Astronomers have captured time-lapsed images of an expanding thermonuclear fireball from a nova that erupted last summer. This is the first time a nova -- which is basically a stellar H-bomb -- has been imaged as the nuclear blast was happening. Observations of this early fireball stage, described in Nature this week, paint a far more complicated picture than simple models previously predicted.
A nova is an explosion on the surface of a dense, compact star called a white dwarf. Much smaller and less violent than a supernova, a nova ejects a thin surface layer of hydrogen gas into space. White dwarfs pull hydrogen in from its close companion star in a binary system. When the hydrogen buildup gets to be about 200 meters deep, the surface gravity of the white dwarf produces sufficient pressures at the bottom of the layer to trigger thermonuclear fusion.
"Like a little stellar mosquito, the white dwarf continually sucks hydrogen from its partner, forming an ocean on its surface. After drawing about as much mass as the entire planet Saturn, the pressure reaches a critical point, then boom!” Peter Tuthill from the University of Sydney describes in a news release. “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.” Pictured to the right, an artist’s conception of matter being drawn from the donor star (right) by a white dwarf (left).
A "new" star suddenly appears where there was none before, then fades in the ensuing weeks as the fireball expands, cools, and dissipates. Nova Delphini 2013 was discovered in August 2013 by an amateur astronomer in Japan. Within 15 hours of the discovery -- and 24 hours of the actual explosion -- astronomers working in "express mode" pointed six telescopes from the Center for High Angular Resolution Astronomy (CHARA) Array toward the nova. The 37-member team led by Gail Schaefer of California's Mount Wilson Observatory measured the size and shape of the fireball, as well as the speed and distance of the ejected gas, on 27 nights over the course of two months.
During the first observation, the physical size of the fireball was about that of Earth’s orbit. It passed Jupiter’s orbit in two weeks, and finally grew to the size of Neptune’s orbit when it was last measured 43 days after detonation. It basically expanded to the size of our solar system: a ferocious 20-fold expansion at a velocity of over 600 kilometers a second.
The fireball was slightly elliptical in shape, they found. The ejecta started as a uniform core, then spread out into a bipolar structure. The outer layers became more diffuse and transparent as it expanded. Watch an animation.
The nova is about 14,800 light years from the sun, which means the explosion actually took place almost 15,000 years ago. Despite the detonation's fury, the white dwarf escaped relatively unscathed and continues to accumulate matter from its companion star for a future performance.
Images: Jimmy Westlake, Colorado Mountain College via NSF (top), David A. Hardy/astroart.org via University of Sydney (middle)