Boffins snap first pics of hot white dwarf nova bursting out of its shell

Astronomers have caught the first pictures of a star in the early stages of going nova after a Japanese amateur skywatcher alerted the community to a new star showing up in the heavens.

On August 14 of last year, Koichi Itagaki reported discovering a new star in the sky called Nova Delphinus 2013 around 14,800 light years from Earth. He put the word out on the Astronomer's Telegram bulletin board, which was spotted by astronomers at the Center for High Angular Resolution Astronomy (CHARA), who started shifting their hardware to catch the star as it went nova and exploded outwards.

"This is the first time astronomers have been able to witness an expanding fireball with such great detail, rather than as a tiny point of light way out in the galaxy," said Georgia State University astronomer Dr. Gail Schaefer. "It was amazing to see the material expanding outward each day after the explosion."

CHARA is a six-telescope array based at Mount Wilson Observatory in Los Angeles, California, partially funded by the National Science Foundation but run by Georgia State, and consists of two batteries of one-meter-wide observatories spaced 330 meters apart. After checking out the new light with three telescopes initially, the entire system was concentrated on Delphinus to catch the light show, Schaefer told The Register.

Novae are much more common and less violent than their flashier supernova cousins, but are still titanic events in their own right. They occur when a white dwarf star gets close enough to another stellar body and begins to accumulate hydrogen from it.

Eventually this layer of hydrogen spreads out over the surface of the white dwarf until it reaches a critical mass of an estimated 200 meters deep. Then the pressure from the white dwarf ignites the hydrogen in an enormous thermonuclear explosion that, in the case of Delphinus, was bright enough to be seen on Earth with the naked eye.

The CHARA observatory caught a partial glimpse of a nova back in 2011, but it was too low in the sky for a Northern Hemisphere telescope to get a really good look. After adjusting the array to focus on Delphinus, Schaefer said that the team got the first complete set of images of a nova from very early on in its birth and for 43 days afterwards.

In a paper published in the journal Nature, the team described how the fireball from Delphinus burst out from the white dwarf's surface at over 1.3 million miles per hour, so that by the time the first pictures came in, the fireball was already the size of Earth's orbit around our sun.

Two days later, the fireball was the size of Mars' orbit, and 12 days after that it had reached out to the equivalent of Jupiter's position from the Sun. By the last day of observation, the fireball was large enough to envelope Neptune, had it been our own star going "boom."

Theoretical models for nova explosions postulate a spherical explosion shell expanding outwards consistently. In this case, however, the team spotted that the nova was actually lopsided, with one plane 13 per cent larger than the other, giving the fireball a slightly oblong appearance.

Other telescopes will now start examining Delphinus to glean more data from the exploding star, and it's possible we may see more from the white dwarf. Once the first batch hydrogen has burned off, the white dwarf may accumulate more and flare off again, possibly within our lifetime.

Recurrent novae are a rare, but well-documented phenomenon. The most frequent example, U Scorpii, has been recorded flaring seven times, with the most recent bursts coming in 1989, 1999, and 2010.

It's observatories like CHARA that will find more. The telescope is so accurate that its creators claim you could site it in Los Angeles and focus on a nickel atop the Eiffel Tower, provided all that pesky line-of-sight business was sorted out. ®