STARS
Last updated: 05/06/2019
A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.
Our Sun is large compared to everything else in the Solar System, but it is actually pretty small when you compare other stars. Our Sun is a yellow dwarf or G2V Star. Other types include:
[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.
[T Tauri Star]- A T Tauri star is a 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.
[Main Sequence Star]- The majority of all stars in our galaxy and even the Universe are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass, and brightness, but they're all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.
[Red Giant Star]- When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star but causes it to increase in size dramatically. The aging star has become a red giant star and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions.
[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 own gravity. A white dwarf shines because it was a hot star once, but there are no fusion reactions happening anymore. A white dwarf will just cool down until it because of the background temperature of the Universe.
[Red Dwarf Star]- Red dwarf stars are the most common kind of stars in the Universe. These are main-sequence stars but they have such low mass that they're much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.
[Neutron Star]- If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn't form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.
[Supergiant Star]- The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.