Nova eruptions by dying stars were thought to be simple, predictable acts of violence. Astronomers could point a telescope at the most recently exploded novae and see an expanding bubble of gaseous debris around each star. Scientists using NASA's Hubble Space Telescope, however, were surprised to find that some nova outbursts may not produce smooth shells of gas, but thousands of gaseous blobs, each the size of our solar system.
Astronomers acquired this new information by focusing the Hubble telescope's cameras on the recurrent nova T Pyxidis, which erupts about every 20 years. Images from ground-based telescopes show a smooth shell of gas surrounding the nova. But closer inspection by the Hubble telescope reveals that the shell is not smooth at all, but a collection of more than 2,000 gaseous blobs packed into an area that is one light-year across. Resembling shrapnel from a shotgun blast, the blobs may have been produced by the nova explosion, the subsequent expansion of gaseous debris, or collisions between fast- and slow-moving gas from several eruptions.
Back to the Drawing Board This new evidence suggests that astronomers may have to rewrite their theory of nova eruptions and accompanying debris.
"Based on these observations, our previously standard view of what nova shells should look like may be fundamentally wrong," says Michael M. Shara, of the Space Telescope Science Institute in Baltimore, Md. "The view is that a nova explosion is the same in all directions, with debris traveling at the same speed, so that a fairly smooth cloud is formed. Instead, we've found this myriad of individual knots [blobs]. This observation suggests that shells of other novae do the same thing, as recently ejected material plows into older, fossil material from previous explosions."
Shara and his colleagues collected this new information from four observations taken by the Hubble telescope's Wide Field and Planetary Camera 2 during a 20-month period from 1994 to 1995. Their results appeared in the July issue of the Astronomical Journal. The scientists selected T Pyxidis because of its closeness to Earth and its long track record of outbursts. T Pyxidis is 6,000 light-years away in the dim southern constellation Pyxis, the Mariner's Compass. Within the last 110 years, T Pyxidis has been very active, erupting in 1966, 1944, 1920, 1902, and 1890.
The nova's active record lured Shara to its debris trail more than a decade ago. His pre-Hubble spectral studies in 1985 using ground-based telescopes showed that the apparently smooth shell was expanding at the rate of 780,000 mph (350 kilometers per second). His recent Hubble observations, however, surprisingly reveal that the material has slowed down considerably since 1985. In fact, the debris is barely moving at all. Images taken months apart show no measurable expansion of the debris. Shara determined that the knots must be moving slower than 90,000 mph (40 kilometers per second). This may seem fast, but actually the gaseous debris was racing through space almost 100 times faster when it was first blown off the nova.
Waves of Violence
Ground-based and Hubble telescope observations have allowed Shara to reconstruct a sequence of a T Pyxidis blast. When the nova erupts, it flings waves of gaseous material at progressively slower speeds: the first wave of hot gas flies through space at 4.5 to 6.7 million mph (2,000 to 3,000 kilometers per second), the last at 446,000 to 670,000 mph (200 to 300 kilometers per second).
About a few weeks after this eruption, the first waves of speedy debris collide with slow-moving fossil material from the previous outburst, possibly forming the gaseous blobs. Shara observed, for example, fast-moving gas from the 1966 eruption plowing into slow-moving material from the 1944 detonation. As the speedy, newly ejected material slams into the older, plodding debris, it heats up, glows brilliantly, and slows almost to a halt. (This explains the tremendous difference in the material's speed between the 1985 and the 1994-95 observations.) Eventually, the bright material fades as it cools down. This collision scenario is like cannonballs zipping through a furnace, heating up and glowing, then cooling and fading. Images of a few blobs brightening and fading over several months were captured by the Hubble telescope.
Stellar "Tree Rings"
The blobs are distributed in eight concentric circles around the exploding star, producing a pattern similar to tree rings. Just as tree rings furnish scientists with information about a tree's life, so the circles of debris around T Pyxidis provide astronomers with a history of this prolific nova.
"We think that we're seeing the collision between pairs of eruptions all the way back to a successive pair generated in the early 1800's," Shara explains. "But we are seeing only the inner, brightest part of the ejected material; there are probably many more knots out there that are too faint for even the Hubble telescope to detect without the nova's future cooperation."
Fortunately, the central star is due for another explosion. Shara is scheduled to take observations with the Hubble telescope within a few days of the next eruption so that he can map the faint, ancient outer debris field, which will be illuminated by the nova's next bright flash. The debris map will show if the recurrent nova has been regularly active for the past thousand years or more, or if its eruptions occur in cycles. It also might offer clues to explain why some novae produce no visible shells at all.
Nova explosions are extremely powerful, equal to a blast of 100 billion billion tons of dynamite. All this punch comes from dying, faint, low-mass stars that have exhausted their hydrogen fuel. Called white dwarfs, these stars have puffed away most of their mass until only their cores are left.
A nova erupts when a white dwarf has siphoned enough hydrogen off a companion star to trigger a thermonuclear runaway. As hydrogen builds up on the surface of a white dwarf, it becomes hotter and denser until it detonates like a colossal hydrogen bomb, leading to a million-fold increase in brightness in one day. This tremendous flash of light prompted astronomers to call these objects novae - Latin for "new" - because they abruptly appeared in the sky. A nova quickly begins to fade in several days or weeks as the hydrogen is exhausted and blown into space.
Most novae spend 10,000 to 100,000 years collecting enough hydrogen from their companions to ignite an explosion. But T Pyxidis detonates several times a century. This nova has such a penchant for outbursts, astronomers believe, because its underlying star is about as massive as a white dwarf can get. A more massive white dwarf would collapse under the crushing force of gravity and become a neutron star or a black hole. Because of its high mass, T Pyxidis only needs to drain one part in 10 million of its companion's hydrogen (roughly the mass of our moon) to start an eruption. (The companion is a red dwarf, a small, cool, faint star.) This can be done in a mere 20 years or so, leading to the fascinating structure the Hubble telescope has now revealed.
Research team members are: Robert Williams, Dave Zurek (Space Telescope Science Institute); Roberto Gilmozzi, (European Southern Observatory); and Dina Prialnik (Tel Aviv University) .
Space Telescope Science Institute, Baltimore, MD
Space Telescope Science Institute, Baltimore, MD