The Life Cycle of a Star

By Alayna Marsh

Stars and Humans

The first definition of a star is "A star is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion)." Stars and humans are alike in many ways, firstly, that they both have lives in which they go through stages of age and growth. When humans are born they are infants, and when stars are born they become a Protostar. Then, when people grow, they become teenagers and adolescent, roughly equivalent to stars in their main sequence. The growth continues, and while humans become elderly, stars pass into their White Dwarf stage. Once maturation is achieved, both humans and stars eventually die.
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  • In the first stage of a star's life, it is called a Protostar. A Protostar is a collection of hydrogen gas and dust in space. A star is born with a purpose, and that purpose is to reach equilibrium. Equilibrium is a state of stable conditions in which all significant factors remain relatively constant over a period of time, and there is little to no susceptibility to change. Protostars are the beginning of the stars' journey to reach this goal.

Main Sequence

Once a Protostar has reached 15 million degrees centigrade, it can pass into the Main Sequence phase of its life. Hydrogen from the Protostar fuses with helium, which causes it to release energy and it stops contracting and letting out light that makes it shine, at that point is the star a Main Sequence star. Fusion is a nuclear reaction where two or more light nuclei are combined or fused into heavier ones, and release radiation and energy in the process. The star remains in the Main Sequence until all the hydrogen has fused to form helium (this take roughly 10 billion years for most stars)

Red Giant

The helium in the star then begins to contract towards the star's center and reactions begin to occur on the outer shell. When it is hot enough for helium to be fused into carbon, the outer layers begin to expand, grow cooler, and their luminosity weakens. The expanding star is now in it's phase of Red Giant. The star then burns until all of the helium core runs out and the outer layers drift away.


After about 1 billion years a a Red Giant, the helium in the core of the star runs out and fusion comes to a halt. The Star then shrinks again and a new helium reachers the core. When the helium starts to burn, the outer layers of the star are blown off in huge chunks called Planetary Nebulae. The center begins to collapse in on itself until they become either a White Dwarf or a Supernovae depending on the size of the star.


The light of a star can tell us many things; how big it is, how far away from Earth it is, its composition, its temperature, and much more. We can tell what elements are in a star by analyzing the light that shines from the star. Using the width and location of 'absorption lines' in the star's spectrum. There are only two elements that were not made from stars: Hydrogen and Helium. Since the first forming of stars, other elements have been fusing together from those original two elements to create heavier elements, forming all the naturally made elements on the periodic table.