EMBARGOED UNTIL: 9:20 a.m. (EST) January 13, 2000

PHOTO NO.: STScI-PRC00-04


AN EXPANDING BUBBLE IN SPACE

Astronomers, using the Wide Field Planetary Camera 2 on board NASA's 
Hubble Space Telescope in October and November 1997 and April 1999, 
imaged the Bubble Nebula (NGC 7635) with unprecedented clarity. 
For the first time, they are able to understand the geometry and 
dynamics of this very complicated system. Earlier pictures taken of 
the nebula with the Wide Field Planetary Camera 1 left many issues
unanswered, as the data could not be fully calibrated for scientific 
use. In addition, those data never imaged the enigmatic inner 
structure presented here.

The remarkably spherical "Bubble" marks the boundary between an 
intense wind of particles from the star and the more quiescent 
interior of the nebula. The central star of the nebula is 40 
times more massive than the Sun and is responsible for a stellar 
wind moving at 2,000 kilometers per second (4 million miles per hour
or 7 million kilometers per hour) which propels particles off the 
surface of the star. The bubble surface actually marks the leading 
edge of this wind's gust front, which is slowing as it plows into 
the denser surrounding material. The surface of the bubble is not 
uniform because as the shell expands outward it encounters regions 
of the cold gas, which are of different density and therefore arrest 
the expansion by differing amounts, resulting in the rippled 
appearance.  It is this gradient of background material that the 
wind is encountering that places the central star off center in the 
bubble.  There is more material to the northeast of the nebula than 
to the southwest, so that the wind progresses less in that direction, 
offsetting the central star from the geometric center of the bubble. 
At a distance of 7,100 light-years from Earth, the Bubble Nebula 
is located in the constellation Cassiopeia and has a diameter of 
6 light-years.

To the right of the central star is a ridge of much denser gas. The 
lower left portion of this ridge is closest to the star and so is 
brightest. It is experiencing the most intense ultraviolet radiation 
as well as the strong wind and is therefore being photoevaporated 
the fastest.  The ridge forms a V-shape in the image, with two 
segments that are aligned at the brightest edge.  The upper of these 
two segments is viewed quite obliquely as it trails off into the back 
of the nebula. The lower segment comes both toward the observer and 
off to the side. This lower ridge appears to lie within the sphere 
described by the bubble but is not actually "inside" the shocked 
region of gas. Instead it is being pushed up against the bubble like 
a hand being pushed against the outside of a party balloon.  While 
the edge of the hand appears to be inside the balloon, it is not. 
As the bubble moves up but not through the ridge, bright blue arcs 
form where the supersonic wind strikes the ridge to form an apparent 
series of nested shock fronts.

The region between the star and ridge reveals several loops and arcs 
which have never been seen before.  The high resolution capabilities 
of Hubble make it possible to examine these features in detail in a 
way that is not possible from the ground. The origin of this 
bubble-within-a-bubble" is unknown at this time.  It may be due to a 
collision of two distinct winds. The stellar wind may be colliding 
with material streaming off the ridge as it is photoevaporated by the 
star's radiation.

Located at the top of the picture are dense clumps or fingers of 
molecular gas which have not yet encountered the expanding shell. 
These structures are similar in form to the columns in the Eagle 
Nebula, except that they are not being eroded as energetically as 
they are in that nebula.  As in the Eagle, the clumps are seen to 
emit light because they are being illuminated by the strong 
ultraviolet radiation from the central star, which travels much 
faster than the shell and has reached the outer knots long before 
the expanding rim will.

Credits: NASA, Donald Walter (South Carolina State University), 
Paul Scowen and Brian Moore (Arizona State University)

Research Team: Donald Walter (South Carolina State University), 
Paul Scowen, Jeff Hester, Brian Moore (Arizona State University), 
Reggie Dufour, Patrick Hartigan and Brent Buckalew (Rice University).

Funding: Space Telescope Science Institute, NASA MUSPIN and NASA URC.


NOTE TO EDITORS - For additional information, please contact:
Donald Walter, Dept. of Physical Sciences South Carolina State Univ., 
P.O. Box 7296 300 College St., Orangeburg, SC 29117, (phone)
803-533-3773, (fax) 803-536-8500, (e-mail) dkw@physics.scsu.edu.