IN CONTEXT
AstroPhysics
1854 German physicist Hermann von Helmholtz suggests that the Sun generates heat through slow gravitational contraction.
1863 English astronomer William Huggins’ spectrum analysis of stars shows they share elements found on Earth.
1905–10 Astronomers in the USA and Sweden analyse stars’ luminosity and group them into dwarfs and giants.
1920 Arthur Eddington argues that stars turn hydrogen into helium through nuclear fusion.
1934 Fritz Zwicky coins the term “supernova” for a massive star’s explosive end.
2013 Deep-sea fossils reveal what may be biological traces of iron from a supernova.
The idea that stars generate energy through the process of nuclear fusion was first proposed by British astronomer Arthur Eddington in 1920. Stars, he argued, were factories for fusing nuclei of hydrogen into helium. A helium nucleus contains slightly less mass than the four hydrogen nuclei required to create it. This mass is converted into energy in accordance with the equation E = mc2. Eddington developed a model of star structure in terms of the balance between the inward pull of gravity and the outward pressure of escaping radiation, but he did not work out the physics of the nuclear reactions involved.
Making heavier elements
In 1939, German-born US physicist Hans Bethe published a detailed analysis of the different pathways that hydrogen fusion might take. He identified two routes – a slow, low-temperature chain that dominates in stars like our Sun, and a rapid, high-temperature cycle that dominates in more massive stars. Between 1946 and 1957, British astronomer Fred Hoyle and others developed Bethe’s ideas to show how further fusion reactions involving helium could generate carbon and heavier elements up to and including the mass of iron. This explained the origin of many of the Universe’s heavier elements. We now know that elements heavier than iron form in supernova explosions – the death-throes of massive stars. The elements needed for life are made in stars.
"Space isn’t remote at all. It’s only an hour’s drive away if your car could go straight upwards."
Fred Hoyle
See also: Marie Curie • Albert Einstein • Ernest Rutherford • Georges Lemaître • Fritz Zwicky