It's bad luck if a black hole crosses your path. Actually it's very bad luck, particularly if you happen to be a star like SDSS J090745.0+024507. Known by some as "The Outcast Star", it had the misfortune tens of millions of years ago to stray a little too close to Sagittarius A*, our galaxy's resident supermassive black hole. Subsequently it was flung outwards at a blistering speed, and it's been travelling ever since. In fact, it's in the process of leaving the galaxy!

Outcast (as I'll call it from here on, for simplicity's sake) is a veritable stellar bullet. A hypervelocity star, screaming through the galactic halo at over 709 kilometres per second. Currently about 50 kiloparsecs (roughly 160 thousand light years) from the galactic core, Outcast is already in the outer reaches of the Milky Way's halo. And it's been travelling for a long time. Even at such a high speed, it would have left the core around 80 million years ago. Around the time dinosaurs still walked on our planet, in fact. To give some perspective, anything moving faster than 30 km/s is normally considered 'high velocity'. It is, in fact, the fastest star ever discovered. This is also the reason we know it was kicked out by A*. Only a supermassive black hole could throw an entire star at such a speed. Even a supernova, while capable of delivering quite a kick, would only be able to manage a speed of around 300km/s.

Gravitational behemoths, supermassive black holes can wreak havoc on anything that gets caught in their titanic grip. Once upon a time, Outcast would probably have had a companion star. The two would have formed a heavy binary system, probably just minding their own business somewhere near the galactic core. Somehow though, they strayed a little too close for comfort. Whenever a two body system encounters another object closely enough, the end result is normally that the smallest of the objects is ejected from the system. In this case, that object happened to be the star, Outcast. It's one time companion might even still be orbiting A*. Assuming a similar mass, it would have ended up orbiting at a radius of around 4000 AU, revolving once around the black hole every 100 years or so. That's a terrifyingly fast orbital velocity. And no wonder. A* is estimated to weigh in at around 4 million solar masses. 4000 AU is certainly much closer than I'd ever like to get to it!

The thing is, while I said before that Outcast was the smallest of the objects in the system, it's not exactly small. Weighing in at around 3 solar masses, it's a late B-type star. A slowly pulsating B-type variable, to be precise, with an effective surface temperature of around ten thousand degrees. A lot hotter and brighter than the Sun. B-type stars typically have a lifespan of around 350 million years, which is quite short for a star. So it goes, the hotter they are, the faster they burn out. That said, there's a fair chance it may yet live long enough to properly leave the galaxy before its inevitable demise. A view of the Milky Way from the outside would probably be so spectacular it would almost be worth the one way ticket to intergalactic oblivion!

It's probably not the only one, either. The authors of one of these papers estimate (albeit tentatively) that between a thousand and ten thousand such hypervelocity stars probably exist in the Milky Way's halo at any given time. You have to wonder how many might be recaptured into the Milky Way's halo, and how many will be lost to the vast recesses of intergalactic space.

A final interesting thing about all of this is that, seeing as we're quite certain this star left the galactic core, there must be a population of young stars in the centre of the galaxy. They must certainly have existed 100 million years ago, before Outcast was forcibly ejected. Actually, younger stars have been observed in close proximity to the galactic centre too. Our neighbouring spiral galaxy, Andromeda, also has young blue stars in its core. And that's puzzling. Puzzling because so close to a hulking black hole, tidal forces should disrupt star formation. Simply, stars should be torn apart long before they can form. So how do stars form in such an extreme environment? Actually, that part is still a mystery. I expect some theorist somewhere is losing sleep trying to solve the problem even as I type this... Whoever they are, I wish them luck!

Illustration:
Photomanipulation by yours truly.
Original images:
Galaxy -- NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team.
Star -- Spica. Photographer unknown.

Brown, W., Geller, M., Kenyon, S., & Kurtz, M. (2005). Discovery of an Unbound Hypervelocity Star in the Milky Way Halo The Astrophysical Journal, 622 (1) DOI: 10.1086/429378


Cesar I. Fuentes, K. Z. Stanek, B. Scott Gaudi, Brian A. McLeod, Slavko B. Bogdanov, Joel D. Hartman, Ryan C. Hickox, Matthew J. Holman (2008). The Hypervelocity Star SDSS J090745.0+024507 is a Short-Period Variable arXiv/astro-ph arXiv: 0507520