How do stars die when they fall into a supermassive black hole?

When stars get too close to a black hole, they go out with a whimper, not a bang.
By | Published: May 30, 2017
This artist’s rendition shows a star passing the event horizon of a black hole, disappearing from the observable universe as it’s swallowed whole.
Mark A. Garlick/CfA

When stars are swallowed by a supermassive black hole, do they go out like a candle or crash into a solid surface? The first option upholds general relativity as is, while the second relies on a modified version of this famous theory. Now, a group of astronomers has found a way to study what happens at a black hole’s event horizon, even though there are no images of this region of space. Their findings? General relativity is safe.


Pawan Kumar of The University of Texas at Austin, along with his graduate student Wenbin Lu and colleague Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics, found a unique way to determine just what happens to stars as they approach extremely massive objects — i.e., black holes. Their results are published in Monthly Notices of the Royal Astronomical Society.


Nearly every galaxy in the universe, including our own, has a central massive object at its center. These massive objects are assumed to be supermassive black holes several millions or even billions of times the mass of our Sun. This is because according to general relativity, objects of a certain mass cannot be held up by any known force, and thus collapse into black holes.


Black holes are singularities with no physical surface area, surrounded by an event horizon. The event horizon acts like a one-way membrane — material can fall in toward the black hole, but once it passes the event horizon, it can no longer send out light that is visible to the rest of the universe because the gravity of the black hole pulls the light back toward itself. Once past the event horizon, the material, in essence, disappears from view.


But what if general relativity isn’t quite right? What if, instead, these central massive objects aren’t collapsed down to a point? If that were the case, the event horizon would have different properties. Kumar and his colleagues theorized that if the central massive object is not a black hole, then the “event horizon” would not act like a one-way membrane, but like some kind of solid surface against which any infalling material would smash. This would produce a visible effect as the infalling star’s gas lit up as a result of the collision, enveloping the massive object and glowing visibly for months or even years.


“Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” said Kumar in a press release announcing their results.


To test these competing ideas, the team turned to observations taken with the 1.8-meter Pan-STARRS telescope in Hawaii over the course of 3.5 years. By calculating how many stars should fall onto supermassive black holes in the nearby universe, the group could determine how many events they should see over the course of the 3.5 years the survey was active. If they saw signs of this theorized glow, it would signal that the event horizon was solid; if not, it would mean that general relativity is correct, and stars simply pass the event horizon and go dark.


“Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true,” explained Lu.


Why is there any debate at all? Currently, telescopes are not able to resolve the region immediately around a compact object to directly observe the event horizon and its properties. But astronomers are continually pushing the boundaries of their instruments, seeking better, closer-in images of black holes.


The Event Horizon Telescope, a combination of several observatories, made its first observations of the area surrounding a supermassive black hole in April, though these data are still undergoing image processing and evaluation.


The Large Synoptic Survey Telescope, currently under construction, will perform surveys like those taken with the Pan-STARRS telescope, but with significantly greater sensitivity to events like the glow that would be left behind by collisions with a solid surface event horizon.


Because of this lack of direct evidence, the event horizon has remained mysterious in nature. And according to Kumar, “Our motive is not so much to establish that there is a hard surface, but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”


After poring through the data returned from the Pan-STARRS telescope, Kumar’s group found no “afterglow” signature of any collisions.


In this case, a lack of signal is a good thing, if you support general relativity. Said Narayan, “Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades. General relativity has passed another critical test.”