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The Milky Way is destined to get a major makeover during theencounter, which is predicted to happen four billion years fromnow. It is likely the Sun will be flung into a new region of ourgalaxy, but our Earth and solar system are in no danger of beingdestroyed. "Our findings are statistically consistent with a head-oncollision between the Andromeda galaxy and our Milky Waygalaxy," said Roeland van der Marel of the Space TelescopeScience Institute (STScI) in Baltimore. The solution came through painstaking NASA Hubble Space Telescopemeasurements of the motion of Andromeda, which also is known asM31. The galaxy is now 2.5 million light-years away, but it isinexorably falling toward the Milky Way under the mutual pull ofgravity between the two galaxies and the invisible dark matter thatsurrounds them both. "After nearly a century of speculation about the futuredestiny of Andromeda and our Milky Way, we at last have a clearpicture of how events will unfold over the coming billions ofyears," said Sangmo Tony Sohn of STScI. The scenario is like a baseball batter watching an oncomingfastball. Although Andromeda is approaching us more than twothousand times faster, it will take four billion years before thestrike. Computer simulations derived from Hubble's data show that it willtake an additional two billion years after the encounter for theinteracting galaxies to completely merge under the tug of gravityand reshape into a single elliptical galaxy similar to the kindcommonly seen in the local universe. Although the galaxies will plow into each other, stars inside eachgalaxy are so far apart that they will not collide with other starsduring the encounter. However, the stars will be thrown intodifferent orbits around the new galactic center. Simulations showthat our solar system will probably be tossed much farther from thegalactic core than it is today. To make matters more complicated, M31's small companion, theTriangulum galaxy, M33, will join in the collision and perhapslater merge with the M31/Milky Way pair. There is a small chancethat M33 will hit the Milky Way first. The universe is expanding and accelerating, and collisions betweengalaxies in close proximity to each other still happen because theyare bound by the gravity of the dark matter surrounding them. TheHubble Space Telescope's deep views of the universe show suchencounters between galaxies were more common in the past when theuniverse was smaller. A century ago astronomers did not realize that M31 was a separategalaxy far beyond the stars of the Milky Way. Edwin Hubble measuredits vast distance by uncovering a variable star that served as a"milepost marker." Edwin Hubble went on to discover the expanding universe wheregalaxies are rushing away from us, but it has long been known thatM31 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 achange in frequency and wavelength of waves produced by a movingsource relative to an observer, to measure how starlight in thegalaxy has been compressed by Andromeda's motion toward us. Previously, it was unknown whether the far-future encounter will bea miss, glancing blow, or head-on smashup. This depends on M31'stangential motion. Until now, astronomers have not been able tomeasure M31's sideways motion in the sky, despite attempts datingback more than a century. The Hubble Space Telescope team, led byvan der Marel, conducted extraordinarily precise observations ofthe sideways motion of M31 that remove any doubt that it isdestined to collide and merge with the Milky Way. "This was accomplished by repeatedly observing select regionsof the galaxy over a five- to seven-year period," said JayAnderson of STScI. "In the 'worst-case-scenario' simulation, M31 slams into theMilky Way head-on and the stars are all scattered into differentorbits," said team member Gurtina Besla of Columbia Universityin New York, N.Y. "The stellar populations of both galaxiesare jostled, and the Milky Way loses its flattened pancake shapewith most of the stars on nearly circular orbits. The galaxies'cores merge, and the stars settle into randomized orbits to createan elliptical-shaped galaxy." The space shuttle servicing missions to Hubble upgraded it withever more-powerful cameras, which have given astronomers along-enough time baseline to make the critical measurements neededto nail down M31's motion. The Hubble observations and theconsequences of the merger are reported in three papers that willappear in an upcoming issue of the Astrophysical Journal. The science team that did the investigation is led by PrincipalInvestigator 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 ofCalifornia, Santa Cruz, Calif.), and T.J. Cox (CarnegieObservatories, Pasadena, Calif). 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