Stars seem to remain unchanging, shining night after night, year after year. But these heavenly bodies, too, are not outside the cycle of life and death.

“Stars shine until they exhaust their fuel supply,” said Professor William Arnett of the University of Arizona. Arnett was in Calcutta recently to deliver the first S. Chandrasekhar lecture organised by the S.N. Bose National Centre for Basic Sciences. Speaking on the topic “The Way Stars Die”, he also elaborated on the existence and formation of black holes.

There are three general end points for stellar evolution, explained Arnett — white dwarfs, neutron stars and black holes. All these possibilities are the result of some general principles of physics, involving quantum mechanics, nuclear physics and Albert Einstein’s general theory of relativity, he said.

The lifespan of a star depends upon its mass. The bigger the star, the quicker it dies. This might seem odd, but the more mass a star has, the hotter it gets. “And the hotter it gets, the quicker it exhausts its fuel supply,” he explained.

Our nearest star — the sun — will exhaust its supply of hydrogen fuel in around four billion years. Its core will then collapse under its own gravity. At the same time, its atmosphere will become unstable and start to expand. This will transform the sun into a giant red star.

As a star’s core crashes inwards, it eventually becomes hot enough to ignite another of its constituent atoms, helium. “Helium atoms fuse together to form carbon. When the helium supply runs out, the centre collapses again and the atmosphere inflates. The sun isn’t massive enough to fully re-ignite its core for a third time. So it goes on expanding, shedding its atmosphere in a series of bursts,” said Arnett.

This dying core will eventually form a white dwarf — a spherical diamond the size of the earth, made of carbon and oxygen. From then on, the sun will gradually fade away, becoming dimmer and dimmer until its light is finally snuffed out.

When a star a few times larger than the sun runs out of fuel, its end is far more spectacular. A massive shock wave radiates through the star, which heats up to around 1 billion ºC. It then explodes as a supernova. “This flash is as bright as a whole galaxy and leaves behind a rapidly spinning neutron star,” said Arnett. Neutron stars consist mostly of neutrons, and hence the name.

Even more dramatic is the death of stars that are over 20 times bigger than the sun. These stellar monsters do not stop collapsing, even when they become neutron stars. There is no force in the universe strong enough to stop them. When they implode, they bend the very fabric of space and time and form the most mysterious phenomenon in the whole cosmos — a black hole.

“The final states of stars are thus white dwarfs (plus ejected planetary nebulae), neutron stars and black holes,” Arnett summed up. And whilst we can see supernovae and white dwarfs, we can only infer about black holes.