From the March 2015 issue

A white dwarf pulls matter from its binary and explodes in a nova. Yet when the mass hits a certain limit, the star bursts in a type Ia supernova and is destroyed. Why the difference?

Suzanne Farkas, Amherst, Ohio
By | Published: March 30, 2015 | Last updated on May 18, 2023
Red giant supernova
This artist’s conception shows a binary star system that produces recurrent novae, and ultimately, the supernova PTF 11kx. A red giant star (foreground) loses some of its outer layers though a stellar wind, and some of it forms a disk around a companion white dwarf star. This material falls onto the white dwarf, causing it to experience periodic nova eruptions every few decades. When the mass builds up to the near the ultimate limit a white dwarf star can take, it explodes as a type Ia supernova, destroying the white dwarf.
Illustration by Romano Corradi and the Instituto de Astrofísica de Canarias
A nova is a relatively small surface explosion from a white dwarf below the Chandrasekhar limit of 1.4 solar masses. Novae can recur on timescales as short as a year. Large telescopes have been around for only about a century, limiting our ability to measure recurrent nova time-scales; nova explosions could well recur after thousands or millions of years.

A type Ia supernova explosion occurs when the white dwarf pulls enough material from its companion star to reach 1.4 solar masses. A likely mechanism is a small thermonuclear flame near the center that propagates so fast that the entire white dwarf incinerates before it can expand and cool. So far there is no clear consensus on how a type Ia (or nova) explodes. A type Ia explosion easily can burn more than half of the white dwarf into iron, so next time you fry something in an iron pot, a good chunk may have synthesized in such explosions over billions of years. Another oddity is that while most of the energy is released in a few seconds, the peak brightness occurs three weeks later!

This delay is because the ejected material is so dense after the explosion that photons can barely escape. Over time, the rapidly advancing ejected material moving at about 22 million mph (10,000 km/s) becomes less dense and more photons escape, resulting in an observable “rise time.” Finally, note that “core collapse” supernovae, the natural evolutionary end for large stars, are much more common than type Ia, but the type Ia are spectacularly bright and seen over much larger distances.

Richard Kessler
University of Chicago