A nova first observed in 2000 is nearly a decade into its stellar explosion, and yet no hydrogen has been detected in the material ejected from the star. This so-called "helium nova" is the first of its kind ever discovered.
Considering hydrogen is the most plentiful element in the universe, astronomers expect to find it pretty much anywhere they look, particularly when stars are involved. And yet the nova in question, classified V445 Puppis, completely lacks the gas. Helium, which is right behind hydrogen as both the second lightest and second most plentiful gas in the universe, has taken hydrogen's place in the composition of V445 Puppis, leading to its designation as a helium nova. Such an object is unique in the history of astronomy, and it might help astronomers better understand how they date the history of the universe.
According to Danny Steeghs, an astrophysicist at the United Kingdom's University of Warwick, V445 Puppis probably came out of a rather unusual kind of binary star system. The star that went nova in 2000 was probably a hydrogen-depleted star that relied on helium to fuel itself and produce light. The other star is likely a white dwarf, the super-dense remnants of a collapsed star that previous went nova. In such systems, the white dwarf sucks in helium from the younger star when it goes nova, which can have one of two possible outcomes.
The first possibility — and the only one observed before V445 Puppis — is that the white dwarf grows in mass and temperature from all the accumulated helium, until it grows beyond what is known as the Chandrasekhar limit, which is about 1.4 times the size of our sun. When this happens, the white dwarf explodes in a special kind of supernova, known as a type Ia supernova.
However, what appears to be happening in the case of V445 Puppis - at least for now - is that the white dwarf is too small to reach the Chandrasekhar limit, and so it will not explode. Instead, the helium radiates out from the binary star system in the bow-tie-like configuration astronomers have observed. Steeghs points out, however, that this may just be a transitional state before V445 Puppis does go supernova, although there's no guarantee that any of us will be around to see such an event.
The subject is of general interest to astronomers because type Ia supernovas are often used to help date different parts of the universe. These supernovas have very consistent luminosities, meaning their brightness is almost entirely determined by their distance from Earth. Thus, type Ia supernovas are hugely useful in determining the distances of their host galaxies, but the supernovas themselves had previously been only poorly understood. Astronomers now hope that, beyond its status as an astronomical curiosity, V445 Puppis will provide a chance to improve our understanding of these supernovas.