Astronomers have learned a great deal about the planets, stars, galaxies, and other matter in space by studying the spectra of these objects. On this page we want to provide some background about the light emitted by astronomical objects and the variety of spectra we observe.

We have to start by introducing the Bohr model of the atom. An atom is the smallest component of an element that retains all the properties of that element. A typical cloud of gas in space contains many atoms, including hydrogen, helium, and trace amounts of heavier elements like oxygen, nitrogen, carbon, and perhaps iron. What do those atoms in those clouds have to do with spectra?

The atoms inside the cloud of gas are made up of a nucleus of positively charged protons and neutrons, which have no charge. Surrounding the nucleus are one or more negatively charged electrons.

In the Bohr model of the atom, electrons are only allowed to exist in specific orbits, or energy levels. To move up to a higher energy level, the electron has to absorb energy from some source. If the electron falls to a lower energy level, it must lose energy.

If you look back at the image of the spectrum of an incandescent bulb, you will see that it appears to be giving off some light of every color. However, if you look back at the spectrum of the Sun, you will see that there are specific wavelengths (or colors) where no light appears to be emitted. Those gaps are referred to as "absorption lines", and they are caused by absorption of specific photons by the electrons in the atoms in a gas. The different types of spectra and how they are created are explained more on the pages below: