When a star is swallowed by a black hole, things can get quite messy. Take, for example, the event known as ASASSN-14li, in which a massive star collided close to a supermassive black hole and paid the final price.
There are two main dangers posed by a black hole. The first is that they own event horizon. Mass curves in space and black holes contain so much mass in such a small space that space-time collapses in on itself creating a cosmic trap. Anything that crosses the event horizon is lost forever. The second is the threat of tidal forces.
When an object of a dense mass such as a planet, a star, or a black hole approaches, the part of the mass that is closer to the mass is pulled in with a slightly greater force than the part of the mass further away from you. Gravity tries to compress the object into an oval or ellipse rather than into a sphere. Since it is these forces between the Earth and the Moon that cause tides in the oceans, they are known as tidal forces. Tidal forces are usually fairly small, but near massive, dense objects such as a black hole, tidal forces can be so strong that they can tear the objects apart, causing what is known as a tidal disturbance event (TDE).
When a star approaches a black hole, tidal forces usually tear the star apart before it reaches the event horizon. How much of a star is consumed by a black hole depends on the star’s original trajectory. In the case of ASASSN-14li, most of the star has not been consumed by the black hole. TDE shredded the star and heated its remains, emitting powerful X-rays and ultraviolet radiation. And while this was bad news for the star, it was great news for astronomers.
NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton telescope captured X-ray spectra of the event, and observations from other observatories provided ultraviolet spectra. Together, this has given astronomers a detailed look at the doomed star’s composition. In this latest work, the team was able to analyze spectra of both material captured in orbit around the black hole and gases flowing away from the black hole. By comparing things like the abundance of nitrogen and carbon in the stellar debris, the team was able to confirm that the star has a mass of about 3 suns.
Earlier this year, astronomers noticed another named TDE Scary Barbie Which could be a star with a mass of 14 suns, but this estimate is based on the event’s overall brightness, not its spectra. The ASASSN-14li is currently the largest TDE with a confirmed mass.
This event is a powerful example of how stars can approach a supermassive black hole. With more events, we may be able to study the clustering of stars around supermassive black holes in other galaxies, which will help us understand how galactic cores might evolve and what role they play in the evolution of stars.
reference: Miller, John M., et al. “Evidence for massive stellar perturbation in the X-ray spectrum of ASASSN-14li.” Astrophysical Journal Letters 953.2 (2023): L23.