It is a popular misconception that black holes behave like cosmic vacuum cleaners, voraciously sucking up any matter in their surroundings. In reality, only things that pass beyond the event horizon, including light, are engulfed and cannot escape, although black holes are messy eaters too. This means that some of an object̵
If that object is a star, the process of being chopped up (or “spaghettified”) by the powerful gravitational forces of a black hole occurs outside the event horizon and some of the star’s original mass is ejected violently towards the star. external. This in turn can form a rotating ring of matter (also known as an accretion disk) around the black hole that emits powerful X-rays and visible light. Those jets are one way astronomers can indirectly infer the presence of a black hole. Now astronomers have recorded the last death spasms of a star destroyed by a supermassive black hole in just such a “tidal disruption event” (TDE), described in a new article published in the Royal Astronomical Society’s Monthly Notices.
“The idea of a black hole ‘sucking in’ a nearby star sounds like science fiction. But that’s exactly what happens in a tidal outage event,” said co-author Matt Nicholl 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 disruption event is the result of the destruction of a star that recedes too close to a supermassive black hole,” said Edo Berger of Harvard University’s Center for Astrophysics, another co-author. “In this case the star was torn apart with about half of its mass feeding on or accumulating in a black hole a million times the mass of the Sun and the other half was ejected outward.”
Death by tidal forces
The idea of being “spaghettified” after falling into a black hole was popularized in Stephen Hawking’s 1988 bestseller, A brief history of time. Hawking imagined an unfortunate astronaut passing beyond the event horizon and finding himself subject to the black hole’s intense gravitational gradient. (The gravitational gradient is the difference in the pull of gravity depending on the orientation of an object.)
If the astronaut fell to his feet first, for example, the traction would be stronger on the feet than on the head. The astronaut would be stretched vertically and compressed horizontally by the tidal forces of the black hole until it resembled a string of spaghetti. From a physical point of view, it is the same reason that the Earth experiences tides: the gravitational pull of the moon attracts the oceans in one way and flattens them in the other. At least it would have been quick; the whole process would take less than a second.
All this is purely hypothetical, the subject of various thought experiments. But on the scale of stars and galaxies, some sort of spaghettification is a real phenomenon, although it occurs outside the black hole’s event horizon rather than inside. These tidal disruption events are probably quite common in our universe, although only a few have been detected to date.
For example, in 2018, astronomers announced the first direct image of the aftermath of a star destroyed by a black hole 20 million times more massive than our Sun, in a pair of colliding galaxies called Arp 299 at around 150 million years old. light from the Earth. They used a combination of radio telescopes and infrared telescopes, including the Very Long Baseline Array (VLBA), to track a particular formation and expansion of the jet of matter ejected in the wake of a star being destroyed by a supermassive black hole at the center of a of colliding galaxies.
However, these powerful flashes of light are often enveloped in a curtain of interstellar dust and debris, making it difficult for astronomers to study them in more detail. The latter event (dubbed AT 2019qiz) was discovered shortly after the star was destroyed last year, making it easier to study in detail, before that curtain of dust and debris had completely formed. Astronomers conducted follow-up observations across the electromagnetic spectrum over the next six months, using multiple telescopes around the world, including the Very Large Telescope (VLT) array and the New Technology Telescope (NTT), both located in Chile. .
“Because we detected it early, we could actually see the curtain of dust and debris rising as the black hole launched a powerful outflow of material with speeds of up to 10,000 km / s,” said Northwestern co-author Kate Alexander. University. “This is a unique ‘peek behind the scenes’ that provided the first opportunity to pinpoint the origin of the obscuring material and follow in real time how it envelops the black hole.”
According to Berger, these observations provide the first direct evidence that the outgoing gas during interruption and accretion produces the powerful optical and radio emissions previously observed. “Until now, 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: Royal Astronomical Society Monthly Notices, 2020. 10.1093 / mnras / staa2824 (About DOI).
Listing image from ESO / M. Kornmesser