Ethan K
April 27, 2021
But… aren’t they all the same?
A star is a star, right? Sure, there are some differences in color when you look up at the night sky. But they are all basically the same, big balls of gas burning up to billions of light years away, right? Well, not exactly. In truth, stars are about as diverse as anything else in our Universe, falling into one of many different classifications based on its defining characteristics.
There are many different age and size and color types that stars are organized in, from tiny brown dwarves to massive red and blue giants, From normal main sequence stars to weird Wolf-Rayet stars. So, let’s go down the list, shall we?
Protostars
A protostar is what comes before a star has formed – a collection of gas that collapsed from a huge molecular cloud. This stage in stellar evolution lasts about 100,000 years. During this period of time just as the collection of gas collapsed to form the protostar, the increase of gravity and pressure causes the protostar to collapse down. This process leaves a pre-main-sequence star which later turns into a main-sequence star when the hydrogen fusion starts.
T Tauri Stars
T Tauri or pre-main-sequence stars are what comes after protostars and before main-sequence stars. These stars symbolize youth and they are named after a young star in the Taurus star-forming region. At this point in stellar evolution, the gravitational pressure that holds the star together is also the source of all its energy. At this stage, they do not have sufficient pressure or a high enough temperature in their cores to onset nuclear fusion. However, they are about the same temperature as main-sequence stars, only brighter because of their larger size. A star will remain in its T Tauri stage for approximately 100 million years before evolving to its next form.
Main-Sequence Stars
Most of the stars in our night sky and in the entire universe are main-sequence; also called dwarfs. Our beloved Sun is a main-sequence star, as well as the brightest star in our night sky – Sirius. These stars can vary in size, brightness, and mass, but they all have the same process happening in their cores. They are converting hydrogen to helium, creating a vast amount of energy.
Dwarf stars are in a state of hydrostatic equilibrium which means that gravity is pulling the star inward while the pressure from the fusion reactions is pushing the star outward. These forces balance each other out so that the star remains a sphere. The size of these stars depends on their mass. Some stars can even become 100 times the mass of the Sun.
Red Giant Stars
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. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.
White Dwarves
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 becomes 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.
Red Dwarves
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. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. 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 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.
Supergiant Stars
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.
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