A black hole destroys a star, then fires off a powerful jet years later — scientists discover the 'burping' feeding pattern behind these cosmic outbursts

A black hole will tear a star to pieces but keep going way after it is dead as some of the matter goes into it, while others are flung off in violent outbursts.

But now, scientists may have discovered a trend which could tell us when this kind of phenomenon happens in the Universe.

According to a study of 20 cases of destruction of stars by supermassive black holes, there are times at which very strong jets appear. They come in two stages of the feeding process of the black hole – one when there is very heavy feeding.


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The other appears hundreds to thousands of days after the star is torn up, when the feeding process slows down to about 2 percent of its peak value.

The research was conducted by Dr. Adelle Goodwin from Curtin University of Western Australia in collaboration with Dr. Andrew Mummery of the Institute for Advanced Study.

A star's destruction does not mean all its material disappears

Black holes have gravity so strong that nothing can escape from inside their event horizon. But a star passing too close to one can be destroyed before reaching that boundary.

The process is called a tidal disruption event. Extreme gravitational forces stretch and tear the star apart, a phenomenon often known as spaghettification.

Not all the resulting debris is swallowed.

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Goodwin explained that roughly half of the star's material may eventually fall into the black hole, while the rest can be thrown back into space through outflows and jets.

These jets can travel enormous distances and, in some cases, influence the environment around a galaxy.

That makes their timing important. A jet appearing months or years after a star's destruction may reveal something about how the black hole continues to feed on the remaining debris.

Radio observations reveal a two-stage feeding pattern

Astronomers have known that black hole jets do not always appear at the same time after a tidal disruption event. Some are detected relatively soon, while others emerge years later.

To investigate the difference, Goodwin and Mummery studied radio observations of 20 tidal disruption events involving supermassive black holes.

Radio telescopes are useful for tracking jets and outflows as they move away from their source. The researchers compared the observed jet activity with the rate at which the black holes were consuming material.

Their analysis pointed to two phases when powerful jets can be launched.

The first takes place when the black hole is feeding at very high rates.

The second occurs after the feeding rate has fallen sharply. In the later phase, the rate is around 2% of the maximum rate at which a black hole can consume material.

This threshold is already known from studies of stellar-mass black holes.

The new findings suggest that the same general feeding pattern may apply to black holes across a vast range of sizes.

Giant and small black holes may share the same trigger

Supermassive black holes can contain hundreds of thousands to billions of times the mass of the Sun.

Stellar-mass black holes are much smaller, generally ranging from around 10 to 50 solar masses.

Despite the difference, the researchers found that supermassive black holes appear to launch jets at similar stages of their feeding cycle to those seen in smaller black holes.

The result could help explain why a jet sometimes appears long after a star has been destroyed.

The debris from the star can continue feeding the black hole, but the conditions associated with jet production may only develop later. When the feeding rate reaches a particular stage, the black hole may launch an outflow.

The research does not mean that every black hole produces an identical jet or that the timing is fixed for every event. The strength and behaviour of individual jets may still depend on the properties of the black hole and the disrupted star.

Predicting jets could help astronomers plan observations

The discovery may have practical value for future studies of black holes.

Tidal disruption events are rare, and following them over several years requires valuable telescope time. If astronomers can estimate when jet activity is likely to occur based on a black hole's feeding rate, they may be able to plan radio observations more efficiently.

Goodwin said the work could help scientists understand not only when jets are launched, but also how powerful they are and whether their properties depend on the black hole.

Dr Sara Webb, an astrophysicist at Swinburne University who was not involved in the research, said the study links the behaviour of supermassive black holes with observations of much smaller stellar-mass black holes.

That comparison matters because astronomers cannot observe a black hole's event horizon directly. Instead, they study the material and radiation produced around it.

Jets provide one way to investigate how matter falling towards a black hole can drive activity far beyond its immediate surroundings.

The researchers will need to examine more events to determine how consistently the two-stage pattern appears and whether the strength of the jets can be linked to specific black hole properties.

Frequently Asked Questions

1. What happens when a black hole tears a star apart?

A star passing too near a black hole will be shredded and pulled apart by tidal forces. Some pieces of the star might get pulled in by the black hole, while some may get flung out.

2. How long can it take for a jet to show up after the destruction of a star?

The results show that strong jets can be formed hundreds to thousands of days after a star is torn apart. Each event differs in timing.

3. Why do researchers observe black hole jets using radio telescopes?

Using radio telescopes, researchers can track jets and outflows as they move away from the black hole. This allows researchers to observe the black hole activity even after a star gets torn apart.

4. Do supermassive and stellar-mass black holes produce jets the same way?

According to the study, both types of black holes can form jets during the same stage of their feeding process. While they vary greatly in size, the feeding rate might be similar.