The article was published at The Conversation.
Daniel Perley is a Reader of Astrophysics at John Moores University.
The curtains on black holes are being drawn back.
In the past few years, we have captured photos of these fearsome creatures and measured the ripples in spacetime that they create when they collide. There is still a lot we don't know about black holes. They form in the first place, which is one of the biggest mysteries.
Some of the best indications of what happens when a black hole forms have been provided by my colleagues and I. The results are published in two journals.
Astronomers believe that most black holes form when the center of a massive star collapses at the end of its life. The star's core uses heat from nuclear reactions to provide pressure. When a star's fuel is exhausted and its nuclear reactions stop, the inner layers of the star collapse under the weight of gravity.
When the star's core condenses into a solid sphere of matter, it stops the collapse. This leads to a powerful rebound explosion that destroys the star and leaves behind an exotic object. The collapse of a dying star will continue until the star is destroyed into a black hole if it is large enough.
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There are explosives theories.
Astronomers are not sure what happens when a star collapses to form a black hole, even though stars collapsing to form neutron stars are now routinely observed throughout the universe. Some models think the star would be swallowed up. Some people think that the collapse of a black hole would cause an explosion.
If the star is rotating at the time of collapse, some of the infalling material may be focused into jets that escape the star at high speed. The jets would pack a big punch if they hit something, and the effects could be dramatic.
Long-duration gamma-ray bursts are the best candidate for an explosion from the birth of a black hole. The events were first discovered in the 1960s by military satellites and have been attributed to jets speeding through the sky. There is a longstanding problem with this scenario, which is that there are abundant radioactive debris that continues to shine for months. Most of the star exploded outward into space, instead of collapsing into a black hole.
While this doesn't mean a black hole can't have been formed in such an explosion, some have concluded that other models provide a more natural explanation. A super-magnetized neutron star can form in such an explosion and produce powerful jets of its own.
Mystery solved?
A new and better candidate event for creating a black hole has recently been uncovered by my colleagues and I. Two times in the past three years, we witnessed a very fast and fleeting type of explosion that originated from a small amount of very fast- moving material slamming into gas in its immediate environment.
The composition of the star that exploded for each of these events could be deduced by using the technique of spectroscopy. The spectrum was very similar to the "Wolf-Rayet stars", a type of star named after Charles Wolf and Georges Rayet, who first detected them. We were able to rule out a normal explosion. The source disappeared as soon as the collision between the fast material and environment stopped.
If the star ejected only a small amount of material, the rest of the object would fall into a black hole.
This is our favored interpretation, but it's not the only possibility. It was a normal supernova explosion, but a vast shell of dust formed in the collision, concealing the radioactive debris from view. It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556
We will need to look for more such objects. These kinds of explosions are hard to study because they are fleeting. We had to use several telescopes in a short period of time to confirm the nature of the explosions, including the Very Large Telescope, the Hubble Space Telescope, and the Nordic Optical Telescope.
When we first discovered these events, we didn't know what they were, but now we have a clear hypothesis: the birth of a black hole.
More data from similar events may soon be able to help us verify or alter this hypothesis and establish a link to other types of unusual, fast explosions that our team and others have been finding. It seems this is the decade we crack the mysteries of black holes.
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