Our sun is a star. It’s bright because we’re close to it. If we could swap out our sun with another star, the dimmest would appear as a tiny dot not even as bright as our full Moon. If we replaced it with Rigel the bright star at the foot of Orion, it would cover a large portion of the sky, burning up the Earth in a few seconds, at 57,000 times the brightness of our own sun.

Most, and perhaps all, of the stars in our night sky have planets like those in our own solar system. In fact, many of the stars closest to Earth have planets.

Stars are like the fireplace we huddle around to stay warm in the cold darkness of night. Stars give off their own light and heat.

Planets merely reflect the light given off by their suns. So, what kinds of stars are there? Let’s start with our own sun for comparison.

Our Own Sun

Our own sun is a bright yellow, main sequence star, about 4.5 billion years old, with a moderate amount of heavier elements like iron, oxygen, and silicon. The name of our sun is Sol.

Spectral Type: G2V
Mass: 1 Sol (333,000 Earths).
Metallicity: 0
Age: 4.5 billion years. (We have another 5.5 billion years before our sun becomes a sub-giant).
Distance: 0 light years (93 million miles from Earth).

The Life of a Star

Stars begin as protostars, then enter the main sequence, become sub-giants and then giants. The most massive even become super-giant stars. After that, stars become either white dwarfs, neutron stars or black holes.

Proto-Stars

Proto-stars are very young stars that are considered pre-main sequence. Their ages range from zero to 100 million years. This is the period when a star is born from dust and gas in interstellar space. This is the period before a star ignites the hydrogen at its core.

Example: T Tauri

Spectral Type: G5V:e.
Mass: 0.53
Radius: ?
Age: 0.4 million years
Distance: 471 light years

Main Sequence Stars

These are stars that are considered mature. They are burning hydrogen at their core. Main sequence stars are fairly common. This group includes:

Red dwarfs—the least massive main sequence stars
Orange dwarfs—slightly more massive
Yellow dwarfs—mid-range mass
Blue dwarfs—more massive
Ultraviolet dwarfs—most massive main sequence stars

READ: 40+ Amazing Facts About the Space

Red Dwarfs

Low mass stars that give off far less light than our own sun, mainly toward the red end of the spectrum.

Spectral Types: M9V–M0V
Mass Range: 0.075–0.45 Sols
Maximum Age: Trillions of years (older than the current universe).
How Common: 76.45% of all main sequence stars

Example: Alpha Centauri C (Proxima Centauri)

Spectral Type: M5.5e.
Mass: 0.122 Sols
Radius: 0.1542 Sols
Metallicity: 21.
Age: 5.3 billion years
Distance: 4.244 light years

Orange Dwarfs

Stars less massive than our own sun that shine mainly in the orange range of the spectrum.

Spectral Types: K9V–K0V.
Mass Range: 0.45–0.8 Sols
Maximum Age: 73 billion years (K9V)
How Common: 12.1% of all main sequence stars

Example: Alpha Centauri B

Spectral Type: K1V
Mass: 0.907 Sols
Radius: 0.8632 Sols
Metallicity: 23
Age: 5.3 billion years
Distance: 4.37 light years

Yellow Dwarfs

Stars similar to our own sun that range from deep yellow to hot, yellow-white.

Spectral Types: G9V–G0V, F9V–F0V
Mass Range: 0.8–1.4 Sols
Maximum Age: 17.5 billion years (G9V); 9.1 billion years (F9V).
How Common: 10.6% of all main sequence stars

Example: Alpha Centauri A

Spectral Type: G2V
Mass: 1.1 Sols
Radius: 1.2234 Sols
Metallicity: 2
Age: 5.3 billion years
Distance: 4.37 light years

Example: Tau Ceti

Spectral Type: G8V
Mass: 0.783 Sols
Radius: 0.793 Sols
Metallicity: –0.55. (Fairly metal poor for a main sequence dwarf)
Age: 5.8 billion years
Distance: 11.905 light years

Blue Dwarfs

Stars more massive than our sun that emit light more toward the blue end of the spectrum.

