of Andromeda displays a of , superimposed with dark that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this it was deduced that Andromeda has a stellar nature. In 1885, a (known as ) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a , and was named accordingly; "Nova 1885".

In 1887, took the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts actually mistook Andromeda and similar spiral nebulae as solar systems being formed. In 1912, used to measure the radial velocity of Andromeda with respect to our —the largest velocity yet measured, at 300 kilometres per second (190 mi/s).

### Island universe

Location of the Andromeda Galaxy (M31) in the Andromeda constellation

In 1917, observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 light-years (3.2×10 AU). He became a proponent of the so-called "island universes" hypothesis, which held that were actually independent galaxies.

Andromeda above the .

In 1920, the between and Curtis took place, concerning the nature of the , spiral nebulae, and the dimensions of the . To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda's significant . In 1922 presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs (1,500,000 ly). settled the debate in 1925 when he identified extra-galactic for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre (100-in) , and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature is not a cluster of stars and gas within our own Galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.

In 1943, was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their , 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 .) Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the Universe.

In 1950, radio emission from the Andromeda Galaxy was detected by and at . The first of the galaxy were made in the 1950s by and collaborators at the . The core of the Andromeda Galaxy is called 2C 56 in the radio astronomy catalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This was detected using a technique called , which is caused by the deflection of light by a massive object.

## General

The estimated distance of the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of . In the 1990s, measurements of both standard as well as stars from the satellite measurements were used to calibrate the Cepheid distances.

### Formation and history

The Andromeda Galaxy as seen by 's

Andromeda was formed roughly 10 billion years ago from the collision and subsequent merger of smaller . This violent collision formed most of Andromeda's (metal-rich) and extended disk. During this epoch, would have been , to the point of becoming a for roughly 100 million years. Andromeda and the had a very close passage 2–4 billion years ago. This event produced high levels of star formation across the Andromeda Galaxy's disk – even some globular clusters – and disturbed M33's outer disk.

Over the past 2 billion years, star formation throughout Andromeda's disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by Andromeda. These interactions have formed structures like . A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.

### Recent distance estimate

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

In 2003, using the infrared (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex in (O/H), an estimate of 2.57 ± 0.06 million (1.625×10 ± 3.8×10 ) was derived.

In 2004, using the method, the distance was estimated to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).

In 2005, an was discovered in the Andromeda Galaxy. The binary is two hot blue stars of O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their . When the and absolute magnitudes are known, the distance to the star can be measured. The stars lie at a distance of 2.52×10~~^~~ ± 0.14×10~~^~~ ly (1.594×10 ± 8.9×10 AU) and the whole Andromeda Galaxy at about 2.5×10~~^~~ ly (1.6×10 AU). This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

In 2005, using the (TRGB) method, the distance was estimated to be 2.56×10~~^~~ ± 0.08×10~~^~~ ly (1.619×10 ± 5.1×10 AU).

Averaged together, these distance estimates give a value of 2.54×10~~^~~ ± 0.11×10~~^~~ ly (1.606×10 ± 7.0×10 AU). And, from this, the diameter of Andromeda at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc). Applying (), this is equivalent to an apparent 4.96 angle in the sky.

### Mass and luminosity estimates

#### Mass

The Andromeda Galaxy pictured in light by

Illustration showing both the size of each galaxy and the distance between the two galaxies, to scale.

Giant halo around Andromeda Galaxy.

Mass estimates for the Andromeda Galaxy's halo (including ) give a value of approximately 1.5×10  (or 1.5 ) compared to 8×10 *M* for the Milky Way. This contradicts earlier measurements, that seem to indicate that Andromeda and the Milky Way are almost equal in mass. Even so, Andromeda's actually has a higher stellar density than that of the Milky Way and its galactic stellar disk is about twice the size of that of the Milky Way. The total *stellar* mass of Andromeda is estimated to be between 1.1×10 *M*., (i.e., around twice as massive as that of the Milky Way) and 1.5×10 *M*, with around 30% of that mass in the central , 56% in the , and the remaining 14% in the .

In addition to it, Andromeda's contains at least around 7.2×10 *M* in the form of , at least 3.4×10 *M* as (within its innermost 10 kiloparsecs), and 5.4×10 *M* of .

Andromeda is surrounded by a large and massive halo of hot gas that is estimated to contain half the mass of the stars in Andromeda. The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda galaxy. The halo is enriched in elements heavier than hydrogen and helium, formed from and its properties are the expected on a galaxy that lies in the *green valley* of the (see ). The supernovae erupt in Andromeda's star-filled disk and eject these heavier elements into space. Over Andromeda's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.

#### Luminosity

Andromeda appears to have significantly more common stars than the Milky Way, seeming to predominate the old stars with ages >7×10 years. The estimated of Andromeda, ~2.6×10 , is about 25% higher than that of our own galaxy. However, the galaxy has a high as seen from Earth and its absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (including to propose it is the second brightest galaxy within a radius of 10 of the Milky Way, after the , with an absolute magnitude of around -22.21 or close)

An estimation done with the help of published in 2010 suggests an (in the blue) of −20.89 (that with a of +0.63 translates to an absolute visual magnitude of −21.52, compared to −20.9 for the Milky Way), and a total luminosity in that of 3.64×10 *L*.

The rate of star formation in the Milky Way is much higher, with Andromeda producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of in the Milky Way is also double that of Andromeda. This suggests that Andromeda 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 Andromeda.

According to recent studies, like the Milky Way, the Andromeda Galaxy lies in what in the is known as the *green valley*, a region populated by galaxies in transition from the *blue cloud* (galaxies actively forming new stars) to the *red sequence* (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between Andromeda and the Milky Way.

