How Black Holes Grow So Big

A black hole: every galaxy has one. Scientists are unsure how these holes exactly come into existence or how they reach their size but one thing is for sure, nothing can escape once it is within their grasp. Anything that comes even remotely close to the range of their event horizon gets sucked in.

Until recently, scientists had no clue as to how black holes came to be, especially during the early years of the universe when the first stars started to develop. Also, there is new evidence that suggests these galactic behemoths devour entire stars much more frequently than previously suspected. Like a lot of scientific discoveries, this one came to be mostly by chance. The scientists that spotted the black holes devouring a star were initially looking into what occurs when two galaxies collide.

During a study in 2015 scientists from the University of Sheffield, led by James Mullaney, realized that from the 15 galaxies being monitored, one underwent serious changes since the last study performed in 2005. What the scientists saw is a strong flash of light known as a Tidal Disruption Event. During a TDE, a star comes too close to a black hole and gets torn apart. As the star dies, the black hole grows.

TDEs are not uncommon, but they are usually found during massive surveys which cover tens of thousands of galaxies. Finding it by chance almost never happens. Chances are the researchers got extremely lucky. Scientists even calculated the odds of something like this and determined that it may happen in one out of 100 attempts. A more probable reason as to why the scientists were able to spot this is that during a galaxy collision, TDEs happen more often.

This is similar to a lung disease survey performed on smokers and nonsmokers. Chances are that you are going to find more people with some form of lung disease in the smoker group than in the non-smoker group. So the chance of stumbling onto an occurrence like the TDE is similar to the chance of finding a nonsmoker with lung disease. Basically, colliding galaxies have a similar chance for TDEs as smokers have of contracting lung disease.

More research is needed if scientists are to better understand black holes. There is a chance that the scientists that spotted the TDE got lucky, but it is unlikely. More galaxy collisions need to be observed in order for the group to make any kind of objective claim. If TDEs indeed happen more often during galaxy collisions, then scientists should be able to find them much more easily from now on.

These findings might not be relevant for the human race, but it may provide answers to how exactly black holes grow. It has been thought that they mostly consume interstellar gas and dust. Consuming entire stars was considered a rare occurrence. Now Mullaney thinks the black holes receive more than a quarter of their size from swallowing stars that drift to close to them, and if this is true scientists just uncovered a highly feasible way for black holes to grow.

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