The Caldwell Objects

20
RA: 20 h 58.8 m
Dec: +44° 20'
Mag: 5.0; 3.8 (O'Meara)
Dim: 100' x 60'
Dist: ~1,800 light-years
Disc: William Herschel, 1786
W HEN SEEN FOR THE FIRST TIME THROUGH A
telescope, few deep-sky objects in the heavens
live up to the poetic names pinned on them by
their discoverers or by later observers. For their
shapes to be seen with the mind's eye, many
nebulae — like the Lagoon (M8), the Crab (M1),
and the Owl (M97) — require either an
impressive photograph or a lot of imagination at
the eyepiece. But not the North America Nebula
in Cygnus. Here is one fuzzy glow that lives up
to its name even upon a casual glance through
binoculars or a rich-field telescope.
Max Wolf first photographed NGC 7000 in
1890 and called it the North America Nebula for
its uncanny resemblance to that continent. Look
at the photograph on page 83 and you will see
why. Among other "geographical" features, the
nebula has a sharply defined eastern seaboard, a
projecting Florida Peninsula, a dark Gulf of
Mexico rimmed by Texas and Central America,
and a West Coast running from Mexico to
Canada. In his book 365 Starry Nights Chet
Raymo writes, "As I look at a photograph of the
North America Nebula, I am reminded of the
explorers who, on horseback and in birchbark
canoe, mapped North America. Our own small
planet has now been thoroughly explored. We
have reached the heart of every dark continent.
We now turn our curiosity to new 'dark
continents' in space and the telescope will be our
'birchbark' canoe."
Your own adventure can start with the 1stmagnitude
star Alpha (α) Cygni (Deneb), one
84
W. H ERSCHEL: [Observed 24 October 1786] Very large
diffused nebulosity, brighter in the middle. 7' or 8'
long. 6' broad and losing itself very gradually and
imperceptibly. (H V-37)
GC/NGC: Faint, most extremely large, diffused
nebulosity.
of the most luminous blue supergiants in the
night sky. Deneb marks the tail of Cygnus, the
Swan, and the top of the Northern Cross. It is also
the northernmost gem in the Summer Triangle,
whose trio of stars hangs high overhead on latesummer
evenings. The North America Nebula
resides only 3° east of Deneb and 1° west of 4thmagnitude
Xi (ξ) Cygni. Immediately west of the
North America Nebula is the famous (though
harder-to-see) Pelican Nebula (IC 5067 and IC
5070), named for its uncanny photographic
resemblance to that fish-eating bird. Together
these remarkable nebulae constitute the visible
portions of a single large ΗII region; a dark dust
lane running from northwest to southeast
separates the landmass from the bird.
Observations of this dust lane by Vytantas
Straizys (Institute of Theoretical Physics and
Astronomy, Lithuania) and colleagues suggest a
distance of about 1,800 light-years for the entire
complex. For a while astronomers believed the
nebula was illuminated by Deneb, but their focus
soon shifted to the hot (spectral type 06) 6th-magnitude
star HD 199579 (20 h 56 m 34.8 s +44° 55' 29")
in the northern quadrant of the Η II region.
Apparently that star emits enough energy to play
a major role in the excitation of the surrounding
nebula.
Some doubt has been shed on HD 199579's
association with the nebula complex, for the
star's distance was made out to be 3,300 lightyears
in 1965. But that distance is probably in
error because it would make the Η II
Deep-Sky Companions: The Caldwell Objects