Types of stars

Types of stars

There are many different types of stars, ranging from tiny brown dwarfs to red and blue super giants. There are even more bizarre kinds of stars, like neutron stars and Wolf-Rayet stars. And as our exploration of the Universe continues, we continue to learn things about stars that force us to expand on the way we think of them. Stars are classified by their spectra (the elements that they absorb) and their temperature.

Here are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K and  M. Blue stars are the hottest, and are called O-type. The coolest stars are red and are called M-type. In order of increasing temperature, the spectral classes are M (red), K (orange), G (yellow), F (yellow-white), A (white), B (blue-white), O (blue).

Protostar

A Protostar is a baby star, an area of material that hasn't yet formed into a fully-fledged star or 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.

Under the act of gravity the temperature of the material increases while the area over which it is spread decreases as gravitational contraction continues, forming a more stellar-like object in the process. During this time, and up until hydrogen burning begins and it joins the main sequence stars.

T Tauri Star

A T Tauri star is 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.

Main Sequence Stars

he 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.  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.

Blue Giant Stars

Blue giants are defined here as large stars with at least a slight blueish coloration. The largest and hottest (O-type) burn through the hydrogen in their cores very quickly causing their outer layers to expand and their luminosity to increase. Their high temperature means they remain blue for much of this expansion. Blue supergiants above about 30 solar masses can begin throw off huge swathes of their outer layers, exposing a super hot and luminous core. These are called Wolf-Rayet stars. These massive stars are more likely to explode in a supernova before they can cool to reach a later evolutionary stage, such as a red supergiant. After a supernova, the stellar remnant becomes a neutron star or a black hole. Example is Rigel, the brightest star in the constellation of Orion.

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. Due to a larger surface area, the surface temperature is actually lower (redder). They eventually eject their outer layers to form a planetary nebula, while the core becomes a white dwarf. Smaller stars do not become red giants because, due to convective heat transport, their cores cannot become dense enough to generate the heat needed for expansion. Larger stars become red supergiants or hypergiants.      Example is Alpha Tauri and Omicron Ceti

White Dwarfs

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’s no fusion reactions happening any more. A white dwarf will just cool down until it because the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

Black Dwarfs

Once a star has become a white dwarf, it will slowly cool to become a black dwarf. As the universe is not old enough for a white dwarf to have cooled sufficiently, no black dwarfs are thought to exist at this time.

Neutron Stars

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.

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