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

  • Astronomers made the discovery after spending years watching black holes
  • Stars involved in tidal disruption events are ripped apart in a matter of hours

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

We already know the notoriously messy eaters swallow everything in their path.

But what astronomers hadn’t realised is the cosmic monsters then ‘burp up’ an eclectic mix of stars, gas, planets and dust that they destroyed years earlier.

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

Traditionally, the objects had only been studied for a few months following a TDE — which happens when stars venture too close to a black hole and are ripped to pieces in a process called spaghettification.

Mysterious: Scientists know black holes are notoriously messy eaters that swallow everything in their path. But what they hadn't realised is that the cosmic monsters 'burp up' a mix of stars, gas, planets and dust that they destroyed years earlier (stock image)

Mysterious: Scientists know black holes are notoriously messy eaters that swallow everything in their path. But what they hadn’t realised is that the cosmic monsters ‘burp up’ a mix of stars, gas, planets and dust that they destroyed years earlier (stock image)

WHAT IS A ‘TIDAL DISRUPTION EVENT’? 

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

These are known as a tidal disruption events, or TDEs, and emit light, radio and other waves for up to a few weeks or months while they occur.

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

Although black holes cannot be observed directly, scientists can watch a TDE because these events emit light, radio and other waves for up to a few weeks or months while they occur. 

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 feeds that stellar material to the black hole.

But what scientists at the Harvard and Smithsonian Center for Astrophysics discovered 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 went on to have ‘burping’ bouts, although they have no idea why. 

‘If you look years later, a very, very large fraction of these black holes that don’t have radio emission at these early times will actually suddenly ‘turn on’ in radio waves,’ lead author Yvette Cendes told Live Science

‘I call it a “burp” because we’re having some sort of delay where this material is not coming out of the accretion disk until much later than people were anticipating.’

Although black holes cannot be observed directly, scientists can watch a TDE because these events emit light, radio and other waves for up to a few weeks or months while they occur

Although black holes cannot be observed directly, scientists can watch a TDE because these events emit light, radio and other waves for up to a few weeks or months while they occur

Although black holes cannot be observed directly, scientists can watch a TDE because these events emit light, radio and other waves for up to a few weeks or months while they occur

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

Scientists know for certain that it’s not coming from inside 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 if the material observed in radio waves is coming from the accretion disk or if it is being stored somewhere closer to the black hole. 

‘Black holes are definitely messy eaters, though.’

The researchers plan to continue monitoring the black holes they have been observing, particularly as some of them are still getting brighter post-TDE.

They are also calling for improved computer modelling to better represent how black holes can ‘burp’ years later, which the experts hope will strengthen the understanding of this strange behaviour.   

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

WHAT’S INSIDE 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 venture too close and cross the so-called event horizon, the point from which no light can escape, you will also be trapped or destroyed.

For small black holes, you would never survive such a close approach anyway. 

The tidal forces close to the event horizon are enough to stretch any matter until it’s just a string of atoms, in a process physicists call ‘spaghettification’.

But for large black holes, like 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 uneventful.

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

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

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

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