Stars are grouped into a spectral class, based on their surface temperatures. The most common and simplest method to do this is the Morgan-Keenan system introduce in 1943 by American astronomers William Morgan and Philip Keenan. This uses letters O, B, A, F, G, K and M to classify stars hottest to coolest. This was based on the original scheme developed by the Harvard College Observatory in the late 1800s, when stars were originally assigned a type A to Q. Updates and developments in astronomy are responsible for throwing the alphabetical tidiness out the window. More recently D was added for white dwarfs (presumably meaning “dead”) and the latest is to add types L and T for brown dwarfs all to the coolest end of the spectrum. At the other end, the hottest of all, was added, W, for Wolf-Rayet stars. So the spectral class order is now W, O, B, A, F, G, K, M, D, L, T.

Wolf-Rayet stars

Wolf-Rayet stars are very rare. There are only 300 known examples in the universe, although some sources suggest there are as many as 500. In context that is still very few. They were discovered in 1867 at the Paris Observatory by French astronomers Charles J E Wolf (1827 – 1918) and Georges A P Rayet (1839 – 1906), hence their name.

They are unusual in that they have strong broad emission lines in their spectra but few absorption lines. They are hot, luminous stars whose strong stellar winds have blown away their outer atmosphere, revealing their inner layers. This last is perhaps their most distinguishing feature. Their emission bands indicate they are surrounded by a rapidly expanding gaseous shell. They have a high metallicity.

More than half of the known Wolf-Rayet stars are members of a binary star system with types O and B as companions. Their temperatures range from 30 000 – 210 000 °C.

Their spectra are characterised by broad emission bands of neutral and ionised helium, but with weak hydrogen lines. Members are in different states of ionisation, broadly subdivided into:

• WN where the emission lines also include hydrogen and nitrogen
• WC whose stars also have emission bands of carbon
• WO whose emission lines also reveal the presence of carbon and helium.

They are massive stars, at least 20 times that of our sun. As with all giant stars they burn their fuels very fast and die very fast and very loudly in the form of a supernova. Their lifespan is measured in so many millions of years. The sun, on the other hand can expect a lifespan of about 10 billion years.

The most massive and luminous star in the known universe is indeed a Wolf-Rayet star. Named R136a1 it resides in the Large Magellanic Cloud where it is the central star of the Tarantula Nebula. It is 265 times the mass of our Sun. It is subtype WN with strong broad emission lines including nitrogen, helium, carbon, and oxygen. A little silicon is also detected and weak or sometimes absent hydrogen lines.

The emission nebula NGC 2359 (aka Thor’s Helmet) has been produced by an extremely hot Wolf-Rayet star (WR7), which resides at its centre. The star, also designated HD 56925, is 15 000 light years away in the constellation Canis Major. It is losing mass at the rate equivalent to the sun every 1 000 years. This is typical of the Wolf-Rayet type and they are unable to exist at this rate for long, which is why they are rarely observed.

Another Wolf-Rayet star, WR104, looks set to go supernova fairly soon (in the next few hundred thousand years). It can be found in Sagittarius at a distance of 8 400 light years.

Wolf-Rayet stars tend to have a high metallicity value because of a higher percentage of their composition will be non-hydrogen gases. Higher metallicity leads to high mass loss, a common feature of this type of star.

Gamma Velorum is a quadruple star system in the constellation Vela. It contains the brightest and closest Wolf-Rayet star, which will also be our nearest supernova experience.

Type O

These stars are known as helium stars. Their temperature ranges from 25 000°C to 40 000°C. Because of their high temperatures they have spectral lines of singly and doubly ionised helium. The spectra also contain lines of doubly and trebly ionised oxygen and ionised nitrogen.

Notable examples: Mintaka, Regor and Zeta Puppis.

Type B

These stars have temperatures in the range 11 000°C to 25 000°C. Their spectra contain no lines of ionised helium but do contain lines of singly ionised oxygen and nitrogen. As with all the star types Type B is further subdivided into ten classes. As the series proceeds from B0 to B9 the oxygen and nitrogen emission lines become fainter, showing that the temperature decreases gradually. Absorption lines of hydrogen begin to appear and become stronger through the series. Some of these stars also have emission lines of hydrogen showing that they have a very rarefied atmosphere. Although this group of stars is proportionately not very abundant their brightness makes them visible from afar. They tend to appear in swarms.

Notable examples: Alnilam (Epsilon Orionis) and Bellatrix (Gamma Orionis) both of course in the constellation of Orion, as well as Rigel.

Type A

The star groups cool down as we go through the list. Type A range in temperature from 7 500°C to 11 000°C. They are a plentiful bunch. Hydrogen lines are strongest in the A0 and A1 class. The spectra contain no helium lines. Lines of singly ionised calcium begin to appear.

Notable examples:  Castor, Fomalhaut, Sirius and Vega.

Type F

The temperatures of this type are between 6 000°C and 7 500°C. The earlier classes F0 and F1 have equally strong hydrogen and singly ionised calcium absorption lines. As we move along in the series hydrogen lines weaken and calcium strengthens. Many fine absorption lines of metals begin to appear.

Notable examples: Canopus and Procyon.

Type G

Further down the scale the temperature range lowers still further to between 5 000°C and 6 000°C. The hydrogen lines continue to weaken but are easily visible. Metallic lines become stronger throughout the series from G0 to G9. Lines of ionised atoms are barely visible. There are two groups of G stars: those with fine absorption lines in the spectra are giants having low pressures in their atmospheres, which is why their lines are so fine; and dwarf stars with high pressures in their atmospheres that have broader absorption lines. The sun is a typical G2.

Notable examples: Alpha Centauri A and Capella.

Type K

Type K stars range in temperature from 3 500°C to 5 000°C. Metallic absorption lines are very strong, especially those of iron. Ionised calcium lines are strongest in K1, having temperatures of between 4 000°C to 5 000°C among the dwarfs and 3 500°C to 4 500°C among the giants. Later types have absorption lines of molecules such as titanium oxide and zirconium oxide.

Notable examples: Aldebaran, Alpha Centarui B, and Arcturus.

Type M

The coolest of the groups is the M type at between 3 000°C to 3 500°C. The spectra contains fluid bands of titanium oxide, which becomes fainter on the red side of the spectrum. There are still many metallic lines. Hydrogen lines are almost invisible except in certain variable stars.

Notable examples: Barnard’s Star and Betelgeuse.

Subtypes

Their temperatures lie in the range 2 000°C to 3 000°C. Type R is very rare. The violet and blue portions of their spectra are fairly bright. They are not as red as M and N types. Two examples are variable stars: R Coronae Borealis, and V Arietus. Type N display fluted bands of corbon compoounds in their spectra. Most of these stars are irregular variables and are very red. An example is W Canis Majoris. Type C are rich in carbon in their atmospheres. They are very red and sometimes classed with their N cousins. Type S are similar to M type but have bands of zirconium oxide and titanium oxide. They are all giants and are usually classified as long-period variables. An example is R Andromedae.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Thursday, 18 March 2021