The phenomenon, known as a tidal disruption event, is caused when a star passes too close to a black hole and the extreme gravitational pull from the black hole shreds the star into thin streams of stellar material -- a process called 'spaghettification'. During this process some of the material falls into the black hole, releasing a bright flare of energy which astronomers can detect, while some continues to orbit the black hole.
Tidal disruption events are rare and not always easy to study because they are usually obscured by a curtain of dust and debris generated in the disruptive process. An international team of scientists led by the University of Birmingham were able to study this event in unprecedented detail because it was detected just a short time after the star was ripped apart.
"The idea of a black hole 'sucking in' a nearby star sounds like science fiction. But this is exactly what happens in a tidal disruption event," says lead author Dr Matt Nicholl, a lecturer and Royal Astronomical Society research fellow at the University of Birmingham. "We were able to investigate in detail what happens when a star is eaten by such a monster."
"When a black hole devours a star, it can launch a powerful blast of material outwards that obstructs our view," explains Samantha Oates, also at the University of Birmingham. "This happens because the energy released as the black hole eats up the streams of stellar material propels the star's debris outwards."
In the case of AT2019qiz, astronomers were able to identify the phenomenon early enough to observe the whole process.
"Several sky surveys discovered emission from the new tidal disruption event very quickly after the star was ripped apart," says Thomas Wevers, an ESO Fellow in Santiago, Chile, who was at the Institute of Astronomy, University of Cambridge, UK, when he conducted the work. "We immediately pointed a suite of ground-based and space telescopes in that direction to see how the light was produced."
The prompt and extensive observations in ultraviolet, optical, X-ray and radio light revealed, for the first time, a direct connection between the material flowing out from the star and the bright flare emitted as it is devoured by the black hole.
"The observations showed that the star had roughly the same mass as our own Sun, and that it lost about half of that to the black hole, which is over a million times more massive," said Nicholl, who is also a visiting researcher at the University of Edinburgh.
"Because we caught it early, we could actually see the curtain of dust and debris being drawn up as the black hole launched a powerful outflow of material with velocities up to 10,000 km/s," said Kate Alexander, NASA Einstein Fellow at Northwestern University in the US. "This unique 'peek behind the curtain' provided the first opportunity to pinpoint the origin of the obscuring material and follow in real time how it engulfs the black hole."
The research helps astronomers better understand supermassive black holes and how matter behaves in the extreme gravity environments around them. The team say AT2019qiz could even act as a 'Rosetta stone' for interpreting future observations of tidal disruption events. ESO's Extremely Large Telescope (ELT), planned to start operating this decade, will enable researchers to detect increasingly fainter and faster evolving tidal disruption events, to solve further mysteries of black hole physics.
See:
https://lecospa.ntu.edu.tw/wpcontent/uploads/2022/09/PhysRevLett.129.061102.pdf
The time for the whole star to cross the Event Horizon (EH) is ∼2𝑅∗/𝑐∼2R∗/c whilst the time to reach the singularity is ∼𝑟𝑠/𝑐∼rs/c. Since 𝑅∗≪𝑟𝑠R∗≪rs for any star that can survive the tidal forces then there would seem to be no issue. It basically depends on the definition of the "volume" inside the event horizon, again. Assuming that you are dealing with the geometric interpretation of black hole volume.
The tidal acceleration on a freely-falling star at the event horizon of a (non-spinning) supermassive black hole is approximately
𝑔tidal≃2𝐺𝑀BH𝑟∗𝑟3𝑠=𝑟∗𝑐6(2𝐺𝑀BH)2 ,gtidal≃2GMBHr∗rs3=r∗c6(2GMBH)2 ,
where 𝑀BH is the mass of the black hole and 𝑟∗ is the radius of the star. i.e. This is the
difference in acceleration between the stellar surface closest to 𝑟=0r=0 and that furthest away. (A more accurate treatment could integrate over the volume of the star).
Thus the tidal acceleration (i.e. tidal force per unit mass) becomes much smaller at the event horizon of larger black holes. If we demand that this tidal acceleration is less than the star's self-gravity, we can obtain a rough condition for survival.
𝑟∗𝑐6(2𝐺𝑀BH)2<𝐺𝑀∗𝑟2∗r∗c6(2GMBH)2<GM∗r∗2
A more detailed theoretical treatment is provided by
Kesden (2012), who also considers spinning black holes. The threshold black hole mass becomes larger for a spinning black hole - approximately 7×108𝑀⊙7×108M⊙ for a maximally spinning black hole - because the event horizon is at a smaller radial coordinate and the tidal forces are commensurately stronger. The picture below shows the dependence of the minimum mass to disrupt a solar-type star prior to crossing the event horizon as a function of the spin parameter of the black hole.
The spaghettification of a star (or tidal disruption event) can cause a lengthy transient brightening of an active galaxy. Some ∼50 of these events have been obserevd (e.g.
Gezari 2021). It turns out that there is indeed a fairly sharp cut-off in the black hole mass distribution for which these tidal disruption events have been seen. This cut-off occurs at 𝑀BH∼10*8𝑀⊙, suggesting indeed that more massive black holes are able to swallow stars whole.
See:
https://astronomy.stackexchange.com...gh-the-event-horizon-of-a-black-hole-unharmed
According to Thomas Wavers, the study co-author and ESO fellow in Santiago, Chile, when an unfortunate star wanders too near a supermassive black hole in the heart of a galaxy, the tremendous gravitational pull of the black hole is shredding the star into thin streams of material. When these thin strands of the material of the star fall into the black hole, they release a brilliant strong flare that astronomers can identify. Spaghettification occurrences are far harder to see and examine. Nonetheless, the researchers have the European Southern Observatory's Very Large Telescope and Fresh Technology Telescope pointed in the right direction after they spotted a new flare of light close to a supermassive black hole a year ago. In a statement, primary study author and lead lecturer Matt Nicholl, who's also a Royal Astronomical Society research fellow at the University of Birmingham in the United Kingdom, explained that a black hole's sucking in a neighboring star seems like ascience fiction. The energy produced by the black hole during its dinner really pushed out the debris of the star in the form of a curtain. Astronomers started to investigate the event right after the star was ripped apart and continued to examine it in detail for six months using various telescopes and devices as the light coming from the star expanded and eventually faded.
Hartmann352
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