Going Nova

Nova is the Latin word for "new," and it seemed to early astronomers an appropriate term for stars that suddenly appeared in the sky. They were obviously new stars. Well, they were new in that they hadn't been seen before. But, in fact, a nova is a phenomenon associated with an old star.

When a binary pair forms, it is very unlikely that they will have the exact same mass. And since the mass of a star determines its lifetime, the stars in a binary pair will (more than likely) be at different points in their evolution. Sometimes, when a white dwarf is part of binary system in which the companion star is still active (on the main sequence or a red giant), the dwarf's gravitational field will pull hydrogen and helium from the outer layers of its companion. This gas accumulates on the white dwarf, becoming hotter and denser until it reaches a temperature sufficient to ignite the fusion of hydrogen into helium. The flare up is brief, a matter of days, weeks, or months, but, to earthly observers, spectacularly brilliant.

Given the right circumstances, a white dwarf in a binary pair can go nova repeatedly, reigniting every few decades as enough material is borrowed from its companion star.

The binary system XZ Tauri consists of two stars separated by approximately the distance between the Sun and Pluto. Images taken over the course of five years reveal that the binary system is associated with an enormous expanding bubble of hot gas.

(Image from NASA and J. Krist, STScl)

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