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SYFY WIRE Bad Astronomy

Astronomers get a front row seat to a star getting torn apart by a huge black hole

By Phil Plait
Artwork depicting a tidal disruption event, when a star is torn apart by a black hole. A stream of matter is puller off, forming a disk around the black hole and creating an outflow of high-speed material. Credit: ESO/M. Kornmesser

When a star goes to battle with a supermassive black hole, the way to bet is kinda obvious. Especially when the black hole is a million times the mass of the star, and is, well, a black hole.

But the specific way the star gets eaten isn't a sure thing. Usually, the star gets torn apart, ripped to shreds by the black hole's immensely powerful gravity, which is why this is called a Tidal Disruption Event, or TDE (astronomers really need to up their nomenclature game). This causes a brain-stompingly huge blast of energy, billions of times brighter than the Sun. But how exactly that happens can change what we see. One problem though is that these explosive events tend to happen a very long way off, making them hard to study.

In 2019, the light from such a doomed encounter reached Earth, and it didn't come from clear across the Universe; it happened a mere 215 million light years away, making this the closest TDE ever seen in optical light*. That's pretty fortunate, because its proximity means it was caught earlier than usual —the intrinsically fainter early days of the event were still bright enough to see; for a more distant galaxy they're too dim to detect.

An image of the host galaxy for the tidal disruption event AT2019qiz, imaged by the Pan-STARRS telescope. The position of the TDE is in the crosshairs, right in the center of the galaxy. The scale bar is about 17,000 light years.

The TDE was first seen by the Zwicky Transient Facility on the evening of 19 September 2019, and given the designation AT2019qiz. It happened in a galaxy with a catalog name of 2MASX J04463790-1013349, a face-on spiral galaxy 215 million light years away in the direction of the constellation Eridanus. That's a decent distance, but actually close by in cosmic terms.

The explosion grew in brightness over several weeks, reaching its peak on 08 October 2019. At that point it was blasting out ten billion times the energy the Sun does, about as much as its entire host galaxy! These are ridiculously powerful events. But then, an entire star was being torn apart by a black hole.

By the way, AT2019qiz is considered to be only a medium powerful one. Some are much brighter. Yikes.

The black hole in the center of the host galaxy probably has a mass about a million times the Sun's (about the same, or a bit less, than Sgr A*, the supermassive black hole in the center of our Milky Way). The way the event behaved indicates the star itself was probably much like the Sun. For a star and black hole like that, the star would have to get within about 70 million kilometers of the black hole to get shredded. That's roughly half the distance of the Earth to the Sun, which is close. The black hole itself is only about 6 million kilometers across.

As the star approached the black hole, things got serious. Gravity gets weaker with distance, so the side of the star closer to the black hole felt more gravity from it then the side farther away. But the force of gravity from a black hole is colossal, so the difference in force across the star was huge: Enough to literally rip it apart.

Can we take a moment to just let that percolate in your brain? An entire star was pulled apart by a black hole like taffy. I still have a hard time grasping the staggering scale of such an event.

Artwork showing a star (center) getting torn apart by a black hole (upper left). Credit: Robin Dienel, courtesy of the Carnegie Institution for Science

The material ripped form the star would then be stretched into a thin stream — astronomers call this spaghettification — which then whipped around the black hole at tremendous speed. Some of that material went all the way around the black hole and slammed into the stream still falling in from the star. That creates a big blast of light. At the same time, much of it fell into a disk around the black hole. Parts of this disk revolve around the black hole at nearly the speed of light, while other parts are slower. The friction generated from this creates a lot of energy, and also powered a huge outburst of material expanding away from the black hole.

The physics is a tad complicated, shockingly, but this expanding bubble created most of the optical light seen. Higher energy light, like ultraviolet and X-rays, are generated close in to the black hole but hits this expanding material and is absorbed by it. As the cloud of material expands it gets more tenuous, eventually allowing that light to escape. That allows astronomers to see what's going on in both the huge bubble as well as right around the black hole.

I have to note that the authors mention that it's possible only 75% of the star was torn away; a quarter of it may still remain, orbiting the black hole as a much smaller and heavily disturbed stellar ball of gas. Too bad it's too far away to see that. Studying what's left after the cataclysm would be amazing.

Artwork depicting a tidal disruption event, when a star is torn apart by a black hole. A stream of matter is puller off, forming a disk around the black hole and creating an outflow of high-speed material. Credit: ESO/M. Kornmesser

Getting in on this event early helped figure all this out. There's a lot of confusing stuff going on at the same time — the expanding debris, the stream of material whipping around the black hole, generation of dust which can obscure the view, and even our viewing angle on the event — which can make it hard to see what's what. Being able to trace the material via the way it emits light from start to eventual finish really makes things easier. Not easy, but easier.

Each TDE is different, with some blasting out radio waves, and others not so much. Some fade rapidly, others glow for years. Decades. Other focus the outflow into twin beams called jets, while some don't. It's a mess.

AT2019qiz may provide vital clues to understanding why each behaves the way it does. In general several per year are detected, most far away. Hopefully, sometime soon in a galaxy not too far away, another star will meet its spaghettified fate, and we'll get an even better view.

*In 2005 one was seen in a galaxy only about 150 million light years away, but it was detected in radio waves, not the kind of light we see. The physics that makes these different flavors of light is different so seeing radio versus optical tells us different important things about what's going on.