The illustration demonstrates the formation mechanism for an emission spectrum and an absorption spectrum. Astronomers measure both types for the light from stars and other celestial bodies and obtain important information about the physical and chemical structure of the observed objects.

Click START to begin, or read on to learn what the illustration shows:

The emission spectrum appears when atoms of the observed body are bombarded by high-speed electrons (emitted by an external source) which collide with atomic electrons creating vacancies in the lower orbits. Such "excited" atoms give off light as other atomic electrons jump downward to fill the vacancies.

Click the START button, and you will see how high-speed electrons fly into the atom and collide randomly with electrons in the atom's orbits. Each time a collision occurs, the target electron is bounced up to a higher orbit. In a while another electron jumps from a higher orbit to fill the vacancy, and a colour line in the atom's spectrum is created. Larger jumps release higher energies and thus create lines in the violet and blue part of the spectrum. Moderate jumps create lines in the green and yellow, and small jumps, releasing small amount of energy, make orange and red lines.

Next, change the mode from "Emission" to "Absorption". Now the atom will be hit by photons of light instead of by high-speed electrons. The spectrum is now a bright rainbow representing light from some outside source such as the photosphere of a star.

Each time a photon strikes an electron, the electron jumps up to a higher shell just as it did when struck by a high-speed electron. However, the photon is absorbed and disappears when an electron takes its energy to make a jump. This causes a dark gap to appear in the bright spectrum.

As before, blue and violet photons energize the largest upward jumps, and the red and orange photons energize the smallest ones.