A Brief History of Astronomy
The Hertzsprung‐Russell Diagram
The brightest naked-eye star is Sirius. Sirius is actually a binary star system with the two components designated Sirius A and Sirius B. Sirius A is an A star and has the same color, hence same surface temperature as its companion; but Sirius B is about 10 magnitudes (or 10,000 times) fainter than A. The two stars therefore must differ in size (using the same argument as for giant and supergiant stars) by a factor √10,000 = 100 times. Because Sirius A is a main sequence star approximately the same size as the Sun, Sirius B is only slightly larger than Earth. The binary orbit of this pair allows determination of their masses. Sirius A has a mass of twice that of the Sun and Sirius B is 1 solar mass. One solar mass in a volume the size of Earth represents an average density of 2,000,000 grams per cubic centimeter — a spoonful of material from Sirius B would weigh (on Earth) two metric tons! Because of their size and color (these hot stars produce more blue than red light, but the spectral sensitivity of the human eye makes them appear white), these compact stars are therefore termed white dwarf stars.
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White Dwarf Stars
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