Black holes are exotic space phenomena whose gravitation pulls are so strong that not even light can escape. They are formed when heavy stars exhaust all their hydrogen and collapse under their own weight.

Nobody has witnessed the birth of a black hole because nobody is sure what kinds of massive stars produce black holes. “Once we start seeing events like this more often, we can actually study the stars that produced them for the first time,” said Tony Piro, a post-doctoral scholar at the California Institute of Technology (Caltech), to this Correspondent in an email.

A paper by Piro published in the May 1 issue of Astrophysical Journal Letters may change that. He has analysed that just before a black hole forms, the dying star may generate a distinct burst of light that could allow astronomers to witness the birth of a black hole for the first time.

When a massive star’s core collapses, its protons and electrons merge to form neutrons. As the collapse intensifies, an extremely dense core called a neutron star is formed.

Particles called neutrinos are also formed simultaneously. They zip through matter at close to the speed of light, taking away a lot of the mass in the core, making its gravitational pull drastically weak. When this happens, the gaseous layers around the core would be propelled away and collide with the outer layers at 1,000 km/hr, causing the latter to heat up and glow.

While this is a potential sign of a black hole’s impending birth, the glow would not be strong enough to be distinguishable from other bright light in the universe.

However, the paper finds that at the moment of the collision itself, a flash 10-100 times brighter than the outer layers’ glow would be visible for 3-10 days.

Commenting on the nature of the glow, Piro said, “The glow tends to be brighter if the star is less massive. But the star is more likely to form a black hole if it's more massive.

“The sweet spot is probably 15 to 30 times the mass of the sun, but nailing this down better will require more work.”

The Caltech-led Palomar Transient Factory supernova-survey is in a good position to track such flashes, and may see up to one such event per year once astronomers fine-tune it.