A Star’s Life

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In the first observation of its kind, scientists at the Weizmann Institute of Science and San Diego State University succeeded in following the evolution of a star the size of 50 suns. As they watched, it exploded and then vanished from view to become a large black hole.

 

A star’s end is predetermined from birth by its size and by the “power plant” that keeps it shining during its lifetime. Stars are fueled by hydrogen nuclei fusing together into helium in the intense heat and pressure of their inner core. When stars like our sun use up all their hydrogen fuel, they burn out relatively quietly in a puff of expansion. But a star that’s eight or more times larger than our 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: The enormous heat and pressure in the star’s center cause the iron nuclei to split apart into their component protons and neutrons, and at some point the star’s core collapses inward, firing the rest of the star’s material rapidly out into space in an explosion known as a supernova.

 

A supernova releases more energy in a few days than our sun will release over its entire lifetime. While a supernova’s outer layers are lighting up the universe with dazzling fireworks, the star’s core is collapsing further and further inward. The gravity created in this collapse becomes so strong that the protons and electrons are squeezed together to form neutrons, and the star’s core is reduced from a sphere 10,000 kilometers around to one with a mere 10-kilometer circumference. But if the exploding star is 20 times or more the mass of our sun, its gravitational pull becomes stronger still: Even light waves are held in place. Such a star – a black hole – is, to all intents and purposes, invisible.

 

Supernovae have been observed with the naked eye since antiquity, and many more have been observed in recent years through ground and space telescopes and via research satellites. Yet until now, none of the exploding stars that scientists have managed to measure had exceeded a mass of 20 suns.

 

Dr. Avishay Gal-Yam of the Weizmann Institute’s Faculty of Physics and Prof. Douglas Leonard of San Diego State University were recently the first to directly observe the process by which a really huge star becomes a black hole. They were looking at a certain region in space using the Keck Telescope on Mauna Kea in Hawaii and the Hubble Space Telescope. The scientists were able to locate a star on the verge of exploding and to measure its mass before the blast. They found that this star’s size was equal to 50 – 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, says Gal-Yam, was drawn into the collapsing core as its gravitational pull increased. Indeed, in subsequent telescope images of that section of the sky, the star seemed to have disappeared. It has now likely become a black hole so dense that even light can’t escape the pull of its immense gravity.