An entirely new class of galaxy-escaping hypervelocity stars

Every once in a while, the Milky Way's supermassive black hole flings a wayward star into intergalactic space at speeds reaching 2 million miles per hour. But astronomers have now discovered a surprising new class of "hypervelocity stars" that can escape the galaxy — and they don't need the galactic core to do it.

Astronomers first discovered hypervelocity stars back in 2005. There may be some 1,000 exile stars originating from the Milky Way, all of them forming near the supermassive black hole at the core of our galaxy. Astronomers estimate that these giant blue stars are tossed out of these central regions every 100,000 years or so.

Hypervelocity stars are typically former members of a binary star system; when a binary swings too close to the supermassive black hole, the intense gravity yanks the binary apart, capturing one star while violently flinging the other outward at tremendous speeds — sometimes reaching 0.2% the speed of light, or 895 km/s, so they're aptly named.

An entirely new class of galaxy-escaping hypervelocity stars

But Lauren Palladino of Vanderbilt University and her colleagues have now discovered an entirely new class of hypervelocity stars, and they behave quite differently. These 20 newly discovered stars are about the same size as our Sun, so they're relatively small. And surprisingly, none of them appear to come from the galactic core.

Needless to say, these hypervelocity stars require a completely different ejection mechanism to escape the gravitational confines of the Milky Way. As astronomer Kelly Holley-Bockelmann noted in a statement, "It's very hard to kick a star out of the galaxy."

An entirely new class of galaxy-escaping hypervelocity stars

It's important to remember that all stars in the galaxy are moving at tremendous speeds — upwards of 1,000 km/s or 2.2 million miles per hour — but not relative to one another. Hypervelocity stars are unique in that they're moving at over a million miles per hour relative to neighboring celestial bodies.

Related: Hypervelocity planets | Has the Milky Way devoured other galaxies?

The researchers found that nearly half of the stars discovered exceed their galactic escape velocities with at least 98% probability, but that each star has less than a 25% chance of being a high-velocity fluke within the sample (which was gleaned from studying Sloan Extension for Galactic Understanding and Exploration [SEGUE] G and K dwarf samples). Consequently, further observations will have to be made to ensure that these are actually hypervelocity stars.

An entirely new class of galaxy-escaping hypervelocity stars

But looking at the data, and based on orbits calculated using the observed six-dimensional positions and velocity, it strongly appears that these object did not originate from the galactic center.

So, without the benefit of the central black hole — with the required mass equivalent to four million suns — how did these stars attain such extreme speeds? The researchers have no idea.

"The big question is: what boosted these stars up to such extreme velocities? We are working on that now," said Holley-Bockelmann.

One possibility, as is the case with the central-region binaries, is a 3-body chaotic gravity assist. Given the number of ternary star systems in the Milky Way (stars that orbit each other), this might actually be what's happening. But can massive stars or medium-sized black holes actually produce the gravitational forces required?

Read the entire study at The Astrophysical Journal: "Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample."

Images: Vanderbilt University/Palladino et al.
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