NGC7635

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.