The spectrum

One of the principal tools of the astronomer is the spectrograph. In brief, a spectrograph is an instrument that spreads light out into a spectrum. A record of the spectrum is called a spectrogram and can be expressed as a graph of intensity vs. wavelength or frequency.

Above is a portion of the solar spectrum between 532.76 and 532.89 nanometres (green) where a chromium line is flanked by two strong iron lines.

[Spectral lines? Well, most spectroscopes (that is a spectrograph that you look through) image the object to be analyzed through a slit, not a pinhole, so you see a band of light red on one side, blue on the other. This band is just the slit smeared out into a spectrum. If a particular wavelength is strongly absorbed, that region of the spectrum is dark, it really looks like a dark vertical line in the band. If you see horizontal lines, it means the slit is dirty and should be cleaned.]

The first spectrum known to man was probably a rainbow. Around 1665 Newton created an artificial rainbow by passing sunlight through a prism. In 1802 William Wollaston observed "color boundaries" then in 1814/15 Joseph Fraunhofer found some 600 lines in the solar spectrum. We now call solar spectral lines Fraunhofer lines. He assigned letters to the more prominent lines and astronomers all know the significance of the "D" lines (the yellow lines of sodium prominent in street lights), the "H" and "K" lines of ionized calcium and the magnesium "b" lines.

As the field of astronomical spectroscopy grew stellar spectra were observed and in 1863 Angelo Secchi classified stars according the strengths of spectral lines of hydrogen from A, the strongest, through B, C,… Saha recognized the reason for the strengths of the spectral features were due to ionization effects and Secchi's scheme was modified and augmented and today I know of seven basic spectral classes and five "extras". (Is this right?) To make a long story short, nobody makes it out of a basic astronomy subject without knowing the spectral sequence in order of decreasing temperature, and knowing a little bit about each class:

Henry Norris Russell came up with the famous mnemonic Oh Be A Fine Girl Kiss Me and you can add Right Now Sweet for the extra three classes. In Australia we use Oh Bring A Fully Grown Kangaroo, My Recipe Needs Some.

Balmer line intensities

The strength of a line depends on how many atoms are (in a state) capable of absorbing the line. This is proportional to the number of atoms and in thermodynamic equilibrium there are two more factors, the Boltzmann (excitation) factor and the Saha (ionization) factor. The Boltzmann factor goes as 10^-5040X/T where X is the excitation energy in electron volts, eV, and T is the temperature. The Balmer lines originate from the n=2 level which lies X=10.2 eV above the ground level. In the sun we have T close to 5040 so we see that only one in ten billion H atoms are in a state capable of absorbing a Balmer line and it is no surprise that the Balmer lines are weak in the sun. Sirius is about twice as hot as the sun so the exponent falls from -10 (sun) to -5 (Sirius). The H lines in Sirius have 100,000 times more atoms capable of absorption than do the solar lines. The hotter the star, the more likely the atom is to absorb the line. But wait, there is a second factor, the ionization factor. Above 7000K the H begins to be ionized and as the temperature rises the Boltzmann factor becomes exponentially larger but the Saha factor becomes exponentially smaller. Beyond 11000K the ionization of H is almost complete and even if all the neutral atoms were in the n=2 level there would be no Balmer lines since there is no more neutral H. So H peaks at spectral class A which of course is no surprise at all since Secchi defined spectral class A as the stars with the strongest Balmer features.

Molecules dissociate (ionize) at temperatures higher than found in G stars so molecular features are the signposts of the cooler stars, especially M stars. But there are some cool stars that do not quite fit into class K or M, a puzzle until it was realized that some stars were unusual in that there was more carbon than oxygen (we write C>O). Now it turns out that carbon monoxide is a very strong molecule so whichever of C or O is more abundant is the only one left to form the other molecular features that appear in the visible. We call these carbon stars and classify them as R and N in order of decreasing temperature.

The seven spectral classes were subdivided into tenths (B0, B1, ..., B8, B9, A0, ...) and firmly established by Annie Cannon who between 1918 and 1924 classified hundreds of thousands of stars. A pretty impressive feat. This Harvard spectral classification system is in use today. The sun's spectral class is G2. Watch out, g Lib is classified as G8.5, pretty precise!

Luminosity classes

Class	Name
I	Supergiant
II	Bright giant
III	Giant
IV	Subgiant
V	Dwarf
VI	Subdwarf

So far we have tried to sort the stars out by their spectra in a temperature sequence. But Antonia Murray in 1897 noted some spectra had sharper spectral lines and Hertzberg in 1905 confirmed that these narrow lined stars were much more luminous than ordinary. In 1937 W. W. Morgan and P. C. Keenan introduced the M-K luminosity classification still in use today. We will see the larger a star is for a given spectral class, the lower the surface gravity and hence pressure. The luminosity class criteria arise from the fact that pressure is related to the width of lines, Balmer lines especially, and to the ionization of the elements.

Note especially that most stars are dwarf stars and we will come to knw these as main-sequence stars. The sun is a G2 dwarf or G2V star. The classes are broken into subclasses a (luminous), ab (bright), b, in order of decreasing luminosity so h CMa is a luminous blue supergiant B5 Ia, z Mon is a G2 supergiant star G2 Ib, and x Tel M1 IIab is a bright red giant star.

Don't confuse the subdwarf class with white dwarf "stars". (The term white dwarf should be used, not white dwarf star.) White dwarfs are really the burned out embers of stars about the size of the Earth and are REALLY DIM. Subdwarf stars are just main sequence stars that are "metal deficient".

Of course the giant stars are REALLY BRIGHT and stand out so while most stars are dwarf (main-sequence) stars the giant stars are the ones we are likely to see. Half of the twenty brightest stars we see are giants while none of the twenty nearest stars are.