What is a spectrum and how are spectra used in astronomy?
A spectrum is a plot of light intensity or power as a function of frequency or wavelength. Spectra is used to determine what stars, nebula and galaxies are made up of.
Gases and molecules give off a certain light wavelength unique to them based upon the frequency of their atoms when excited. Astronomers and scientists use these wavelengths to determine what gases make up whatever object they are looking at.
Other types of spectroscopy are also used to determine the make up of distant objects, such as X-ray and radio spectroscopy. They're used to detect Black Holes, strong gravity fields, Neutron stars, compact stars and interstellar mediums.