Black hole "foraging" live! How do stars become "spaghetti"?

Black holes turn stars into spaghetti: a cosmic crime scene revealed

Astronomers have successfully reconstructed the final fate of a star, which was destroyed and partially swallowed by a giant black hole, as if it had been turned into a long, thin strand of "spaghetti".

An image of a star being torn apart by the immense gravitational pull of a massive black hole.

(Image credit: NASA/CXC/M. Weiss)

A cosmic observation has revealed details about how black holes "feed." In a new study, astronomers examined the destruction of a star by a supermassive black hole, revealing how these cosmic beasts ingest matter that passes close to them. The results show that a significant amount of this matter is not swallowed by the black hole, but ejected.

The bloody event, which took place 215 light-years from Earth and was first observed in October 2019, marks the aftermath of a sun-like star being destroyed by a black hole more than a million times its mass. It is the closest stellar "spaghettification" event ever observed by astronomers, caused by the tidal forces of a massive black hole.

This so-called tidal disruption event, which occurred in a spiral galaxy within the Andromeda constellation, is the first event bright enough in visible light to allow astronomers to study in detail the behavior of the material after the star was torn apart.

By observing the polarization of light during the event, researchers at the University of California, Berkeley, concluded that most of the stellar material was ejected from around the black hole at speeds of up to 22 million miles (about 35 million kilometers) per hour. The ejection event, named AT2019qiz, produced a cloud of gas. New polarized light observations show that the cloud of gas has a spherically symmetrical shape. The width of the gas cloud is equivalent to 200 times the average distance from the Earth to the Sun, which means its radius is 100 times larger than the radius of our planet's orbit, and its outer edge is about 930 million miles (about 1.5 billion kilometers) from the central black hole.

"It is one of the most incredible things in the universe that a supermassive black hole can tear apart a star with its enormous tidal forces," said Wenbin Lü, an astronomer at the University of California, Berkeley, and co-author of a new paper describing the observations, in a statement. "These stellar tidal disruption events are one of the very few ways that astronomers can learn about the existence of supermassive black holes at the centers of galaxies and measure their properties. However, the complex processes behind tidal disruption remain poorly understood due to the extremely high computational cost of numerically simulating these events."

This new discovery may explain why astronomers have not observed high-energy radiation, such as X-rays, in other tidal disruption events. This radiation is caused by matter from the star being dragged into a thin disk around the black hole, where it is heated and produces high-energy radiation, and also when the matter enters the black hole. However, this radiation is obscured by the gas cloud ejected by the powerful gas flow.

When a star strays into an extremely close orbit near a black hole, a tidal disruption event occurs, and simulations show that the destroyed stellar material orbits the cosmic behemoth and eventually falls onto the surface of the black hole. (Image credit: NRAO/AUI/NSF)

"This observation rules out a class of theoretical solutions and provides stronger constraints on the behavior of gas around black holes," Kishore Patra, a graduate student in astronomy at the University of California, Berkeley and lead author of the paper, said in a statement. "We have seen evidence for other winds emanating from these events, and I think this polarization study solidifies that evidence, because without enough wind, it would be impossible to get a spherically symmetric geometry."

"What's interesting here is that a significant portion of the stellar material that spirals toward the black hole does not end up falling into the black hole, but is ejected away from it," Patra added.

The result appears to contradict a theory proposed by many astronomers that when a star is destroyed by a black hole, a highly asymmetric accretion disk is formed. Such an accretion disk would exhibit highly polarized light - something that was not observed in this tidal disruption event.

A second set of observations in November 2019 showed that the light from the event was only slightly polarized. The team said this finding suggests that the ejected gas cloud was thin enough to reveal the asymmetric gas structure around the black hole.

"The accretion disk itself is hot enough to emit most of its light in the X-ray range, but that light has to travel through this cloud, where it is scattered, absorbed, and re-emitted multiple times before it can escape the cloud," Patra said. "At each of these passes, the light loses some of the photon's energy, ultimately dropping to energy levels in the ultraviolet and optical spectrum. The last scattering determines the photon's polarization state. So by measuring the polarization, we can infer the geometry of the surface where the last scattering occurred."

Petra added that the "deathbed scenario" the team observed for this star may not apply to those "special" tidal disruption events, in which matter is ejected from the poles of the black hole at nearly the speed of light. Further polarization studies of tidal disruption events are needed to answer this question.

"Polarization studies are very challenging, and there are only a limited number of people worldwide who are really skilled in this technique," Petra said. "So for tidal disruption events, this is uncharted territory."

Both sets of observations were made using the 3-meter (10-foot) Shane Telescope at Lick Observatory near San Jose, California. The telescope is equipped with the Kast spectrometer, an instrument that can determine the polarization state of light across the entire spectrum.

The team's paper will be published in the September issue of the Monthly Notices of the Royal Astronomical Society.

BY:Robert Lea

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