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Milky Way On Collision Course With Andromeda


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Report this Jun. 10 2012, 2:24 pm

For rock-'em, sock'em cosmic action, you can't beat the collision of two galaxies. It's a scene that has played out countless times since the beginning of the universe. Our own Milky Way, for example, isn't the serene, stately pinwheel it appears to be. Over the eons it has gobbled up many close associates. In fact, right now the Milky Way is devouring a small system called the Sagittarius Dwarf Elliptical Galaxy, or SagDEG for short.

View of M31 in 3¾ billion years Some 4¾ billion years from now, the Andromeda Galaxy (M31) will loom large in the night sky as its fateful collision with our Milky Way nears. NASA / ESA / Z. Levay / R. van der Marel / T. Hallas / A. Mellinger

But it turns out that these skirmishes are merely warm-ups for the upcoming main event: a head-on collision of the Milky Way with the equally massive Andromeda Galaxy (Messier 31) in about 4 billion years.

Astronomers have long known that M31 is relatively nearby, just 2½ million light-years distant, and moving toward us at about 250,000 miles (400,000 km) per hour. That's because both of these massive spiral systems are members of the Local Group, a gravitationally bound collection of three biggies (these two and M33), along with about 50 much smaller dwarf galaxies that include the two Magellanic Clouds. I liken the large trio as "anchor stores" in a medium-size intergalactic mall.

So is the Andromeda Galaxy destined to collide with the Milky Way? No one could say for sure because the answer depended on side-to-side movement across the sky, what astronomers term proper motion. A lateral shift of 60 miles (100 km) per second one way or the other makes all the difference between a near miss, glancing blow, or head-on collision. Yet this amount of proper motion corresponds to a positional change of just 27 millionths of an arcsecond per year — far beyond the ability of ground-based telescope's to measure, even over decades-long baselines.

Difficult — but not impossible — for the Hubble Space Telescope and the unprecedented positional precision made possible by its latest-generation instruments.

A series of three papers, submitted to the Astrophysical Journal last week, reveals how astronomers finally puzzled it all out. First, a trio led by Sangmo Tony Sohn (Space Telescope Science Institute) used dedicated HST observations over several years to measure proper motions in three locations within M31. The average of these turns out to be about 125 miles (200 km) per second. In the second paper, a team led by Roeland van der Marel (also at STScI) modeled the total mass of the Local Group, to see what other gravitational influences might be in play.

HST target fields in M31 Three small boxes denote locations targeted by astronomers to determine the proper motion (lateral movement) of the Andromeda Galaxy. The underlying gray shading represents the density of stars in the galaxy's disk. S.T. Sohn & others / ApJ

Finally, van der Marel and four colleagues simulated the possible outcomes when these titans of the Local Group finally meet. "Most likely, the Milky Way and M31 will merge first," they write, with a 41% chance that they'll strike head-on (their centers no farther than 80,000 light-years apart). In that case, M33 will settle into an orbit around them. However, they add that there's a small chance (9%) that M33 will strike the Milky Way first — or it might get flung from the Local Group altogether (7%).

The first round of this intergalactic smashup won't happen for another 4 billion years, and another 2 billion years will pass before the two (or three) systems merge into a single giant elliptical galaxy. Although our Sun will still be around then, still a billion years shy of its red-giant phase, life on Earth (as we know it) will not.

An STScI press release notes that the hundreds of billions of stars inside each galaxy will be jostled into new orbits during the encounter but won't collide with each other — there's just too much empty space between them. The simulations also suggest that our solar system will likely end up much farther from the galactic core than it is today.

As Americans, we sometimes suffer from too much pluribus and not enough unum. - Arthur Schelsinger, Jr.


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Report this Jun. 10 2012, 4:28 pm

I find it facinating when this will happen. At least I know now that Andromeda is a nearby galaxy. Too bad for the Milky Way.

Data's Girl

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Report this Jun. 11 2012, 9:43 pm

Our sun will go out at around the same time, so unless humanity has figured out a way to go somwhere else in that period of time, we won't be around to see that anyway...

tai nasha no karosha :)


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Report this Jun. 11 2012, 10:06 pm

Ancient news but interesting news. 
Thanks for sharing.

He felt that his whole life was some kind of dream and he sometimes wondered whose it was and whether they were enjoying it. -Douglas Adams (1952-2001)


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Report this Jun. 12 2012, 11:07 am

We may have to prepare to move to a brighter neighborhood.  I wonder: if we all transported (via ships or transporters) to the larger galaxy, would our civilization then survive the impact?

Either wya, we'd better be planning ahea


Var Miklama--Zakdorn, engineer. "A sound mind in a FULL body!" "Time, like latinum, is a limited quantity in the galaxy."


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Report this Jun. 14 2012, 5:34 pm

Actually, if the sun were to still some how be "normal" at that point it seems like it would a highly survivable situation.  I suppose asteroid impact activity might increase with all of the gravitational tugs going on, and the night sky would be brighter with more stars in it. Otherwise, things at the local level sound like they would be relatively normal. Am I missing something?


Edit: Oh yes, a lot of gaseous interactions, but then is their really all that much gas in our solar system outside of the gas giants that would just grow a bit bigger?

As Americans, we sometimes suffer from too much pluribus and not enough unum. - Arthur Schelsinger, Jr.

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