Death of a Star

What happens to a star when it runs out of fuel and dies? In the 1920s, scientists assumed that when a star burns off all its energy supply its light fades, leaving behind the burnt-out and dense remains known as a white dwarf. Subrahmanyan Chandrasekhar was the first to show how the fate of a star lies in its own birth mass.

On a long sea voyage from India to England in 1930, Chandrasekhar passed the time by developing a theory that proposed that a stable white dwarf couldn’t be the fate of stars above a certain critical mass. According to his calculations, stars more than 1.4 times the mass of the Sun, which became known as the Chandrasekhar limit, must collapse under the force of their own weight, and be destined for a more spectacular fate.

It would take a generation of scientists to pinpoint precisely the fate of these larger stars, but in time it was proved that they do indeed go out with a bang, dying in a mammoth explosion called a supernova. If the original star was up to 2-3 times the mass of the Sun, the collapsed corpses left behind from the explosion end up as highly dense neutron stars. Stars that are more than 2-3 times the mass of the Sun suffer an even more exotic death – the force of gravity becomes so strong that matter disappears entirely into a black hole.

Chandrasekhar adopted a highly unusual approach to his research, investigating a fresh field of study each decade, such as how stars die, how radiation passes through a star’s atmosphere and the theory of black holes. Each decade he followed a similar routine; writing a series of papers that solved the unsolved problems in that field, before finally publishing a book that summarized his results and presented the whole field in a new and clearer light. However, it is mainly for his earliest triumph, inspired on the journey that began his voyage through the stars, that Chandrasekhar was awarded the 1983 Nobel Prize in Physics.

Alchemy in the Stars

William A. Fowler, 1/2 of the prize

From the hydrogen and oxygen in the water we drink to the calcium in the bones that we are made of, chemical elements are the raw materials of life as we know it. But where do these elements come from? The 1983 Nobel Prize in Physics rewarded William Fowler’s efforts to show how all the natural elements in the Periodic Table are forged under extreme conditions during the course of a star’s lifetime.

The idea that stars can create elements within the intense temperatures and pressures found in their cores was first proved in 1939, when Hans Bethe showed that the Sun generates its heat and light by squashing hydrogen atoms together to form helium, releasing tremendous amounts of energy in the process. Almost 20 years later, Fowler, together with his colleagues Geoffrey and Margaret Burbidge and Sir Fred Hoyle, provided the definitive outline of how stars create all the elements from this primordial hydrogen and helium.

As a star ages, it eventually exhausts its hydrogen supply and begins to fuse its helium into carbon, once again releasing energy. Stars bigger than the Sun, which shine more furiously and under greater pressure, can fuse heavier elements than helium, creating all the elements up to iron. Creating heavier elements than iron requires the input of the vast amounts of energy produced when a large star dies in a tremendous supernova explosion. Under these extreme conditions, iron atoms can absorb more and more of the uncharged subcomponents of an atom, known as neutrons, leading to the creation of all the elements up to uranium. The explosion also blasts these elements across the universe to create more stars, planets and eventually life as we know it.

Fowler has continued to fine-tune our understanding of the nuclear reactions that form the elements within stars, both with theoretical calculations of the steps involved in the nuclear reactions and with experiments using particle accelerators to study the behaviour of elements. Through this shuttling between theory and experiment, Fowler is helping to shed further light on the formation of the chemical elements in the universe to help us understand how, as the astronomer Carl Sagan has famously said, “we are made of starstuff.”