Object description:
Large, bright-edged nebulae and their more evolved and
smaller relatives, the comet-like globes, are fascinating dynamical structures
formed by the interplay of cold, dark molecular clouds and hot, blue ionizing
stars. In the northern hemisphere, many of these dark clouds were cataloged in
the 1920s by E. E. Barnard and later by Beverly Lynds. In the southern sky,
such catalogs did not become available until the British Schmidt telescope
became operational in the 1970s. The first author of a southern catalog of 1101
dark clouds (1986) was Malcolm Hartley, known for his comet observations. Our
image shows DCld 343.0+02.8 from his
catalog, which lies in the star-forming region Gum 55 and has the popular name
"dark tower".
Typically, the head
of the globule faces one or more hot O- and B-type stars. The intense radiation
from the stars heats the cold surface of the molecular cloud and releases the
hydrogen gas, which condenses on the dust grains. This is immediately ionized
by the intense ultraviolet light from the stars, forming a red HII emission
nebula. The dust is also blown away by the violent radiation pressure (stellar
wind) from the stars, forming a comet-like shape of the molecular cloud. The
ionizing stars involved in the formation of the dark tower are members of the
open cluster NGC 6231, the center of the
Scorpius-OB association, which here lies outside the field of view. In addition
to the emission regions, five small reflection nebulae are visible within the
globule. |