Stars are enormous spheres of ignited gas that light the cosmos, and seed it with the materials for rocky worlds and living beings. They involve many distinct types, sizes and colors that may not be clear enough for the simple human eye.

The amazing light sources are usually classified according to their spectral types. Despite emitting all colors of light, spectral classification only notices the peak of these emissions as a label of the star's surface temperature. By using this system, scientists have managed to discover that blue start, which they like to call the O-type, are defiantly the hottest.

Credit: Michael Shainblum, via Flickr

The coolest starts on the other hand are called M-type and are red. I find this discovery a little bit disturbing since I unwillingly relate the color red to fire and blue to water, but science doesn't care, does it?

The spectral classes are, in order of increasing level of temperature, M (red), K (orange), G (yellow), F (yellow-white), A (white), B (blue-white) and of course O (blue).

This bland categorization is often abandoned and replaced with a more descriptive alternative. As the coolest stars are invariably the smallest, they are called red dwarfs. Conversely, the hottest stars are often called blue giants.

Physical characteristics vary from a star to another and include the surface temperature, luminosity, mass, radius (size), lifetime, prevalence in the cosmos and finally the point in the stellar evolutionary cycle.

1. Yellow Dwarf Stars

  • Lifetime: 4 - 17 billion years
  • Evolution: early, middle
  • 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%

This category includes our precious Sun, Alpha Centauri A and Kepler-22. Yes, these are all star names. The designation "yellow dwarf" might be considered a little bit imprecise by some scientists, but these stars do appear yellow when observed through the Earth's atmosphere.

Yellow Dwarf Stars
Credit: makelessnoise, via Flick

2. 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%

This category's members are relatively smaller, cooler and have a longer life span. They involve Alpha Centauri B and Epsilon Eridani. These beautiful light sources are main sequence stars like their bigger counterparts, and are fusing hydrogen in their cores.

Orange Dwarf Star
Credit: R.J. Hall, via Wikimedia Commons

3. Red Dwarf Stars

  • Lifetime: 73 - 5500 billion years
  • Evolution: early, middle
  • Temperature: 1,800 - 3,500 °C
  • Luminosity: 0.0001 - 0.08
  • Spectral Types: M
  • Radius: 0.12 - 0.7
  • Mass: 0.08 - 0.45
  • Prevalence: 73%

Red dwarfs, the smallest type of main sequence stars, are barely hot enough to retain the nuclear fusion reactions crucial to the utilization of their hydrogen fuel. They include Bernard's Star, the Gliese 581 and the Proxima Centauri.

The Red dwarf stars are the universe's most common due to their slow rate of fusion and efficient circulation of fuel. Their life spans are thought to exceed 13.8 billion years, a duration no human mind is able to imagine.

Red Dwarf Star
Credit: Kilo 66, via Flickr

4. Brown Dwarfs

  • Lifetime: unknown (long)
  • Evolution: not evolving
  • Temperature: 0 - 1,800 °C
  • Radius: 0.06 - 0.12
  • Spectral Types: L, T, Y (after M)
  • Luminosity: ~0.00001
  • Mass: 0.01 - 0.08
  • Prevalence: unknown (many)

The relatively small size of Brown dwarfs does not allow them to generate enough heat for hydrogen fusion. These stars are usually of the same size of Jupiter's but 13 times heavier. They are thought to be in a continuing cooling phase which makes them harder to identify.

Brown Dwarf Star
Credit: European Southern Observatory, via Flickr

5. 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%

These are large stars with a slight blue coloration, although that may be scientifically controversial. Their high temperature allow them to retain their blue color and eventually leads to their transformation into red giants, supergiants or hypergiants due to the non-stop cooling they endure.

Blue Giant Dwarf Star
Credit: schneidercater, via Flickr

6. Red Giant Stars

  • Lifetime: 0.1 - 2 billion years
  • Evolution: late
  • Temperature: 3,000 - 5,000 °C
  • Spectral Types: M, K
  • Luminosity: 100 - 1000
  • Radius: 20 - 100
  • Mass: 0.3 - 10
  • Prevalence: 0.4%

Two examples of this category are Arcturus and Aldebaran. These folks are thought to be in a late evolutionary stage. The accumulation of helium inside them causes an contraction of the core which increases the internal level of temperature. This triggers the hydrogen fusion in the service layers involving the growth in size and luminosity of the star.

Red Giant Dwarf StarCredit: NASA's Marshall Space Flight Center, via Flickr

7. 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 - 1650
  • Mass: 10 - 40
  • Prevalence: 0.0001%

Due to the contraction of their cores, the Red Supergiant Stars have swelled up. Nevertheless, they are thought to evolve from the blue giants and supergiants with 10-40 solar masses. Two of this kind are Antares and Betelgeuse. With time, these amazing creatures eventually destroy themselves in supernova, probably out of boredom, leaving behind a black hole or a neutron star.

Red Supergiant Dwarf StarCredit:, via Flickr

8. 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%

White Dwarfs generate from the remnants of stars less than 10 solar masses, which enter a transformation phase led by Pauli's exclusion principle. They include Van Maanen's star and Sirius B. According to theory, 97% of stars eventually become white dwarfs. These last for trillions of years and end up transforming into black dwarfs after loosing their internal temperature.

White Dwarf StarsCredit: NASA's Marshall Space Flight Center, via Flickr

9. 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. The thing that can take trillions of years. As the age of the universe is thought not be even that long, no black dwarf is thought to exist yet.

Black Dwarf StarCredit: NASA's Marshall Space Flight Center, via Flickr

10. 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 - 3.2
  • Prevalence: 0.7%

When stars larger than about 10 solar masses exhaust their fuel, their cores dramatically collapse to form neutron stars. The massive collapse intensively throws off the outer layers of  the star in a supernova explosion.

Neutron Star
Credit: NASA's Marshall Space Flight Center, via Flickr
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