Andromeda Galaxy is the farthest object in the sky that we can see with the naked eye. But if you don’t know where to look., you won’t notice it. But if it were much more brighter, its appearance in the sky would be sensational. The moon is a good reference about what we’d see in the image below:
Another amazing image of a “brighter” Andromeda Galaxy, the Moon as a reference again:
Andromeda is larger than our galaxy, the Milky Way. In fact, it is is the largest galaxy of the Local Group, the galaxy group that includes the Milky Way. It is approximately 220,000 light years across (Milky Way was considered to be about 100,000–120,000 light-years, but according to the recent findings, it may be 150,000–180,000 light-years). 2006 observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion (1012) 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 1.5×1012 solar masses, while the Milky Way is estimated to be 8.5×1011 solar masses.
Andromeda received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda.
Collision with the Milky Way
But, in the far, far future, humans (assuming humans still exist, of course) will see Andromeda galaxy much more clearly, and in more amazing way - because it is on a direct collision course with the Milky Way. It is expected the Andromeda galaxy to directly collide with the Milky Way in about 4 billion years. Here’s how the Earth’s sky would look shortly before they start to collide:
A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or perhaps even a large disc galaxy. Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System 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 the Andromeda Galaxy.
Here is a video published by NASA depicting the collision between our home galaxy, the Milky Way and the Andromeda galaxy. Hubble Space Telescope observations indicate that the two galaxies, pulled together by their mutual gravity, will crash together about 4 billion years from now. Around 6 billion years from now, the two galaxies will merge to form a single galaxy. The video also shows the Triangulum galaxy, which will join in the collision and perhaps later merge with the Andromeda/Milky Way pair.