In 2019, astronomers observed the closest example of a star being crushed, or “spaghettified,” after getting too close to a massive black hole.
This tidal disruption of a Sun-like star by a black hole 1 million times its mass occurred 215 million light-years from Earth. Fortunately, it was the first such event bright enough for UC Berkeley astronomers to study the optical light from the star’s death, specifically the polarization of the morning, to learn more about what happened after the star was torn apart.
Their observations from October 8, 2019, indicate that much of the star’s material was blown away at high speeds – up to 10,000 kilometers per second – to form a spherical cloud of gas that blocked most of the high-energy emissions produced as the black hole swallowed the rest of the star.
Earlier, other optical light observations from the explosion, AT2019qiz, revealed that much of the star’s mass had been ejected outwards in a strong wind. But the new data on the polarization of the light, which was essentially zero at visible or optical wavelengths when the event was brightest, tells astronomers that the cloud was likely spherically symmetric.
“This is the first time anyone has deduced the shape of the gas cloud around a tidal star,” said Alex Filippenko, a professor of astronomy at UC Berkeley and a research team member.
The results support one answer to why astronomers don’t see high-energy radiation like X-rays from many of the dozens of tidal disruption events observed so far: X-rays that are produced by material stripped from the star. And dragged into the accretion disk around the black hole before falling in, they are obscured from view by gas blown outward by the black hole’s strong winds.
“This observation rules out a class of solutions that have been suggested theoretically and gives us stronger constraints on what happens to the gas around the black hole,” said UC Berkeley graduate student Kishore Patra, lead author of the study. “People have seen other evidence of wind coming from these events, and I think this polarization study will definitely strengthen that evidence in the sense that you wouldn’t get a spherical geometry without enough wind. The interesting fact here is that a significant fraction of the material in a star that spirals moves inward, it doesn’t end up falling into the black hole—it’s blown away from the black hole.”