Sometimes even the biggest stars can be bullied. And in this new Hubble Space Telescope observation, a large star has been ejected from its place of birth, probably after being catapulted by the overwhelming gravitational force of a couple of larger stellar siblings.
This runaway star is traveling at high speed, fast enough to be ejected from its galaxy entirely. There’s no turning back, it is destined for a lonely future charging toward the void between the galaxies. The star is currently 375 light-years from its suspected birthplace (a giant star cluster called R136 inside the Large Magellanic Cloud) and flying through its galaxy at 400,000 kilometers per hour — a speed that would cover the distance between the Earth and the moon in just an hour.
But how did the massive star on the outskirts of the 30 Doradus Nebula get blasted away? There is no known mechanism that allows a star to travel at these speeds by itself, so astronomers studying the Hubble images believe there were another two stars implicit in sending the third to a lonely death.
30 Doradus (also known as the Tarantula Nebula) is a chaotic breeding ground for the most massive stars in the Large Magellanic Cloud, located about 170,000 light-years from Earth. In fact, several stellar heavyweights topping 100 solar masses (nearly the maximum mass for a star) are known to live there, so this nebula is an ideal place to study how and why these stars grow so big.
In such a fertile and dynamic stellar nursery, it’s hardly surprising that some of these monsters cross paths from time to time. However, this is the first runaway star to be detected powering away from the nebula at such high speeds.
Using the new Cosmic Origins Spectrograph (COS) that was installed during Hubble’s Service Mission 4 (in May 2009), astronomers calibrated the instrument by studying this young massive star. Astronomers have known about the star for some time (since its original discovery in 2006 by astronomers using the Anglo-Australian Telescope at Siding Spring Observatory, Australia) and suspected it was a little out of place. It is located far from any other young blue stars in the R136 cluster, a sure sign that something was amiss.
The COS instrument confirmed this in July 2009 when it observed extremely hot stellar winds emanating from the star. This suggests the star was only one or two million years old, again, another piece of evidence that confirmed the rogue star’s youth.
These powerful stellar winds indicate the star is also very big, approximately 90 times the mass of our sun. Its mass and extreme winds also indicate the star will have a pretty short lifespan, exploding as a supernova and ending up as a black hole in only a few million years.
Interestingly, the COS team think there are more rogue stars out there, all dislodged from the nebula and flung away. But why is this happening?
There are two possible mechanisms that result in hyper-velocity stars. The first is thought to occur when a binary star system consists of one massive star and one smaller star. When the larger of the pair explodes as a supernova, the smaller star is flung away (like a spinning hammer thrower releasing the ball and chain, sending it across the field at high speed). The second mechanism entails a complex interaction with the gravity of other stars; their gravitational fields tugging at, then flinging a star away at hyper-velocity speeds.
It isn’t thought the stars in the nebula are old enough to explode as supernovae, so the supernova-binary system explanation probably didn’t cause this particular speeding star. The rogue star was most likely accelerated by the second mechanism, treated like a massive pinball by its larger stellar siblings.
Source: Hubble Space Telescope