Who is the force in the universe that can "tear apart" stars?

Astronomers witnessed 18 voracious black holes tearing apart and devouring stars, a terrifying discovery that more than doubles the number of tidal disruption events found in the local universe.

(Image credit: ESO/M. Kornmesser)

Image: A star being torn apart in a violent tidal disruption event

For stars that venture too close to black holes, it's a cosmic jungle out there. A team of researchers from MIT has discovered 18 new examples of black holes tearing stars apart and feasting on their remains.

The result more than doubles the number of terrifying, star-shredding tidal disruption events (TDEs) seen in the local universe. The findings could help astronomers better estimate the number of tidal disruption events occurring throughout the universe, bringing their occurrence rates closer to theoretical predictions.

When a star passes too close to a black hole, the gravity of the black hole generates huge tidal forces in the star, so much so that the stellar body is stretched vertically and squeezed horizontally, a process called "spike formation." Matter from the star forms a flat disk around the black hole, with some of the matter accreting to the center of the black hole, while other stellar matter is drawn to the poles by strong magnetic fields and ejected in jets at nearly the speed of light.

Previously, astronomers believed that black holes would tear apart and devour stars while blasting out high-energy jets in galaxies that had recently undergone intense periods of star birth known as starbursts.

However, the research means that tidal disruption events may occur in a wider range of galaxies and help explain the extreme physics seen around such events.

“People have come up with really fancy solutions to these puzzles, and now we’ve gotten to the point where we can solve them all,” team member Erin Kara, an assistant professor of physics at MIT, said in a statement.

How did the search for tidal disruption events begin?

After discovering the closest tidal disruption event ever seen near Earth, the MIT team set out to look for more black holes swallowing stars. They saw a flare in the galaxy NGC 7392, about 137 million light-years from Earth.

This, they say, opens up a whole new way to spot actively feeding black holes. It involves using infrared light and an algorithm that looks for patterns in infrared data that indicate brief, or "transient," bursts of radiation. The technique was done using historical data collected by NASA's Wide-field Infrared Survey Explorer (NEOWISE), which has been searching Earth's skies for brief bursts of infrared light since its launch in 2009.

The team then cross-referenced the transients they had discovered with a catalog of galaxies within 600 million light-years of Earth and found that the infrared bursts could be traced back to about 1,000 galaxies.

The researchers zoomed in on these galaxies to try to see if the signals they detected came from tidal disruption events, or if they could be triggered by some other violent event, such as a supernova explosion from a dying massive star. It's also possible that the signals could come from the supermassive black holes at the centers of galaxies as they feed on infalling dust and gas.

(Photo credit: Zwicky Transient Facility/R.Hurt (Caltech/IPAC)

An artist's depiction of an X-ray beam produced by a black hole devouring a star. The beam would then blast away matter, releasing other types of light.

This ultimately led to the discovery of 18 legitimate TDE signals, which are the result of the gravitational influence of black holes creating tidal forces in stars, ultimately leading to some bloody stellar deaths.

Surprisingly, the team found that tidal disruption events seem to occur in a range of galaxies across the sky, including those filled with thick clouds of dust.

"If you look up at the sky and you see a bunch of galaxies, it's pretty typical for tidal disruption events to occur in all of them," said Megan Masterson, a graduate student at MIT's Kavli Institute for Astrophysics and Space Research and the lead author of the study.

Hiding the Crimes of the Universe

Overall, this research helps resolve some big questions about tidal disruption events.

Previously, astronomers had seen this star-crushing phenomenon primarily in post-explosion galaxies, where the star factories had just shut down.

Such galaxies are quite rare, and scientists were puzzled as to why tidal disruption events seemed to be limited to these galaxies. This study alleviated that confusion by finding gory tidal disruption events in multiple types of galaxies.

The reason there don't seem to be tidal disruption events in other galaxies is that post-explosion galaxies have depleted their gas and dust as they created lots of new stars, so there's no thick gas and dust, while other galaxies still have lots of gas and dust. Gas and dust are good at absorbing or blocking optical and X-ray light, but infrared light can pass through these materials more easily.

"It's not that they occur in only one type of galaxy, as people have assumed based on optical and X-ray searches alone," Masterson added. "If you want to understand tidal disruption events in general and use them to probe the population structure of supermassive black holes, you need to observe in the infrared."

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The MIT researchers' findings could also explain why tidal disruption events don't seem to radiate as much energy as theory predicts. The team thinks the lack of energy is due to dust absorbing the tidal disruption event's optical and X-ray radiation, as well as extreme ultraviolet radiation.

Combining their newly discovered 18 tidal disruption events with previously observed events of the same nature, the MIT team estimates that the galaxy experiences a tidal disruption event about once every 50,000 years, which is consistent with previous theoretical predictions.

"This gives us confidence that we don't need all this exotic physics to explain what we're seeing," Kara concluded. "We have a much better understanding of the mechanics behind how stars are ripped apart and eaten by black holes. We are understanding these systems much better."

BY:Robert Lea

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