Astronomers watch the whole process as a huge star dies

WASHINGTON, Aug 7 - Astronomers have taken the rare opportunity to observe the explosive death of an enormous star from start to finish, gaining a new understanding of how this kind of violent event can transpire in more ways than previously known.

The observations began in March when China's Einstein Probe space telescope detected a momentary flare of X-rays caused when a powerful shock wave triggered by the explosion in the collapsing stellar core tore through the doomed star's surface.

This is called a shock breakout. It is thought to occur in every supernova but is notoriously hard to spot because of how brief it is. In fact, this one was the first since 2008 to be observed.

"It requires serendipity to catch it in real time," said astronomer Brendan O'Connor, a postdoctoral fellow at Carnegie Mellon University in Pittsburgh and lead author of one of two scientific papers describing the supernova published in the Astrophysical Journal Letters.

"You can think of the shock like radar — as the shock plows through the star's outer layers and any material in the vicinity, it leaves an imprint on the signal that we detect in X-rays. We can use these X-rays to give us an unprecedented, close-up view of the star at the brink of collapse," said astronomer Jillian Rastinejad, a NASA Einstein Fellow at the University of Maryland and lead author of the other study.

Researchers quickly marshaled a battalion of telescopes, including the orbiting Chandra X-ray Observatory and a host of ground-based facilities, to continue the observations for almost three months until the supernova's location passed behind our sun, from the perspective of Earth.

The star was located roughly 500 million light-years from Earth in a relatively nearby galaxy. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The star was estimated to be around 30 times more massive than the sun, prior to the supernova.

"This would be among the most massive stars, probably larger than Betelgeuse," Rastinejad said, referring to one of the brightest stars in the night sky, a stellar behemoth believed to be nearing a supernova death, located in the Milky Way about 500 to 600 light-years from Earth.

The blast is thought to have left behind a black hole, an exceptionally dense object with gravity so strong not even light can escape. Before the explosion, this was a Wolf-Rayet star, a rare and gigantic kind that had shed its outer layers of hydrogen and helium through powerful stellar winds.

The star's explosion was classified as a "broad-lined Type Ic" supernova, involving a star with stripped outer layers and with the material ejected by the blast moving at an extreme velocity — in this case a bit more than 10% of the speed of light.

NO GAMMA-RAY BURST

The supernova displayed many characteristics of powerful stellar explosions often associated with a phenomenon called a gamma-ray burst — an intense flash of gamma-ray radiation, the highest-energy form of light — but in this case there was no evidence of one.

"One of the central unanswered questions in the field is why some collapsing massive stars launch jets of material near the speed of light that escape the star and produce gamma-ray bursts, while apparently similar stars do not," O'Connor said.

"The fast-moving matter in the jet collides with itself in discrete shocks to produce the gamma-rays we see in gamma-ray bursts," O'Connor said.

One possibility is that this jet was "choked" — prevented from surging into space — either by the surface of the star or by dense material that had surrounded the star in the final stages of its life.

"The existence of choked jets was theorized decades ago, but yet to be conclusively identified. The difference between a successful and choked jet is whether the jet is moving fast enough, or is powerful enough, to break out of the surrounding stellar material," O'Connor said.

This represented the first supernova of its type observed without a gamma-ray burst and jet of material.

"The finding shows that the most massive stars die in more ways than astronomers previously thought," Rastinejad said.

"These extreme supernovae are laboratories for astrophysicists to study how the laws of physics behave in extreme environments — think high densities, high temperatures, material that is several times the mass of our sun — that we can't recreate here on Earth. By studying them, we learn more about the laws of our universe," Rastinejad said. REUTERS