In the first observation if its kind, scientists at the Weizmann Institute of Science in Rehovot and San Diego State University in California have watched what happens when a star the size of 50 suns explodes.

An exploding star [Illustrative].
Photo: AP

As the scientists continued to track the spectacular event, they found that most of the star's mass collapsed in on itself, resulting in a massive black hole.

While supernovae (exploding stars) have been viewed with everything from the naked eye to high-tech research satellites, no one had previously directly observed a giant star going nova. Dr. Avishay Gal-Yam of Weizmann's physics faculty and Prof. Douglas Leonard of San Diego State University recently located and calculated the mass of a gigantic star on the verge of exploding, following through with observations of the blast and its aftermath.

Their findings have given support to the popular scientific theory that stars ranging from tens to hundreds of times the mass of our sun all end up as black holes.

Until now, none of the exploding stars scientists managed to observe had a mass of more than 20 suns. Gal-Yam and Leonard were looking at a specific region in space using the Keck Telescope on Mauna Kea in Hawaii and the Hubble Space Telescope. Identifying the about-to-explode star, they calculated its mass to be equal to 50 to 100 suns. Continued observation revealed that only a small part of the star's mass was flung off in the explosion. Most of the material, said Gal-Yam, was drawn into the collapsing core. Indeed, in subsequent telescope images of that section of the sky, the star seems to have disappeared. In other words, the star has now become a black hole - so dense that light can't escape.

The death of a star is predetermined from birth by its size and by the "power plant" that keeps it burning during its lifetime. Stars, among them our sun, are fueled by hydrogen nuclei fusing together into helium in the intense heat and pressure of their inner cores. A helium nucleus is a bit lighter than the sum of the masses of the four hydrogen nuclei that went into making it and, from Einstein's theory of relativity (E=MC2), we know that the missing mass is released as energy.

When stars like our sun finish off their hydrogen fuel, they burn out relatively quietly in a puff of expansion. But a star that's eight or more times larger than the sun makes a much more dramatic exit: Nuclear fusion continues after the hydrogen is exhausted, producing heavier elements in the star's different layers. When this process progresses to the point that the core of the star has turned to iron, another phenomenon takes over; in the enormous heat and pressure in the star's center, the iron nuclei break apart into their component protons and neutrons. At some point, this causes the core and the layer above it to collapse inward, firing the rest of the star's material rapidly out into space in a supernova flash.

According to Gal-Yam, a supernova releases more energy in a few days than our sun will release over its entire existence, and the explosion is so bright that one occurring hundreds of light years away can be seen from Earth even in the daytime. While a supernova's outer layers light up the universe with dazzling fireworks, the star's core collapses further and further inward. The gravitational forces involved in this collapse are so strong that the protons and electrons are squeezed together to form neutrons, and the star's spherical core is reduced from a circumference of 10,000 kilometers to only 10 kilometers. Just a crate-full of material from such a neutron star would weigh as much as the Earth.

But when a star 20 times the mass of our sun or more collapses, he continued, its gravitational pull is so powerful that even light waves can't escape it. Such a star is practically invisible.