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Rogue stars sail in intergalactic space

Animation of a rogue star

Illustration of a rogue star being ejected from the galaxy after tangling with the Milky Way’s central black hole.

IT’S VERY DIFFICULT to knock a star out of our Milky Way galaxy. In fact, the main mechanism that astronomers have come up with that can give a star the three-million-plus kilometre-per-hour kick it takes involves tangling with the supermassive black hole at the Milky Way’s core.

So far astronomers have found 16 of these “hypervelocity” stars. Although they are travelling fast enough to eventually escape galaxy’s gravitational grasp, they have actually been discovered while they are still inside the galaxy.

Now, astronomers report in a recent issue of the Astronomical Journal that they’ve identified a group of more than 675 stars on the outskirts of the Milky Way, which they argue are hypervelocity stars that have been ejected from the galactic core.

They selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.

“These stars really stand out. They are red giant stars with high metallicity which gives them an unusual colour,” says Vanderbilt University Assistant Professor Kelly Holley-Bockelmann who conducted the study with graduate student Lauren Palladino.

In astronomy and cosmology, “metallicity” is a measure of the proportion of chemical elements other than hydrogen and helium that a star contains. In this case, high metallicity is a signature that indicates an inner galactic origin—older stars and stars from the galactic fringes tend to have lower metallicities.

The researchers identified the candidates by analysing millions of stars catalogued in the Sloan Digital Sky Survey.

Illustration of a supermassive black hole

Illustration of a supermassive black hole, like the one thought to reside at the core of our Milky Way galaxy.

Encounter with a black hole

“We figured that these rogue stars must be there, outside the galaxy, but no one had ever looked for them. So we decided to give it a try,” said Holley-Bockelmann, who is studying the behaviour of the black hole at the centre of the Milky Way galaxy.

Astronomers have now found evidence for giant black holes at the centres of many galaxies. They estimate that the Milky Way’s central black hole has a mass of four million solar masses. They calculate that the gravitational field surrounding such a supermassive black hole is strong enough to accelerate stars to hypervelocities.

The typical scenario involves a binary pair of stars that get caught in the black hole’s grip. As one of the stars spirals in towards the black hole, its companion is flung outward at a tremendous velocity. A second scenario takes place during periods when the central black hole is in the process of ingesting a smaller black hole. Any star that ventures too close to the circling pair can also get a hypervelocity kick.

Even travelling at hypervelocities, it would take a star about 10 million years to travel from the Milky Way’s central hub to its outskirts 50,000 light years away.

Adapted from information issued by Vanderbilt University. Images courtesy Michael Smelzer / Vanderbilt University / Jenni Ohnstad / NASA Jet Propulsion Laboratory.

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Hypervelocity star leaves home

Illustration of a star being ejected from the Milky Way

In this illustration, the hot, blue star HE 0437-5439 has been tossed out of the centre of our Milky Way galaxy with enough speed—2.5 million kilometres per hour—to escape the galaxy's gravitational clutches.

  • Star is moving at 2.5 million kilometres per hour
  • Hubble shows that it originated in the core of the Milky Way
  • Now on the Milky Way’s outskirts and heading outward

Using the Hubble Space Telescope, astronomers have studied a “super-hot blue star” leaving our Milky Way galaxy three times faster than the speed of the Sun.

The “hypervelocity” star—known as HE 0437-5439—is presently 200,000 light-years from the galactic centre, and shooting outward at around 2.5 million kilometres per hour.

It also appears to be bafflingly youthful.

So what’s a nice young star doing in a place like that?

The astronomers say the most likely scenario is that it was once part of a triple-star system that lived in the inner parts of our galaxy, and which one day came too close the Milky Way’s central black hole.

They think one of the stars was captured by the black hole, but that the other two were boosted onto a trajectory that sent them zooming up and out of the Milky Way.

Illustration of a triple-star system being split up by coming too close to a black hole

This illustration shows how a triple-star system could be split up by coming too close to a black hole. One star gets captured, the other two head off together with extra speed. Those two stars then merged to become a single "blue straggler" star.

Somewhere along the way, the two stars merged to form one, much bigger and hotter star.

All of the 16 known hypervelocity stars seem to have come from the Galaxy’s inner region, but HE 0437-5439 is the first to have been pinned down as coming from the galactic core.

“Using Hubble, we can for the first time trace back to where the star came from by measuring the star’s direction of motion on the sky,” said astronomer Warren Brown of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts. “Our measurements point directly to the Milky Way centre.”

An age-old problem

The star’s apparent youth is another puzzle. In 2008, a different team of astronomers found a chemical match between the light spectrum of HE 0437-5439 and stars that live in the Large Magellanic Cloud (LMC), a neighbouring galaxy only 65,000 light-years from HE 0437-5439’s present position.

Hubble image of HE 0437-5439

HE 0437-5439 is 200,000 light-years from the Milky Way's core, heading outward at 2.5 million kilometres per hour.

This implied that HE 0437-5439 might have been travelling in the opposite direction, having come from the LMC.

But using Hubble to measure the star’s movement through space—by comparing images taken 3.5 years apart—it’s now clear that it originated in the inner Milky Way.

At its speed, the star should have taken around 100 million years to reach its current location. But stars this big and hot tend to “burn out” quickly, in perhaps just 20 million years.

So how did it go so far without burning out?

This is where the triple-star origin comes in. With one star captured by the black hole, the other two headed off together at a great rate of knots into the wild black yonder. One of them aged a little quicker, which in stellar terms means it puffed up and became a red giant, engulfing and merging with its sibling.

The result was what astronomers call a “blue straggler“, a “re-born” young star that comes from a stellar merger.

The astronomers are now working on finding the origin of four other lone stars on the far outskirts of the Milky Way.

Story by Jonathan Nally, Editor,

Image credits: illustrations, NASA / ESA / G. Bacon, A. Field and Z. Levay (STScI); science, NASA / ESA / O. Gnedin (University of Michigan, Ann Arbor), and W. Brown (Harvard-Smithsonian Centre for Astrophysics, Cambridge, Mass.)

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