Mysteries of a Runaway Star

Posted on August 3, 2010

By Dr. Jim Logan

NASA’s Hubble Space Telescope has clocked a massive blue star nine times the size of our sun hurtling away from our galaxy at an incredible 1.6 million miles an hour.  The star, dubbed HE 0437-5439, is one of sixteen so-called hypervelocity stars. Cosmic detectives, all members of the Hubble team, appear to have solved several fundamental mysteries surrounding this wayward blue giant.

The universe is never boring. Just when it seems almost understandable, it serves up another head scratcher. This stellar runaway is in intergalactic space, almost 200,000 light years from any other star, far above the plane of the galactic disk. Where did it come from? How did it get there? And why is it going so fast?

The Hubble team used the telescope’s Advanced Camera for Surveys to take two observations of the star three and one-half years apart. By measuring the star’s position relative to eleven background galaxies, an accurate position could be pinpointed. By determining the change in position, the star’s velocity could be calculated. This massive star is the ultimate speed demon. It is going almost 450 miles per second, three times faster than our sun is orbiting around the Milky Way and twice as fast as any object would have to travel to escape the galaxy’s gravitational field.

By plotting the star’s position, speed and heading, it becomes possible to extrapolate backward in time to determine its place of origin. A hundred million years ago, it was likely hustling through the center of our galaxy when it passed too close to a massive black hole.

But wait. Don’t black holes devour stars that get too close rather than toss them out of the galaxy? So the answer can’t be that simple. Plus there’s another twist. Recall our wayward exile has been travelling for at least one hundred million years. A star that size and that blue should have burned itself out after only 20 million years – far shorter than the time it took for it to get to its current location. 

By what possible mechanism could a star that size get thrown out of the galaxy AND be transformed by a fountain of youth? Obviously something out of the ordinary happened.

Astronomers and astrophysicists believe that all 16 known hypervelocity stars originated in the very heart of the galaxy. For every 100 million stars in the Milky Way, a single hypervelocity star is generated.

According to the experts, the most likely scenario for HE 0437-5439’s blue color and its incredible speed is this: It was once part of a triple-star system that consisted of two closely orbiting stars and a third outer member that was gravitationally bound to the group. The massive black hole at the center of our galaxy stripped away the outer member of the tight binary group thus transferring the doomed star’s momentum to the remaining stellar duo, catapulting them out of the galaxy. As each star of the pair evolved, the larger of the two aged more quickly and puffed up into a red giant eventually engulfing its partner. The two stars spiraled in together forming one giant superstar – our miraculous blue renegade.

What fate awaits our massive solitary stellar traveler?  

Size does matter and in this case size is destiny. HE 0437-5439 will undoubtedly live up to the reputation of all blue giants. They are referred to as James Dean stars because these superstars live fast and die young. Our galactic exile will shine like a cosmic beacon across a vast sea of emptiness for a few more million years consuming its own internal fuel at an ever-increasing rate. At the end it will expand rapidly in a futile attempt to find the last remnants of fuel for its nuclear furnaces. Then, in a cosmic instant, it will collapse under its own weight and die a spectacular and cataclysmic death as a supernova. For a few moments, it will outshine the entire galaxy it escaped so many years before. Then it will go dark. Dust to dust.

The ashes of this once magnificent star will slowly dissipate into the cold intergalactic void…forever.

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