Stellar Spectra

We saw in Unit 2 how the Fraunhofer spectrum has told us so much about the composition of the Sun. We can gain similar information about the composition of other stars by examining their spectra, i.e. the intensity of the different wavelengths of light they emit.

Of course we need very sensitive instruments to do this. A machine is used called a spectrograph.

Courtesy of J.B. Kaler University of Arizona

Light collected by the telescope passes through a diffraction grating, this does exactly the same job as a prism splitting the blended light into its different wavelengths. This light falls on electronic sensors, very similar to those inside a digital camera, which measure the wavelength and intensity of the star's spectrum.

There are lots of different types of star. They vary greatly in composition, size and temperature. We know now that stars change with time, evolving from one type to another, so we can also deduce the age of stars.

The main types of star are divided into a number of classes depending on their spectra and temperature.

Class	Spectrum	Colour	Temperature
O	ionized and neutral helium, weakened hydrogen	bluish	above 31,000 K
B	neutral helium, stronger hydrogen	blue-white	9750-31,000 K
A	strong hydrogen, ionized metals	white	7100-9750 K
F	weaker hydrogen, ionized metals	yellowish white	5950-7100 K
G	still weaker hydrogen, ionized and neutral metals	yellowish	5250-5950 K
K	weak hydrogen, neutral metals	orange	3800-5250 K
M	little or no hydrogen, neutral metals, molecules	reddish	2200-3800 K

Oh, Be A Fine Girl Kiss Me is the famous way of remembering these classes. There are actually a few others in non-visible wavelengths.

Each of these classes is then divided into 10. Our Sun is class G2. Have a good look at Orion. Betelgeuse is a class M star whilst Rigel is a class B star. One can clearly see the difference in colour even with the naked eye.

For more information about stellar spectra have a look at http://www.astro.uiuc.edu/~kaler/sow/spectra.html