'Monster' Stars Do Not Collide: Spectacular Crash Avoided

Collisions between the remains of monstrous stars could be among the most spectacular phenomena in the Universe. Unfortunately, it's not likely that we'll be able to see these crashes any time soon. Astronomers have discovered that the first of these stellar collisions won't occur for billions of years.

"Monster" stars can reach masses between 200 to 300 times that of our own Sun. In fact, astronomers didn't even think that stars this large could exist until relatively recently. Three years ago, star clusters in the Magellanic Clouds were discovered to house these "impossible" stars--monsters with masses that made our own Sun seem tiny in comparison.

Needless to say, astronomers were intrigued by the findings. Astrophysicists in particular wondered if collisions between their remnants could occur. The gravitational waves resulting from such an event would be powerful enough so that even our current detectors could sense them--and at distances much larger than our stellar black holes.

When large stars "die," they can pass in two different ways. Material from these stars can be blown into space, or the stars can collapse under their own gravity into a black hole.  Researchers have demonstrated in the past that it's possible for some supermassive stars to form black holes, which means that the universe could play host to binary systems of supermassive stars which later transformed into binary systems of two black holes.

It's these binary systems that hold the potential for a collision. Objects orbiting in tight binary systems composed of neutron stars or ordinary black holes lose their energy over time. Eventually, the two objects grow closer and closer, eventually colliding into one another. This collision can result in an explosion, accompanied by the emission of gravitational waves. In order to see whether such a collision would be likely, though, researchers took a closer look at these supermassive stars.

"In a supermassive binary star system, the situation is different," said Krzysztof Belczynski, one of the researchers, in a news release. "We know that the components of such a system must be formed at a relatively large distance from each other. We also know that supermassive stars do not expand, so there cannot be a common envelope phase. This means that there is no physical mechanism that would effectively cause the orbit to tighten!"

In fact, the researchers found that the only way for these huge remnants to lose energy is through the emission of gravitational waves. However, the waves emitted by such a system of widely separated stars or black holes are very weak. This means that the energy loss is extremely slow.

"It will take many tens of billions of years, perhaps hundreds of billions of years, for the black holes to collide," said Daniel Holz of the University of Chicago in a news release. "That is many times longer than the amount of time which has passed since the Big Bang, so we stand practically no chance of detecting the gravitational waves from such a collision in the heavens."

The findings were presented at the 10^th Edoardo Amaldi Conference on Gravitational Waves held in Warsaw.