A star is a star, right? Of course, there are some differences in terms of color when you look at the night sky. But they are all, in principle, the same big balls of burning gas, millions, billions of light years away, right? Well, not quite. In truth, stars are as diverse as everything in our universe, boiling down to one of many classifications based on their characteristics.
In general, there are many different types of stars, from tiny brown dwarfs to red and blue supergiants. There are even stranger kinds of stars, like neutron and Wolf-Rayet stars, and theoretical quark stars. And as we continue to explore the Universe, we continue to study everything about the stars that makes us expand our worldview. Let's take a look at the different types of stars.
Protostars:
A protostar is what happens before the formation of the star itself. A protostar is an object composed of gas that has collapsed from a giant molecular cloud. The phase of stellar evolution - a protostar - lasts about 100,000 years. Over time, gravity and pressure increase, causing the star to collapse (contract). All the energy release of the protostar comes only from the heating caused by gravitational contraction - thermonuclear reactions have not yet begun.
A size chart showing our Sun (left) in comparison with known huge stars.
Image: earthspacecircle.blogspot.ca
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T Taurus Stars:
The T Tauri star is the stage in the formation and evolution of a star right before becoming 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 do not have enough pressure and temperature in their cores to trigger thermonuclear fusion, but they do not look like main sequence stars in that they are brighter than them, because they are larger than them. T Tauri stars have large sunspot areas, and they have intense X-ray flares and extremely powerful stellar winds. Stars have been in the T Tauri stage for about 100 million years.
Main sequence stars:
Most of the stars in our galaxy, and even in the universe, are main sequence stars. Our Sun is a main sequence star, just like our closest neighbors Sirius and Alpha Centauri A. Main sequence stars can vary greatly in size, mass, and brightness, but they all do the same thing: they convert hydrogen to helium in their cores by releasing huge amount of energy.
The main sequence star is in hydrostatic equilibrium. Gravity pulls the star inward, the pressure of light from all thermonuclear reactions in the star pushes outward. These outward and inward forces balance each other and the star maintains a spherical shape. The size of the main sequence stars will depend on their mass, which determines the amount of gravity pulling it inward.
The lower mass limit for a main sequence star is about 0.08 solar masses, or 80 Jupiter masses. This is the minimum amount of gravitational pressure needed to trigger nuclear fusion reactions in the core. In theory, stars can grow up to 100 solar masses.
Red giant:
When a star has used up all of its core hydrogen reserves, thermonuclear reactions stop and the star no longer builds up outward pressure to counteract the inward gravitational pressure pulling the star together. A shell of hydrogen around the core starts the continuation of the star's life, but the star will dramatically increase in size. The aging star has become a red giant, and could be 100 times the size of a main sequence star. When its hydrogen fuel is consumed, helium, and then heavier elements, will begin to be processed in thermonuclear reactions. A star in the red giant phase will only last a few hundred million years before it runs out of fuel and becomes a white dwarf.
White dwarf:
When a star has completely depleted the hydrogen fuel in its core, it will experience a lack of mass to process heavier elements in thermonuclear reactions, and will enter the white dwarf phase. The pressure of light outward from thermonuclear reactions will stop, and the star will collapse (shrink) under the influence of its own gravity. The white dwarf shines only because it was once a hot star, but since thermonuclear reactions no longer occur in it, it cools down to the background temperature of the universe. This process will take hundreds of billions of years, so the white dwarfs are actually not very cool yet.
Red dwarf:
Red dwarfs are one of the most common types of stars in the universe. They are main sequence stars, but they have so little mass that they are much colder than our Sun. But their feature is different. Red dwarfs are able to store hydrogen fuel by mixing it in their core, and therefore they can save their fuel much more than other stars. Astronomers believe that some of the red dwarfs can burn fuel for up to 10 trillion years. The smallest red dwarfs have approximately 0.075 solar masses, and their mass can be as much as half the mass of the Sun.
Neutron stars:
If the mass of a star is about 1.35 - 2.1 solar masses, then it will not turn into a white dwarf when it dies. Instead, the star will die in a catastrophic event called a supernova, and the remaining core will become a neutron star. As its name suggests, a neutron star is an exotic type of star that is composed entirely of neutrons. This is due to strong gravity, when the star is compressed so much that all the protons and electrons are squeezed together to form neutrons. If the stars are even more massive, then they turn into black holes after a supernova explosion.
Supergiants:
The largest stars in the universe are supergiants. These are monsters with a mass tens of times greater than the mass of the Sun. Unlike the relatively stable star of the Sun, supergiants consume their hydrogen fuel at an incredible rate, and all of their fuel will be completely used up in a few million years. Supergiants live fast and die young, exploding in supernovae; completely destroying yourself in the process.
As you can see, stars come in many sizes, colors and types. Knowing what explains this and what the different stages of a star's life look like is important when it comes to understanding our universe. It also helps when it comes to our ongoing efforts to explore the local stellar neighborhood, not to mention the hunt for extraterrestrial life!