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MILKY WAY |
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INTERSTELLER MEDIUM The stars define the Milky Way’s shape and structure, but what lies between them is just as important. Space is not entirely empty: a volume about the size of a matchbox contains about half a dozen hydrogen atoms and the odd dust grain. Over the vast distances in space, these tiny amounts add up to 10 per cent of our Galaxy’s mass. There is enough gas alone to make 20 billion stars like the Sun. This mixture of dust and gas – the interstellar medium – is always churning giving birth to stars and absorbing some of their material when they die. The matter returned by a drying star is subtly different from that which made it, so the make-up of the interstellar medium is constantly evolving. Between the Stars The interstellar medium is far from uniform. Most of the gas is spread out in what is called the warm intercloud medium, where "warm" is 8,000° C (hotter than the Sun). Throughout this are bubbles of thin gas, where temperatures exceed 1 million ° C, created by the energy if supernovas or groups of young, hot stars. There are also cold clouds of hydrogen atoms – often curving, filament – like structures that outline the shells of ancient gas bubbles. Finally, there are very dark, dense clouds of molecules of gas and dust – molecular clouds – in which stars are born. |
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