Cosmologists long tried to fathom the stars' substance, but it took an Irish scientist to make the discovery, writes Mary Mulvihill.
What is the stars, what is the stars? Joxer it was who first asked this in 1925 in Sean O'Casey's play, Juno and the Paycock .
Three years later another Dublin man had the answer: the stars is mostly gas, Joxer, namely three-quarters hydrogen and one-quarter helium. And a pretty shocking answer it was too, because for thousands of years people had presumed that the stars, including our Sun, were mostly made of iron.
People have stared at and speculated about the heavens since time began. In the 5th century BC, a Greek philosopher, Anaxagoras, suggested that stars made of the same stuff as the Earth. With no other information to go on, it seemed a reasonable suggestion, and for nearly 2,500 years people accepted it.
In the 1860s, however, a new scientific instrument called a spectroscope began to revolutionise our understanding of the heavens.
With a spectroscope, scientists could study the chemical composition of things by analysing the light coming from them. Amazingly, this meant that scientists could begin to study the composition of stars and planets, without ever leaving the Earth.
Over the next 50 years, the data accumulated, and a puzzling picture emerged. The planets seemed similar to Earth, as Anaxagoras had suggested, but not the stars. Scientists could find no trace of iron or any of the other terrestrial elements in the light coming from the Sun, only evidence of two simple gases, hydrogen and helium.
Enter a young Dublin-born physics student, Sir William Hunter McCrea who made sense of it all. In 1928 McCrea, who was completing a PhD at Cambridge University, did calculations that proved conclusively that the Sun was three-quarters hydrogen and one-quarter helium, with only traces of a few other elements.
Soon it was clear that hydrogen, the simplest of all chemical elements, is the main constituent in all stars. Thanks to McCrea's calculations, this provided evidence for the Big Bang theory about the formation of the universe.
The particles that formed just after the bang were so hot that the protons and neutrons could hardly come together. Only the simplest chemicals formed, for the most part hydrogen, which needs only one proton plus some helium, with two protons and a neutron or two.
As the universe expanded, it cooled, and after a billion years or so, the hydrogen and helium gases could condense to form galaxies and stars. The gravity at the centre of the stars was so great that helium atoms were crushed together to form atoms of some of the heavier elements that come further down the periodic table, such as carbon and oxygen. These in turn would later be crushed together to form even heavier atoms. But most of the matter still remained as hydrogen.
McCrea's findings laid the basis for our modern understanding of how stars form, and the origins of the solar system, galaxies and even the universe. Ironically, McCrea himself did not believe in a Big Bang, and preferred to think the universe formed from several small bangs.
Awarded numerous international honours, Sir William, or Bill as he was known, was professor at Queen's University Belfast from 1936-44, and later at the University of London and Sussex University.
This modest, courteous, grand old man of astronomy, who died in 1999, helped develop Armagh and Dunsink observatories. He was born in Ranelagh 100 years ago, on December 13th 1904.