Want to Know How A Black Hole Swallows Stars? Ask a Computer!

a computer model of a black hole eating a star
A simulated black hole eating a star. Credit: NASA; S. Gezari (The Johns Hopkins University); and J. Guillochon (University of California, Santa Cruz

We're all fascinated with black holes. We ask astronomers about them, we read about them in the news. and they show up in TV shows and movies. However, for all our curiosity about these cosmic beasts, we still don't know everything about them. They flout the rules by being hard to study and detect. Astronomers are still figuring out the exact mechanics of how stellar black holes form when massive stars die.

All this is made tougher by the fact that we haven't seen one up close. Getting near one (if we could) would be very hazardous. No one would survive even a close brush with one of these high-gravity monsters. So, astronomers do what they can to understand them from a distance. They use light (visible, x-ray, radio, and ultraviolet emissions) that come from the region around the black hole to make some very shrewd deductions about its mass, spin, its jet, and other characteristics. Then, they feed all this into computer programs designed to model black hole activity.Computer models based on actual observational data of black holes help them to simulate what happens at black holes, particularly when one gobbles something up.

What Does a Black Hole Computer Model Show Us?

Let's say that somewhere in the universe, at the center of a galaxy like our own Milky Way, there's a black hole. Suddenly an intense flash of radiation flares out from the area of the black hole.

What has happened? A nearby star has wandered into accretion disk (the disk of material spiraling into the black hole), crossed the event horizon (the gravitational point of no return around a black hole), and is torn apart by the intense gravitational pull. The stellar gases are heated up as the star is shredded and that flash of radiation is its last communication to the outside world before it is lost forever.

The Tell-Tale Radiation Signature

Those radiation signatures are important clues to the very existence of a black hole, which does not give off any radiation of its own. All the radiation we see is coming from the objects and material around it. So, astronomers look for the telltale radiation signatures of matter being gobbled up by black holes: x-rays or radio emissions, since the events that emit them are very energetic. 

After studying black holes in distant galaxies, astronomers noticed that some galaxies suddenly brighten up at their cores and then slowly dim down. The characteristics of the light given off and the dim-down time came to be known as signatures of black hole accretion disks eating nearby stars and gas clouds and giving off radiation. It was, as one astronomer said, "Like a black hole putting up a sign that said, "Here I am!!""

Data Make the Model

With enough data on these flareups at the hearts of galaxies, astronomers can use supercomputers to simulate the dynamic forces at work in the region around a supermassive black hole. What they've found tells us much about how these black holes work and how often they light up their galactic hosts.

For example, a galaxy like our Milky Way with its central black hole might gobble up an average of one star every 10,000 years.

The flare of radiation from such a feast fades very quickly, so if we miss the show, we might not see it again for quite a long time. But, there are many galaxies, and so astronomers survey as many as possible to look for radiation outbursts.

In the coming years, astronomers will be deluged with data from such projects as Pan-STARRS, GALEX, the Palomar Transient Factory, and other upcoming astronomical surveys. There will be hundreds of events in their data sets to explore. That should really boost our understanding of black holes and the stars around them. Computer models will continue to play a large part in delving into the continuing mysteries of these cosmic monsters.