An international team of astronomers have finally caught a glimpse of an expanding thermonuclear fireball from an exploding white dwarf. It's the first time this kind of data has ever been captured, allowing the researchers to study how the fireball evolves and rapidly expands into space.
Above: Artistic impression by David Hardy/ astroart.org/NASA.
Not to be confused with a supernova, a nova explosion happens after a white dwarf has drawn a sufficient amount of hydrogen from its companion star. Bright novae go off every few years, but this is the first time that astronomers were able to track the process right from the get go. Here's what they learned about the extraordinary Nova Del 2013.
Like a Stellar Mosquito
This particular nova is located about 14,800 light-years from here in the constellation of Delphinus (the Dolphin), so technically speaking it exploded some 15,000 years ago. The astronomers observed the event on August 14th, 2013, and the results of their analysis have been published in the latest edition of the journal Nature.
As noted, novae are a phenomenon exclusive to systems in which a white dwarf, a kind of dead star, draws hydrogen from its parent star.
Image: NASA/Casey Reed.
"Like a little stellar mosquito, the white dwarf continually sucks hydrogen from its partner, forming an ocean on its surface," noted Sydney Institute for Astronomy's Peter Tuthill in a statement. "After drawing about as much mass as the entire planet Saturn, the pressure reaches a critical point, then boom! The stellar surface turns into one titanic hydrogen bomb, hurling a fireball out into space and propelling a formerly dim, obscure star system into prominence as a nova in our night skies."
Once the hydrogen reaches a depth of around 660 feet (200 meters), the intense gravity produces enough pressure to trigger thermonuclear fusion. It's like a stellar atomic bomb, one that can be seen many light years away.
A Rapidly Expanding Fireball
The data showed that the ferocity of the initial expansion was intense, engulfing a region the size of the Earth's orbit within a day, and passing Jupiter's orbit in less than two weeks. At the 43-day mark, the fireball had expanded to the size of Neptune's orbit.
Fascinatingly, despite the fury of the detonation on the white dwarf's surface, the star escaped relatively unharmed. It will continue to accumulate more matter for a repeat performance at a future date.
Astronomers aren't entirely sure how these explosions arise on the white dwarf's surface, though they suspect the blast must happen all over the star at once. But the process is complex and not well understood.
"We found the initial nova explosion wasn't spherical, giving the fireball a slightly elliptical shape," noted Australian National University's Michael Ireland in an ABC News article. "This happens because the white dwarf's atmosphere is spinning, and there's a disk of accreted material falling onto it from the companion star, so there's a lot happening to prevent the system from being spherical when it goes bang."
This could be a clue to understanding how material is ejected from the surface of the white dwarf during the explosion.
Unexplained Multiple Shells
What's more, mysterious multiple shells were seen as the nova exploded. A main shell expanded at about 600 kilometers per second, but there were also semi-transparent shells further out going even faster. The astronomers observed the optically thick inner shell and the transparent outer shells expanding at the same time — and they're not entirely sure what they are.
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"I think there's a very tenuous outer shell, and as you look deeper and closer into the star, things get denser until eventually you can't see through things," says Ireland.
"It looks like there are multiple shells, but it could be a continuance of thicker stuff near the white dwarf and thinner stuff further out."
After about 30 days, the authors saw the outer, cooler layers begin to brighten.
"Suddenly the nova seemed to get bigger, as the tenuous outer shell became the dominate feature," says Ireland.
"That's almost certainly because warm dust had started to condense out of the gas as it cools, emitting in infrared,"
Ireland next wants to find the shock waves from the explosion.
"Hopefully, that will tell us how density changes from the outer layers to the inner layers as the explosion proceeds," says Ireland.
To capture this fine level of detail from a distance of 15,000 light-years, the astronomers had to use some very specialized equipment. The Sydney researchers collaborated with the team running the CHARA array in southern California. This array combines the light from six optical telescopes in a process called interferometry to create very high resolution images.
"The technical challenge posed requires magnification equivalent to watching a flower in my Spanish hometown of Algeciras unfold from here in Sydney, a distance of 12,000 kilometers away," added team member Vicente Maestro.
Team leader Gail Schaefer from Georgia State University was on-hand when the data first started to come in.
"It was hugely exciting to see the nova grow a little bigger than before with each night's observation. This is the first time astronomers have been able to witness an expanding fireball as if it were in the local neighborhood, rather than way out in the galaxy," Dr Schaefer said.
The new data shows in detail exactly how the fireball evolves and the gas expands and cools. The process appears to be more dynamic and complicated than previously thought, particularly in the strange and unpredicted way the gas expands outwards.
Read the entire study at Nature: "Early evolution of the extraordinary Nova Del 2013 (V339 Del)".