The Caldwell Objects

appears to be the edge of a
rapidly expanding shock wave
plowing through the surrounding
ΗII region. Using one's imagination,
these shock waves
appear to be washing onto the
nebula's shores like tidal waves
intercepting a coastal plain.
EGGs (evaporating gaseous
globules), similar to those
found in the Eagle Nebula
(Ml6), have also been detected.
But those in NGC 7635 are
larger, in both angular and
physical terms, than those in
the Eagle. EGGs are solarsystem-sized
globs of matter in
which con-
ditions seem ripe for star
birth and, possibly, planet
formation.
Undoubtedly anyone
who has pointed a widefield
telescope at M52 has
also received weak
photon hits from NGC
7635. But far fewer have
seen the Bubble itself.
Many sources identify the
Bubble with NGC 7635
while failing to clarify a
very important point: the
Bubble is only a
parr of NGC 7635. The Bubble is a dim 3'
ring of vapors surrounding (though not
centered on) SAO 20575, which
punctuates the south-central rim of the
somewhat California-shaped NGC 7635;
its brightest segment is an insignificant
arc about 1’ in length. By contrast, the
fully elongated form of NGC 7635
measures 15' x 8' and extends from the
southeast to the
The Caldwell Objects
11
northwest. The distinction between NGC 7635 and
the Bubble is critical to visual observers, especially
those with modest telescopes. Without this
knowledge the 15' x 8' dimensions of NGC 7635
might easily be mistaken for those of the Bubble,
leading one to search fruitlessly for something five
times larger than it really is. Be warned: a roughly
circular glow
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