The ultimate fate of a star ripped apart by a black hole

In 2019, astronomers observed the closest example yet of a star being shredded, or “spaghettified,” after approaching too close to a massive black hole.

This tidal disruption of a Sun-like star by a black hole 1 million times more massive than itself took place 215 million light-years from Earth. Fortunately, this was the first such event that was bright enough for astronomers at the University of California, Berkeley to study the optical light from the star’s death, specifically the light’s polarization, to learn more about what happened after the star ripped apart became.

Their October 8, 2019 observations suggest that much of the stellar material was blown away at high speeds — up to 10,000 kilometers per second — forming a spherical cloud of gas that blocked most of the high-energy emissions produced by the black hole devoured the rest of the star.

Previously, other observations of the optical light from the explosion, dubbed AT2019qiz, showed that much of the star’s matter was blown outward 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 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 member of the research team.

The results support an answer to why astronomers don’t see high-energy radiation like X-rays from many of the dozens of tidal disturbances observed so far: the X-rays produced by material ripped off the star and into an accretion disk around the black hole pulled before falling inward are obscured by gas being blown outward from the black hole by strong winds.

“This observation rules out a class of solutions that have been proposed theoretically and gives us a stronger constraint on what happens to gas around a black hole,” said Kishore Patra, a UC Berkeley doctoral student and lead author of the study. “People have seen other evidence that wind comes from these events, and I think this polarization study definitely reinforces that evidence, in the sense that without a sufficient amount of wind, you wouldn’t get spherical geometry.” The interesting fact here is that a significant portion of the material in the star that spirals inward does not eventually fall into the black hole — it is blown away by the black hole.

Polarization reveals symmetry

Many theorists have hypothesized that the post-perturbation stellar debris forms an eccentric, asymmetric disk, but an eccentric disk is expected to have a relatively high degree of polarization, which would mean that perhaps several percent of all light is polarized. This was not observed in this tidal disturbance event.

“One of the craziest things a supermassive black hole can do is shred a star apart with its tremendous tidal forces,” said team member Wenbin Lu, an assistant professor of astronomy at UC Berkeley. “These stellar tidal disturbances are one of the few ways astronomers can detect the existence of supermassive black holes at the centers of galaxies and measure their properties. However, due to the extreme computational effort involved in numerically simulating such events, astronomers still do not understand these complicated processes after a tidal disturbance.”

A second set of observations on November 6, 29 days after the October observation, revealed that the light was very slightly polarized, about 1%, indicating that the cloud had thinned sufficiently to reveal the asymmetric gas structure around the to visualize the black hole around it. Both observations come from the 3-meter Shane Telescope at Lick Observatory near San Jose, California, which is equipped with the Kast spectrograph, an instrument that can determine the polarization of light across the entire optical spectrum. The light becomes polarized – its electric field oscillating mainly in one direction – as it scatters electrons in the gas cloud.

“The accretion disk itself is hot enough to emit most of its light in X-rays, but that light has to get through that cloud, and there’s a lot of scattering, absorption, and re-emission of light before it can escape that cloud,” said patra “In each of these processes, light loses some of its photon energy down to ultraviolet and optical energies. The final scattering then determines the polarization state of the photon. So from measuring the polarization we can deduce the geometry of the surface where the final scattering takes place.”

Patra noted that this deathbed scenario may only apply to normal tidal disturbances – not “strange orbs” where jets of relativistic material are ejected from the black hole’s poles. Only more measurements of the polarization of the light from these events will answer that question.

“Polarization studies are very challenging and very few people around the world are familiar enough with the technique to use it,” he said. “So this is new territory for tidal disturbances.”

Patra, Filippenko, Lu and UC Berkeley researcher Thomas Brink, graduate student Sergiy Vasylyev, and postdoctoral researcher Yi Yang reported their observations in an article accepted for publication in the journal Monthly Bulletins of the Royal Astronomical Society.

A cloud 100 times larger than Earth’s orbit

The UC Berkeley researchers calculated that the polarized light was emitted from the surface of a spherical cloud with a radius of about 100 astronomical units (au), which is 100 times farther from the star than Earth is from the Sun. An optical glow of hot gas emanated from a region at about 30 AU.

The 2019 spectropolarimetric observations – a technique that measures polarization across many wavelengths of light – were of AT2019qiz, a tidal disturbance event located in a spiral galaxy in the constellation of Eridanus. The zero polarization of the entire spectrum in October indicates a spherically symmetric gas cloud – all polarized photons cancel each other out. The slight polarization of the November measurements indicates a small asymmetry. Because these tidal disturbances occur so far away at the centers of distant galaxies, they appear only as a point of light, and polarization is one of the few clues to the shape of objects.

“These disruptive events are so far away that you can’t really resolve them, so you can’t study the geometry of the event or the structure of these explosions,” Filippenko said. “But studying polarized light actually helps us derive some information about the distribution of the matter in this explosion, or in this case how the gas – and possibly the accretion disk – is shaped around this black hole.”