In the last page, you examined the spectra of 14 stars. You probably noticed two features that all the spectra had in
common. All the spectra have similar overall shapes, and all have peaks and valleys of different heights.
These are the very same features astronomers use to classify stars. In fact, it was through classifying
stars that astronomers eventually realized what those features mean, and how they relate.
The Hydrogen Atom
To begin to understand what the peaks and valleys mean, let's take a close look at a hydrogen atom. A hydrogen atom has one
proton and one electron. Its electron can only occupy certain energy levels; think of energy levels as unequallyspaced steps of a
ladder. The higher up an electron is on the ladder, the more energy it has. Astronomers use the letter 'n' and
a number to designate each energy level. The lowest energy level is called the 'n=1' level, the second lowest level 'n=2', the
third 'n=3', and so on.
Electrons can move from one level to another, but the atom's total energy must always be conserved. So, if an
electron moves down from the 2^{nd} energy level to the 1^{st} (n=2 to n=1), then the atom conserves energy
by emitting a photon of light. The emitted photon has an energy equal to the difference between the 2nd and 1st energy levels.
An electron can only move up from the 1st to 2nd energy level if it gains the right amount of energy.
The electron typically gains this energy if the atom absorbs a photon of light with the right energy.
The animation to the right shows a hydrogen atom that starts with its electron in the n=1 energy level. It absorbs a
photon to go to n=2, then emits a photon of the same energy to go back to n=1. Because a photon's wavelength is determined
by its energy, if you know the energy a photon has, you know its wavelength. To go up from n=1 to n=2, an electron must
emit a photon with an energy of 10.2 electronVolts (1.63 x 10^{18} Joules)  this photon has a wavelength of
1216 Ångstroms.
To go down from n=2 to n=1, the atom must absorb a photon of 1216 Ångstroms.
Question 4. To go up in energy level from n=1 to n=2, what wavelength of light must a hydrogen atom
absorb? How do you know? 
