Different Types of Stars in the Universe

Different Kinds of Stars

Stars are enormous spheres of fusing gas that light the cosmos and seed it with the materials for rocky worlds and living beings. They come in many different types and sizes, from smouldering white dwarfs to blazing red giants.

Stars are often classified according to their spectral type. Although they emit all colours of light, spectral classification considers only the peak of this emission as an indicator of the star's surface temperature. Using this system, blue stars are the hottest and are called O-type. The coolest stars are red and are called M-type. 

What Are the 7 Main Spectral Types of Stars?

In order of increasing surface temperature, the spectral classes of stars are:

• M (red)

• K (orange)

• G (yellow)

• F (yellow-white)

• A (white)

• B (blue-white)

• O (blue)
  
  

This bland categorization may be abandoned for a more descriptive alternative. As the coolest stars (red) are usually the smallest, they can be called red dwarfs. Conversely, the hottest stars are often called blue giants. However, it is common for stars to become red giants later in their evolutionary cycle.

Stars of different types exhibit a wide range of physical features. These include their temperature, luminosity (brightness), mass (weight), radius (size), lifetime, prevalence in the cosmos, and point in the stellar evolutionary cycle. The best way to understand these features is to compare them with the features of our nearest stellar companion, the Sun. 

The following stars are described relative to the Sun, e.g., a mass of "2" means "twice the mass of the sun."

Yellow Dwarf Stars

• Lifetime: 4 - 17 billion years

• Evolution: early, middle (i.e., they only change near the end of their life)

• Temperature: 5,000 - 7,300 °C

• Spectral Types: G, F

• Luminosity: 0.6 - 5.0

• Radius: 0.96 - 1.4

• Mass: 0.8 - 1.4

• Prevalence: 10% (i.e., one in ten stars are yellow dwarfs)

The Sun, Alpha Centauri A (one of our nearest neighbours), and Kepler-22 (a star with an Earth-sized exoplanet) are yellow dwarfs. These stellar cauldrons are in the prime of their lives because they are fusing hydrogen in their cores. This places them on the "main sequence" of stellar evolution, where the majority of stars are found. The name "yellow dwarf" may be imprecise, as these stars typically have a whiter colour. However, they do appear yellow when observed through the Earth's atmosphere. 

Orange Dwarf Stars

• Lifetime: 17 - 73 billion years

• Evolution: early, middle

• Temperature: 3,500 - 5,000 °C

• Spectral Types: K

• Luminosity: 0.08 - 0.6

• Radius: 0.7 - 0.96

• Mass: 0.45 - 0.8

• Prevalence: 11% 

Alpha Centauri B and Epsilon Eridani (a nearby star with an exoplanet) are orange dwarf stars. These are smaller, cooler, and live longer than yellow dwarfs like our Sun. Like their larger counterparts, they are main sequence stars fusing hydrogen in their cores. 

Red Dwarf Stars

• Lifetime: 73 - 5500 billion years

• Evolution: early, middle

• Temperature: 1,800 - 3,500 °C

• Spectral Types: M

• Luminosity: 0.0001 - 0.08

• Radius: 0.12 - 0.7

• Mass: 0.08 - 0.45

• Prevalence: 73% 

Proxima Centauri (our nearest neighbour), Barnard's Star (a nearby star) and Gliese 581 (a star with a planetary system) are all red dwarfs. They are the smallest kind of main sequence star. Red dwarfs are barely hot enough to maintain the nuclear fusion reactions required to use their hydrogen fuel. However, they are the most common type of star, owing to their remarkably long lifetime that exceeds the age of the universe (13.8 billion years). This is due to a slow rate of fusion and an efficient circulation of their hydrogen fuel via convective heat transport.

Brown Dwarfs

• Lifetime: unknown (long)

• Evolution: not evolving

• Temperature: 0 - 1,800 °C

• Spectral Types: L, T, Y (after M)

• Luminosity: ~0.00001

• Radius: 0.06 - 0.12

• Mass: 0.01 - 0.08

• Prevalence: unknown (many) 

Brown dwarfs are substellar objects that never accumulated enough material to become stars. They are too small to generate the heat required for hydrogen fusion. Brown dwarfs constitute the midpoint between the smallest red dwarf stars and massive planets like Jupiter. They are about the same size as Jupiter but they must be at least 13 times heavier to qualify as a brown dwarf.

As brown dwarfs gradually cool they become difficult to identify and it is unclear how many exist. Their cold exteriors emit radiation beyond the red region of the spectrum and, to the human observer, they appear magenta rather than brown. 

Blue Giant Stars

• Lifetime: 3 - 4,000 million years

• Evolution: early, middle

• Temperature: 7,300 - 200,000 °C

• Spectral Types: O, B, A

• Luminosity: 5.0 - 9,000,000

• Radius: 1.4 - 250

• Mass: 1.4 - 265

• Prevalence: 0.7% 

Not all blue giants are main sequence stars. The largest and hottest (O-type) fuse all of the hydrogen in their cores very quickly, which causes their outer layers to expand and their luminosity to increase. Their high temperature means they remain blue for much of this expansion (e.g., Rigel), but eventually they may cool to become a red giant, supergiant, or hypergiant.

