Life Cycle of Stars

The Journey of Star

Stars have a life cycle much like that of humans. A star is born, travels through the equivalent of infancy, adulthood, and old age, and then dies. The protostar phase is similar to a fetus. Main sequence stars represent the period of infancy through adulthood. A red giant star is entering old age, and a supernova or a white dwarf is the death of a star.
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Pre-Birth: Interstellar Medium

Space is filled with gas (mostly hydrogen and helium) and dust (carbon and silicon) called interstellar medium. Sometimes, interstellar medium forms a big cloud called a nebula. Most stars are formed from the gas and dust in nebulae.


A protostar is a star in the process of being formed. A protostar is comparable to a fetus. Gravity pulls atoms of interstellar medium together, usually in a nebula. If a protostar is going to survive, it must achieve equilibrium. This is the balance between gravity and gas pressure in a star. Gravity pulls matter towards the center of the star and gas pressure, which depends on temperature and density, sends heat and light away from the core.

The Birth of a Star

Nuclear fusion powers stars. During nuclear fusion, two nuclei will combine to form a larger, more stable nucleus. In order for this process to begin, the critical temperature must be reached in a protostar. Most stars fuse hydrogen into helium, but when the hydrogen is gone helium is fused into carbon. Giant stars may even fuse carbon into heavier elements.

The image below shows how hydrogen fuses to form helium.

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Main Sequence Star

The main sequence (the longest period of a stars life) represents the period of infancy through adulthood. During the main sequence, stars are fusing hydrogen into helium while maintaining equilibrium. The size of a star determines how long the main sequence phase will be. Some small stars may even die before they reach the main sequence.

Red Giant Star

A red giant star is entering old age. Now helium is fusing in the core and larger elements such as carbon and beryllium are formed. The star begins to collapse inwards, and the outer shell expands to trap heat inside the core. A different nuclear process, neutron capture, may occur. Red giants are larger and brighter than main sequence stars, but not as hot.
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Death of a Star

Stars can die in several different ways. Sometimes the outer layers of a red giant star are blown off by stellar winds, creating a planetary nebula. The remaining star is called a white dwarf, or a very small, hot, and dense star. Or, a star may explode in a supernova. The movement of atomic particles in the core creates a shockwave, and the energy scatters the star out into space. The size of a star determines how it will die.

The Light of a Star

Astronomers can determine the composition of a star by looking at its emission spectra. Different gases and elements in stars produce different bands of light at different wavelengths. The Doppler Effect causes a phenomenon called blue-shift and red-shift in stars. Because stars are moving, the waves become compacted in front of the object from which the waves originate. If light is moving towards an observer, the wavelength is shortened (this is called blue shifted). When light is traveling away from an observer, the wavelength is lengthed (red shifted).

Below is the emission spectrum of helium.

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The Origin of Elements

Elements are created in different ways in our universe. Hydrogen was formed moments after the Big Bang. In stars, hydrogen fuses to make helium, carbon, beryllium, and lithium. Calcium and iron were formed by the same process of nuclear fusion in large stars. Some of the heavier elements, such as gold, and other elements, like oxygen and nitrogen, were formed by supernovae.

Works Cited

"Life Cycle of a Star." Life Cycle of a Star. Web. 23 Mar. 2015. <>.

"NUCLEAR FUSION." Schoolphysics ::Welcome::. Web. 25 Mar. 2015. < physics/text/Nuclear_fusion/index.html>.

"Red Giant." Wikipedia. Wikimedia Foundation. Web. 25 Mar. 2015. <>.

"Stellar Evolution." Wikipedia. Wikimedia Foundation. Web. 10 Mar. 2015. <>.

Research Guide: Our Cosmic Connection to the Elements

All images in the public domain.