It is a common misconception that black holes behave like space vacuum cleaners, plundering all kinds of matter in their vicinity. In fact, only things that transcend the horizon of events ̵
If this object is a star, the process of fragmentation (or “spaghettization”) by the powerful gravitational forces of a black hole occurs outside the event horizon and part of the star’s initial mass is thrown out strongly. This in turn can form a rotating ring of matter (also known as an accretion disk) around the black hole, which emits powerful X-rays and visible light. These jets are one way for astronomers to indirectly infer the presence of a black hole. Astronomers have now recorded the last death shot of a star shattered by a supermassive black hole in just such a “tidal development event” (TDE), described in a new article published in the Monthly Notices of the Royal Astronomical Society.
“The idea of a black hole sucking up a nearby star sounds like science fiction. But that’s exactly what happens at high tide,” said co-author Matt Nichol of the University of Birmingham. “We were able to investigate in detail what happens when a star is eaten by such a monster.”
“A tidal event is the result of the destruction of a star that deviates too close to a supermassive black hole,” said Edo Berger of the Center for Astrophysics at Harvard University, another co-author. “In this case, the star was torn by about half of its mass feeding – or rising – into a black hole a million times the mass of the Sun, and the other half was thrown out.”
Death by tidal forces
The idea of ”spaghetti” after falling into a black hole was popularized in Stephen Hawking’s best-selling book from 1988, A brief history of time. Hawking imagined an unfortunate astronaut to cross the horizon of events and be the subject of the intense gravitational gradient of the black hole. (The gravitational gradient is the difference in the strength of gravitational attraction depending on the orientation of the object.)
If the astronaut first fell with his feet, for example, the pull will be stronger on the legs than the head. The astronaut would be stretched vertically and compressed horizontally by the tidal forces of the black hole until they looked like a string of spaghetti. From the point of view of physics, this is the same reason why the Earth experiences tides: the gravitational pull of the Moon pulls the oceans in one direction and flattens them on the other. At least it would be fast; the whole process will happen in less than a second.
All this is purely hypothetical, the subject of various thought experiments. But on the scale of stars and galaxies, a kind of spaghetti is a real phenomenon, albeit one that occurs outside the black hole event horizon, not inside. These tidal events are probably quite common in our universe, although only a few have been discovered so far.
For example, in 2018, astronomers announced the first direct image of the effects of a star shattered by a black hole 20 million times more massive than our Sun in a pair of colliding galaxies called Arp 299, about 150 million light-years from Earth. They used a combination of radio and infrared telescopes, including a very long base array (VLBA), to track the formation and expansion of a jet of matter ejected by a star shattered by a supermassive black hole at the center of one of the colliding galaxies.
However, these powerful bursts of light are often shrouded behind a curtain of interstellar dust and debris, making it difficult for astronomers to study them in more detail. This latest event (called AT 2019qiz) was discovered shortly after the star was shattered last year, which made it easier to study in detail before this curtain of dust and debris was fully formed. Astronomers made follow-up observations in the electromagnetic spectrum over the next six months using a number of telescopes around the world, including the VLT and the New Technology Telescope (NTT), located in Chile.
“Because we caught it early, we could actually see the curtain of dust and debris close when the black hole launched a powerful spill of material at speeds of up to 10,000 km / s,” said co-author Kate Alexander of Northwestern University. “It’s a unique ‘peek behind the curtain’ that gave the first opportunity to determine the origin of the blackout material and track in real time how it absorbs the black hole.”
According to Berger, these observations provide the first direct evidence that the leaking gas during destruction and growth produces the powerful optical and radio emissions previously observed. “So far, the nature of these emissions has been much debated, but here we see that the two regimes are linked through a single process,” he said.
DOI: Monthly Bulletin of the Royal Astronomical Society, 2020. 10.1093 / mnras / staa2824 (For DOI).
Image of ESO / M. Kornmeser