Andromeda Galaxy

The Andromeda Galaxy, also commonly known as Messier 31 (M31), NGC 224, rarely as Flamsteed 58 or historically as the Great Andromeda Nebula, is a large barred spiral galaxy (SA(s)b). M31 is located in the constellation of Andromeda about 2,450,000 light-years (751,153 parsecs) away from the Earth, making it the nearest large galaxy to the Milky Way. It is roughly 2 times larger than the Milky Way, about 220,150 ly (67,500 pc) in diameter. However, it has approximately 10 times more stars than the Milky Way, about 1 trillion. Based on this number of stars, the mass of M31 is estimated to be 1.5 trillion solar masses.

The Andromeda Galaxy is one of few galaxies that can be seen with the naked eye. It is visible as as a faint "nebulous smear" or a small "cloud". It is visible from both of the hemispheres and shines brightly during certain times of the year. Even in places not far away from light pollution, it is still visible in the sky. The galaxy is in the constellation of Andromeda, hence its name. It has an apparent length of about 3.167° or about 6 times the Moon's diameter. The coordinates for this object in the night sky are 00h 42m 44.3s, +41° 16′ 9″.

The Andromeda Galaxy is predicted to collide with the Milky Way in about 4.5 billion years, smashing large gas clouds together and trigger a new era of star formation, maybe even producing a quasar as a result. The resulting galaxy will probably be an elliptical galaxy, popularly known as "Milkdromeda" or "Milkomeda".

Observation
The Andromeda Galaxy has been easily visible for nighttime observers well back into prehistory since it was easily visible and bright. The first formal mention of the galaxy was by the Persian astronomer Abdul Rahman al-Sufi (عبدالرحمن الصوفی), also known as Azophi, in his book titled "Book of Fixed Stars" (كتاب صور الكواكب) around 964. He referred to it as a "nebulous smear" or "small cloud". Star charts of that period labelled it as the "Little Cloud".

In 1612, a few years after the invention of the telescope, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. The first suggestion that the galaxy was outside the Milky Way was by Pierre Louis Maupertuis in 1745, and he called the galaxy an "island universe". In 1764, Charles Messier cataloged the galaxy as object Messier 31 (M31) and incorrectly credited Simon Marius as the discoverer.

In 1785, William Herschel observed a reddish hue in the core region of M31 and believed it is the nearest of all the "great nebulae". Based on the color and brightness of the "nebula", he incorrectly guessed that it was less than 2,000 times the distance of Sirius, or about 17,200 ly. In 1850, William Parson made the first drawing of M31's spiral structure.

In 1864, William Huggins discovered that the light spectrum of M31 are different from that of a standard gaseous nebula. M31's spectrum is very similar to that of stars and from this, he deduced that M31 has a stellar nature. In 1885, a supernova, now known as SN 1885A, was observed in M31, the first one observed in that galaxy. Since M31 was still considered to be a nearby object, the supernova was thought to be just a nova.

In 1888, Isaac Roberts took one of the first photographs of M31. He also confused M31 and other similarly-shaped "spiral nebulae" as circumstellar disks around protostars as they were being formed.

Island universe theory
In 1745, Pierre Louis Maupertuis was the first to suggest that M31 was outside the Milky Way. He called M31 an "island universe". In 1755, Immanuel Kant proposed that the Milky Way is one of many galaxies, in his book "Universal Natural History and Theory of the Heavens". He argued that a structure like the Milky Way would look like a spiral nebula viewed from above and like an elliptical if viewed from an angle. He concluded that the elliptical "nebulae" like M31 were galaxies similar to the Milky Way, a hypothesis that could not be disproven at the time.

In 1917, Heber Curtis observed a nova within the galaxy. His observation prompted him to search the photographic record and he discovered 11 more novae. Curtis noticed that these novae were 10 magnitudes fainter than those that occurred elsewhere. Based on this, he came up with a distance of 500,000 ly. He became a supporter of the "island universe theory", which held that spiral nebulae were actually galaxies outside our own.

The Great Debate
A Great Debate took place on April 26, 1920, at the Smithsonian Museum of Natural History, between the astronomers Harlow Shapley and Heber Curtis. The debate concerned the nature of the spiral nebulae and the size of the universe.

Shapley was arguing in favor of the Milky Way as the entirety of the universe. He backed up his claim with the argument that if M31 was not part of the Milky Way, then its distance must have been around 100 million light years, a distance most astronomers would not accept. His claim was backed up by astronomer Adriaan van Maanen, who claimed he had observed the Pinwheel Galaxy rotating, and that if it were in fact a distinct galaxy, its rotational velocity would be so huge and exceed the universal speed limit—the speed of light.

On the other hand, Curtis supported that M31 and other "spiral nebulae" were in fact separate galaxies or "island universes"—outside our own. He proved this by showing that there were more novae in M31 than in other sections of the galaxy. From this, he could ask why there were more novae in one small section of the galaxy than the other sections, if M31 were not a separate galaxy but a nebula. Besides that, he also cited dark lanes present in other galaxies which are similar to the dust clouds found in the Milky Way, explaining the Zone of Avoidance.

After the debate, later in the decade, Edwin Hubble measured M31's distance using Cepheid variables, and discovered that it was far outside the Milky Way, proving that Curtis was correct. Later, it was proved that van Maanen's observations were incorrect and that one cannot actually see the Pinwheel Galaxy rotate, even by a small bit, during an entire human lifespan.

Aftermath
In 1943, Walter Baade became the first person to resolve stars in the core region of M31. He also confirmed two distinct populations of stars based on their metallicity—young, high-velocity stars (Type I) and older, red stars (Type II)—which was previously noted by Jan Oort. The terms 'Type I' and 'Type II' were later used for stars in our own Milky Way. Baade also discovered two different types of Cepheid variables, which increased the distance estimate by a factor of 2.

Seven years later in 1950, Robert H. Brown and Cyril Hazard at Jodrell Bank Observatory, England, detected radio emission from M31. Later that decade, the first radio maps were made by John Baldwin and others at the Cambridge Radio Astronomy Group (later changed its name to the Cavendish Astrophysics Group).

In 2009, the first planet in M31 may have been discovered. It was detected using a method called 'microlensing'.