## Structure

The Andromeda Galaxy seen in by the , one of 's four

Image of the Andromeda Galaxy taken by in infrared, 24 (Credit:/–/K. Gordon, )

Play media

A Tour of Andromeda Galaxy

A image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s . Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.

Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the of spiral galaxies. However, data from the survey showed that Andromeda is actually a , like the Milky Way, with the Andromeda's bar oriented along its long axis.

In 2005, astronomers used the 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 220,000 light-years (67,000 pc) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21,000 to 37,000 pc) 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 Andromeda. The galaxy could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the as a function of radial distance from the core. The rotational velocity has a maximum value of 225 kilometres per second (140 mi/s) at 1,300 (82,000,000 ) from the core, and it has its minimum possibly as low as 50 kilometres per second (31 mi/s) at 7,000 (440,000,000 ) from the core. Further out, rotational velocity rises out to a radius of 33,000 (2.1×10 ), where it reaches a peak of 250 kilometres per second (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000 (5.1×10 ). These velocity measurements imply a concentrated mass of about 6×10  in the . The total mass of the galaxy increases out to 45,000 (2.8×10 ), then more slowly beyond that radius.

The of Andromeda are outlined by a series of , first studied in great detail by and described by him as resembling "beads on a string". his studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy. His descriptions of the spiral structure, as each arm crosses the major axis of Andromeda, are as follows:

Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 (820,000,000 ) and that can be followed outward from a distance of roughly 1,600 (100,000,000 ) from the core. Alternative spiral structures have been proposed such as a single spiral arm or a pattern of long, filamentary, and thick spiral arms.

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites and . This can be seen by the displacement of the from the stars.

In 1998, images from the 's demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a . The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 (2.0×10 ) (10 kiloparsecs) from the core, nicknamed by some astronomers the *ring of fire*. This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in Andromeda is concentrated there.

Later studies with the help of the showed how Andromeda's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above. Those arms, however, are not continuous and have a segmented structure.

Close examination of the inner region of Andromeda with the same telescope also 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 the galaxy in 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. It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.

Studies of the extended halo of Andromeda show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "", 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 100–200 low-mass galaxies during the past 12 billion years. The stars in the extended halos of Andromeda and the Milky Way may extend nearly one-third the distance separating the two galaxies.

## Nucleus

HST image of the Andromeda Galaxy core showing possible double structure. */ photo*

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. In 1991, the was used to image Andromeda's inner nucleus. The nucleus consists of two concentrations separated by 1.5 (4.9 ). 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 measured at 3–5 × 10 in 1993, and at 1.1–2.3 × 10 *M* in 2005. The of material around it is measured to be ≈ 160 km/s.

image of the center of Andromeda. A number of X-ray sources, likely X-ray binary stars, within Andromeda's central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an around the central black hole. The eccentricity is such that stars linger at the orbital , creating a concentration of stars. P2 also contains a compact disk of hot, 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 is the remnant of a small galaxy "cannibalized" by Andromeda, this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to 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

Andromeda Galaxy - in high-energy X-ray and ultraviolet light (released 5 January 2016).

Artist's concept of the Andromeda Galaxy core showing a view across a disk of young, blue stars encircling a supermassive black hole. */ photo*

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 Andromeda.

Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the 's (ESA) orbiting observatory. *et al.* hypothesized that these are candidate black holes or , 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 associated with the Andromeda Galaxy. The most massive of these clusters, identified as , nicknamed Globular One, has a greater luminosity than any other known globular cluster in the of galaxies. It contains several million stars, and is about twice as luminous as , the brightest known globular cluster in the . 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 that was consumed by Andromeda in the distant past. The globular with the greatest apparent brightness is which is located in the south-west arm's eastern half. Another massive globular cluster -named *037-B327*-, discovered in 2006 as is heavily reddened by the Andromeda Galaxy's , was thought to be more massive than G1 and the largest cluster of the Local Group; however, other studies have shown it is actually similar in properties to G1.

Star cluster in the Andromeda galaxy.

Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years

In 2005, astronomers discovered a completely new type of star cluster in Andromeda. 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.

In 2012, a , a radio burst emanating from a smaller black hole, was detected in the Andromeda Galaxy. The progenitor black hole is located near the galactic center and has about 10 . Discovered through a data collected by the 's probe, and subsequently observed by 's and , the , and the , the microquasar was the first observed within the Andromeda Galaxy and the first outside of the Milky Way Galaxy.

## Satellites

Like the Milky Way, the Andromeda Galaxy has , consisting of 14 known . The best known and most readily observed satellite galaxies are and . Based on current evidence, it appears that M32 underwent a close encounter with 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 in the core region, which lasted until the relatively recent past.

M110 also appears to be interacting with Andromeda, and astronomers have found in the halo of Andromeda a stream of metal-rich stars that appear 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 the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.

## Collision with the Milky Way

The Andromeda Galaxy is approaching the at about 110 kilometres per second (68 mi/s). It has been measured approaching relative to our Sun at around 300 kilometres per second (190 mi/s) as the Sun orbits around the center of our galaxy at a speed of approximately 225 kilometres per second (140 mi/s). This makes Andromeda one of about 100 galaxies that we observe. Andromeda's tangential or side-ways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to directly collide with the Milky Way in about 4 billion years. A likely outcome of the collision is that the to form a giant or perhaps even a large . Such events are frequent among the galaxies in . The fate of the and the in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.

## Amateur observing

The Andromeda Galaxy is bright enough to be seen with the , even with some light pollution. Andromeda is best seen during nights in the , when from mid-latitudes the galaxy reaches and can be seen almost all night. From the , it is a object and does not reach a high altitude over the northern horizon, thus making it difficult to observe. can reveal some larger structures and its two brightest satellite galaxies, M32 and M110. An can reveal Andromeda's disk, dark dust lanes, the large star cloud , and even some of its brightest globular clusters.