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Visible to the naked eye on a clear night, the Andromeda galaxy was first thought to be just a cloud of gas or some kind of nebula. In 1923, Edwin Hubble proposed that it was actually another galaxy--distinct from our own--and our understanding of the size of the universe expanded tremendously. Until then, we thought the entire universe was the Milky Way; now we understand that our galaxy is just one of at least 100 billion other galaxies.
Although they're both spiral-shaped galaxies, Andromeda is about two times larger than the Milky Way, and believed to contain twice as many stars (200-400 billion versus the 100-200 billion in our own). Together with the Milky Way and 44 smaller galaxies, we make up a galactic cluster known as the Local Group. Although lots is known about our closest major galactic neighbor, astronomers are still learning a lot about it.
For example, using the Hubble Space Telescope, they recently discovered that Andromeda is surrounded by a ball of gas that's so giant, it nearly reaches halfway toward the Milky Way. The gas is dark, but if we could see it in the night sky, it would appear about 100 times larger than our moon. Andromeda is also heading towards us at about 250,000 miles per hour (400,000 kph), but even at this speed, the two galaxies won't collide for another four billion years.
Andromeda Galaxy Is Surrounded by a Supersized Gas Halo (Space.com)
"One of the Milky Way's closest galactic neighbors is surrounded by a much bigger halo of gas than previously thought, new observations from the Hubble Space Telescope reveal."
Hubble's High-Definition Panoramic View of the Andromeda Galaxy (NASA)
"The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird's-eye view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor."
Andromeda Galaxy Will Collide With Milky Way (CBS)
"Andromeda galaxy will actually collide with the Milky Way in about 2 billion years, according to the most recent Hubble Space Telescope measurements of the motion of Andromeda and the Milky Way."