The Andromeda Galaxy is the nearest major galaxy to our own Milky Way Galaxy. Together, these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. Continue reading for more.
According to a team of astronomers reporting in 2010, M31 was formed out of the collision of two smaller galaxies between 5 and 9 billion years ago. A paper published in 2012 has outlined M31's basic history since its birth. According to it, Andromeda was born roughly 10 billion years ago from the merger of many smaller protogalaxies, leading to a galaxy smaller than the one we see today.
At 3.4, the apparent magnitude of Andromeda Galaxy is one of the brightest of any 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.
The Andromeda Galaxy is estimated to be 7.1x1011 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.
Astronomers are continuing to calculate the distance to Andromeda, and in 2003, they calculated that Andromeda is 2.57 million light-years away. However, one year later, in 2004, astronomers recalculated Hubble's Cepheid variable data, and determined that Andromeda was 2.51 million light-years. So, the agreed distance of 2.54 million light-years is an average of the distances measured so far.
1. Milky Way Collision
The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 mi/s). We measure it approaching relative to our sun at around 300 kilometres per second as the sun orbits around the center of our galaxy at a speed of approximately 225 kilometres per second. This makes Andromeda one of the few blueshifted 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 we expect it to directly collide with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.