You are now leaving the Milky Way

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!

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

ResearchBlogging.orgBrown, 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

ResearchBlogging.orgCesar 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

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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15 Responses to You are now leaving the Milky Way

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  3. islamovz says:

    That blue star looks huger than our galaxy.
    восстановление зрения

  4. invaderxan says:

    Re: Slow way to travel…
    Now there’s an interesting concept. Olaf Stapledon… I might have to try and find a copy of that book! :)

  5. invaderxan says:

    Re: Slow way to travel…
    That sounds similar to the concept behind the Shkadov Thruster actually — although as a stellar engine, it’s doubtful that such a construction could ever achieve galactic escape velocity.
    Unfortunately, I suspect hitching a ride with a hypervelocity star would be a difficult task. The two options are either attempting to survive your planet being torn away by the supermassive black hole’s gravity as it’s slung outwards, or catching a hypervelocity star on it’s way out. Sadly, neither of these seems easily attainable…
    All the same, it’s very true that given the correct velocity, a red dwarf could easily survive long enough to make an intergalactic journey — assuming it wasn’t simply cast into intergalactic space of course.

  6. Anonymous says:

    Re: Slow way to travel…
    PS It’s my comment about hyperspace and A* above. High-speed stars remind me of Olaf Stapledon’s old novel “Star Maker” and his discussion of intergalactic travel – the Milky Way community of worlds tried to launch a star (plus Dyson swarm) towards another Galaxy, but the star objected and went nova in its displeasure, thus beginning the war between planet-life and stars. Stapledon’s physics is hokey but I do wonder about the stars…

  7. Anonymous says:

    Slow way to travel…
    Hi InvaderXan
    It’s a slow way to travel between Galaxies, but cheap. Wonder how fast a star can get up to? If it could push 0.005 c then a trip to M31 would take about 500 million years. If the star one’s planet was orbitting was a red-dwarf then it’s ~trillion year lifespan would allow a trek of ~5 billion light-years.

  8. invaderxan says:

    Re: Super-Massive Black-Hole…
    Very cool rock song title IMHO. I like Muse. :)
    Most figures for A*’s mass seem to be approximately 4 million M☉, mind you. I think the most accurate was about 3.8 million M☉… Though that’s just from memory.
    It is interesting that the larger the event horizon’s radius, the less dense the black hole technically is. I believe it was Michio Kaku who pointed out that a black hole with an event horizon the size of the observable Universe would have approximately the same density as the observable Universe… Which is rather a mind bender!

  9. Anonymous says:

    Super-Massive Black-Hole…
    Aside from being a very odd rock-song title, I’ve got to wonder just how big the Core BH will be. Every time I hear news from the Core I also hear a new figure for its mass. Ulvi Yurtsever wrote a paper on just how BHs evolve over time some years ago – apparently their interiors become nicer places to visit as the singularities stabilise and no longer cause disruptive tidal forces. Igor Novikov has also discussed their interiors and computes that one of the inner singularities encountered on the way in might actually be survivable, while the other one fades out. The tidal force of the remaining singularity goes to infinity BUT the total impulse imparted to an infalling body is finite, so conceivably one might survive an infall. Total infall time into a 4 million solar mass black-hole once you cross the event horizon is a surprising 1 minute or so. Thus you get to see the sights before hitting the singularity… but what happens then? In one possible scenario you end up in an anti-gravity ‘space’ that then spits you out of a new black-hole, maybe in this Universe or another.
    Anyone for a trip to ‘Hyperspace’?

  10. 6_bleen_7 says:

    Thanks—that’s actually larger than I’d have guessed.

  11. invaderxan says:

    Re: Sounds like A* is for “asshole” ;)
    Heheh… Ah, it’s all part of the dog-eat-dog world of stellar dynamics. :P

  12. beepbeep says:

    Sounds like A* is for “asshole” ;)
    I totally empathize with Outcast, FWIW !
    This silly moment was brought to you by Unemployed Liberal Arts Degree Holders, Ltd. You may now return to your productive scientific discussions.

  13. invaderxan says:

    Just as an FYI, some rough back-of-the-envelope calculations tell me that a 3 solar mass star orbiting at around 4000AU around A* would have an effective hill sphere of about 36AU. Which is more gravitational influence than I’d have initially expected!
    So discounting all other effects (like the tital disruption when Outcast was ejected, for instance), in theory any planet in a currently stable prograde orbit around the star should remain there if it’s orbit is within that distance.
    Though celestial mechanics isn’t exactly my forte, and there’s very likely something I’ve overlooked…

  14. invaderxan says:

    Insane speed, isn’t it? :)
    And yeah, I’m pretty sure any planets would be rapidly ripped away, but I can’t say for certain. Plus, I have no idea how big A*’s hill sphere would be — but I’m guessing it would be gigantic!

  15. 6_bleen_7 says:

    If the other member of the binary is orbiting Sagittarius A* at 4000 AU (6 × 1011 km), then assuming a perfectly circular orbit, its linear speed has to be about 1200 km/s! I haven’t done the math, but I get the feeling that a star in that orbit would have a really tough time holding on to planets, on account of the steepness of the gravity gradient.

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