NASA's Hubble Shows Milky Way is Destined for Head-on Collision with Andromeda Galaxy

Article ID: 589867

Released: 31-May-2012 1:00 PM EDT

Source Newsroom: Space Telescope Science Institute (STScI)

  • Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), and A. Mellinger

    CRASH OF THE TITANS This photo illustration depicts a view of the night sky just before the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy. About 3.75 billion years from now, Andromeda's disk fills the field of view and its gravity begins to create tidal distortions in the Milky Way. The view is inspired by dynamical computer modeling of the future collision between the two galaxies. The two galaxies collide about 4 billion years from now and merge to form a single galaxy about 6 billion years from now.

  • Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger

    ILLUSTRATION SEQUENCE OF THE MILKY WAY AND ANDROMEDA GALAXY COLLIDING. This series of photo illustrations shows the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. The sequence is inspired by dynamical computer modeling of the inevitable future collision between the two galaxies. [First Row, Left Panel: Present Day]: This is a nighttime view of the current sky, with the bright belt of our Milky Way. The Andromeda galaxy lies 2.5 million light-years away and looks like a faint spindle, several times the diameter of the full Moon. [First Row, Right Panel: 2 Billion Years]: The disk of the approaching Andromeda galaxy is noticeably larger. [Second Row, Left Panel: 3.75 Billion Years]: Andromeda fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda. [Second Row, Right Panel and Third Row, Left Panel: 3.85-3.9 Billion Years]: During the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters. [Third Row, Right Panel: 4 Billion Years]: After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped. [Fourth Row, Left Panel: 5.1 Billion Years]: During the second close passage, the cores of the Milky Way and Andromeda appear as a pair of bright lobes. Star-forming nebulae are much less prominent because the interstellar gas and dust has been significantly decreased by previous bursts of star formation. [Fourth Row, Right Panel: 7 Billion Years]: The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky. Scoured of dust and gas, the newly merged elliptical galaxy no longer makes stars and no nebulae appear in the sky. The aging starry population is no longer concentrated along a plane, but instead fills an ellipsoidal volume. NOTE: These illustrations depict the view from about 25,000 light-years away from the center of the Milky Way. The future view from our solar system will most likely be markedly different, depending on how the Sun's orbit within the galaxy changes during the collision.

  • Credit: NASA, ESA, R. van der Marel and T. Brown (STScI), and the Digitized Sky Survey

    HALO OF ANDROMEDA GALAXY USED TO MEASURE ITS DRIFT ACROSS SPACE. This composite image shows a region in the halo in the neighboring Andromeda galaxy that astronomers used to precisely measure the galaxy's sideways motion on the sky. This has allowed them to predict a direct collision between Andromeda and the Milky Way about 4 billion years from now. The inset image on the left is from a 2002 Hubble Space Telescope deep exposure that captured the light from 300,000 stars in Andromeda's halo, a vast spherical cloud of stars surrounding the galaxy's bright disk. Embedded in the image are numerous background galaxies that are much father away than Andromeda. Astronomers compared this region to pictures of the same area taken seven years later. They measured the tiny amount of sideways drift in the halo stars relative to the stationary background galaxy field. The same measurements were done for two other fields in the galaxy as well. This is similar to measuring the drift of a boat relative to a background shoreline.

  • Credit: NASA, ESA, and A. Feild and R. van der Marel (STScI)

    MEASURING THE DRIFT OF THE ANDROMEDA GALAXY. This illustration shows one of the regions in the neighboring Andromeda galaxy where astronomers aimed the Hubble Space Telescope to make precise measurements of the galaxy's lateral motion. As the galaxy drifts through space, the stars will appear to uniformly move against the far more distant background galaxies that remain fixed on the sky.

  • Credit: NASA, ESA, and A. Feild and R. van der Marel (STScI)

    COLLISION SCENARIO FOR MILKY WAY AND ANDROMEDA GALAXY. This illustration shows the inevitable collision between our Milky Way galaxy and the Andromeda galaxy approximately 4 billion years from now. The galaxies are moving toward each other under the inexorable pull of gravity between them. A smaller galaxy, Triangulum, may be part of the smashup.

  • Credit: NASA, ESA, and A. Feild and R. van der Marel (STScI)

    FATE OF THE SUN. This illustration is a before and after comparison of the size of our Milky Way galaxy at present, and after it fully completes a merger with the neighboring Andromeda galaxy 10 billion years from now. The merged galaxies will blend together to create an elliptical galaxy of aging stars. Our Sun now orbits in the Milky Way's disk. But after the merger, it likely will be tossed into a looping orbit that will bring it both nearer to the center and farther into the outskirts of the newly formed elliptical galaxy.

Newswise — 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 four 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."

Edwin 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 have 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 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 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.

The science team that did the investigation is led by Principal Investigator R.P. van der Marel (Space Telescope Science Institute [STScI], Baltimore, Md.), and further consists of S.T. Sohn and J. Anderson (STScI), G. Besla (Columbia University, New York, N.Y.), M. Fardal (University of Massachusetts, Amherst, Mass.), R.L. Beaton (University of Virginia, Charlottesville, Va.), Thomas M. Brown (STScI), P. Guhathakurta (UCO/Lick Observatory, University of California, Santa Cruz, Calif.), and T.J. Cox (Carnegie Observatories, Pasadena, Calif).

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

http://hubblesite.org/news/2012/20
http://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html

For more information about the Hubble Space Telescope, visit:

http://www.nasa.gov/hubble

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.


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