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

Crash of the Titans: Andromeda Galaxy and the Milky Way Collision

Science Illustration Credit: NASA, ESA,
Z. Levay and R. van der Marel (STScI), and A. Mellinger
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Nighttime Sky View of Future Galaxy Merger

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

Scientists Reflect on Expected Collision
Between the Milky Way and the Andromeda Galaxy

This video discusses what the science team's challenges and techniques were in their quest to determine the fate of the Milky Way galaxy. They reflect on the encounter's possible influence on the solar system, and how the Hubble Space Telescope was vital to this research.

Credit: NASA, ESA, M. Estacion, F. Summers, G. Bacon, B. Moster, J. Anderson, R. van der Marel, and S.T. Sohn (STScI)

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).

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

J.D. Harrington
Headquarters, Washington
202-358-5241
jharring@nasa.gov

Roeland van der Marel
Space Telescope Science Institute, Baltimore, Md.
410-338-4931
marel@stsci.edu

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Rare Ultra-blue Stars Found in Neighboring Galaxy's Hub

Andromeda Galaxy

Credit for the Hubble images: NASA, ESA, B.F. Williams (University of Washington, Seattle), D. Lang (Princeton University, N.J.), J. Kalirai (Space Telescope Science Institute, Baltimore), and J. Dalcanton (University of Washington, Seattle)

Peering deep inside the hub of the neighboring Andromeda galaxy, NASA's Hubble Space Telescope has uncovered a large, rare population of hot, bright stars.

Blue is typically an indicator of hot, young stars. In this case, however, the stellar oddities are aging, Sun-like stars that have prematurely cast off their outer layers of material, exposing their extremely blue-hot cores.

Astronomers were surprised when they spotted these stars because physical models show that only an unusual type of old star can be as hot and as bright in ultraviolet light.

While Hubble has spied these ultra-blue stars before in Andromeda, the new observation covers a much broader area, revealing that these stellar misfits are scattered throughout the galaxy's bustling center. Astronomers used Hubble's Wide Field Camera 3 to find roughly 8,000 of the ultra-blue stars in a stellar census made in ultraviolet light, which traces the glow of the hottest stars. The study is part of the multi-year Panchromatic Hubble Andromeda Treasury survey to map stellar populations across the Andromeda galaxy.

"We were not looking for these stars. They stood out because they were bright in ultraviolet light and very different from the stars we expected to see," said Julianne Dalcanton of the University of Washington in Seattle, leader of the Hubble survey.

The team's results are being presented today at the American Astronomical Society meeting in Austin, Texas.

The telescope spied the stars within 2,600 light-years of Andromeda's core. After analyzing the stars for nearly a year, Dalcanton's team determined that they were well past their prime. "The stars are dimmer and have a range of surface temperatures different from the extremely bright stars we see in the star-forming regions of Andromeda," said team member Phil Rosenfield of the University of Washington.

As these stars evolved, puffing up to become red giants, they ejected most of their outer layers to expose their blue-hot cores. When normal Sun-like stars swell up to become red giants, they lose much less material and therefore never look as bright in the ultraviolet.

"We caught these stars when they're the brightest, just before they become white dwarfs," said team member Leo Girardi of the National Institute for Astrophysics's Astronomical Observatory of Padua. "It is likely that there are many other similarly hot stars in this central part of Andromeda at earlier stages of their lives. But such stars are too dim for Hubble to see because they're mixed in with a crowd of normal stars."

The astronomers have proposed two possible scenarios to explain why these blue stars evolve differently. According to Rosenfield, the most likely scenario is that the stars are rich in chemical elements other than hydrogen and helium. Observations with ground-based telescopes have shown the stars in the galaxy's hub have an abundant supply of "heavy elements," which makes it easier for stars to eject lots of material into space late in life.

In this scenario radiation from the star is more efficient at pushing on gas laced with heavy elements, which drives away the material, like wind moving a thick sail. Although all the stars in the core are enriched in heavy elements, the bright blue stars may contain especially high amounts, which help trigger the mass loss.

The study also shows that the number of blue stars decreases with distance from the core, tracing the drop in the amount of heavy elements.

Another possible explanation is that the blue stars are in close binary systems and have lost mass to their partners. This mass loss would expose the stars' hot cores. The astronomers were surprised to find that the ultra-blue stars are distributed in the galaxy in the same way as a population of binary stars with similar masses that were found in X-ray observations by NASA's Chandra X-ray Observatory.

The astronomers' next step is to create simulations of these stars to try to determine which scenario is the one that leads them on a different evolutionary path.

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

Julianne Dalcanton
University of Washington, Seattle, Wash.
206-685-2155
jd@astro.washington.edu

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