(checked and revised version)

FOR RELEASE:  10:00 a.m. PDT, June 10, 1998


HUBBLE SPACE TELESCOPE SEES EXPANDING NOVA SHELLS


Astronomers are announcing today that they have found expanding gas clouds
thrown off by nuclear eruptions in stars. The report is being presented by
Drs. Fred Ringwald, Jerome A. Orosz, Richard A. Wade, and Robin Ciardullo
of Penn State University, in University Park, PA, to the American
Astronomical Society meeting in San Diego, CA. The result is of interest
because it provides reliable distances to these stars. 

These Hubble Space Telescope images, taken with the refurbished
Wide-Field/Planetary Camera (WFPC2), are of two novae, stars that had
eruptions in 1984 (left) and 1991 (right).  The stars' names are QU
Vulpeculae, in the constellation Vulpecula (the Little Fox), 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, called nova shells. 

Both images were taken in the red light of hydrogen alpha, where nova
shells are known to glow brightly.  They are among the most detailed
pictures ever taken of nova shells so soon after an eruption, thanks to
Hubble's high resolution.  Until now, nova shells have been hard to study,
since they are faint and very small.  These two shells do appear to be
tiny, barely one second of arc in apparent diameter, the angle covered by
a dime over two miles away. 

This is only because the nova shells are far away, though. Since it is
known how fast the shells are expanding, and when the novae erupted, nova
shells are useful milestones of distances in space, which are ordinarily
difficult to measure.  Ringwald's team find that QU Vul is 13,900
light-years away, and that V351 Pup is 18,400 light-years away. 

By learning about their distances, astronomers can use this information to
estimate distances to novae that are much further away, even in other
galaxies.  With enough galaxies, this may show the size of the entire
Universe. 

The shell of QU Vul is expanding at over 700 miles per second, and the
shell of V351 Pup is expanding at 1250 miles per second.  This is known
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, IN, and their
collaborators.  The mass of the gas in nova shells is thought to be on the
order of 30 times the mass of Earth. 

Both shells' true diameters are about half a trillion miles. 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. 

Structure is apparent in the nova shells, including the two bright spots
in the outer ring that may be ``polar blobs'' in the shell of QU Vul,
which is known to be edge-on.  Just visible also may be edge-on
``equatorial'' and ``temperate'' rings. There is a definite asymmetry in
the shell of V351 Pup also, perhaps also from polar blobs, approximately
perpendicular to those shown in the figure of QU Vul.  Structure in nova
shells is thought to be formed in, or shortly after, the eruptions
themselves, although exactly how is unknown. 

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
radio telescope 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 Dr.
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 therefore provide unique laboratories for gas dynamics in
space, since they change over just a few years, not millenia as with other
astronomical gas clouds.  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 NASA. 

PHOTO CREDIT:  Penn State University and NASA. 


EDITORS: This greyscale gif-format image can be obtained over the Internet
via http://www.astro.psu.edu/users/ringwald/novashells.gif