September 18, 1997
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.
"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."
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.
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.
"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.
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).
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).
Photos, captions and press release are available via the World Wide Web.
The image on the left, taken by a ground-based telescope, shows shells of gas around the star that were blown off during several eruptions.
Closer inspection by the Hubble Space Telescope (right-hand image), however, reveals that the shells are not smooth at all. In fact, this high-resolution image shows that the shells are actually more than 2,000 gaseous blobs packed into an area that is 1 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-moving and slow- moving gas from several eruptions. False color has been applied to this image to enhance details in the blobs.
The ground-based image was taken Jan. 19, 1995 by the European Southern Observatory's New Technology Telescope in La Silla, Chile. The Hubble telescope picture is a compilation of data taken on Feb. 26, 1994, and June 16, Oct. 7, and Nov. 10, 1995, by the Wide Field and Planetary Camera 2.
T Pyxidis is 6,000 light-years away in the dim southern constellation Pyxis, the Mariner's Compass.
Credits: Mike Shara, Bob Williams, and David Zurek (Space Telescope Science Institute); Roberto Gilmozzi (European Southern Observatory); Dina Prialnik (Tel Aviv University); and NASA.