Stars are born in vast clouds of dust and gas, and take many years to evolve into the brilliant objects that illuminate our night skies. Stars are immense balls of hydrogen and helium, which shine by the same nuclear reactions that power hydrogen bombs. The Sun around which the Earth and other planets revolve is a typical middle-aged star. At its core hydrogen is being converted to helium at a rate of four million tonnes per second, which releases vast amounts of energy. Though this sounds an enormous figure, astronomers know that there is enough of this ‘nuclear fuel’ to power the Sun for another five billion years. It has been known for some time that stars are formed in nebulae – clouds of dust and gas – some of which can be seen in the night sky. To the naked eye, the Great Nebula in Orion appears as part of the mighty hunter’s sword. But many nebulae are cold and dark and cannot be seen with even the most powerful optical telescopes.
In 1983 a satellite called IRAS – the Infra-red Astronomical Satellite - was launched. It revealed that many nebulae extend further than they appear to in original light, the outer regions of dust and gas glowing at the longer infra-red wavelengths. The dust is a vital ingredient in starbirth, even though it only accounts for 0.1 per cent of the material between the stars in our Galaxy. Some of it is made of ice, it contains ice crystals, silicon and even organic compounds, such as formaldehyde. But most of it is carbon in the form of small dust grains. Other molecules have been revealed by radio telescopes that can, ‘tune into’ their atomic vibrations.
Moment of starbirth
The dust and gas gradually begins to come together under the influence of its own gravity, helped by other processes that compress the gas – such as collision with the expanding shells of material thrown off by dying stars. Massive stars may undergo a tremendous supernova explosion, in which most of their material is throw off into space, adding greatly to the dust and gas already in the vicinity, and further compressing the gas. Eventually, in regions where the density of material is sufficiently great, the dust and gas condenses into a vast swirling concentration – a proto-star.
The proto-star continues to contract and as it does so the temperature at its centre increases. The more massive the proto-star, the higher its central temperature. Some proto-stars are so small that they will never become hot enough for nuclear reactions to begin. But a proto-star of sufficient mass will, after many tens of thousand of years, ‘go nuclear’ (start to shine), when it central temperature reaches ten million degrees Celsius. This is the moment of starbirth.
The new-born star emits large quantities of radiation, which can disrupt the formation of other stars in the vicinity. However, after its violent birth, the star settles down to a period of relative stability. Any remaining dust and gas in close proximity to the star may clump together to form planets, comets and meteoroids, such as in our own Solar System.