Black holes keep ‘burping up’ stars they destroyed years earlier – and astronomers have no idea why

US
By James
  • Astronomers made the discovery after years of looking at black holes
  • Stars involved in tidal disturbances are torn apart within hours

By Sam Tonkin

Updated:

Black holes are among the most talked about objects in the universe, but scientists still have so much to learn when it comes to understanding their mysterious behavior.

We already know that the notoriously messy eaters gobble up everything in their path.

But what astronomers hadn’t realized is that the cosmic monsters then “burp up” an eclectic mix of stars, gas, planets and dust that they had destroyed years before.

This surprise only came to light because experts decided to monitor black holes for years after they had been involved in tidal disturbance events (TDEs).

Traditionally, the objects were studied just a few months after a TDE — which happens when stars venture too close to a black hole and are ripped apart in a process called spaghettification.

Mysterious: Scientists know that black holes are notoriously messy eaters that gobble up everything in their path. But what they hadn’t realized is that the cosmic monsters are “burping up” a mix of stars, gas, planets and dust they destroyed years before (stock image)

WHAT IS A ‘TIDE DISTURBANCE EVENT’?

When a star gets too close to a black hole, it undergoes ‘spaghettification’: it is stretched vertically and compressed horizontally by the strong gravitational field.

These are known as tidal disturbance events, or TDEs, and emit light, radio, and other waves for several weeks or months as they happen.

This is because the elongated material eventually spins around the black hole and heats up, creating a flash that astronomers can detect millions of light years away.

While black holes cannot be directly observed, scientists can watch a TDE because these events emit light, radio and other waves over several weeks or months as they happen.

When they happen, some of the leftover gas and dust from a destroyed star is flung away from the black hole.

The rest then forms a thin Frisbee-like structure around it, called an accretion disk, which gradually carries the stellar material towards the black hole.

But what scientists at the Harvard and Smithsonian Center for Astrophysics found is that some of this material can then reappear between two and six years after a TDE.

They found that up to half of the 24 black holes they observed “regurgitated,” though they have no idea why.

“If you look years later, a very, very large portion of these black holes, which have no radio emission in these early times, will actually suddenly ‘turn on’ in radio waves,” said lead author Yvette Cendes. Living Science.

“I call it a ‘burp’ because we have a kind of lag where this material doesn’t come out of the accretion disk until much later than people expected.”

While black holes cannot be directly observed, scientists can watch a TDE because these events emit light, radio and other waves over several weeks or months as they happen.

While black holes cannot be directly observed, scientists can watch a TDE because these events emit light, radio and other waves over several weeks or months as they happen.

The question is, where is it stored before being ‘burped’ out again?

Scientists are sure it’s not coming from a black hole because the objects have an event horizon where gravity is so strong that not even light can escape.

Cendes added: “We don’t fully understand whether the material seen in radio waves comes from the accretion disk or if it is stored somewhere closer to the black hole.

“Black holes are definitely messy eaters, though.”

The researchers plan to continue monitoring the black holes they observed, especially as some of them continue to brighten after the TDE.

They also call for improved computer models to better represent how black holes may “burp” years later, which the experts hope will increase understanding of this strange behavior.

The new research has been published in the preprint database arXiv but not yet peer reviewed.

WHAT’S IN A BLACK HOLE?

Black holes are strange objects in the universe that get their name from the fact that nothing can escape their gravity, not even light.

If you get too close and cross the so-called event horizon, the point where no light can escape, you will also be captured or destroyed.

For small black holes you would never survive that close anyway.

The tidal forces near the event horizon are enough to stretch any matter into just a string of atoms, in a process physicists call “spaghettification.”

But for large black holes, such as the supermassive objects at the cores of galaxies like the Milky Way, which weigh tens of millions if not billions of times the mass of a star, crossing the event horizon would be straightforward.

Because it should be possible to survive the transition from our world to the world of black holes, physicists and mathematicians have long wondered what that world would look like.

They’ve turned to Einstein’s general equations of relativity to predict the world inside a black hole.

These equations work well until an observer reaches the center or singularity, where in theoretical calculations the curvature of space-time becomes infinite.

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