While there are several star classification systems in use today, the Morgan–Keenan (MK) system is both the easiest to master, and the one that makes the most sense to amateur observers. Using the letters O, B, A, F, G, K and M, stars are easily classified from hottest (O) to coolest (M). The temperature of each spectral class is then further subdivided from hot to cool by the simple addition of a number, where 0 is the hottest and 9 the coolest. For instance, the hottest stars in class A are A0, and then A1, A2, etc all the way to A9, the coolest A type star.
This list subsequently shows the main star types using the Morgan–Keenan system. It also provides some quick facts about each type of star, as well as a few details on the physical properties of each class. Also, note that stellar luminosities, radii and masses are given relative to the Sun’s luminosity, radius and mass.
Although the relative colors of each broad category of star is mentioned, barring the very brightest example of each color, all stars generally appear white. The reason being that the colors of stars are usually too dim to activate color vision in human eyes.
Main Sequence Stars
Main sequence stars are powered by the fusion of hydrogen (H) into helium (He) in their cores, a process that requires temperatures of more than 10 million Kelvin. Around 90 percent of the stars in the Universe are main sequence stars, including our sun. Main sequence stars typically range from between one tenth to 200 times the Sun’s mass.
-Spectral Type: O, B
-Life Cycle: On the main sequence
-Typical temperature: ~30,000K
-Typical luminosity: ~100 to ~1,000,000
-Typical radius: ~2.7 to ~10
-Typical mass: ~2.5 to ~90
-Typical age: < ~40 million years
Examples of blue stars include 10 Lacertae, AE Aurigae, Delta Circini, V560 Carinae, Mu Columbae, Sigma Orionis, Theta1 Orionis C, Zeta Ophiuchi.
Blue stars are typically hot, O-type stars that are commonly found in active star forming regions, particularly in the arms of spiral galaxies, where their light illuminates surrounding dust and gas clouds making these areas typically appear blue. Blue stars are also often found in complex multi-star systems, where their evolution is much more difficult to predict due to the phenomenon of mass transfer between stars, as well as the possibility of different stars in the system ending their lives as supernovas at different times.
Blue stars are mainly characterized by the strong Helium-II absorption lines in their spectra, and the hydrogen and neutral helium lines in their spectra that are markedly weaker than in B-type stars. Because blue stars are so hot and massive, they have relatively short lives that end in violent supernova events, ultimately resulting in the creation of either black holes or neutron stars.
S-Spectral Type: G
-Life Cycle: On the main sequence
-Typical Temperature: ~5,200K to ~7,500K
-Typical Luminosity: ~0.6 to ~5.0
-Typical Radius: ~0.96 to ~1.4
-Typical Mass: ~0.8 to ~1.4
-Typical Age: ~4 to ~17 billion years
Examples of yellow dwarf stars include Alpha Centauri A, Tau Ceti, 51 Pegasi.
G-type stars are often mistakenly referred to as yellow dwarf stars. Our Sun is an example of a G-type star, but it is in fact white, since all the colors it emits are blended together. Nonetheless, even though all the Sun’s visible light is blended to produce white, its visible light emission peaks in the green part of the spectrum, but the green component is absorbed and/or scattered by other frequencies both in the Sun itself, and in Earth’s atmosphere.
Typical G-type stars have between 0.84 and 1.15 solar masses, and temperatures that fall into a narrow range of between 5,300K and 6,000K. Like the Sun, all G-type stars convert hydrogen into helium in their cores, and will evolve into red giants as their supply of hydrogen fuel is depleted.
Spectral Type: K
Life Cycle: On the main sequence
Typical Temperature: ~3,700K to ~5,200K
Typical Luminosity: ~0.08 to ~0.6
Typical Radius: ~0.7 to ~0.96
Typical Mass: ~0.45 to ~0.8
Typical Age: ~15 to ~30 billion years
Examples of orange dwarf stars include Alpha Centauri B, Epsilon Indi.
