What is spaghettification in astrophysics

Death from spaghetti

Star death at the black hole: Astronomers have for the first time observed how a sun-like star is torn apart by a supermassive black hole. The star was torn into thin threads of matter - spaghettified - and then devoured by the gravitational giant. About half of the stellar material ended up in the black hole, while the other half formed an enveloping cloud of dust and gas.

In 2014, a star in the heart of the Milky Way just managed to avoid its death: it approached the central black hole of our galaxy, was not torn apart. But not all stars are so lucky. Some come so close to such a gravitational giant that the tremendous tidal forces tear the star apart.

"When an unfortunate star wanders too close to a supermassive black hole in the center of a galaxy, the extreme gravitational pull of the black hole tears the star into thin threads of matter," explains co-author Thomas Wevers of the University of Cambridge. The star is to a certain extent "spaghetti". This “feast” of a supermassive black hole becomes visible through powerful bursts of rays that can be seen over millions of light years.

A distant sun is torn apart

Astronomers have now observed what happens at such a “tidal disruption event” more closely and in detail than ever before. The event, baptized AT2019qiz, was detected and reported on September 29, 2019 by an automated telescope specializing in short-lived cosmic events. A little later, other telescopes also had their sights set on the burst of rays, which glowed around 215 million light-years from Earth in the constellation Eridani.

Over a period of six months, the team around Wevers and first author Matt Nicholl from the University of Birmingham was able to follow the events at the distant black hole in different wave ranges of light. "The observations showed that the star had roughly the same mass as our own sun and that it lost about half of it to the black hole, which is over a million times more massive," says Nicholl.

First lighting up, then darkening

At the beginning of the event, the black hole's strong gravity expanded the star's photosphere, creating strong outbursts of star material. This was snatched from the star at up to 10,000 kilometers per second, as the spectral analyzes showed. From their data, the astronomers conclude that the star lost around 75 percent of its mass as a result of this spaghettification.

The matter sucked up by the dying star causes a bright burst of rays through its acceleration and the interaction with the gravitational force of the black hole, which increases significantly over the course of several days. After a peak, however, this radiation emission weakens significantly. “The temperature suddenly drops around the maximum brightness,” report the researchers.

"Unique view behind the curtain"

For the first time, astronomers were able to observe more closely what causes this darkening. "Because we caught him early, we were able to watch a curtain of dust and debris build up as the black hole triggered the powerful ejection of material," says co-author Kate Alexander of Northwestern University in Evanston. "This unique look behind the curtain gave us the first chance to determine the origin of the covering material."

The observations confirm the hypothesis that the outflow of matter from the star creates debris for this cocoon. At the same time, they explain why tidal disruption events are optically bright, but often only emit weak X-rays: this radiation is swallowed by the cocoon.

How the end of the star at the black hole happened. © European Southern Observatory (ESO)

Mystery of the missing energy

The observation of AT2019qiz could also help to clear up the so-called "problem of the lack of energy". "This is based on the fact that the luminosity of up to 10 to the power of 51 ergs observed at most tidal disruption events is several orders of magnitude lower than the amount of energy that would actually have to be released when sucking in a considerable amount of stellar mass through the supermassive black hole," said Nicholl and his colleagues.

According to one hypothesis, most of this energy goes into accelerating the streams of matter from the dying star. “But that's not enough. to explain the missing amounts of energy, say the researchers. In addition, the observations at AT2019qiz did not match. Another explanation would be that the residual energy is not released directly during the eruption, but only after it: "AT2019qiz could radiate the rest of the expected energy by only accreting a few percent of solar masses per year into the black hole," the astronomers speculate .

“Rosetta Stone” for other such events

Further observations should now show whether this is true: "The proximity of AT2019qiz makes it an ideal source for testing these scenarios through continuous monitoring," said Nicholl and his team. In their opinion, AT2019qiz could even serve as a “Rosetta Stone” for the interpretation of future observations of such tidal disruption events. (Monthly Notices of the Royal Astronomical Society, 2020; doi: 10.1093 / mnras / staa2824)

Source: European Southern Observatory (ESO), University of Birmingham

October 13, 2020

- Nadja Podbregar