composite image of a star

What are the different types of stars in the universe?

Last Updated: October 12, 2021

While there are ~1024 stars in the universe, there are less than 20 types of stars. In this article, we’ll explore star types. 

How are stars categorized?

Stars are categorized by their mass, temperature, spectra and brightness. In 1890, E.C. Pickering of Harvard College Observatory classified stars in order of their decreasing surface temperature. The categories were O, B, A, F, G, K, and M and each category was associated with a different star color: blue, blue-white, white, yellow-white, yellow, orange, and red.

In 1911, Danish astronomer Ejnar Hertzsprung and US astronomer Henry Norris Russell independently created scatter plots of star temperature versus luminosity or spectral type versus absolute magnitude. These plots are known as the Hertzsprung-Russell Diagram or HR Diagram and describe star classifications and star evolution. 

In 1943 William Wilson Morgan and Philip C Keenan refined star classification by adding luminosity subcategories. Each spectral class (O, B, A, F, G, K and M) was subdivided into categories of temperature with 0 being the hottest and 9 being the coolest.

What are the star categories?

hertzsprung russell diagram

The Hertzsprung Russell diagram show 4 clusters of stars: 

  1. Main Sequence (the diagonal swath from upper left to lower right)
  2. Giants (immediately above Main Sequence) 
  3. Supergiants (above Giants) 
  4. White Dwarfs (below Main Sequence)

Main Sequence Stars

Approximately 95% of the stars in the universe are main sequence stars. These relatively young stars fuse hydrogen (H) into helium (He) in their cores. The inward pull of gravity equals the outward hydrostatic pressure of the fusion reaction, so main sequence stars are roughly spherical. The size of main sequence stars depends upon their mass and they typically range from 0.10-200 Solar masses.

There are seven spectral types of main sequence stars. 

O Type Stars

These stars are known as blue stars and they are ~0.00001% of the stars in the universe. They have temperatures of ~40,000K, radii of ~10 solar radii, masses of ~50 solar masses and luminosity of ~100,000 solar luminosities. These stars live for ~10 million years. 10 Lacertae is an O type star. 

B Type Stars

These stars are also known as blue stars and they are ~0.1% of the stars in the universe. They have temperatures of ~20,000K, radii of ~5 solar radii, masses of ~10 solar masses and luminosity of ~1000 solar luminosities. These stars live for ~100 million years. Spica is a B type star. 

A Type Stars

These white stars are ~0.7% of the stars in the universe. They have temperatures of ~8500K, radii of ~1.7 solar radii, masses of ~2.0 solar masses and luminosity of ~20 solar luminosities. These stars live for ~1000 million years. Sirius A is an A-type star.

F Type Stars

These yellow-white, stars are ~2% of the stars in the universe. They have temperatures of ~6500K, radii of ~1.3 solar radii, masses of ~1.5 solar masses and luminosity of ~4 solar luminosities. These stars live for ~3000 million years. Proxima Centauri is an F type star.

G Type Stars

These stars are known as yellow dwarfs and they are ~3.5% of the stars in the universe. They have temperatures of ~5700K, radii of ~1.0 solar radii, masses of ~1.0 solar masses and luminosity of ~1 solar luminosity. These stars live for ~10,000 million years. Our Sun and Alpha Centauri A are G type stars. 

K Type Stars

These stars are known as orange dwarfs (or red dwarfs) and they are ~8% of the stars in the universe. They have temperatures of ~4500K, radii of ~0.8 solar radii, masses of ~0.7 solar masses and luminosity of ~0.2 solar luminosities. These stars live for ~50,000 million years. Alpha Centauri B is a K type star. 

M Type Stars

These stars are known as red dwarfs and they are ~80% of the stars in the universe. They have temperatures of ~3200K, radii of ~0.3 solar radii, masses of ~0.2 solar masses and luminosity of ~0.01 solar luminosity. These stars live for ~200,000 million years. Proxima Centauri is an M type star. M type stars are the most common because their small size and low temperature result in extremely slow hydrogen consumption and increased longevity.

the different types of stars

Giant Stars

When main sequence stars with masses from ~0.25-10 solar masses exhaust their supplies of hydrogen they begin burning helium. The pressure of fusion causes them to expand and they evolve into red giants. 

Red giants are ~0.4% of the stars in the universe. Their spectra may be G, K, or M. Their temperatures range from ~3000K-10,000K, radii range from ~10-50 solar radii, masses range from ~1-5 solar masses and luminosity ranges from ~50-1000 solar luminosities. These stars live for ~1000 million years. Aldebaran and Arcturus are red giants. 

Blue giants have also evolved off the main sequence and are extremely rare. Their spectra may be O, B, or A. Their temperatures range from ~10,000K-33,000K+, radii from ~5-10 solar radii, masses from ~21-150 solar masses and luminosities are ~10,000 solar luminosities. These stars live for ~1000 million years. Mintaka is a blue giant.

Supergiant Stars

When main sequence stars with masses of ~10+ solar masses exhaust their supplies of hydrogen, they begin burning helium. The pressure of fusion causes them to expand and they evolve into supergiants. Supergiants that create heavy elements may evolve into supernovas. 

Supergiants are ~0.0001% of the stars in the universe. They may be of any spectral type. Their temperatures range from ~4000K-40,000K, radii range from ~30-500 solar radii, masses range from ~10-70 solar masses and luminosities range from ~30,3330-1,000,000 solar luminosities. These stars live for ~10 million years. Alnitak is a blue supergiant. Antares and Betelgeuse are red supergiants.

Dead stars

Stars that no longer undergo fusion belong to their own spectral class; D. There are 4 types. 

White Dwarfs

When main sequence stars with masses less than ~0.25 Solar masses exhaust their supplies of hydrogen, they collapse into white dwarfs. These dying remnants of imploded stars are ~5% of the stars in the universe. Their temperatures are <10,000K, radii <0.01 solar radii, mass <1.4 solar masses and luminosities <0.01 solar luminosities. Sirius B and Procyon B are white dwarfs. White dwarfs may become novae. 

Neutron Stars

Neutron stars are produced when the cores of massive stars are compressed past the white dwarf stage during a supernova event. Neutron stars are ~0.7% of the stars in the universe. Their temperatures are ~600,000K, radii 5-15 km, mass ~1.4-3.2 solar masses and extremely low luminosities.

Black Dwarfs

These hypothetical stars are thought to form when white dwarfs radiate all of their energy.

Black Holes

When stars of masses >3 solar masses die in a supernova explosion, the dead core collapses into a gravitational singularly called a black hole.

Failed Stars

Brown dwarfs form when the gravitational collapse of large clouds of hydrogen gas does not result in a large enough mass to ignite the hydrogen core. Their spectral class may be M, L. T. or Y. Their temperatures are ~300K-2800K, radii ~0.06-0.12 solar radii, mass ~0.01-0.08 solar masses (13-80 Jupiter-masses) and luminosity ~0.000001 solar luminosities. 

Conclusion

Red dwarfs are ~80% of the stars in the universe and they are 1 of 7 types of main-sequence stars contributing to ~95% of all stars. White dwarfs contribute ~5% and giants and supergiants contribute another ~0.04001%.

Tanya C Forde

Written by Tanya C. Forde

Hi! I’m Tanya C. Forde, Msc (earth sciences). I was raised under the dark sky of rural Alberta and have been fascinated by astronomy since childhood. I began my exploration of the night sky with naked-eye viewing before moving on to binoculars and then telescope ownership. I also write for Sewn By Tanya.

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