When Galaxies Collide: The Merger of the Milky Way and Andromeda
For dozens of millennia, humans have looked toward the heavens to see a milky band of light dividing the night sky. This vast collection of stars, gas and dust spanning the sky, which we now know as the Milky Way galaxy, has always seemed to be a permanent backdrop to the night sky - a stationary sphere against which the moon and planets moved.
However, the Milky Way is not as permanent as it seems. Our galaxy is on a collision course with the neighboring Andromeda galaxy. The inevitable collision will not only alter the night sky, but will likely fling our small Sun out to the furthest reaches of the galaxy, perhaps even ejecting us altogether.
However, this collision is not going to happen for about four billion years, so it's not something we have to worry about for a while.
Our Soon-To-Be Neighbor
Galactic collisions are a relatively common event in the universe. Observations of distant galaxies using the Hubble Space Telescope and other ground-based and orbiting observatories have found many examples of colliding galaxies in different stages of merging. There is also evidence that some globular clusters in our own galaxy, such as Omega Centauri in the constellation Centaurus, are the result of previous galactic mergers.
The impending collision with Andromeda was discovered by measuring the Doppler shift of light from some of the estimated one trillion stars in our neighboring galaxy. In much the same way as the pitch of a motorcycle engine rises when it is coming toward you and drops when it is moving away, the color of visible light shifts toward the red end of the spectrum when it is moving away from an observer and blue when it is moving toward the observer.
The light from most other galaxies is red-shifted, meaning that they are moving away from us - a discovery that led to the Big Bang Theory. Andromeda and the nearby Triangulum galaxies, on the other hand, are blue-shifted. Calculations based on this Doppler shift have revealed that Andromeda is moving towards us at about 120 km/sec.
A Galactic Merger
Although astronomers have known for years that the Milky Way and Andromeda are on a collision course, it wasn't until 2012 whether the collision would be a direct hit or a glancing blow. This is because the sideways motion of Andromeda relative to us had not been accurately measured.
A good analogy would be a batter in a baseball game watching an incoming pitch leaving the pitcher's hand (readers outside the Americas can feel free to substitute appropriate cricket terminology here). Though he can immediately see how fast the ball is moving, he cannot yet judge whether the pitch will be in the strike zone or curve outside.
By making precise observations of several thousand bright stars in Andromeda using the Hubble Space Telescope over nearly a decade, astronomers were able to precisely measure the galaxy's sideways motion and predict that it is destined to collide head-on with our Milky Way galaxy, in approximately 4 billion years. The collision itself is likely to be a very gradual affair, lasting at least 1.5 to 2 billion years before the galaxies are fully merged.
The Future of the Solar System
Although the idea of two galaxies colliding seems like a frightening prospect, there is little danger of collision between any of the stars within each galaxy. The sheer distances between stars - even in relatively dense regions such as the galactic centers - is so vast that an actual collision is highly improbable.
However, the effect of gravity, both from the newly-introduced stars and the supermassive black holes at the center of each galaxy, will rearrange the sky considerably as the two galaxies merge. Simulations of the event predict that our Sun, currently located in the Orion Spur between the Perseus and Sagittarius Arms of the galaxy, will likely end up much farther from the galactic center, and will possibly be thrown out of the galaxy altogether.
As for the consequences of this event for life on our planet - well, there likely won't be any. Four billion years from now, our Sun will be well on its way to becoming a red giant, swelling to many times its current size and possibly swallowing up our Earth as it reaches its maximum diameter. All life on the planet will have been gone for many billions of years. If any descendants of current Earthly life are still around, they won't be watching these events from our planet.
Sources and Further Information
- The Collision Between The Milky Way And Andromeda
T.J. Cox, Abraham Loeb (Harvard/CfA) We use a N--body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies...
- Distant future of the Sun and Earth revisited
Klaus-Peter Schroder, Robert C. Smith - We revisit the distant future of the Sun and the solar system, based on stellar models computed with a thoroughly tested evolution code.
- NASA's Hubble Shows Milky Way is Destined for Head-on Collision with Andromeda Galaxy
NASA astronomers announced Thursday they can now predict with certainty the next major cosmic event to affect our galaxy, Sun, and solar system: the titanic collision of our Milky Way galaxy with the neighboring Andromeda galaxy.