Spectrum

From Slackerpedia Galactica

A single particle of light -- a photon -- is characterized primarily by its wavelength. Two photons with different energies have different wavelengths. Many sources of light give off many many many photons, which often have different wavelengths from one another. The wavelength distribution of photons given off by a light source is its spectrum.

A beam of sunlight dispersed by a prism is spread out into a rainbow of colors, showing the part of the Sun's spectrum visible to our eyes. If we had eyes to see it, this rainbow would extend from radio waves, through microwaves, infrared light, visible light, ultraviolet, X-ray, and into gamma rays. The full range of possible photon wavelengths from radio light to gamma rays is known as the electromagnetic spectrum.

Astronomers spend a lot of time measuring spectra, mainly because it's hard to reach out and chip off a piece of quasar to bring back to the lab for study. Light is all we have, so by measuring how much light an object emits at a given wavelength we can learn about why it is emitting light. We can take its temperature, measure its composition, or learn something about the physics of the system. Is it made of a hot gas? Does it have dust? Are there relativistic particles flying around? All of these produce unique fingerprints in a spectrum -- some in visible light, some in radio waves, and some in gamma rays.

Conveniently, similar types of objects have similar spectra. For example, stars may have spectral signatures of particular chemical elements, like hydrogen or calcium. Sometimes these signatures change depending upon the temperature of the star. Stars with similar temperatures and chemical compositions have similar spectra. Galaxies that are themselves made of lots of stars can have spectra that resemble those of young stars, or old stars, or in between stars, or all kinds of stars, depending upon whether the galaxy is making new stars, or stopped a long, long time ago.