Apparent λmax Actual λmax

Solar radiation and the Atmosphere When measured above the Earth atmosphere, solar radiation is not quite black body radiation but it is a close approximation. As the radiation passes through the atmosphere prominent parts of that radiation is absorbed. Although most of the light e.m. radiation is absorbed at longer wavelengths UV absorption is particularly important for life on earth. The effect of atmospheric absorption is to push λmax to the left making the Sun (and other stars appear slightly hotter

Classification of stars by spectral class In the nineteenth century it was possible to classify the stars by the characteristics of their spectra rather than simply by their temperature. Many classes of stars were identified depending on features of their spectra. This was systematised by Annie Cannon (1910 Harvard)

The classification picks out features in the spectrum of stars but these are almost entirely determined by the temperature of the star (Payne and Saha 1920). O B A F G K M (N) Temperature To make for a finer classification system the letters are suffixed with numbers G0….G2 …G3 …etc up to 9 effectively making the system a continuous one accounting for subtle changes in spectra from star to star.

Notice how the prominence of lines changes with the spectral type ( i.e. effectively the surface temperature) of the star. The V “luminosity class just means that these are “normal stars” in mid “life”.

What causes stellar spectra Photosphere of Star Black body radiation is emitted from the surface of the star but has to pass through gaseous layers immediately above it. This results in absorption at particular frequencies Stellar “Atmosphere”

Reminder-line spectra Suppose a hydrogen atom (possible energy levels n1,n2….) is sitting in the stellar atmosphere. It has an electron in n=2. It can absorb light which has the exact energy to create a transition to n=3 n=4 n=5 etc. This leads to absorption of light at only specific frequencies.

Transitions to higher levels from n=2 in the hydrogen atom are referred to as the Balmer series name n=2 to n=3 Hα n=2 to n=4 Hβ n=2 to n=5 Hγ

The Balmer series Hδ Hγ Hβ Hα Stellar spectra are much more complicated than this. Many elements are be involved. These elements can be in different states of ionisation. For instance a helium atom may exist as He, He+ or He2+ Astronomers label these states with roman numerals e.g. He I is neutral helium He II is helium which is singly ionised He+