Spectral Types: A9V–A0V, B9V–B0V.
Mass Range: 1.4–16 Sols
Maximum Age: 4.3 billion years (A9V); 1.56 billion years (B9V)
How Common: 0.73% of all main sequence stars

Example: Sirius (Alpha Canis Majoris)

Spectral Type: A1V
Mass: 2.063 Sols
Radius: 1.711 Sols
Metallicity: 0.50
Age: 242 million years
Distance: 8.6 light years

Example: Alkaid (Eta Ursae Majoris)

Spectral Type: B3V
Mass: 6.1 Sols
Radius: 3.4 Sols
Age: 10 million years
Distance: 103.9 light years

Ultraviolet Dwarfs

Very massive main sequence stars that emit light largely in the blue and ultraviolet range.

Spectral Types: O9V–O5V.
Mass Range: ≥ 16
Maximum Age: 9.77 million years (O9V).
How Common: 0.13% of all main sequence

Example: HD 93129 B

Spectral Type: O3.5V(f)z.
Mass: 52 Sols
Radius: 13 Sols
Age: 0.9 million years
Distance: 7,500 light years

Subdwarfs

Below the main sequence are a class of stars that are less luminous than main sequence stars. This is because they don’t have as much of the extra elements found in stars like our sun. They have almost no oxygen, iron, carbon, lithium and other elements that make our own sun burn brighter.

READ: 49 Breathtaking Facts About the Moon You Can't afford To Miss

Their luminosity class is “VI.” So, a sub-dwarf star of the same temperature as our sun would have a spectral type of G2VI. Out of roughly 200 billion stars in our Milky Way galaxy, roughly half of them are subdwarfs, making them the most numerous type known.

Example: Groombridge 1830

Spectral Type: G5VIp
Mass: 0.661 Sols
Radius: 0.681 Sols
Age: 5.0 billion years
Distance: 29.7 light years

Example: Mu Cassiopeiae

Spectral Type: G5VIb
Mass: 0.74 Sols
Radius: 0.791 Sols
Age: 4.5 billion years
Distance: 24.6 light years

Brown Dwarfs

These are failed stars that have insufficient mass to ignite their nuclear cores.

They glow only from the heat of compression. Some of them glow in the visible range, but most of their light is in the infrared range (beyond red). Because they are so dim, they are difficult to detect. They may be far more common than red dwarfs, but we’re still learning about them.

Example: Luhman 16A

Spectral Type: L8
Mass: 0.032 Sols
Radius: ?
Age: ?
Distance: 6.516 light years

Example: Epsilon Indi Ba

Spectral Type: 0V
Mass: 0.066 Sols
Radius: 0.08 Sols
Age: 1.3 billion years
Distance: 11.81 light years

Sub-Giants

These are stars that have evolved away from the main sequence on their way to becoming red giants—mainly K, G, and F types.

Spectral Types: M9IV–O7IV.
How Common: Moderately rare

Example: Gamma Cephei

Spectral Type: K1IV CN1
Mass: 1.41 Sols
Radius: 4.93 Sols
Age: 3.25 billion years
Distance: 44.9 light yeats

Example: Beta Scorpii Aa

Spectral Type: 5IV (one star in a six-sun system).
Mass: 15.0
Radius: 6.3
Age: 6.3 million
Distance: ~400 light

Giants

These are stars that have stopped burning hydrogen as their main fuel and are now using helium to create their energy.

Spectral Types: M9III–O9III.
How Common: Very rare

Example: Aldeberan (Alpha Tauri)

Spectral Type: K5III
Mass: 1.16 Sols
Radius: 44.13 Sols
Age: 6.4 billion years
Distance: 65.3 light years

Example: Thuban A (Alpha Draconis A)

Spectral Type: K5III
Mass: 2.8 Sols
Radius: 3.4 Sols
Distance: 303 light years

Super-Giants

These are very massive stars that are burning helium or carbon in their cores. If the star is massive enough to form an iron core, it will likely end its life as a supernova.

