Extended Spectral Types 

Of Stars

Protostar

A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy released by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

T Tauri Star

A T Tauri star is the stage in a star’s formation and evolution right before it becomes a main-sequence star. This phase occurs at the end of the protostar phase when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main-sequence stars; they’re about the same temperature but brighter because they’re larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Stars

Main Sequence stars are young stars. They are powered by the fusion of hydrogen (H) into helium (He) in their cores, a process that requires temperatures of more than 10 million Kelvin. Around 90 percent of the stars in the Universe are main-sequence stars, including our sun. The main sequence stars typically range from between one-tenth to 200 times the Sun’s mass. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.

White Dwarf Star

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its gravity. A white dwarf shines because it was a hot star once, but no fusion reactions are happening anymore. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have cooled down that far yet.

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