Radio image of the galactic centre. (SARAO)

Bizarre Star Found Hurtling Out of Our Galaxy Centre Is Fastest of Its Kind Ever Seen

By Michelle Starr

July 31, 2019

Astronomers have discovered a new main sequence star that's the fastest of its kind ever found in the Milky Way. It's hurtling through the galaxy at an eye-popping speed of around 1,700 kilometres per second, and it's racing away from the centre of the galaxy.

This also makes it the first star that astronomers can confidently identify as having been ejected from the galactic centre. That means it was probably booted out by an interaction with the supermassive black hole therein, the colossus Sagittarius A* (Sgr A*).

"We actually discovered the star serendipitously," astronomer Daniel Zucker of Macquarie University told ScienceAlert.

"We've been conducting a survey called S5 … to observe stars in stellar streams, which are structures around our galaxy created by star clusters and dwarf galaxies as they are torn apart by the Milky Way's tidal forces."

According to Zucker, the spectra of stars can tell us about their temperatures, compositions, and ages, as well as how fast they are moving toward or away from us (their radial velocity).

"As a side project, professor Sergey Koposov of Carnegie Mellon University was examining spectra from the S5 survey to look for stars with high radial velocities; he was very surprised to find that one star - which he dubbed S5-HVS1 - was moving away from us at a speed of over 1,000 km/s!"

Follow-up observations and calculations confirmed the result, giving an insanely fast velocity of 1,700 km/s.

S5-HVS1 is pretty interesting. It's a main-sequence, or "living" star that is still undergoing hydrogen fusion in its core; in fact, it's relatively young, up to just 500 million years old of its estimated 1 billion-year lifespan.

It's an A-type star around 2.35 times the mass of the Sun, and shining quite brightly. These characteristics make it a real oddball among these speeding stars, known as hypervelocity stars, or HSVs.

That's because, according to a 2015 analysis, those HSVs astronomers have found in the main sequence tend towards O- and B-type stars: very hot and massive 'live fast and die young' stars that don't live more than a few tens of millions of years.

There are also hypervelocity 'dead' stars, or neutron stars, like the previous speed record-holder, RX J0822−4300, with an apparent velocity over 1,500 kilometres per second – a record speed when it was first calculated in 2006.

More recent research subsequently found it to be significantly slower, but faster dead stars were identified last year anyway: two white dwarfs clocking in at around 2,200 kilometres per second.

But hypervelocity dead stars have a clearer origin: when a dying star goes supernova, the explosion can be asymmetrical, ejecting the star itself out into space at insane speeds. For the white dwarfs, a double detonation, where both stars go kaboom, is believed to be responsible. 

Before S5-HVS1 (the discovery of which is still awaiting peer review), the current undisputed fastest known main sequence star in the galaxy was US 708, at 1,200 kilometres per second. It's an O-type. S5-HVS1 blows it out of the water.

But without an explosion, how do main sequence stars get kicked into such insane speeds? Well, that's where the black holes come in.

Astronomers think that the hypervelocity main sequence stars identified to date could have been ejected into space via three-body exchange interactions, where one of the bodies is a black hole, and the other two are stars in a binary system.

(Take a breather and watch this beautiful animation of a three-body system set to sound.)

"Three-body exchange interactions among stars and a massive black hole inevitably unbinds stars from a galaxy," wrote astronomer Warren Brown of the Harvard Smithsonian Centre for Astrophysics in 2015.

"Because stars have finite sizes, only a massive compact object can explain stars ejected at 1,000-km s−1 velocities."

S5-HVS1's position, roughly 29,000 light-years away from Earth, and the speed with which it's moving, indicates it was kicked out of the galactic centre about 4.8 million years ago, with a great deal of force.

"Our model of its orbit suggests that its initial velocity was fairly similar to its current velocity - roughly 1,800 km/s," Zucker said.

"To get it up to that speed would require a transfer of roughly 6 x 10^42 Joules of kinetic energy - for reference, a kick like that would accelerate the Earth to 0.997 times the speed of light!"

It's been hurtling across space ever since - but the mechanism that kicked it out is a little less clear.

For a three-body exchange interaction to have occurred, according to the paper, one of the stars would have had to have been relatively low mass, less than the mass of the Sun, locked in a short-period orbit with S5-HVS1 - lasting between 3 and 40 days.

Although these binaries would be rare, they are possible. An accretion event a few million years ago could have kicked off star formation in the galactic centre, producing the S5-HSV1 binary. Its trajectory is curiously aligned with a disc of these stars, which could indicate that's where it originated.

"The basic idea (sometimes referred to as the Hills mechanism) is that a binary star system (two stars orbiting each other) gets close to a super massive black hole, and one of the stars gets captured by the black hole," Zucker explained. 

"As the captured star is pulled into orbit around the black hole, the other star is flung off into space at high speed - in the case of a black hole with a mass several million times that of the Sun, the ejected star could reach a velocity of ~1,000 km/s or more."

Still another possibility is that an intermediate black hole in the galactic centre merged with Sgr A* a few million years ago. The dynamical friction at the last stages of its inspiral could have kicked a bunch of stars out of the galactic centre.

Although there is little evidence of such an event, it could be investigated by searching for more hypervelocity stars ejected at the same time as S5-HVS1.

There's also still more to learn about the star itself. A new release of Gaia data - a project to map the galaxy in three dimensions at the highest level of accuracy and detail ever - is expected at the end of 2021.

"Further observations of the star, and, in particular, the next Gaia data release, will allow us to measure the star's 3D position and velocity with greater accuracy, and hence better model its orbit; this will allow us to precisely pinpoint the star's origin," Zucker said.

"In addition, the discovery of more such HVS will give us clues about what's going on at the Galactic Centre, and enable us to get a better idea of the galaxy's shape and mass distribution."

The research has been submitted to the Monthly Notices of the Royal Astronomical Society, and is available on the pre-print website arXiv.

Update 1 August 2019: Article updated with comments from Daniel Zucker.