The branch of physics that deals with the production, measurement and interpretation of electromagnetic spectra, such as those produced by stars, nebulae and comets. Using spectroscopy, astrophysicists can dissect light to learn more about the nature of a heavenly object, including the chemical elements and compounds that may be present. Spectrograph An instrument that breaks up light into various wavelengths and then measures them. The electromagnetic spectrum is the complete range of frequencies of electromagnetic waves from the lowest to the highest frequency including, in order, radio, infrared, visible light, ultraviolet, X-ray, gamma ray and cosmic ray waves. The defining feature of a comet is its long tail.
Comets often have two tails, a sweeping dust tail and a thinner, straighter plasma tail.
The dust tail, as its name implies, is caused by the dust particles released from the ices of the nucleus when they are vaporized. A comet's tail can extend 150 million kilometers and always points away from the sun.