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Astronomy: Double stars
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About half of the stars in the Universe are double stars, two companions of a binary system. The stars may vary in orbital period (and thus their distance from each other) from hours to many thousand of years. Seen from Earth some double stars eclipse each other which explains a reduction in brightness of the system. Periodically one star passes in front of the other. Close pairs may even exchange mass. A binary pair of dwarfs can be extremely close. A white dwarf can draw off gass from a cool (red) dwarf, which causes a spiralling accretion disk around the white dwarf. The point where the attracted mass hits the disk is a bright hot spot. And instability periodically causes surges in brightness of as much as six magnitudes. This is called a dwarf nova. Related subject>> How are a star and planets formed? |
 The blue and yellow star of Albireo |
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