ASTROBOB: Satellite catches black hole tearing apart star
Black holes are ever fascinating. The fact that something so massive could imprison matter and crush it into an infinitely small point seems totally out of this world. How could it be?
And yet they exist. We have plenty of proof and at least one photo, an image made earlier this year of the supermassive black hole in the galaxy M87.
Now for the first time, NASA’s extraterrestrial-planet-hunting satellite, Transiting Exoplanet Survey Satellite (TESS) has watched a black hole tear a star apart in what astronomers call a tidal disruption event.
The star, named ASASSN-19bt, was discovered by the All-Sky Automated Survey for Supernovae (ASA-SN). ASAS-SN is a worldwide network of 20 automated telescopes that repeatedly scans the sky looking for flaring stars, comets and anything else that goes wink in the night.
The network discovered the event on Jan. 29 this year, an alert was issued and astronomers quickly trained other telescopes on that spot in the sky to gather as much data in as many different wavelengths (colors) of light as possible.
When it comes to unusual outbursts in the sky, it’s crucial to get as many instruments pointed there as possible. Some cosmic events are very brief, and if you don’t look now, they’re gone in a flash.
By good fortune, TESS had already been looking in the same area because it monitors large swaths of sky called sectors for 27 days at a time looking for repeat changes in the brightness of stars that may indicate orbiting planets. When a planet passes in front of a star — an event called a transit — the star’s brightness drops slightly.
TESS first saw ASASSN-19bt on Jan. 21, over a week before the event was bright enough for ASAS-SN to detect it. But because the satellite only transmits data to Earth every two weeks, and it takes time to process and analyze, astronomers had to wait until March 13 to see the results.
“The early TESS data allow us to see light very close to the black hole, much closer than we’ve been able to see before,” said Patrick Vallely, a co-author of the scientific paper published on the topic just last week. “They also show us that ASASSN-19bt’s rise in brightness was very smooth, which helps us tell that the event was a tidal disruption and not another type of outburst, like from the center of a galaxy or a supernova.”
Date from the Swift satellite, which studies stars in ultraviolet light, discovered that the temperature of the material whirling around the black hole dropped from around 71,500-35,500 degrees over a few days as the stellar debris cooled. Astronomers think the supermassive black hole that generated ASASSN-19bt weighs around 6 million times the Sun’s mass. It sits at the center of a galaxy called 2MASX J07001137-6602251 located around 375 million light-years away in the constellation Volans the Flying Fish.
Tidal disruptions are incredibly rare, occurring once every 10,000 to 100,000 years in a galaxy the size of our own Milky Way. Supernovae, by comparison, happen every 100 years or so, though we see fewer because galactic gas obscures them. In total, astronomers have observed only about 40 tidal disruptions to date.
The destroyed star may have been similar in size to our sun. Now it’s been reduced to a stream of hot gas spinning around the hole’s event horizon before it crosses that boundary and swirls into oblivion. Nothing that passes over the event horizon can ever return to view. To do so, it would have to travel faster than the speed of light, an impossibility.
It’s important to remember that despite their enormous gravity, black holes don’t go around sucking things up. Only objects that happen to stray very close to the hole will be latched on, torn apart and pulled in. If our own sun became a black hole we’d miss the light for sure, but Earth and the other planets would keep on orbiting just as they do now.
Bob King is a photographer, author and amateur astronomer. Read more on his blog at astrobob.areavoices.com.