Astronomers spy fastest speeding star

A giant explosion hurled this sun out of the Milky Way

In this artist’s illustration, a hypervelocity star is hurtling through space beyond the Milky Way.

Ruth Bazinet/CfA

Some stars are in an awful rush to get out of our galaxy. Astronomers have just clocked one hurtling away from the Milky Way at roughly 4.3 million kilometers (2.7 million miles) per hour. That makes it the fastest moving star to be ejected into the region between galaxies. Scientists refer to this area as intergalactic space.

Located about 28,000 light-years away from Earth, the escapee has been designated US 708. It appears  in the constellation Ursa Major (or Big Bear). And it might have been blown out of our galaxy by an exploding star known as a type 1a supernova. That’s the conclusion of Stephan Geier and his co-workers. Geier is an astronomer at the European Southern Observatory in Garching, Germany. This team reported its findings March 6 in Science.

US 708 is one of roughly a couple dozen suns known as hypervelocity stars. All travel so fast they can escape our galaxy, the Milky Way.

Astronomers suspect that most hypervelocity stars leave the Milky Way after a close brush with the supermassive black hole that sits at the center of our galaxy. A black hole is a region of space that is so dense that neither light nor matter can escape the pull of its gravity. That gravity also can slingshot into space any stars that skirt the black hole’s edge.

Discovered in 2005, US 708 differs from other known hypervelocity stars. Most of them are similar to our sun. But US 708 “has always been an oddball,” says Geier. This star has had most of its atmosphere stripped away. He says that suggests it once had a very close companion star.

In its new study, Geier’s team measured the speed of US 708. The astronomers also calculated its route through space. With this information, they could trace its path back to somewhere in the disk of the Milky Way. That’s well away from the galactic center and its supermassive black hole.

In fact, US 708 might not have needed the black hole to get it up to speed. Instead, Geier’s team suggests, it might once have orbited very close to a white dwarf — the white-hot core of a long-dead star. As US 708 traveled around the white dwarf, the dead star would have stolen its helium. (Helium is part of the fuel that keeps a sun burning.) The buildup of helium on the white dwarf eventually would have triggered an explosion, called a supernova. That likely would have destroyed the white dwarf and jet-propelled US 708 right out of the Milky Way.

“That’s pretty remarkable,” says Warren Brown. He’s an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “You don’t normally think of supernovas popping off their companion stars at over 1,000 kilometers [620 miles] per second.”

Brown discovered the first hypervelocity star in 2005. His team recently used the Hubble Space Telescope to track the motion of 16 more, including US 708. They reported their findings online February 18 on arXiv.org. (Many scientists use this online server to share their recent research.) US 708 was probably launched from the outskirts of the Milky Way, Brown’s team says. Indeed, they calculate that the star came from much farther from the center of the galaxy than Geier suggests. Still, the basic conclusion is the same. US 708 “pretty clearly doesn’t come from the center of the galaxy,” Brown affirms.

Stars like US 708 could give researchers a better handle on what causes type 1a supernovas. These are among the most powerful explosions in the universe.

The speed at which US 708 is departing the Milky Way would depend on the mass of the white dwarf that exploded. So astronomers might be able use US 708’s speed to determine that white dwarf’s mass. This could help them better understand how and why white dwarf stars explode. “If this scenario works,” Geier says, “we have a better means to study type 1a supernovas than before.”

Currently, all astronomers can do is observe a supernova’s stellar fireworks and then try to piece together what happened. “It’s like you have a crime scene,” Geier says. “Something killed the white dwarf and you want to figure it out.”

Power Words

(for more about Power Words, click here)

astronomy    The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

atmosphere   The envelope of gases surrounding Earth, another planet or a star.

black hole  A region of space having a gravitational field so intense that no matter nor radiation (including light) can escape.

constellation  Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

galaxy  A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gravity  The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

helium  An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -458 degrees Fahrenheit (-272 degrees Celsius).

hypervelocity   An adjective for stars that move across space at unusual speed — enough speed, in fact, that they can escape the gravitational hold of their parent galaxy.

intergalactic space   The region between galaxies.

light-year  The distance light travels in one year, about 9.48 trillion kilometers (almost 6  trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

mass  A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

matter  Something which occupies space and has mass. Anything with matter will weigh something on Earth.

Milky Way  The galaxy in which Earth’s solar system resides.

star  The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun  The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy.

supernova  (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

type 1a supernova  A supernova that results from some binary (paired) star systems in which a white dwarf star gains matter from a companion. The white dwarf eventually gains so much mass that it explodes.

velocity   The speed of something in a given direction.

white dwarf  A small, very dense star that is typically the size of a planet.It is what is left when a star with a mass about the same as our sun’s has exhausted its nuclear fuel of hydrogen, and collapsed.

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