A star is a vast sustained nuclear explosion at the centre of an enormous sphere of superheated gas (or plasma). The Sun is the nearest star to Earth. Massive clouds of hydrogen gas (often laced with trace amounts of heavier elements) start condensing through various mechanisms until they’re dense enough for gravity to start drawing the cloud in upon itself. As it contracts, the temperature and pressure begin to rise. After a while the cloud will be so dense and heavy that it forces itself into a round shape (in the same way as gas giant planets like Jupiter), but as more material falls inwards, the pressure and temperature at the core continue to rise. Eventually it is so hot and dense at the core that nuclear fusion reactions begin, converting the hydrogen into helium. The enormous amounts of energy released push back against the weight, stopping the cloud from collapsing any further, and heating up the main volume to extreme temperatures. At this point, those parts of the cloud which are still far from the surface of the new star are blown away by radiation pressure, revealing the newborn star.
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