Asked by Ben Sulivan
Many stars are made of about 90 per cent hydrogen, with most of the remainder being helium and a very small fraction of heavier elements.
Stars form from the sudden collapse of clouds of interstellar matter and during its lifetime a star burns its hydrogen to form helium. In fact, in order for a star to form, it must be able to start the process of burning hydrogen. Whether or not a star can begin this process is dependant on the collapsing interstellar cloud’s mass.
The original conditions of the interstellar cloud can determine a star’s type, how bright it is and how long it’s likely to last. Stellar composition can vary – newer stars are found to have a relatively higher percentage of heavier elements. However, for their birth they need conditions that enable them to burn their hydrogen.
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