The Caldwell Objects

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If NGC 5248's estimated distance of 74 million
light-years is accepted, then the galaxy is indeed
a physically large system with a diameter of
123,000 light-years and a total mass of 140 billion
Suns. The spiral is receding from the solar system
at a speed of 1,156 km per second. We see the
galaxy inclined 45° from face on. Its extremely
bright nucleus lies in a spherical lens laced with
dust. Its two bright, dust-laden inner spiral arms
look like water fleeing from a reciprocating lawn
sprinkler, and they outline the central lens. In
photographs taken with large telescopes, five
very faint arms can be seen branching, like lacy
filaments, from the central arms that amateurs
can see. Two of the outer arms on the western
side appear stubby, while two arms at the
northwestern end wrap halfway around the
galaxy; the remaining outer arm begins at the
southeastern end of the galaxy's bright portion
and spirals south through west before branching
at its northwestern end. The brightest features in
these otherwise very faint arms are composed of
relatively young stars.
In 1998 Johan H. Knapen (University of
Hertfordshire, England) and his colleagues
imaged NGC 5248 with an adaptive-optics
system and a near-infrared camera on the
Canada-France-Hawaii Telescope, which sits on
top of Hawaii's Mauna Kea volcano. The
unprecedented, highly detailed images revealed
a tightly wound and dusty spiral structure near
the galaxy's quiescent nucleus. It ends near the
inner edge of a starburst ring, which surrounds
the nucleus at a radius of about 3". Near-infrared
observations made two years earlier by Debra M.
Elmegreen (Vassar College) and her colleagues
already had revealed hot spots more or less
evenly spaced around this cir-cumnuclear ring.
Both groups' near-infrared images show details
similar to those seen in Hubble Space Telescope
ultraviolet images of
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the galaxy's nuclear region; the HST images show
hot spots on small spiral arcs that collectively
make up the starburst ring. The spacing of the
hot spots implies that large-scale gravitational
collapse is occurring along the periphery of the
ring, where large populations of "super" star
clusters have formed.
NGC 5248 is a bit of a challenge to locate,
though it is not difficult to see. The galaxy is
tucked into the extreme southwestern corner of
Boötes, about 7° southwest of 4th-magni-tude
Upsilon (υ) Boötis in the Herdsman's
western knee. The spiral can also be found about
8½° east-southeast of 3rd-magnitude Epsilon (ε)
Virginis, orVindemiatrix. (In Roman days
Vindemiatrix was Vindemiator; both names
mean "grape gatherer.") To star-hop to the galaxy
use your widest field of view and start from
Vindemiatrix. First move 2° southeast to a
solitary 6th-magnitude star. Another 2° hop in
roughly the same direction will bring you to 5thmagnitude
59 Virginis. Place 59 Virginis in the
northernmost edge of your widest field of view,
then sweep 5° farther to the east. NGC 5248
should be near the center
Deep-Sky Companions: The Caldwell Objects