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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.

Graph showing the relative strength of absorption lines versus spectral class and temperature. The vertical axis plots the relative strength of lines in arbitrary units. The horizontal axis plots both spectral class and temperature in degrees Kelvin. The spectral classes start at O on the left, then B, A, F, G, K, and M on the right. The temperature scale starts at >30,000 at left, then 30,000-10,000, 10,000-7500, 7500-6000, 6000-5200, 5200-3700, and 3700-2400 on the right. Six curves are plotted, each peaking as follows (from left to right): ionized helium peaks at spectral type O, neutral helium peaks at B, hydrogen peaks at about A, ionized metals peak between F and G, neutral metals peak at K, and molecules peak beyond M at right.
This graph shows the strengths of absorption lines of different chemical species (atoms, ions, molecules) as we move from hot (left) to cool (right) stars. The sequence of spectral types is also shown.
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.

Practice Key Terms 2

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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