ANDROMEDA GALAXY

The Andromeda Galaxy /ænˈdrɒmɨdə/ is a spiral galaxy approximately 2.5 million light-years (2.4×10¹⁹ km) from Earth^[4] in the Andromeda constellation. Also known as Messier 31, M31, or NGC 224, it is often referred to as the Great Andromeda Nebula in older texts. The Andromeda Galaxy is the nearest spiral galaxy to our Milky Way galaxy, but not the closest galaxy overall. It gets its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. The Andromeda Galaxy is the largestgalaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, the Andromeda Galaxy may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and could be the most massive in the grouping. The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (10¹²) stars: at least twice the number of stars in the Milky Way galaxy, which is estimated to be 200–400 billion.

The Andromeda Galaxy is estimated to be 7.1×10¹¹ solar masses. In comparison a 2009 study estimated that the Milky Way and M31 are about equal in mass, while a 2006 study put the mass of the Milky Way at ~80% of the mass of the Andromeda Galaxy. The two galaxies areexpected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy.

At an apparent magnitude of 3.4, the Andromeda Galaxy is one of the brightest Messier objects, making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope.

Observation history

The Persian astronomer Abd al-Rahman al-Sufi wrote a tantalizing line about the chained constellation in his Book of Fixed Stars around 964, describing it as a "small cloud".^[16][17] Star charts of that period have it labeled as the Little Cloud.^[17] The first description of the object based on telescopic observation was given by German astronomer Simon Marius on December 15, 1612.^[18] Charles Messier 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 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 color and magnitude of the nebula, he incorrectly guessed that it was no more than 2,000 times the distance of Sirius.

William Huggins in 1864 observed the spectrum of M31 and noted that it differed from a gaseous nebula. The spectra of M31 displayed a continuumof frequencies, 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 (known as S Andromedae) 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, and was named accordingly "Nova 1885".

The first photographs of M31 were taken in 1887 by Isaac Roberts 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 of this object with respect to our solar system was measured in 1912 by Vesto Slipher at the Lowell Observatory, using spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (190 mi/s), moving in the direction of the Sun.

Island universe

In 1917, American astronomer Heber Curtis observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes 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 (3.2×10¹⁰ AU). He became a proponent of the so-called "island universes" hypothesis, which held thatspiral nebulae were actually independent galaxies.

In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. 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 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 450,000parsec, which is about 1,500,000 ly. Edwin Hubble 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.

M31 plays an important role in galactic studies, since it is the nearest spiral galaxy (although not the nearest galaxy). In 1943 Walter Baade 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, 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.) 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 and Cyril Hazard at Jodrell Bank Observatory using the 218-ft Transit Telescope, and was announced in 1950 (Earlier observations were made by radio astronomy pioneer Grote Reber in 1940, but were inconclusive, and were later shown to be an order of magnitude too high). The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group. 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.

Mass

Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately 1.23×10¹² M_☉ (or 1.2 trillion solar masses) compared to 1.9×10¹² 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'sspheroid actually has a higher stellar density than that of the Milky Way.

Luminosity

M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31, ~2.6×10¹⁰ L_☉, is about 25% higher than that of our own galaxy. However, the galaxy has a high inclination as seen from Earth and its interstellar dust 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's the second brightest galaxy within a radius of 10 megaparsecs of the Milky Way, after the Sombrero Galaxy) , the most recent estimation (done in 2010 with the help of Spitzer Space Telescope) suggesting an absolute magnitude (in the blue) of −20.89 (that with a color index of +0.63 translates to an absolute visual magnitude of −21.52^[b], compared to −20.9 for the Milky Way), and a total luminosity in that wavelength of 3.64×10¹⁰L_☉

The rate of star formation in the Milky Way is much higher, with M31 producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of supernovae 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.

According to recent studies, like the Milky Way, the Andromeda Galaxy lies in what in the galaxy color-magnitude diagram 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 both Andromeda and the Milky Way.