The Andromeda Galaxy, or M31, is the nearest large neighbor of our Milky Way, though it sits some 2.5 million light-years away. That makes it the most distant object regularly visible with the naked eye. The Andromeda Galaxy is a staggering 23 billion km away.
In the 1920s, the distant galaxy became part of the Great Debate between American astronomers Harlow Shapley and Heber Curtis. At the time, astronomers thought the Milky Way composed the whole universe, and the strange patches known as nebulae lay inside of them. Curtis had spotted various novae in Andromeda Galaxy, and argued instead that it was a separate galaxy.
In the decades ahead, other astronomers started noticing supernovae exploding in Andromeda Galaxy, too. One astronomer in particular, Heber Curtis, used the known brightness of these explosions to calculate the distance to Andromeda Galaxy. He estimated that this “spiral nebulae” was an unprecedented 500,000 light-years away, which would put it well outside the confines of our Milky Way.
Our understanding of the size of the Andromeda Galaxy has grown bigger in recent years. In 2015, observations from the Hubble Space Telescope found that a halo of material surrounding Andromeda Galaxy is six times larger and 1,000 times more massive than what was previously measured.
You can see the Andromeda Galaxy best in autumn, at its highest in the south around 8pm. But it is visible from the northern hemisphere throughout much of the year. Under dark, Moon-free skies, your unaided eye should be able to find the Andromeda Galaxy as a faint misty patch a short distance from the band of the Milky Way.
Like the Milky Way, M31 or Andromeda Galaxy is a giant spiral-shaped disk of stars, with a bulbous central hub of older stars. M31 has long been known to have a bright and extremely dense grouping of a few million stars clustered at the very center of its spherical hub. As seen from large ground-based telescopes, the starlight blends to resemble a single, bright, almost point-like source. Previous ground-based observations gave little hint of the true structure of the core, which is now revealed by Hubble.
A spiral galaxy like the Milky Way, Andromeda Galaxy contains a concentrated bulge of matter in the middle, surrounded by a disk of gas, dust, and stars and an immense halo. Though Andromeda Galaxy contains approximately a trillion stars to the 250 billion in the Milky Way. Our galaxy is actually more massive, because it is thought to contain more dark matter.
Andromeda’s close proximity to Earth makes it a convenient target to observe for extrapolations about other spiral galaxies. In recent years, scientists have done detailed studies of black holes, stars and other objects within the galaxy. This included a stunning mosaic of Andromeda Galaxy images taken by the Hubble Space Telescope in 2015.
In 2015, scientists released the most detailed photo of Andromeda Galaxy ever using a mosaic of images from the Hubble Space Telescope. The image included 7,398 exposures taken over 411 pointings of the telescope. The image revealed more than 100 million stars within the galaxy, as well as dust structures and other features. At the time, scientists said the images would help with extrapolating the structure of spiral galaxies that are even farther from Earth, making them more difficult to view in such detail.
We now know the Andromeda Galaxy truly is an island universe distinct from our own. But it won’t always be that way. As Slipher’s observations first showed, over the next five billion years or so, the Milky Way and Andromeda Galaxy will draw uncomfortably close together.
When you consider that most of the stars we see are just tens or hundreds of light-years away, the distance to the Andromeda Galaxy becomes phenomenal. This is about the farthest thing in the Universe that you can see using just your eyes.
But while our Galaxy has up to 400 billion stars, Andromeda Galaxy is thought to contain a trillion. With so many more stars, it could mean that there’s more chance of life existing over there, in our similar but larger galactic sibling.
By some estimates, the Andromeda Galaxy contains roughly one trillion stars. And it stretches more than 200,000 light-years in diameter. That’s significantly bigger than the Milky Way, which more recent estimates suggest is 150,000 light-years across (though the exact boundary of where either of these galaxies “end” is a bit nebulous). Astronomers are still struggling to get an accurate count. But our galaxy also appears to have roughly a quarter to a half as many stars as Andromeda Galaxy.
The Andromeda Galaxy, and our Milky Way’s closest neighbor, is the most distant object in the sky that you can see with your unaided eye. But only on a clear night from a location with a very dark sky. The galaxy is a beautiful spiral, but one fact you may not be aware of. We’re safe for a few billion years, but Andromeda Galaxy is headed our way and on a collision course with the Milky Way.
Andromeda Galaxy and the Milky Way are heading on a collision course that will alter the structure of the two galaxies forever. The galaxies are rushing closer to one another at about 70 miles per second (112 kilometers per second). Astronomers estimate that Andromeda Galaxy will collide with the Milky Way in 4 billion years, with the merger concluding 6 billion years from now.
Galaxy collisions are a normal part of the universe’s evolution. In fact, both Andromeda Galaxy and the Milky Way bear signs of having already crashed into other galaxies. Andromeda Galaxy boasts a large ring of dust in its center, giving it an interesting shape. Astronomers believe this dust may have formed when it swallowed an existing galaxy.
A problem with the collision scenario is that the remnant core should be torn apart by the massive black hole hypothesized to dwell at the exact center of M31. The suspected black hole would be located in the middle of the dimmer peak uncovered by HST.
However, while a collision between two galaxies might seem like it could only end in destruction, galaxy mergers often lead to extreme bursts of star formation. This too will be visible from our solar system — although, humans probably won’t survive long enough to see it.