Orange dwarf stars are K-type stars on the main sequence that in terms of size, fall between red M-type main-sequence stars and yellow G-type main-sequence stars. K-type stars are of particular interest in the search for extraterrestrial life, since they emit markedly less UV radiation (that damages or destroys DNA) than G-type stars on the one hand, and they remain stable on the main sequence for up to about 30 billion years, as compared to about 10 billion years for the Sun. Moreover, K-type stars are about four times as common as G-type stars, making the search for exoplanets a lot easier.
-Spectral Type: K, M
-Life Cycle: Early main sequence
-Typical Temperature: = ~4,000K
-Typical Luminosity: ~0.0001 to ~0.08
-Typical Radius: = ~0.7
-Typical Mass: ~0.08 to ~0.45
-Typical Age: Undetermined, but expected to be several trillion years
Examples of red dwarf stars include Proxima Centauri, TRAPPIST-1.
Red dwarfs account for the bulk of the Milky Way‘s stellar population, but since they are very faint, no red dwarf stars are visible without optical aid. Typically, red dwarf stars that are more massive than 0.35 solar masses are fully convective, which means that the process of converting hydrogen into helium occurs throughout the star, and not only in the core, as is the case with more massive stars.
In this way, the nuclear fusion process is slowed down and at the same time greatly prolonged, which keeps the star at a constant luminosity and temperature for several trillion years. In fact, the process of nuclear synthesis happens so slowly in these that the Universe is not old enough for any known red dwarf star to have aged into an advanced state of evolution.
Giants and Supergiants
Giants and supergiants form when a star runs out of hydrogen and begins burning helium. As the star’s core collapses and gets even hotter, the resulting heat subsequently causes the star’s outer layers to expand outwards. Low and medium-mass stars then evolve into red giants. However, high-mass stars 10+ times bigger than the Sun become red supergiants during their helium-burning phase.
These high mass stars fuse helium into carbon and oxygen at a faster rate, but during periods of slow fusion the star can contract in on itself and become a blue supergiant. They are blue because their temperature are spread over a smaller surface area, making them hotter and blue in color. This process can oscillate over time, though, and its not unusual for such stars to undergo transformation between red and blue supergiant phases during their lifetimes before eventually going supernova.
-Spectral Types: O, B, and occasionally, A-type stars
-Life Cycle: Evolved off the main sequence
-Typical temperature: ~10,000K to ~33,000K+
-Typical luminosity: ~10 000
-Typical radius: ~5 to ~10
-Typical mass: ~2 to ~150
-Typical age: ~10 to ~100 million years
Examples of blue giant stars include Iota Orionis, LH54-425, Meissa, Plaskett’s star, Xi Persei, Mintaka.
The term “blue giant star” has no scientific definition, and is commonly applied to a wide variety of stars that have all evolved off the main sequence. However, for practical reasons, stars with luminosity classifications of III and II (bright giant and giant) respectively, are referred to as “blue giant stars” purely for convenience, but only when that stars is hot enough to be called a blue star, which is usually above around 10,000K. Nonetheless, the term blue giant is often mis-applied to some stars simply because they are big and hot.
In practice however, big stars are referred to as “blue giants” when they inhabit a specific region of the H-R diagram (top of page), rather than because the star meets a specific set of criteria.
-Spectral Types: OB
-Life Cycle: Evolved off the main sequence
-Typical Temperature: ~10,000K to ~50,000K
-Typical Luminosity: ~10,000 to ~1,000,000
-Typical Radius: ~20+
-Typical Mass: ~20 to ~1 000
-Typical Age: = ~10 million years
Examples of blue supergiant stars include UW Canis Majoris (UW CMa) – blue-white (O-type) supergiant; Rigel (ß Orionis) – blue-white (B-type) supergiant; Zeta Puppis (Naos) – blue (O-type) supergiant; 29 Canis Majoris; Alnitak; Alpha Camelopardalis; Cygnus X-1; Tau Canis Majoris; Zeta Puppis.
