Andromeda Galaxy

The Andromeda galaxy is the closest large galaxy close to our own Milky Way. It is likely that our galaxy, the Milky Way, much like the Andromeda galaxy.
The Great Andromeda Galaxy and the Milky Way are the two largest galaxies in the Local Group.
The galaxy M31 is the 31st entry in the catalog of diffuse sky objects, established by the French astronomer Charles Messier. M31 is about 2.4 to 2.9 million light years from our galaxy, meaning that its light takes about 2.4 to 2.9 million years to reach us.
This galaxy is one of the few galaxies visible to the naked eye, when weather conditions are favorable.
Its apparent diameter is pretty huge since it is equivalent, as seen from Earth diameters of 5 full moon. Its diameter is 170 000 light years. The two galaxies, the Milky Way and Andromeda are approaching at a speed of around 300 km/s and could collide in a few billion years.

This hypothesis not confirmed, is likely.

Image: Galaxy M31 has an apparent magnitude of 3.38, making it visible to the naked eye in dark.
The glow of Andromeda is none other than the light accumulated hundreds of billions of stars that compose it. The stars we see in the foreground of this picture of Andromeda, are actually stars in our own galaxy.
Bottom right you can see his companion, the dwarf galaxy NGC 205 and M110.
The big bright spot very close to 10 H of the disk of M31, M32 is another galaxy companion.
Credit: Robert Gendler (robgendlerastropics.com)

The detailed ultraviolet image of the Andromeda galaxy was produced by the telescope installed on the NASA Swift satellite.
This image is composed of 330 individual images covering an area of 200 000 light years in diameter.
It contains about 20 000 UV emission from stars, most of whom are young hot stars.
We can also see dense star clusters because these objects radiate heavily in the ultraviolet spectrum.
The Andromeda Galaxy confirms the growth pattern of galaxies with "cannibalism," according to a study published September 16, 2009, in the British scientific journal Nature.
Remnants of dwarf galaxies absorbed or dislocated by Andromeda, were observed.

"We have detected stars and structures that are probably remnants of dwarf galaxies destroyed by the tidal effects of M31," explains Alan McConnachie (NRC Herzberg Institute of Astrophysics, Victoria, Canada) and colleagues. The Triangulum Galaxy is surrounded by a stellar structure "that provides evidence of a recent encounter with M31," they are. Millions of stars would then be propelled out of the Triangle found.
This "confirms directly the basic principles of the hierarchical model of galaxy formation" by incorporation of smaller galaxies, the astronomers conclude.

NB: 1 light-years = 9 500 billion km, (symbol: ly), is a unit of length equal to just under 10 trillion kilometers (or about 6 trillion miles).