When a star wanders too close to a black hole, it faces a dramatic end that serves as a stark demonstration of the fascinating and destructive forces found across the universe.
Black holes are regions in space whose gravitational pull is so strong that not even light can escape. Their immense gravity can pull in a star and tear it apart, sending its pieces spiralling around the black hole in a spectacular display of light and energy.
This rare event, known as a tidal disruption, not only reveals the hidden presence of a black hole but also helps scientists understand how these cosmic giants affect the evolution of galaxies and the universe as a whole.
What happens when a black hole destroys a star?
As the star’s pieces get pulled towards the black hole, they start to form a bright, glowing ring around it, called an accretion disc.
Ariel Chitan, a PhD candidate in physics and astronomy at Western University in Ontario, Canada, told The National that “the disc around the black hole gets so hot that it shines brightly and releases X-ray radiation”.
“Astronomers can observe tidal disruption event to get key information about the black hole’s light,” she said.
“These detections can tell us how massive the black hole is or, even, how fast the black hole is spinning.”
From Earth, scientists can detect these X-rays, which act like a beacon, helping them identify where black holes are lurking. Since black holes are invisible, observing this glowing disc is one of the few ways scientists can locate them.
How black holes grow over time
Watching a black hole pull in the remains of a star gives scientists clues about how black holes grow over time.
This is especially important for the supermassive black holes – ones that are more than 100,000 times the mass of the Sun – found at the centre of most galaxies, including our Milky Way.
“A current open question is how do black holes, supermassive ones in particular, become so massive?” said Ms Chitan.
“One possible answer is that they accumulate this mass after devouring stars, for example via accretion.
“However, the time it takes for a black hole to become supermassive in this way, might be longer than even the lifetime of the universe.”
“Observing more and more tidal disruption events can help us to understand if this is a feasible growth mechanism for black holes or not,” said Ms Chitan.
How black holes affect galaxies
Black holes do not only destroy stars, they can also change the structure of entire galaxies. In some cases, the energy released from a tidal disruption event can cause the black hole to produce powerful jets of energy that move through space.
“These jets, moving near the speed of light, punch through the galaxy and alter its structure,” said Ms Chitan.
“Images of such galaxies often show a massive jet extending far out from the galaxy.”
Examples of such jets can be seen in galaxies like M87, where the central black hole ejects a jet that stretches thousands of light-years, heating and displacing surrounding gas.
In Centaurus A, jets extend beyond the galaxy, pushing out gas and suppressing star formation in some regions.
“These jets can drive gas out of the galaxy and heat what remains. Since cold gas is crucial for star formation, this process, known as quenching, prevents new stars from forming,” said Ms Chitan.
“When a galaxy can no longer form stars, it is called a quiescent or ‘dead’ galaxy.”
What tidal disruptions teach us about black holes
With newer, more advanced telescopes, astronomers are now better able to observe these star-destroying events, helping them to learn more about black holes than ever before.
Recent technology, like Nasa’s Nicer (Neutron Star Interior Composition Explorer), which is installed on the International Space Station, has allowed scientists to closely study these events.
Nicer’s X-ray data, combined with images from telescopes like the Very Large Telescope, located in Chile, and Nasa's Chandra observatory, has provided details about black holes’ temperature, rotation and growth, helping scientists piece together how black holes grow and impact the galaxies around them.
Scientists used the Nicer space telescope in 2018 and 2019 to study the J1820 black hole, located in the Milky Way, about 10,000 light-years from Earth.
Philip Uttley, an astrophysicist at the University of Amsterdam, said when the findings of the study were published in 2019: “Nicer's observations of J1820 have taught us something new about stellar-mass black holes and about how we might use them as analogues for studying supermassive black holes and their effects on galaxy formation.
“We’ve seen four similar events in Nicer’s first year, and it’s remarkable. It feels like we’re on the edge of a huge breakthrough in X-ray astronomy.”
Last year, the Nicer space telescope experienced technical issues, and its observations have been significantly reduced. In August, Nasa sent a repair kit to the space station that astronauts can use during a future spacewalk.
