IMAGES
What does a star look like, when we take a picture of it? How close must it be to the Earth for us to resolve structure (size, sunspots)? What else can an image tell us about a star?

• Images taken in different colors can tell us whether a star emits most of its energy at red (cool) or blue (hot) wavelengths.
• Images taken over a long period of time check whether a star shows variation (gets brighter and fainter with time, in a repeatable pattern).
• Images can occasionally capture unique moments in a star's life, such as the death throes of a supernova explosion.

SPECTRA
What can a spectrum (a plot of light intensity versus wavelength, or frequency) tell us about a star?

Astronomers quickly recognized that different stars had dramatically different absorption lines in their spectra, when they began to observe them. Some had strong absorption lines due to hydrogen and little else, some had no hydrogen lines and many lines due to iron, calcium and other elements. The different spectral types were assigned letters with type "A" having the strongest hydrogen absorption lines, type "B" the next strongest, and so on down the alphabet (skipping lots of letters for various reasons).

[NMSU, N. Vogt]

There is a simple relationship between the wavelength of the peak of the flux for a star, λ, and the stellar temperature, T, called Wien's Law. Hotter stars have spectra which peak at bluer (shorter) wavelengths.

The strong correlations between the presence of various spectral lines and stellar color suggested that the underlying cause was linked to atomic physics. Consider the absorption lines caused when a gas of hydrogen atoms absorbs photons with an energy that corresponds to an electron jumping from the first excited state to the second excited state in the hydrogen atom. For this to happen, there must be some hydrogen atoms in the gas with their electrons in the first excited state.

Suppose we are talking about the atmosphere of a star.

• In cold stars with LOW surface temperatures, the sluggish atoms and molecules in the atmosphere do not have enough energy to move around as fast as those in a hotter gas. There will be fewer energetic collision between atoms to catapult electrons into excited states, so essentially all the hydrogen atoms will have their electrons in the ground state. Even if there are many hydrogen atoms, there will be no tell-tale absorption features.

• In hot stars with HIGH surface temperatures, the atoms in the atmosphere fly around very quickly. There are many energetic collisions, but a large fraction of the hydrogen atoms are ionized and have lost their electrons (with no chance of producing absorption lines).

• For stars with just the right surface temperature such that collisions continuously populate the first excited state with electrons, there will be lots of photons caught that excite the electrons to the second excited level, and there will be strong hydrogen absorption lines.

• Therefore, a lack of hydrogen absorption lines in a star does not necessarily mean the star's atmosphere is devoid of hydrogen; it could also mean that the star has a low or very high surface temperature. These temperature effects are far and away the most important things when determining spectral types.

• This effect can be turned on its head, and we can use spectral types to assign surface temperatures for stars. Once it was recognized that differences in spectral type were due mostly to differences in temperatures of the stars the spectral sequence was reordered by temperature.

[NMSU, N. Vogt]