Lecture 4. On to the stars
Stars have different temperatures and absolute luminosities
Inverse square law of brightness with distance
Electrons in atoms

Things to look up:
what is a spectrum ?
Apparent brightness depends on both absolute luminosity and distance: see also page 374 in Kuhn
If all stars were the same luminosity as the Sun we could work out their distance from their apparent brightness via the inverse square law (see also p 375 in Kuhn)
But not all stars are the same as the Sun: they have different colours i.e. they have different temperatures see e.g. Figure 12.12 in Kuhn
Temperature can be measured from the spectra of a star either from the wavelength of maximum intensity of the blackbody radiation, or more accurately, from the spectral lines
more on atoms and radiation, including a nice introduction to spectral lines and a good java animation of absorption and emission lines. A nice introduction to the Bohr atom.

For those with more science background there are more detailed notes on formation of spectral lines and the Bohr model atom and how atoms produce their spectra

more on electrons in atoms on pages 106-112 in Kuhn
Clusters of stars are all at the same distance, so their apparent brightness is determined only by absolute luminosity
Plot temperature and apparent brightness: This is called the Hertzsprung-Russell diagram - see also p377-382 in Kuhn
Most stars lie on the 'Main sequence', where temperature luminosity are tightly linked: high temperature implies high luminosity and vica versa. We can find a star with spectrum like the sun, and use this to get the distance to the whole cluster. Then we know the absolute luminosities ofthe other types of stars. This is called spectroscopic parallax
We can now measure distances to lots of stars directly using trigonometric parallax