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. If it was much brighter, its appearance in the sky would be sensational. The moon is a good reference for what we’d see if our galaxy’s neighbor was much brighter in the image below:
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A brighter Andromeda galaxy in the Earth’s sky
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 galaxy received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess, the daughter of Cepheus and Cassiopeia, king and queen of ancient Ethiopia in the Greek mythology.
Andromeda is on a collision course 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 a more amazing way – because it is on a direct collision course with our galaxy, 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 two galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or even 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.
Update February 10, 2019: We finally know when our Milky Way will crash into the Andromeda Galaxy
According to a study published this month in The Astrophysical Journal, the collision between our Milky Way and fellow spiral galaxy Andromeda will occur about 4.5 billion years from now. The new research is based on observations made by the European Space Agency’s (ESA) Gaia spacecraft.