Chandrasekhar limit

The Chandrasekahar limit (1.44 solar masses) is the maximum size of a white dwarf, beyond which the electron gas pressure cannot resist gravitational collapse. White dwarfs form from the shrinking central core of a star that throws off outer layers in a series of ejections that form a planetary nebula. Note the central stars in the images below.

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Super Nova

Type Ia supernovas occur when a nearby companion star dumps material onto a white dwarf and pushes it over the Chandrasekhar limit. We believe that all Type1a supernovas are of the same brightness. They are used as 'standard candles' to estimate intergalactic distances.