SCIENCE

THE BIRTH OF A STAR

Introduced by Dr Stephen Davis, University of Bath, on 23 January 1998

Stars are formed from the clouds of Interstellar Medium congealing by gravitational attraction into increasingly large lumps, which it does under the influence of radiated energy and particles from other stars already formed in the cloud.
Interstellar Medium consists of particles of carbon, ice and silicates, often much less than 1 micron (1/1000 mm) in size, hydrocarbon molecules and ionised hydrogen containing traces of carbon monoxide, ammonia, water vapour and hydrocarbons. The dust provides the surfaces on which chemical reactions take place.
As the particles impact on the lump their energy is converted into heat. Eventually, after perhaps a million years, the lump becomes hot enough in the centre for nuclear fusion to start, making it luminous. It emits radiation at infra-red and radio wavelengths around 1 mm, and the consequent cooling opposes further growth, so that it comes into 'hydrostatic equilbrium' - the state our Sun is now in.The luminosity of the star is related to its temperature, from dull red-hot to blue-hot, and this relationship is shown by a Hertzsprung-Russell diagram. (Fig.1)
Fig.1

The Interstellar Medium forms a star when its local average density achieves a critical density, which is very low in terrestrial terms, e.g. 105 (100,000) molecules per cubic metre. (Air at sea level contains 1025 , 1 followed by 25 zeros.) It may be compressed to this density by a shock wave or by radiation from another star in the cloud.
Once formed nuclear fusion in the young star produces heavier elements, up to the atomic weight of iron, from the hydrogen and helium of which it is mostly composed. If the nuclear reaction causes it to explode into a supernova, the enormous energy released produces elements heavier than iron, which are spread through space and which provide all those which occur on earth and in our bodies.
During the discussion, the speaker described his work on constructing detectors for measuring the spectra of stars at radio wavelengths. This involves assembling two films at such a distance apart that four extra atoms of the spacing material can make the detector unsatisfactory! It is then operated, cooled by liquid helium to a few degrees above absolute zero for three weeks at a time, at the focus of a radio telescope on top of an extinct volcano in Hawaii. The telescope has a 25m-diameter mirror curved to an accuracy of 1 part in 10,000 and has to withstand hurricane force winds.
Don Lovell