A star with a mass approximately that of our own Sun, that has exhausted all its thermonuclear fuel and contracted to a size roughly equal to the size of the Earth. The density of matter in these very old, Earth-sized stellar corpses is extremely high. For example, a teaspoonful of white-dwarf material brought to the Earth would weigh as much as a truck.
Cataclysmic variables form a diverse class of close binary star systems composed of a white dwarf star and a cooler star. The stars in these binaries are so close that the time it takes for them to orbit about each other is measured in hours rather than days or years. Material is lost from the cooler star and is accreted by the white dwarf through a disk. As material spirals through this disk to the surface of the white dwarf it heats and becomes luminous; so luminous in fact that it outshines both the white dwarf and the cooler star.
Stars can be classified into various groups or spectral types, according to the appearance of their spectra. The Guest Investigator Puzzle lab allows the user to classify various mystery stars into several categories based on the appearance of their spectra. Features in a star's spectrum give astronomers clues as to the composition of the stellar atmosphere (which chemical elements are present), as well as other physical parameters such as the temperature of the stellar atmosphere. Late-type stars are cool, with surface temperatures around 3,000 degrees Kelvin. At these cool temperatures, atoms can stick together in molecules.
Early-type stars have hot surface temperatures in excess of 35,000 Kelvin. Their spectra show features caused by the presence of Helium and Silicon atoms in their atmospheres.
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