The Caldwell Objects

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Bubble Nebula
NGC 7635
Type: Emission Nebula
Con: Cassiopeia
RA: 23 h 20.7 m
Dec: +61° 12'
Mag: 10.0 (O'Meara)
Dim: 15' x 8' (nebula); 3' x 3' (bubble)
Dist: 7,100 light-years
Disc: William Herschel, 1787
W. HERSCHEL: [Observed 3 November 1787] A star of 9th magnitude
with very faint nebulosity of small extent about it. (HIV-52)
GC: Very faint, a star of 9th magnitude involved a little excentric.
NGC: Very faint, a star of 8th magnitude involved a little excentric.
S AILING CLOSE TO THE C ASSIOPEIA-
Cepheus border, about 6° west-northwest of Beta
(β) Cassiopeiae (Caph), is a whimsical pairing of
deep-sky objects: the illustrious 7th-magnitude
open cluster M52 and, about ½° to its southwest,
the dauntingly dim emission nebula NGC 7635
(Caldwell 11). Once included in the Catalogue of
Galactic Planetary Nebulae by Lubos Perek and
Lubos Kohoutek, the Bubble Nebula has since
been reclassified as an emission nebula. It found
its way into the first edition of Sky Atlas 2000.0 as
a planetary nebula, but more modern atlases,
such as the Millennium Star Atlas, Uranometria
2000.0, and the second edition of Sky Atlas 2000.0,
portray it correcdy as a diffuse emission nebula.
Detailed photographs of the Bubble help us
understand the confusion surrounding this
object. Red-light-sensitive plates taken with the
Palomar Mountain 200-inch telescope
52
show a magnificent loop of gas centered on, and
seemingly blowing away from, a nondescript
central star. But the Bubble is not a planetary
nebula; it is a hollow cavity in the ΗII region
known as NGC 7635, and it was formed by
radiation pressure streaming out from a hot blue
central star (9th-magnitude SAO 20575, with a
spectral type of O6.5). Hubble Space Telescope
images of the Bubble suggest a celestial ballet of
windswept veils dancing playfully around that
9th-magnitude sun.
A closer look at the images reveals dense
knots embedded within the nebula's central
cavity. The surfaces of these knots are being
boiled away by the region's flood of intense
ultraviolet radiation. They also have been
shocked by the central star's 2,000-km-per-second
wind. By contrast, high-density knots outside the
sharp northern rim of the Bubble show no shockwind
effects. The rim thus
Deep-Sky Companions: The Caldwell Objects