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[BLOG-LIKE POSTING] On the impending union of the Milky Way and Andromeda galaxies

Our Milky Way Galaxy and the Andromeda Galaxy 2.6 million light years away from us are the two largest components of the Local Group, a cluster of more than four dozen galaxies and assorted debris bound by mutual gravitation. (It’s still open to debate whether the Milky Way Galaxy is the more massive of the two, or whether the most massive is the Andromeda Galaxy. The most recent estimates of star numbers and total mass I’ve seen suggest that the Andromeda Galaxy might have more, but less massive, stars than our galaxy, and that the two galaxies are roughly equal in mass.)

For some time, measurements of the movement of the Andromeda Galaxy relative to our own have suggested that the two largest galaxies in the Local Group are set to collide. What was unknown was the way in which it would collide. Would it be a head-on collision, or something more glancing, or perhaps even a near miss? Universe Today’s Nancy Atkinson explains that it’s going to be a direct collision.

Astronomers have known for years that our Milky Way and its closest neighbor, the Andromeda galaxy, (a.k.a M31) are being pulled together in a gravitational dance, but no one was sure whether the galaxies would collide head-on or glide past one another. Precise measurements from the Hubble Space Telescope have now confirmed that the two galaxies are indeed on a collision course, headed straight for a colossal cosmic collision.

No need to panic for the moment, as this is not going to happen for another four billion years. And while astronomers say it is likely the Sun will be flung into a different region of our galaxy, Earth and the solar system will probably just go along for the ride and are in no danger of being destroyed.

“In the ‘worst-case-scenario’ simulation, M31 slams into the Milky Way head-on and the stars are all scattered into different orbits,” said team member Gurtina Besla of Columbia University in New York, N.Y. “The stellar populations of both galaxies are jostled, and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits. The galaxies’ cores merge, and the stars settle into randomized orbits to create an elliptical-shaped galaxy.”

The simulations Besla was talking about came from precise measurements by Hubble, painstakingly determining the motion of Andromeda, looking particularly at the sideways motion of M31, which until now has not been able to be done.

“This was accomplished by repeatedly observing select regions of the galaxy over a five- to seven-year period,” said Jay Anderson of STScI.

[. . .]

Of course, the collision is not like a head-on between two cars that takes place in an instant. Hubble data show that it will take an additional two billion years after the encounter for the interacting galaxies to completely merge under the tug of gravity and reshape into a single elliptical galaxy similar to the kind commonly seen in the local universe.

Astronomers said the stars inside each galaxy are so far apart that they will not collide with other stars during the encounter. However, the stars will be thrown into different orbits around the new galactic center. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.

There’s also the complication of M31′s small companion, the Triangulum galaxy, M33. This galaxy will join in the collision and perhaps later merge with the M31/Milky Way pair. There is a small chance that M33 will hit the Milky Way first.

The astronomers working on this project said that they were able to make the precise measurements because of the upgraded cameras on Hubble, installed during the final servicing mission. This gave astronomers a long enough time baseline to make the critical measurements needed to nail down M31′s motion.

Bad Astronomy’s Phil Plait did a good job outlining the mechanics of the collision.

Over the next few billion years, Andromeda — currently a barely naked-eye object in the northern sky — will grow larger as it approaches. In just under 4 billion years, the mutual gravity from the two galaxies will start to play havoc on each other. The Milky Way and Andromeda are about the same mass, so the effects they will have on each other will be profound. Stars on the outskirts of both galaxies will be drawn out, and long tails or streamers of stars and gas will be flung out.

Then, over a hundred or so million years, the galaxies will physically collide. Stars are small and so far apart on galactic scales that the odds of two stars colliding (or even getting close enough to affect each other or any planets) are actually incredibly small. But gas clouds are huge, light years across, so head-on collisions between them is inevitable. They’ll crash into each other, collapsing, and furiously form new stars. These stars will light up the gas, and from a distance the two galaxies will be seen have long strings of fiercely glowing red gas along their arms, like the Antenna galaxies, shown here.

The two galaxies will probably pass right through each other, pulling apart even as chaos reigns inside each. But the pull of gravity will not be denied. They’ll slow as they draw apart, eventually stop, and fall back in to each other. At that point they’ll merge, becoming a single, larger galaxy. It will probably be an elliptical galaxy, a big fuzzy cotton ball, as opposed to the spiral that each galaxy is now. That will take about two billion years after the initial collision, or six billion years from now.

Interestingly, the Sun will still be around then. It’ll be different, having used up most of its nuclear fuel, and on its way to becoming a red giant. But it’s possible the Earth and other planets will still exist! So it’s possible someone (maybe not resembling humans too much, but still) may yet be around to watch this event unfold.

The Sun’s orbit around the galaxy will change, though. Right now we orbit the Milky Way’s center in a roughly circular path, taking over 200 million years to complete one orbit. According to the models the astronomers developed using the Hubble observations, during the collision the Sun will be flung into a looping elliptical orbit around the new galaxy’s center, taking it farther out than we are now. That may be a good thing: both the Milky Way and Andromeda have supermassive black holes in their cores, and these black holes will merge eventually as well. It’s unclear what will happen when this occurs (though we may become an active galaxy, spewing out huge amounts of energy), but I suspect it’s best to be as far from that as possible when it does!

These new results make me pretty happy. We knew that a collision was inevitable, but the timing has always been a question. In my book Death from the Skies! I wrote a chapter on this event, but based on what was known at the time (just a few years ago!) it was supposed to happen in 1-2 billion years. These new results double that to 4 billion, which means I have a firmer number to quote. Moreover, we didn’t know if it would be a glancing blow at first or a head-on collision, and it looks now like it’s headed right at us.

Plait’s YouTube account hosts a nifty animation of the collision as expected.

By this time, the Earth will be uninhabitable, the warming and expansion of our Sol helping push our homeworld into a Venus-like state. (If nothing interferes, that is.) Everyone commenting on this is right to note that the galaxy formed by this collision would be a spectacular thing to see in our night skies.

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Written by Randy McDonald

June 2, 2012 at 4:00 am

One Response

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  1. This is great stuff. Thanks for sharing this.

    RichardB1001

    June 2, 2012 at 4:08 am


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