NASA'S Hubble Shows Milky Way is Destined for Head-on Collision

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May 31, 2012

J.D. Harrington 
Headquarters, Washington 
202-358-5241 
j.d.harrington@xxxxxxxx 

Ray Villard 
Space Telescope Science Institute, Baltimore 
410-338-4514 
villard@xxxxxxxxx 

RELEASE: 12-159

NASA'S HUBBLE SHOWS MILKY WAY IS DESTINED FOR HEAD-ON COLLISION

WASHINGTON -- 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. 

The Milky Way is destined to get a major makeover during the 
encounter, which is predicted to happen four billion years from now. 
It is likely the sun will be flung into a new region of our galaxy, 
but our Earth and solar system are in no danger of being destroyed. 

"Our findings are statistically consistent with a head-on collision 
between the Andromeda galaxy and our Milky Way galaxy," said Roeland 
van der Marel of the Space Telescope Science Institute (STScI) in 
Baltimore. 

The solution came through painstaking NASA Hubble Space Telescope 
measurements of the motion of Andromeda, which also is known as M31. 
The galaxy is now 2.5 million light-years away, but it is inexorably 
falling toward the Milky Way under the mutual pull of gravity between 
the two galaxies and the invisible dark matter that surrounds them 
both. 

"After nearly a century of speculation about the future destiny of 
Andromeda and our Milky Way, we at last have a clear picture of how 
events will unfold over the coming billions of years," said Sangmo 
Tony Sohn of STScI. 

The scenario is like a baseball batter watching an oncoming fastball. 
Although Andromeda is approaching us more than two thousand times 
faster, it will take 4 billion years before the strike. 

Computer simulations derived from Hubble's 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. 

Although the galaxies will plow into each other, 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. 

To make matters more complicated, M31's small companion, the 
Triangulum galaxy, M33, 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 universe is expanding and accelerating, and collisions between 
galaxies in close proximity to each other still happen because they 
are bound by the gravity of the dark matter surrounding them. The 
Hubble Space Telescope's deep views of the universe show such 
encounters between galaxies were more common in the past when the 
universe was smaller. 

A century ago astronomers did not realize that M31 was a separate 
galaxy far beyond the stars of the Milky Way. Edwin Hubble measured 
its vast distance by uncovering a variable star that served as a 
"milepost marker." 

Hubble went on to discover the expanding universe where galaxies are 
rushing away from us, but it has long been known that M31 is moving 
toward the Milky Way at about 250,000 miles per hour. That is fast 
enough to travel from here to the moon in one hour. The measurement 
was made using the Doppler effect, which is a change in frequency and 
wavelength of waves produced by a moving source relative to an 
observer, to measure how starlight in the galaxy has been compressed 
by Andromeda's motion toward us. 

Previously, it was unknown whether the far-future encounter will be a 
miss, glancing blow, or head-on smashup. This depends on M31's 
tangential motion. Until now, astronomers had not been able to 
measure M31's sideways motion in the sky, despite attempts dating 
back more than a century. The Hubble Space Telescope team, led by van 
der Marel, conducted extraordinarily precise observations of the 
sideways motion of M31 that remove any doubt that it is destined to 
collide and merge with the Milky Way. 

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

"In the worst-case-scenario simulation, M31 slams into the Milky Way 
head-on and the stars are all scattered into different orbits," said 
Gurtina Besla of Columbia University in New York. "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 space shuttle servicing missions to Hubble upgraded it with ever 
more-powerful cameras, which have given astronomers a long-enough 
time baseline to make the critical measurements needed to nail down 
M31's motion. The Hubble observations and the consequences of the 
merger are reported in three papers that will appear in an upcoming 
issue of the Astrophysical Journal. 

For images, video, and more information about M31's collision with the 
Milky Way, visit: 

http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html 

http://hubblesite.org/news/2012/20 

For more information about NASA's Hubble Space Telescope, visit: 

http://www.nasa.gov/hubble 

	
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