Spectral Types: M9I–O9I.
How Common: Extremely rare

Example: Rigel A (Beta Orionis A)

Spectral Type: B8Ia
Mass: 21 Sols
Radius: 78.9 Sols
Age: 8 million years
Distance: 860 light years

Example: Betelgeuse (Alpha Orionis)

Spectral Type: M1Ia
Mass: 6 Sols
Radius: 955 Sols
Age: 8.3 million years
Distance: 725 light years

White Dwarfs

White dwarfs are the end result of very old stars that has gone past their red giant phase and shrugged off their outer layers, leaving only the collapsed core which is typically about the size of our planet Earth, but dense enough for a tablespoon of its mass to weigh several tons.

Spectral Types: Commonly start with letter “D” for degenerate star
Duration: Trillions of years
How Common: ~5% of the count of the main sequence stars

READ: 30+ Remarkable Facts About the Planet Jupiter

Example: Sirius B

Spectral Type: DA2
Mass: 1.018 Sols
Radius: 0.0084 Sols
Age: 228 million years
Distance: 8.6 light years

Example: Procyon B

Spectral Type: DQZ
Mass: 0.602 Sols
Radius: 0.012 Sols
Age: 1.37 billion years
Distance: 11.46 light years

Black Dwarfs

A black dwarf is the end result of a white dwarf that has cooled down so that it no longer emits any visible light. So far, the universe is not old enough for any white dwarfs to have cooled to this extent.

Spectral Types: None (no light emitted).
Duration: Unknown. Probably as long as the universe
How Common: 0% of all stars. The universe isn’t old enough,

Neutron Stars

These are stars that have a mass between 10–29 solar masses (Sols) before they explode as a supernova. The shock of the explosion compresses the core to become more dense than a white dwarf. Some neutron stars give off pulses of energy; these are called pulsars.

How Common: Only about 2,000 are known in the Milky Way and Magellanic

Clouds. This amounts to ~0.000001% of the stars.

Example: PSR J0108−1431

Spectral Type: Pulsar
Mass: ?
Radius: ?
Age: 166 million
Distance: 424 light

Example: PSR B1257+12

Spectral Type: Pulsar
Mass: 1.4 Sols
Radius: 10 kilometers (~0.000015 Sols).
Age: ~2 billion
Distance: 2,300 light years

Black Holes

A black hole is the super-massive end result of a collapsed star after a supernova explosion.

Spectral Types: None (no light directly emitted).
Duration: Unknown. May last as long as the universe
How Common: Unknown because they don’t emit any light. We only know of some black holes because of their influence on other, nearby objects, or from the material falling into them from nearby

Example: Cygnus X-1

Spectral Type: None
Mass: 14.8
Radius of Event Horizon: ~300
Age: 5 million
Distance: ~6,500 light

Other Types

There are numerous other types of stars, including novae, recurrent novae, and supernovae. Within some of the categories, above, there are also some stars that vary in brightness on a fairly regular basis. Delta Scuti variables, for instance, vary with such precision, that they are used as a crude measuring stick to nearby galaxies.

Sources:

Astronomy for Beginners. (ND). “10 closest stars to earth known to have planets.” Retrieved on May 1, 2019 from https://astronomyforbeginners.com/astronomy/ closest-stars-to-earth/

Garrison, R.F. (1994). “A Hierarchy of Standards for the MK Process.” Retrieved on May 1, 2019 from https://ui.adsabs.harvard.edu/abs/1994ASPC…60….3G/abstract

Johnson, H.L., and Morgan, W.W. (1953). “Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas.” Retrieved on May 1, 2019 from https://ui.adsabs.harvard.edu/abs/1953ApJ…117..313J/abstract

SpaceSoftware. (ND). Stars in the NeighborHood software. Retrieved on May 1, 2019 from https://spacesoftware.wordpress.com/