The result is of interest because it provides reliable distances to these stars.
Pictures of the expanding gas clouds, which were taken with the Hubble Space Telescope, are available for viewing on the World Wide Web at http://www.astro.psu.edu/users/ringwald.
A grayscale PostScript image is available for downloading at http://www.astro.psu.edu/users/ringwald/novashells.ps.
Thanks to Hubble's high resolution, they are among the most detailed pictures ever taken of nova shells -- expanding clouds of gas thrown off by nuclear eruptions in novae stars.
These images, taken with Hubble's refurbished Wide-Field/Planetary Camera (WFPC2), are of two novae stars that had nuclear eruptions in 1984 and 1991. The stars' names are QU Vulpeculae, in the constellation Vulpecula (the Fox with a Duck), and V351 Puppis, in the constellation Puppis (the Afterdeck of Argo, the Ship). The rings around the stars are expanding clouds of gas thrown off by the eruptions. The stars in which the eruptions occurred are still visible in the centers of both expanding clouds.
Both images were taken in the red light of hydrogen alpha, where nova shells are known to glow brightly. Until now, nova shells have been hard to study because they are faint and appear to be very small because they are far from Earth. These two nova shells, for example, are so far away that they appear to be tiny -- barely one second of arc in apparent diameter, which is the angle covered by a dime when viewed from over two miles away.
Since it is known how fast the shells are expanding and when the novae erupted, nova shells are useful milestones for calculating distances in space, which are ordinarily difficult to measure. Ringwald's team found that QU Vul is 18,300 light-years away, and V351 Pup is 14,800 light-years away.
"We know that the shell of QU Vul is expanding at over 2,000 miles per second and the shell of V351 Pup is expanding at 3,200 miles per second from analyses of the light, previously published in the scientific literature, by Dr. Massimo Della Valle at the University of Padova, Italy, Dr. George Sonneborn at NASA/Goddard Space Flight Center in Greenbelt, Md., and Dr. Steve Shore at Indiana University at South Bend, Ind., and their collaborators," Ringwald explains. "The mass of the gas in nova shells is thought to be on the order of 30 times the mass of Earth and the true diameter of both of these shells is about half a trillion miles," he says.
During the nova eruptions, both QU Vul and V351 Pup were just visible to the unaided eye, at fifth and sixth magnitudes, respectively. They have since faded greatly: V351 Pup is about one million times fainter than the eye can see, and QU Vul is only about six times brighter than that.
"Interestingly, structure is apparent in the nova shells, including what may be polar blobs in the shell of QU Vul, which is known to be edge-on," Ringwald says. Just visible also are also "equatorial" and "temperate" rings. "There is also a definite asymmetry in the shell of V351 Pup, perhaps also with polar blobs, approximately perpendicular to those shown in the figure of QU Vul. This structure is thought to be formed in, or shortly after, the eruptions themselves, although exactly how is a still-unsolved problem," Ringwald comments.
The researchers say the Hubble image of QU Vul supports the idea that this structure is formed early, since its polar blobs match the orientation of those seen with a radiotelescope shortly after the eruption, when the gas was much hotter than it is now. A radio map of the shell of QU Vul was published by A. R. Taylor and collaborators in the journal Nature in 1988; the observations were carried out with the Very Large Array radio telescope in New Mexico.
"Nova shells provide unique laboratories for gas dynamics in space, since they change over just a few years, not millennia as with other astronomical gas clouds," Ringwald says. "By studying nova shells, we can see the wonders of the universe unfold before our very eyes."
This work was supported by Space Telescope Science Institute, which is operated by AURA, Inc., under contract with the National Aeronautics and Space Administration (NASA).
PHOTO CREDIT: Penn State and NASA.