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Runaway stars

Martin Meredith

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While most stars follow a steady course, orbiting with the rest of the Milky Way, since 2005 a couple of dozen stars have been discovered that have a trajectory and velocity that will eventually allow them to leave our galaxy altogether. I came across them in an article with the exciting title Proper motions and trajectories for 16 extreme runaway and hypervelocity stars by Warren Brown (the discoverer of the first such stars) and colleagues.

It is difficult to explain how a star can achieve 'galactic escape velocity'. The leading model, which apparently provides a plausible explanation for most hypervelocity stars, involves a binary star coming close to the super-massive black hole at the centre of our galaxy, leading to one member of the pair being ejected! Most of these stars are quite faint (range 16-20), making them ideal candidates for electronically-assisted observing. Given the rarity of these beasts and the undoubtably exotic nature of their ejection, who could resist having a go at observing them? Not me. 

There are two in particular that I want to observe in the coming months, both in Ursa Major, and tonight was the first session when the constellation was high enough (spring would be better but I am impatient). Unfortunately, the dew on the scope froze up before I had a chance to capture the second of the two, but even at 1200 km/s -- the fastest unbound star in our galaxy -- I guess it will still be around for a while ;-)

The one I did manage to track down goes under the designation HVS 5 and has a radial velocity consistent with the central black hole scenario (that is, it can be traced back to the centre of the galaxy). It can be found at 9h 17m 59.47, +67 22 38.3, not too far from M81/M82 and a degree or so from Arp 300. As you can see, it is quite faint (around mag 17.7) and lies at the end of a sickle-shaped asterism. There isn't much to see, but considering what this star has been through and what lies ahead, it is surely worth a look.


There is an interesting story behind the other star that is on my runaway star observing programme, but that's for another day when I manage to capture it...

Thanks for looking.





Edited by Martin Meredith
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Thanks for the comments. Rob, as I understand it there is no requirement for a high proper motion as in Barnard's Star but there is some measurable PM over a 6 or 7 year interval. I think the survey that discovered these stars was confined to the outer halo, so they're already quite distant (> 50 kpc). They do mention that PM measurements with Gaia will provide better constraints on the origin of these stars. 

There's an intriguing finding in there too: a surprisingly large number of runaways found so far are clustered together in Leo. 


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