Nova eruptions by dying stars were thought to be simple,predictable acts of violence. Astronomers could point a telescopeat the most recently exploded novae and see an expanding bubbleof gaseous debris around each star. Scientists using NASA's HubbleSpace Telescope, however, were surprised to find that some novaoutbursts may not produce smooth shells of gas, but thousands ofgaseous blobs, each the size of our solar system.
Astronomers acquired this new information by focusing the Hubbletelescope's cameras on the recurrent nova T Pyxidis, which eruptsabout every 20 years. Images from ground-based telescopes show asmooth shell of gas surrounding the nova. But closer inspectionby the Hubble telescope reveals that the shell is not smooth atall, but a collection of more than 2,000 gaseous blobs packedinto an area that is one light-year across. Resembling shrapnelfrom a shotgun blast, the blobs may have been produced by thenova explosion, the subsequent expansion of gaseous debris, orcollisions between fast- and slow-moving gas from severaleruptions.
Back to the Drawing Board This new evidence suggests that astronomers may have to rewritetheir theory of nova eruptions and accompanying debris.
"Based on these observations, our previously standard view ofwhat nova shells should look like may be fundamentally wrong,"says Michael M. Shara, of the Space Telescope Science Institutein Baltimore, Md. "The view is that a nova explosion is the samein all directions, with debris traveling at the same speed, sothat a fairly smooth cloud is formed. Instead, we've found thismyriad of individual knots [blobs]. This observation suggeststhat shells of other novae do the same thing, as recently ejectedmaterial plows into older, fossil material from previousexplosions."
Shara and his colleagues collected this new information from fourobservations taken by the Hubble telescope's Wide Field andPlanetary Camera 2 during a 20-month period from 1994 to 1995.Their results appeared in the July issue of the AstronomicalJournal. The scientists selected T Pyxidis because of itscloseness to Earth and its long track record of outbursts. T Pyxidis is 6,000 light-years away in the dim southernconstellation Pyxis, the Mariner's Compass. Within the last 110years, 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 morethan a decade ago. His pre-Hubble spectral studies in 1985 usingground-based telescopes showed that the apparently smooth shellwas expanding at the rate of 780,000 mph (350 kilometers persecond). His recent Hubble observations, however, surprisinglyreveal that the material has slowed down considerably since 1985.In fact, the debris is barely moving at all. Images taken monthsapart show no measurable expansion of the debris. Sharadetermined that the knots must be moving slower than 90,000 mph(40 kilometers per second). This may seem fast, but actually thegaseous debris was racing through space almost 100 times fasterwhen it was first blown off the nova.
Waves of Violence
Ground-based and Hubble telescope observations have allowed Sharato reconstruct a sequence of a T Pyxidis blast. When the novaerupts, it flings waves of gaseous material at progressivelyslower speeds: the first wave of hot gas flies through space at4.5 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 persecond).
About a few weeks after this eruption, the first waves of speedydebris collide with slow-moving fossil material from the previousoutburst, possibly forming the gaseous blobs. Shara observed, forexample, fast-moving gas from the 1966 eruption plowing intoslow-moving material from the 1944 detonation. As the speedy,newly ejected material slams into the older, plodding debris, itheats up, glows brilliantly, and slows almost to a halt. (Thisexplains the tremendous difference in the material's speedbetween the 1985 and the 1994-95 observations.) Eventually, thebright material fades as it cools down. This collision scenariois like cannonballs zipping through a furnace, heating up andglowing, then cooling and fading. Images of a few blobsbrightening and fading over several months were captured by theHubble telescope.
Stellar "Tree Rings"
The blobs are distributed in eight concentric circles around theexploding star, producing a pattern similar to tree rings. Justas tree rings furnish scientists with information about a tree'slife, so the circles of debris around T Pyxidis provideastronomers with a history of this prolific nova.
"We think that we're seeing the collision between pairs oferuptions all the way back to a successive pair generated in theearly 1800's," Shara explains. "But we are seeing only the inner,brightest part of the ejected material; there are probably manymore knots out there that are too faint for even the Hubbletelescope to detect without the nova's future cooperation."
Fortunately, the central star is due for another explosion. Sharais scheduled to take observations with the Hubble telescopewithin a few days of the next eruption so that he can map thefaint, ancient outer debris field, which will be illuminated bythe nova's next bright flash. The debris map will show if therecurrent nova has been regularly active for the past thousandyears or more, or if its eruptions occur in cycles. It also mightoffer clues to explain why some novae produce no visible shellsat all.
Nova explosions are extremely powerful, equal to a blast of 100billion billion tons of dynamite. All this punch comes fromdying, faint, low-mass stars that have exhausted their hydrogenfuel. Called white dwarfs, these stars have puffed away most oftheir mass until only their cores are left.
A nova erupts when a white dwarf has siphoned enough hydrogen offa companion star to trigger a thermonuclear runaway. As hydrogenbuilds up on the surface of a white dwarf, it becomeshotter and denser until it detonates like a colossal hydrogenbomb, leading to a million-fold increase in brightness in oneday. This tremendous flash of light prompted astronomers to callthese objects novae - Latin for "new" - because they abruptlyappeared in the sky. A nova quickly begins to fade in severaldays or weeks as the hydrogen is exhausted and blown into space.
Most novae spend 10,000 to 100,000 years collecting enoughhydrogen from their companions to ignite an explosion. But T Pyxidis detonates several times a century. This nova has such apenchant for outbursts, astronomers believe, because itsunderlying star is about as massive as a white dwarf can get. Amore massive white dwarf would collapse under the crushing forceof gravity and become a neutron star or a black hole. Because ofits high mass, T Pyxidis only needs to drain one part in 10million of its companion's hydrogen (roughly the mass of ourmoon) to start an eruption. (The companion is a red dwarf, asmall, cool, faint star.) This can be done in a mere 20 years orso, leading to the fascinating structure the Hubble telescope hasnow revealed.
Research team members are: Robert Williams, Dave Zurek (Space Telescope Science Institute); Roberto Gilmozzi, (European Southern Observatory); and Dina Prialnik (Tel Aviv University).
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The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
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