Each of these spectral classes, except possibly for the Y class which is still being defined, is further subdivided into 10 subclasses designated by the numbers 0 through 9. A B0 star is the hottest type of B star; a B9 star is the coolest type of B star and is only slightly hotter than an A0 star.
And just one more item of vocabulary: for historical reasons, astronomers call all the elements heavier than helium metals , even though most of them do not show metallic properties. (If you are getting annoyed at the peculiar jargon that astronomers use, just bear in mind that every field of human activity tends to develop its own specialized vocabulary. Just try reading a credit card or social media agreement form these days without training in law!)
Let’s take a look at some of the details of how the spectra of the stars change with temperature. (It is these details that allowed Annie Cannon to identify the spectral types of stars as quickly as three per minute!) As [link] shows, in the hottest O stars (those with temperatures over 28,000 K), only lines of ionized helium and highly ionized atoms of other elements are conspicuous. Hydrogen lines are strongest in A stars with atmospheric temperatures of about 10,000 K. Ionized metals provide the most conspicuous lines in stars with temperatures from 6000 to 7500 K (spectral type F). In the coolest M stars (below 3500 K), absorption bands of titanium oxide and other molecules are very strong. By the way, the spectral class assigned to the Sun is G2. The sequence of spectral class es is summarized in [link] .
Absorption lines in stars of different temperatures.
| Spectral Classes for Stars | ||||
|---|---|---|---|---|
| Spectral Class | Color | Approximate Temperature (K) | Principal Features | Examples |
| O | Blue | >30,000 | Neutral and ionized helium lines, weak hydrogen lines | 10 Lacertae |
| B | Blue-white | 10,000–30,000 | Neutral helium lines, strong hydrogen lines | Rigel, Spica |
| A | White | 7500–10,000 | Strongest hydrogen lines, weak ionized calcium lines, weak ionized metal (e.g., iron, magnesium) lines | Sirius , Vega |
| F | Yellow-white | 6000–7500 | Strong hydrogen lines, strong ionized calcium lines, weak sodium lines, many ionized metal lines | Canopus, Procyon |
| G | Yellow | 5200–6000 | Weaker hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of ionized and neutral metals | Sun , Capella |
| K | Orange | 3700–5200 | Very weak hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of neutral metals | Arcturus, Aldebaran |
| M | Red | 2400–3700 | Strong lines of neutral metals and molecular bands of titanium oxide dominate | Betelgeuse , Antares |
| L | Red | 1300–2400 | Metal hydride lines, alkali metal lines (e.g., sodium, potassium, rubidium) | Teide 1 |
| T | Magenta | 700–1300 | Methane lines | Gliese 229B |
| Y | Infrared Absorption by sodium and potassium atoms makes Y dwarfs appear a bit less red than L dwarfs. | <700 | Ammonia lines | WISE 1828+2650 |
To see how spectral classification works, let’s use [link] . Suppose you have a spectrum in which the hydrogen lines are about half as strong as those seen in an A star. Looking at the lines in our figure, you see that the star could be either a B star or a G star. But if the spectrum also contains helium lines, then it is a B star, whereas if it contains lines of ionized iron and other metals, it must be a G star.