Blue supergiant stars are scientifically known as OB super giants, and generally have luminosity classifications of I, and spectral classifications of B9 or earlier. Blue super giant stars are typically larger than the Sun, but smaller than red super giant stars, and fall into a mass range of between 10 and 100 solar masses.
Typically, type-O and early type-B main sequence stars leave the main sequence in only a few million years, since they burn through their supply of hydrogen very quickly due to their high masses. These stars start the process of expansion into the blue super giant phase as soon as heavy elements appear on their surfaces, but in some cases, some stars evolve directly into Wolf–Rayet stars, skipping the “normal” blue super giant phase.
– Life Cycle: Evolved off the main sequence
– Spectral Type: M, K
– Prevalence: ~0.4%
– Typical Temperature: ~3 300 – ~5 300K
– Typical Luminosity: ~100 -~1000
– Typical Radius: ~20 – ~100
– Typical Mass: ~0.3 – ~10
– Typical Age: ~0.1 – ~2 billion years
Red giant stars are smaller and less massive that red super giants, generally weighing in at between 0.3 to 8 solar masses. In these stars, of which the RBG-branch stars are the most common, hydrogen is still being fused into helium, but in a shell around an inert helium core. Other types of red giant stars include the red-clump stars, in which helium is being fused into carbon, and the asymptotic-giant-branch (AGB) stars, in which helium burning occurs in a shell around a degenerate core of carbon and oxygen, as well as in a shell that surrounds the inner helium-burning shell.
-Life Cycle: Evolved off the main sequence
-Spectral Type: K, M
-Prevalence: ~ 0.0001%
-Typical temperature: ~3,500 to ~4,500K
-Typical luminosity: ~1,000 to ~800,000
-Typical radius: ~100 to ~1650
-Typical mass: ~10 to ~40
-Typical age: ~3 million to ~100 million years
Red supergiant stars are stars that have exhausted their supply of hydrogen at their cores, and as a result, their outer layers expand hugely as they evolve off the main sequence. Stars of this type are among the biggest stars known in terms of sheer bulk, although they are generally not among the most massive or luminous. In rare cases, red supergiant stars are massive enough to fuse very heavy elements (including iron) that are arranged around the core in a way that somewhat resembles the layers of an onion, only without sharp divisions. Red supergiants that create heavy elements eventually explode as type-II supernovas.
The following are dead stars, which no longer have fusion processes taking place in their cores:
-Life Cycle: No longer producing energy
-Spectral Type: D
-Typical temperature: ~8,000K to 40,000K
-Typical luminosity: ~0.0001 to ~100
-Typical radius: ~0.008 to ~0.2
-Typical mass: ~0.1 to ~1.4
-Typical Age: Largely undetermined, but estimated to be between ~100,000 years to ~10 billion years
White dwarf stars are the cores of low and intermediate mass (typically lower than 3 solar masses) stars that have blown off their outer layers late in their lives. These stellar remnants no longer produce energy to counteract their mass, and are supported against gravitational collapse by a process called electron degeneracy pressure. While the theoretical maximum mass of a white dwarf star cannot exceed 1.4 solar masses (Chandrasekhar limit), this value does not include the effects of rotation. In practice, this means that rapidly spinning white dwarf stars can exceed the maximum mass limit by a significant margin.
Some types of white dwarfs, most notably carbon-oxygen stars, can also survive several nuclear explosions on their surfaces when the mass of accreted material pulled from normal companion stars exceed a critical level.
– Life Cycle: No longer producing energy
– Spectral Type: D
– Prevalence: ~0.7%
– Typical temperature: ~ 600,000K
– Typical luminosity: Typically very low due to their small size
– Typical radius: ~5 to ~15 km
– Typical mass: ~1.4 to ~3.2
– Typical Age: Largely undetermined, but estimated to be between ~100,000 years to ~10 billion years
Examples of neutron stars include PSR J0108-1431 (closest neutron star); LGM-1 (first recognized radio-pulsar); PSR B1257+12 (first neutron star discovered with planets); SWIFT J1756.9-2508 (a millisecond pulsar with a stellar-type companion with planetary range mass); PSR B1509-58 (source of the “Hand of God” photograph taken by the Chandra X-ray Observatory); PSR J0348+0432 (most massive neutron star with a well-constrained mass of 2.01 ± 0.04 solar masses).
