composite image of a star

What are the different types of stars in the universe?

Last Updated: January 25, 2023

Have you ever looked up at the night sky and wondered about the different types of stars that make up the universe? From bright, blue giants to dim, red dwarfs, stars come in all shapes and sizes.

Whilst scientists estimate that there are ~1024 stars in the universe, there are less than 20 types of stars.

In this article, we’ll take a closer look at the various types of stars and what sets them apart from one another. We’ll explore the fascinating starry spectrum, and learn why scientists have categorized them that way.

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. 

Some examples of O-type stars include:

  • 9 Sagittarii
  • 10 Lacertae
  • AE Aurigae
  • BI 253
  • Delta Circini

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. 

Some examples of O-type stars include:

  • Algol
  • Algorab
  • Alperatz
  • Spica
  • Beta Centauri
  • Rigel.

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.

Some examples of A-type stars include:

  • Sirius
  • Altair
  • Vega
  • Fomalhaut
  • Denebola

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 represent about ~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 and are among the largest stars in the universe. 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.

Neutron stars are incredibly dense and compact objects that are made almost entirely of neutrons, which are subatomic particles that make up the nucleus of atoms. Neutron stars have incredibly strong gravitational fields, which can be billions of times stronger than that of Earth. This resulted in them having some of the strongest magnetic fields in the universe. It is so strong that anything approaching those stars would be ripped apart long before it could reach the surface.

To add to their super exotic nature, they also spin incredibly fast, with some neutron stars completing a rotation in just a few milliseconds. Some neutron stars, known as pulsars, emit beams of light and radiation that sweep across space like a lighthouse. 

Black Dwarfs

Black Dwarf stars are a theoretical type of star that is thought to exist in the far future of a star’s lifecycle. These stars are the final stage of a star’s evolution after it has exhausted all of its fuel and shed its outer layers.

It is believed that a Black Dwarf would be incredibly dense and would have a very low temperature. However, these stars have not yet been observed in the universe as the time it would take for a star to reach this stage is longer than the current age of the universe.

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. Black holes are extremely dense regions in space where gravity is so strong that nothing, not even light, can escape. These cosmic powerhouses don’t emit any light, so they are invisible to our normal means of detection. To spot them, scientists have to investigate and analyse their effects on nearby matter.

There are different types of black holes, depending on their masses: the smallest are called stellar black holes and can be as small as a few suns, while the biggest are called supermassive black holes and can be billions of times more massive than the sun. Black holes are also believed to be responsible for powering some of the hottest and brightest phenomena in the universe, like gamma-ray bursts.

Related reading: Interesting facts about black holes

Despite the fact that black holes are a rather stealthy structure in space, scientists still managed to find a way to image one. The first-ever direct image of a black hole was captured in April 2019 by the Event Horizon Telescope (EHT), which is a network of telescopes around the world. The black hole imaged is located in the centre of the galaxy M87, and has a mass of 6.5 billion times that of the Sun.

Have look at the incredible image below. How mind blowing? 

M87 black hole direct image

Brown Stars (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. 

What type of star is the sun?

The Sun is a G-type main-sequence star that generates its energy from the fusion of hydrogen to helium deep in its core. It is a yellow dwarf and has a surface temperature of about 5,800 kelvins (9,980 degrees Fahrenheit / 5,520 degrees Celsius). 

Despite being an average star it is special as it supports life on Earth through the process of photosynthesis by plants, which produces oxygen as a byproduct. The sun emits light and many other forms of electromagnetic radiation including ultraviolet rays, x-rays, microwaves and radio waves.

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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