An unusual blue star, racing away from the Milky Way at 2.5 million kilometres per hour, was put on its path by an encounter with the supermassive black hole at the centre of our Galaxy, which resulted in one star being destroyed and another two merging
A total of 16 ‘hypervelocity’ stars have thus far been discovered speeding through the Galaxy’s halo, but this is the first time that any have been traced directly back to the centre of the Galaxy. The star in question, HE 0437-5439, was first discovered in 2005 by a survey conducted by the European Southern Observatory, and is already 200,000 light years from the centre of the Galaxy. To ascertain whether HE 0437-5439 really did originate from the Milky Way, or whether it had instead been ejected from the nearby Large Magellanic Cloud, a group of astronomers used a method developed by team-member Jay Anderson of the Space Telescope Science Institute in Baltimore to measure the star’s motion. Using the Hubble Space Telescope’s Advanced Camera for Surveys, they snapped two pictures of the star taken three-and-a-half years apart. Despite HE 0437-5439’s tremendous velocity, its huge distance meant that it only appeared to move by 0.04 of a pixel in the images. They measured this motion against eleven faint background galaxies, and were able to draw a line back to where it had come from.
“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,” says team-member Warren Brown of the Harvard-Smithsonian Center for Astrophysics. “Its motion points directly from the Milky Way centre.”
Given HE 0437-5439’s breakaway speed, it must have been flung out 100 million years ago to have reached its current distance, and it is just going to keep on going – it’s velocity is twice the escape velocity of the Galaxy.
“There is no star that travels that quickly under normal circumstances – something exotic has to happen,” says Brown. The puzzle doesn’t end there. HE 0437-5439 has a mass nine times that of the Sun, which means that it should have used up all its nuclear fuel and possibly even exploded as a supernova after just 20 million years. Why does it still exist after 100 million years? The answer lies in its ‘exotic’ origin
A hundred million years ago, a triple star system wandered too close to the 4.3 million solar mass black hole in the galactic centre. Theoretical models have shown that in such circumstances one of the three stars will be captured by the black hole, and probably eventually destroyed, while its momentum is transferred to the other two stars that are flung away at high speed. Only that in the case of HE 0437-5439, the two ejected stars merged, creating what is known as a blue straggler – a star in which the hydrogen has been efficiently mixed up so that it can survive longer than other stars of similar mass (causing them to linger on the main sequence of hydrogen-fusing stars, hence the term ‘stragglers’).
Such stars are rare; it is expected that on average the black hole will fling out stars every 100,000 years. “ For every 100 million stars in the Galaxy lurks one hypervelocity star,” says Brown. But they don’t just tell us about where they have come from; they can also tell us about where they are going. Surrounding the Milky Way is a giant halo of invisible dark matter, the gravity of which affects the trajectory of hypervelocity stars, so by measuring the motion of these stars we can detect where the densest clumps of dark matter are, and judge the halo’s overall shape.
The team’s findings on HE 0437-5439 are published online in the 20 July edition of Astrophysical Journal Letters.
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