Main sequence stars are the most common type in the Universe. Main sequence stars are stable. They fuse hydrogen nuclei together to form helium nuclei, releasing energy and emitting light.

A main sequence star is a star in the stable part of its life cycle. They are the most common type of star in the universe. Our star, the Sun, is in the main sequence phase. It is about halfway through this stage, and ultimately will become a red giant in roughly five billion years.

All main sequence stars are in equilibrium, meaning the outward pressure caused by the fusion reactions is balanced by the force of gravity pulling the star together. The pressure and temperature of a main sequence star increase as you get closer to its center. The length of time that a star spends at this stage in its life depends on how much mass the star has. Counterintuitively, massive stars have a shorter lifespan than smaller stars. Large, massive stars use up their nuclear fuel at a much faster rate than smaller stars. Stars can range in size from about a tenth of the size of our sun all the way up to hundreds of times as big. The color of a star also varies depending on its size. Larger stars are hotter and they emit more blue light; smaller stars are smaller and emit more red light.

The main sequence stage occurs after a stellar nebula collapses due to the force of gravity. As the nebula collapses, the internal temperature increases. When the core of the newly-formed protostar reaches a certain temperature, nuclear fusion starts. Nuclear fusion is a nuclear reaction that releases energy by fusing together smaller, lighter nuclei into a larger, heavier nucleus. This process releases photons of energy. These photons are absorbed and reabsorbed multiple times before leaving the star. The amount of energy that is released can be calculated using Einstein’s famous equation, E=mc2, where E is the amount of energy, m is the change in mass and c is the speed of light.

Most main sequence stars are nearly completely composed of hydrogen and helium. Some have a small percentage of heavier elements, such as carbon or oxygen. Scientists can analyze the composition of a main sequence star by studying the light that they emit.

Stages in a Star’s Life Cycle

(a star with a similar mass to our Sun)

  1. Stellar Nebula

  2. Main Sequence Star

  3. Red Giant Star

  4. Planetary Nebula

  5. White Dwarf

  6. Black Dwarf

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