The new observations, reported in the journal Nature, show these eruptions are far more complicated than previously thought.
"Bright novae go off every few years, but this is the first to have occurred in good weather since the development of reliable optical telescope interferometers," said one of the study's authors, Dr Michael Ireland of the Australian National University. "Everything has come together to give us a really good look at one."
A nova is the thermonuclear explosion of hydrogen on the surface of a dead star called a white dwarf. When a white dwarf comes into a very close orbit with a companion star, it can suck hydrogen off the other star onto its own surface.
Once this hydrogen ocean reaches a depth of around 200 meters, gravity produces enough pressure to trigger thermonuclear fusion -- essentially a stellar atomic bomb -- that is visible many light years away.
On Aug. 14, 2013 a nova exploded about 14,800 light-years away in the constellation Delphinus. The 'new' star was subsequently named Nova Delphinus 2013.
"This nova was bright enough to be seen with the unaided eye," said Ireland.
Within hours of its discovery, Ireland and colleagues pointed the telescopes of the CHARA array in California toward the expanding nova fireball.
The CHARA Array combines the light from six optical telescopes in a process called interferometery, to create very high resolution images. The first measurements, which were the earliest ever obtained for a nova, showed the fireball was already as big as Earth's orbit around the sun.
This dramatically expanded to the size of Neptune's orbit by the time of the final observations 43 days after the initial blast.
One of the big questions about novae is how the explosion happens. Astronomers suspect the blast must happen all over the star at once, but the process is very complex and not well understood.
"We found the initial nova explosion wasn't spherical, giving the fireball a slightly elliptical shape," said Ireland.
"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 provides clues to understanding how material is ejected from the surface of the white dwarf during the explosion.
"The interesting thing for me, were the multiple shells seen as the nova explodes," said Ireland.
"There's a main shell expanding at about 600 kilometres per second, but then there were also semi-transparent shells further out going even faster. So we could see both the optically thick inner shell, and the transparent outer shells expanding at the same time."
Ireland and colleagues still don't know what these shells are.
"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," said 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."
Read more at Discovery News
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