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When Galaxies Collide: The Merger of the Milky Way and Andromeda

Updated on October 4, 2012
Illustration showing what Earth's night sky might look like in 3.75 billion years.
Illustration showing what Earth's night sky might look like in 3.75 billion years. | Source
As imaged by the Hubble Space Telescope, the galaxies NGC2207 and IC2163 are in the process of colliding, some 80 million light years away.
As imaged by the Hubble Space Telescope, the galaxies NGC2207 and IC2163 are in the process of colliding, some 80 million light years away. | Source

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

Ultraviolet imaging of Andromeda from the Spitzer Space Telescope.
Ultraviolet imaging of Andromeda from the Spitzer Space Telescope. | Source

Galactic Collisions

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.

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    • livelonger profile image

      Jason Menayan 4 years ago from San Francisco

      Really interesting, and not nearly as frightening when you mention that collision won't be happening for another 4 billion years. Our planet is only 4.5 billion years old, and we've only existed for a few hundred thousand years, so I suspect we'll have figured out an adequate escape plan in time for the fireworks. ;)

    • Chuck Field profile image

      Chuck Field 4 years ago

      Great article! I wonder what happens when two black holes crash into each other...

    • Phil Riddel profile image

      Phil Riddel 4 years ago from Scotland

      Fascinating stuff: it's a shame that, barring unprecedented advances in medical science, none of us will live to see it, but we can always watch the simulations; this would be the better option for the impatient in any case if it's going to take over a billion years...

    • Sustainable Sue profile image

      Sustainable Sue 4 years ago from Altadena CA, USA

      Both black holes would be vacuums, right? Would they repel each other like two magnets do? Or would the stronger one suck the weaker one in over time?

      Then how would the orbits of the planets in each galaxy shift? If the black holes repel each other, each one is bound to pick up a few planets (and a lot of dust) from the other, even as each one loses some in passing. On the other hand, if one black hole absorbs the other, will it then be left as the center of a giant galaxy too big to hold onto?

    • Chuck Field profile image

      Chuck Field 4 years ago

      I suspect that the black holes would attract to each other as each are intense gravity sources. Likely the bigger one would prevail, but since nothing, including light escapes a black hole; I come full circle and wonder how the bigger would suck up the smaller.

    • scottcgruber profile image
      Author

      scottcgruber 4 years ago from USA

      Great question, Chuck and Sue!

      It seems that when black holes collide, they ricochet like spinning billiard balls. This result had been predicted in computer models, then was observed in galaxy SDSSJ0927+2943 in 2008. A collision between two supermassive black holes at the center of this galaxy caused one to be ejected from the galaxy at 1,600 miles per second.

      http://www.universetoday.com/13958/supermassive-bl...

    • Chuck Field profile image

      Chuck Field 4 years ago

      Black hole bumper cars... so cool.

    • Sustainable Sue profile image

      Sustainable Sue 4 years ago from Altadena CA, USA

      I like the ricochet description of the article you linked us to, Scott. So this smaller black hole is slapped out of the orbit of the bigger one and goes spinning off into space, gathering space dust and rogue star systems as it goes, slowing down slightly as it gets "older." It's like a space vacuum cleaner.

      Now I wonder what it does with all the debris inside of itself? Maybe the debris consolidates and gets spit out as another star system. (lol) There or somewhere else? Like the black hole is a tunnel into another part of space that shoots the consolidated debris out as a new star system somewhere else, thereby continually cleaning and then forming new galaxies throughout what we call "space," but have no idea of the size of.

    • scottcgruber profile image
      Author

      scottcgruber 4 years ago from USA

      Now that's an interesting question. The idea of black holes as tunnels is a useful device in science fiction, but I'm not sure how it works in reality.

      All a black hole really is is a bit of matter that is so dense that its gravity doesn't allow light to escape. And since we know from General Relativity that gravity bends spacetime, the spacetime around a black hole is so severely twisted that all directions of time and space point into it. It isn't actually a hole in the physical sense, more like a hole in the physics sense - a point beyond which all the laws we know about the Universe break down.

      Though it's interesting to speculate about what lies beyond the event horizon, we can never really know. And I don't recommend diving into one to find out...

    • cclitgirl profile image

      Cynthia Calhoun 4 years ago from Western NC

      How cool! I recently watched a show on PBS about our galaxy colliding with Andromeda, but your detailed explanation here is awesome! If only we could predict 4 billion years into the future, haha, and see the fate of the Sun and it's wee little planets. Perhaps we'll meet some new neighbors by then and will have colonized more planets. Ha! Great read! Thanks for sharing this info.

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