Neutron stars are the collapsed cores of massive stars (between 10 and 29 solar masses) that were compressed past the white dwarf stage during a supernova event. In this state, the entire mass of the stellar remnant consists of neutrons, particles that are marginally more massive than protons, but carry no electrical charge. Neutron stars are supported against their own mass by a process called “neutron degeneracy pressure”, but the process of gravitational collapse into a black hole may continue if the remnant has more than 3 solar masses. However, neutron stars with very high spin rates may be able to resist collapsing into black holes even if they have substantially more than 3 solar masses.
Note that while Pulsars are often referred to as a class of star, pulsars are merely energetic neutron stars that emit huge quantities of radiation in various frequencies.
Black dwarfs are hypothetical stars that are theorized to be white dwarfs that have radiated away all their leftover heat and light. However, white dwarfs live for an extremely long period of time, with many of the ones detected so far being in excess of 10 billion years, meaning that no black dwarfs have had enough time to form in the Universe’s 13.8 billion year history. If these theoretical stars could one day exist, however, none are expected to be found within the remaining lifetime of the Sun. They would also be incredibly difficult to detect due to a lack of radiation, although they would still retain mass, with their gravitational influence thus providing a clue to their origins in space.
While smaller stars may become a neutron star or a white dwarf after their fuel begins to run out, larger stars with masses more than three times that of our sun may end their lives in a supernova explosion. The dead remnant left behind with no outward pressure to oppose the force of gravity will then continue to collapse into a gravitational singularity and eventually become a black hole, with the gravity of such an object so strong that not even light can escape from it.
However, there are a variety of different black holes. For instance, ‘stellar-mass’ black holes are the result of a star around 10 times heavier than the Sun ending its life in a supernova explosion, while ‘supermassive’ black holes found at the centre of galaxies may be millions or even billions of times more massive than a typical stellar-mass black hole. Well known examples of black holes include Cygnus X-1, and Sagittarius A.
Failed stars, known as brown dwarfs, form just like stars and result from the gravitational collapse of large clouds of hydrogen gas. Unlike stars, however, brown dwarfs do not have sufficient mass to ignite and fuse hydrogen in their cores, the process that powers stars such as our sun. They therefore don’t shine, and can be small, sometimes only a little larger than gas giants such as Jupiter.
– Spectral Type: M, L, T, Y
– Life Cycle: Non-main sequence
– Prevalence: ~1% to ~10%
– Typical temperature: ~300K to ~2,800K
– Typical luminosity: ~0.00001
– Typical radius: ~0.06 to ~0.12
– Typical mass: ~0.01 to ~0.08
– Typical age: Undetermined, but suspected to be several trillion years
Examples of brown dwarfs include Gliese 229 B, 54 Piscium, Luhman 16. Note that while brown dwarf stars exist in large numbers, Luhman 16 is the closest known example, being only 6.5 light years away.
Also commonly referred to as “failed stars”, brown dwarfs are sub-stellar objects that fill the gap between the most massive gas planets, and the least massive true stars. Typically, brown dwarf stars fall into the mass range of 13 to 80 Jupiter-masses, with sub-brown dwarf stars falling below this range, and the least massive red dwarf stars falling above it. Note though that brown dwarf stars mostly do not emit visible light, but where they do, they can occur in a wide range of colors. Human vision would likely perceive most stars of this type as deep red or dark magenta.
While brown dwarf stars are not massive enough to initiate and sustain a process of converting hydrogen into helium in their cores, some brown dwarfs are capable of sustaining a process in which deuterium (2H) and lithium (7Li) are converted into various isotopes if the stars’ masses are above 13 and 65 Jupiter masses, respectively.