Hypervelocity stars, such as the on referenced in Tom’s post, are stars which are moving so fast they have achieved escape velocity from the galaxy.  Sounds cool, but they’re also known as “exiled stars”, so maybe not so much.  They achieve velocity through gravity assist; by getting too close to something of high mass (like a supermassive black hole), and getting a sling-shot out of the galaxy.  The star maintains its shape and life cycle, but any planets that were around it would’ve been lost by the sling.  These are incredible forces you’re dealing with.  There is nothing gentle about the process.

Chandra image of Sgr A*, NASA/CXC/MIT/F. Baganoff, et al

We use the same process when we sling our spacecraft around planets to give them a speed-boost.  It works VERY well.

How common is such an event we don’t know.  We’ve found 16 so far.  The original companion planets of the stars may be on their way out of the galaxy, also, but since they aren’t shiny we don’t see them.  They aren’t still circling their star, that’s for certain.  They all may eventually be captured, but it won’t be in this galaxy.  They’re outta here.

Theorized in 1988 and discovered in 2005, not much yet is known about HVS.  So far, the theory runs that there may be as many as 1,000 of them in the Milky Way Galaxy.  Considering there are 100 billion stars in the Milky Way, that’s not a whole lot.  Still, they sure aren’t hanging around waiting for us to take their picture.  We could be getting new ones all the time.

Some scientists think our HVS could be stars our galaxy originally captured from an orbiting dwarf galaxy which got too close.  Some neutron stars are inferred to be at high velocity, but that’s the result of an asymmetrical supernova.  Not only did it supernova, it supernovaed messily.  An example of that would be the neutron star RX J0822-4300, which moves at 0.5% of light speed, or about 1500 km/s.  That speed doesn’t grab you?  The 125X1400mm shell fired out of a tank travels at about 1700 m/s.  Or about 6120 km/h.  Hmmm, that’s kilometers per second vs kilometers per hour.  These puppies aren’t wasting time.

The first HVS discovered is SDSS J090744.99+024506.8.  Its “other” name is The Outcast Star.  I don’t know which is worse.  One thing is for sure; there are a lot of strange, strange things in the universe.  Some of them in our own back yard.