Hydrogen burning fuses hydrogen nuclei into helium in stars, creating heavier elements in their cores. Star formation occurs in interstellar gas clouds, with the birth of a star occurring when hydrogen ignites. The more massive the star, the faster it burns its fuel, with the hydrogen burning phase lasting up to nine billion years in stars like our Sun.
Hydrogen burning is a process that occurs in every star, whereby hydrogen nuclei are fused into helium at high temperatures and pressures. It is the most common type of process known as stellar nucleosynthesis. After the Big Bang, the universe consisted of about 75% hydrogen and 25% helium. Today the proportions aren’t all that different, but there are new elements: the universe is about 74% hydrogen, 24% helium, and 2% other elements. These other elements, the most common being oxygen (1%), carbon (.4%), neon (.1%), iron (.1%) and nitrogen (.1%) are all products of stellar nucleosynthesis – the synthesis of heavier elements in stellar cores. Elements heavier than iron are created in supernovae.
Star formation occurs in dense clouds of gas in interstellar space. These are called H II regions or star nurseries. Eventually, a high concentration of mass appears in an area the size of our solar system. This is called a Bok globule. When the temperature and pressure at its center reach a certain level (about 10 million degrees Kelvin), the hydrogen is ignited and large amounts of heat and light are produced. This is the birth of a star.
When a star is engaged in hydrogen burning, it is said to be on the main sequence and is called a dwarf star. Our Sun is a yellow dwarf. Main sequence stars are the most common stars in the universe, mainly due to the time it takes for hydrogen to burn. Only a small percentage of the nuclei in the stellar core are fused into helium per year. If hydrogen burned rapidly, most of the hydrogen in the universe would have already been consumed by nuclear reactions and converted into heavier elements, making the formation of water (H2O) – and therefore life – difficult if not impossible.
How a star evolves after its formation depends on its mass. The more massive the star, the faster it burns its fuel. In the most massive stars, the burning of hydrogen is mostly completed after only a few million years and the next step begins: the burning of helium. In stars like our Sun, the hydrogen burning phase is expected to last nine billion years. In stars one-tenth the mass of the Sun, hydrogen combustion can last up to a trillion years! Such stars are significantly cooler than our Sun.
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