Blue supergiants above about 30 solar masses can begin to 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. 

Red Giant Stars

• Lifetime: 0.1 - 2 billion years

• Evolution: late (i.e., they were a different type of star earlier in life)

• Temperature: 3,000 - 5,000 °C

• Spectral Types: M, K

• Luminosity: 100 - 1000

• Radius: 20 - 100

• Mass: 0.3 - 10

• Prevalence: 0.4% 

Aldebaran and Arcturus are red giants. These stars are in a late evolutionary phase. Red giants would previously have been main sequence stars (such as the Sun) with between 0.3 and 10 solar masses. 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 (see below).

For red giants, the accumulation of helium (from hydrogen fusion), which is a heavier element than hydrogen, causes a contraction of the core that raises the internal temperature. This triggers hydrogen fusion in the outer layers of the star, causing it to grow in size and luminosity. However, due to it now having a larger surface area, the surface temperature is actually lower (redder). Red giants eventually eject their outer layers, which form a planetary nebula, while the core becomes a white dwarf (see below). 

Red Supergiant Stars

• Lifetime: 3 - 100 million years

• Evolution: late

• Temperature: 3,000 - 5,000 ºC

• Spectral Types: K, M

• Luminosity: 1,000 - 800,000

• Radius: 100 - 2000

• Mass: 10 - 40

• Prevalence: 0.0001% 

Betelgeuse and Antares are red supergiants. The largest of these types of stars are sometimes called red hypergiants. One of these is 1708 times the size of our Sun (UY Scuti) and is the largest known star in the universe. UY Scuti is about 9,500 light years away from the Earth.

Like red giants, these stars have swelled due to the contraction of their cores. However, red supergiants typically evolve from blue giants and supergiants with between 10 and 40 solar masses. Higher mass stars shed their layers too quickly, becoming Wolf-Rayet stars, or exploding in supernovae. Red supergiants eventually destroy themselves in a supernova, leaving behind a neutron star or black hole. 

White Dwarfs

• Lifetime: 1015 - 1025 years

• Evolution: dead, cooling

• Temperature: 4,000 - 150,000 ºC

• Spectral Types: D (degenerate)

• Luminosity: 0.0001 - 100

• Radius: 0.008 - 0.2

• Mass: 0.1 - 1.4

• Prevalence: 4% 

Stars with less than 10 solar masses eventually shed their outer layers, forming planetary nebulae. However, they typically leave behind an Earth-sized core of less than 1.4 solar masses. This core is so dense that some particles within its mass (electrons) cannot be physically compressed any further (i.e., they become "degenerate"). This physical law (Pauli's exclusion principle) prevents the stellar remnant from collapsing any further.

This core, or remnant, is called a white dwarf, and examples include Sirius B (see above) and Van Maanen's star. More than 97% of stars are theorized to become white dwarfs. These super hot structures will remain hot for trillions of years before cooling to become black dwarfs. 

Black Dwarfs

• Lifetime: unknown (long)

• Evolution: dead

• Temperature: < -270 °C

• Spectral Types: none

• Luminosity: infinitesimal

• Radius: 0.008 - 0.2

• Mass: 0.1 - 1.4

• Prevalence: ~0% 

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

• Lifetime: unknown (long)

• Evolution: dead, cooling

• Temperature: < 2,000,000 ºC

• Spectral Types: D (degenerate)

• Luminosity: ~0.000001

• Radius: 5 - 15 km

• Mass: 1.4 - 2.3 (although 1.1 - 1.4 solar masses is possible)

• Prevalence: 0.7% 

When stars larger than about 10 solar masses exhaust their fuel, their cores dramatically collapse to form neutron stars. The collapse throws off the outer layers of the star in a supernova explosion. If the core still has a mass above about 1.4 solar masses, electron degeneracy will be unable to halt the collapse. Instead, the electrons fuse with protons to produce neutral particles called neutrons, which are compressed until they can no longer occupy a smaller space (i.e., becoming degenerate).

The stellar remnant, composed almost entirely of neutrons, is so dense that it occupies a radius of about 12 km. Due to conservation of angular momentum, neutron stars are often left in a rapidly rotating state called a pulsar. 

When Does a Star Become a Black Hole?

Stars larger than 20 solar masses and with cores larger than about 2.3 solar masses are likely to become black holes instead of neutron stars. For a black hole to form, the density (and associated pressure) in the core must become great enough to overcome neutron degeneracy, causing a further collapse into a gravitational singularity. 

While stellar classification is more precisely described in terms of spectral type, this does little to fire the imaginations of those who will become the next generation of astrophysicists. There are many different types of stars in the universe, and it is no surprise that those with the most exotic sounding names receive the greatest levels of attention.

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