Superfast Star Shot Out of Milky Way
This Hubble Space Telescope photo shows the superfast star HE 0437-5439 (with arrow) as it is being booted from the MIlky Way galaxy.
Credit: NASA/ESA/Z. Levay (STScI)/O. Gnedin/W.Brown Full Story

This story was updated at 11:05 a.m. ET.

A super-hot blue star hurtling through space has been shot completely out of the Milky Way, new Hubble Space Telescope photos reveal.

The star is streaking across space at a blistering speed of 1.6 million mph (2.5 million kph) ? three times faster than our sun's orbital velocity in the Milky Way. Hubble observations confirm that the stellar speedster hails from the Milky Way's core, settling some confusion over where it originally called home. [Photo of the ejected star.]

Astronomers think the star is a survivor from a triple-star system that traveled through the bustling center of our Milky Way galaxy 100 million years ago, but made the perilous mistake of wandering too close to the galaxy's giant black hole, which captured one of the stars and flung the other two out of the Milky Way. The two ejected stars then merged to form a super-hot, blue star.

While it may seem a little farfetched, astronomers using NASA's Hubble Space Telescope say it is the most likely scenario for the so-called hypervelocity star, known as HE 0437-5439, one of the fastest ever detected.

"Using Hubble, we can for the first time trace back to where the star comes from by measuring the star's direction of motion on the sky," said astronomer Warren Brown of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of the study and a member of the Hubble team that observed the star. "Its motion points directly from the Milky Way center. These exiled stars are rare in the Milky Way's population of 100 billion stars. For every 100 million stars in the galaxy lurks one hypervelocity star."

Since the first discovery in 2005, astronomers have found 16 hypervelocity stars, most of which are thought to be exiles from the heart of our galaxy. But, the new Hubble result is the first direct observation that links a high-flying star to a galactic center origin.

What this tells us

The movements of these breakneck stars could reveal the shape of the dark matter distribution surrounding our galaxy.

"Studying these stars could provide more clues about the nature of some of the universe's unseen mass, and it could help astronomers better understand how galaxies form," said Oleg Gnedin of the University of Michigan in Ann Arbor. "Dark matter's gravitational pull is measured by the shape of the hyperfast stars' trajectories out of the Milky Way."

The HE 0437-5439 stellar outcast is already trekking deep in the Milky Way's distant outskirts, high above the galaxy's disk, about 200,000 light-years from the center. By comparison, the diameter of the Milky Way's disk is approximately 100,000 light-years.

Astronomers used Hubble data to measure the runaway star's direction of motion and determine the Milky Way's core as its starting point. Brown and Gnedin's team then calculated how fast the star had to have been ejected in order to reach its current location.

"The star is traveling at an absurd velocity, twice as much as the star needs to escape the galaxy's gravitational field," said Brown, who discovered the first unbound, hypervelocity star in 2005. "There is no star that travels that quickly under normal circumstances ? something exotic has to happen."

But wait, there's more

After studying the star, researchers noticed that some of the calculations did not seem to add up.

Based on the speed and position of HE 0437-5439, the star would have to be 100 million years old to have journeyed such a distance from the Milky Way's core. Yet its mass ? nine times that of our sun ? and blue color mean that it should have burned out after only 20 million years ? far shorter than the transit time it took to get to its current location.

The most likely explanation for the star's blue color and extreme speed is that it was part of a triple-star system that was involved in an ill-fated encounter with the Milky Way's monster black hole.

This concept for imparting an escape velocity on stars was first proposed in 1988. The theory predicted that the Milky Way's black hole should eject a star about once every 100,000 years.

Brown suggests that the triple-star system contained a pair of closely orbiting stars and a third outer member that was also gravitationally tied to the group. The close encounter enabled the black hole to wrench the outer star away from the tight binary system.

The doomed star's momentum was then transferred to the stellar twosome, boosting the duo to escape velocity from the galaxy. As the pair rocketed away, they carried on with normal stellar evolution.

The more massive companion star evolved more quickly, puffing up to become a red giant and enveloping its partner in the process. The two stars spiraled together, merging into one superstar ? a blue straggler.

"While the blue straggler story may seem odd, you do see them in the Milky Way, and most stars are in multiple systems," Brown said.

The mysterious blue straggler

This vagabond star has puzzled astronomers since its discovery in 2005 by the Hamburg/European Southern Observatory sky survey.

Astronomers had proposed two possibilities to solve the age problem. The star either defied the normal aging process by becoming a blue straggler, or it was flung out of the Large Magellanic Cloud, a neighboring galaxy.

In 2008, a team of astronomers thought they had solved the mystery. They found a match between the exiled star's chemical makeup and the characteristics of other stars in the Large Magellanic Cloud.

The rogue star's position is also close to the neighboring galaxy ? only 65,000 light-years away. The new Hubble result now settles the debate over the star's birthplace.

Astronomers using the sharp vision of Hubble's Advanced Camera for Surveys were able to make two separate observations of the wayward star 3 1/2 years apart. Team member Jay Anderson of the Space Telescope Science Institute in Baltimore, Md., developed a technique that measures the star's position relative to each of 11 distant background galaxies, which form a frame of reference.

Anderson then compared the star's position in images taken in 2006 with those taken in 2009 in order to calculate how far the star moved in relation to the background galaxies. The star appeared to move, but only by 0.04 of a pixel (picture element) against the background.

"Hubble excels with this type of measurement," said Anderson. "This observation would be challenging to do from the ground."

The team is now trying to determine the homes of four other unbound stars, all located on the fringes of the Milky Way galaxy.

"We are targeting massive 'B' stars, like HE 0437-5439," said Brown, who has discovered 14 of the 16 known hypervelocity stars. "These stars shouldn't live long enough to reach the distant outskirts of the Milky Way, so we shouldn't expect to find them there. The density of stars in the outer region is much less than in the core, so we have a better chance to find these unusual objects."

The results of this study were published online July 20 in The Astrophysical Journal Letters.