Supernovae, gigantic stellar explosions, are not only used as cosmic yardsticks by cosmologists, they are also important chemical element factories in our Universe. So far, astrophysicists know of two physical processes giving rise to these bursts: one is the core collapse of a massive star at the end of its lifetime, the other the thermonuclear detonation of an old white dwarf star.

An international team of researchers, including scientists from the Max Planck Institute for Astrophysics, have now identified a third type of these stellar explosions, arising from a helium-rich, old stellar system (Nature, 20 May 2010).

Depending on certain chemical elements identified in the light of supernovae, these stellar explosions are classified as Type Ia, Ib, Ic or Type II. As the light curves of Type Ia supernovae are very characteristic and uniform, astronomers use them as "standard" candles in extragalactic astronomy to determine the distance to their host galaxies.

These supernovae are thought to arise when a white dwarf star, the burnt-out remnant of a normal star such as our Sun, approaches the so-called Chandrasekhar limit by accreting material from a binary companion. The dense core of mainly carbon and oxygen then ignites and releases so much energy that the star explodes as a supernova.

The other process leading to a supernova explosion is the gravitational collapse of the core of massive, short-lived stars at the end of their lifetimes. Astronomers believe that these are observed as Type Ib/c or Type II supernovae, which are associated with young stellar populations.

Most of the stellar material is ejected due to the enormous amounts of energy released in the explosion, leaving behind a remnant with only a fraction of the initial mass of the star.

In January 2005, a faint supernova (SN 2005E) appeared in the halo of the nearby galaxy NGC 1032, and an international team of astronomers collected observations of this supernova from telescopes around the world.

Surprisingly, the measurements of the chemical composition and amount of material expelled in the burst fit neither of the two known explosion mechanisms. The lack of any recent star formation activity near the supernova location and the very small mass ejected in the explosion (only about one third of the mass of the Sun) do not agree with an exploding giant star, i.e., a core collapse origin.

The alternative, an exploding old white dwarf star that had a long time to travel from its star formation birthplace out to the halo, does not agree with the observations either, as the light spectrum indicates a different chemical composition.

The material expelled by the supernova contains a higher fraction of calcium and titanium than any supernova observed so far. These elements are produced in nuclear reactions involving helium rather than the carbon and oxygen found in the center of white dwarf stars.

Computer models have now shown that the supernova most likely occurred in an interacting system of two close white dwarf stars, where the helium shell of one white dwarf is drawn onto the other one. "Once the receiving star has accumulated a certain amount, the helium starts to burn explosively," explains Paolo Mazzali, (Max Planck Institute for Astrophysics) who performed the calculations together with David Arnett (University of Arizona).

"The unique processes producing certain chemical elements in these explosions could solve some of the puzzles related to chemical enrichment. This could, for example, be the main source of titanium."

The supernova SN 2005E might be only one of a new subset of dim supernovae arising from this distinct physical class of explosions. Several similar supernova events have been identified in evolved elliptical galaxies, whose light curves, environments and ejected mass are best described by the helium detonation process.

"When we observed SN 2005E it soon became clear that we were seeing a new type of supernova," says Hagai Perets (Weizmann Institute, now at the Center for Astrophysics, Harvard University), the lead observer.

"As these kinds of supernovae are relatively faint, they are difficult to detect. But if they are actually not all that rare, they might provide an answer to some fundamental physics puzzles about the production of chemical elements in the universe."

Unusual supernovae are a specialty of this astronomer team. Only a few months ago they reported the first confirmed observation of another very peculiar type of supernova, which does not leave behind any remnant. Depending on their mass, stars end their lives as white dwarfs, neutron stars or black holes.

Extremely massive stars, however, might disappear completely in the supernova explosion at the end of their lifetime. In these so-called pair-instability supernovae, energetic light particles are converted into electron-positron pairs, which cannot counteract the gravitational collapse. The violent contraction triggers a nuclear explosion that rips the star apart completely.

The astronomers identified such a supernova, SN 2007bi, in a nearby dwarf galaxy, and published their findings in the journal Nature in December 2009.