Not all supernovae are formed from the catastrophic collapse of massive stars. Another common scenario occurs when two stars are born together but go through their life stages at different times. Binary systems are very common — about two-thirds of all stars are in binary or multiple systems. But even if stars form from a common protostellar disk, they can have very different masses.
The mass of a star determines its lifespan and ultimate fate. A more massive star has greater gravitational pressure in its core, leading to an increased rate of fusion. The faster a star burns through its supply of usable hydrogen in its core, the sooner it enters into its red-giant phase.
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If one star in a binary pair is more massive than another, then it will become a red giant and eventually lose its atmosphere into a planetary nebula, all while its companion is steadily burning hydrogen in its main sequence lifetime. The more massive star will leave behind a white dwarf — the unfused core of carbon and oxygen.
But after billions of years, the companion too can become a red giant. The diffuse, extended atmosphere of the new red giant can begin to funnel onto the white dwarf companion, building up on the carbon and oxygen surface.
If a critical density is reached, the gravitational pressure of the new material can overwhelm the electron degeneracy pressure that supports the white dwarf; in an instant, the carbon and oxygen fuse in a massive explosion known as a Type 1a supernova.
These kinds of supernovae are very useful in cosmology because their absolute brightness can be calibrated, allowing astronomers to measure the distance to the galaxy hosting such an event.
"We Don't Planet" is hosted by Ohio State University astrophysicist and COSI chief scientist Paul Sutter with undergraduate student Anna Voelker. Produced by Doug Dangler, ASC Technology Services. Supported by The Ohio State University Department of Astronomy and Center for Cosmology and AstroParticle Physics. You can follow Paul on Twitter and Facebook.