The Caldwell Objects

NGC 253 is the brightest member of the Sculptor
Group of galaxies, the closest such system to our
Local Group. NGC 253 travels through space
with NGC 247 (Caldwell 62), NGC 300 (Caldwell
70), and NGC 55 (Caldwell 72) and has a recessional
velocity of 241 km per second. Handbooks
often compare NGC 253 to M31 in Andromeda,
but the two really do not have that much in
common. NGC 253 measures 54,000 light-years in
diameter and has a total mass of about 75 billion
Suns, making it about 2½ times smaller and 4
times less massive than M31. As of March 2000,
14 globular clusters had been identified around
NGC 253.
Most galaxies form stars slowly but not NGC
253. Like M82 in Ursa Major, NGC 253 is a
starburst galaxy, meaning its nuclear region has
recently hosted massive and rapid bursts of star
birth. Each furious episode generates a
"superwind" that sends vast amounts of energy
and mass streaming into the intergalactic
medium, which it heats up. In a 1994 Astrophysical
Journal paper, a team led by Yoshiaki
Sofue (University of Tokyo) used Anglo-
Australian Observatory photographs by David
Malin to find 1,000-light-year-long dust features
(called "arcs") that appear to be associated with
two or more even larger dark "loops." One
spectacular bubble may have been formed when
a supernova exploded in NGC 253's halo. The
entire galaxy's nuclear region appears to be
boiling with invisible turbulence.
Midinfrared studies of NGC 253 have found
its nuclear region to have three major
components: an unusually luminous active
nucleus; a "supercluster" of stars; and a large,
diffuse envelope containing an older population
of supernova remnants and lower-mass stars.
The Very Large Array has revealed a family of
compact sources of radio waves inside a
starburst ring; they are most likely supernova
remnants. The VLA also found an ΗII region
The Caldwell Objects
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that resembles the inner part of 30 Doradus in
the Large Magellanic Cloud (see Caldwell 103).
This enormous nebula contains approximately
105 million solar masses in the form of stars as
well as about 600 million solar masses of ionized
gas.
The Hubble Space Telescope imaged NGC
253 in remarkable detail. Hubble's close-up of the
galaxy's innermost core shows streaming bands
of dark clouds that nearly obliterate the view of a
multitude of stellar furnaces below. The entire
scene looks gloomy. What Hubble actually
imaged is a dusty region of furious star
formation; the dim lights peeking through the
dust are members of an extremely compact star
cluster. Hubble also revealed numerous young
star clusters tracing out a 163-light-year-wide
ring around the galaxy's active nucleus. Hubble
confirmed that starbursts can spawn dense
clusters of stars. NGC 253's most recent starburst
probably has been going on for 20 to 30 million
years.
To zoom in on this extragalactic drama, look
7½ ° south-southeast of 2nd-magnitude Beta (β)
Ceti (Deneb Kaitos) or 4¾° northwest of 4thmagnitude
Alpha (α) Sculptoris. That's where
you'll find NGC 253 — just 2° northwest of the
South Galactic Pole. From more southerly
locations the galaxy is a true deep-sky wonder
and can be seen with the naked eye on very clear
nights. Doing so requires a few minutes of
dedication and averted vision. (From Hawaii
NGC 253 can be seen this way on the best of
nights when on the meridian.) The galaxy is well
placed among the stars for naked-eye viewing
because it sits between the southern tips of two
pronounced asterisms of 5th- to 6th-magnitude
stars "beneath" the tail of Cetus. The galaxy is the
only object in the gap between the tips, down to
at least 9th magnitude, so its light cannot be
confused with that of any naked-eye star; you
either see the
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