THACKERAY'S GLOBULES IN IC 2944
Strangely glowing dark clouds float
serenely in this remarkable and beautiful image
taken with NASA's Hubble Space Telescope. These
dense, opaque dust clouds - known as "globules"
- are silhouetted against the red glow of
hydrogen gas and bright stars in the busy star forming
region, IC 2944. These globules were first found
in IC 2944 by astronomer A.D. Thackeray in 1950.
Although globules like these have
been known since Dutch-American astronomer Bart
Bok first drew attention to such objects in 1947,
little is known about their origin and nature, except
that they are generally associated with large areas
of star-formation, called "HII regions"
due to the presence of hydrogen gas.
The largest of the globules in this
image is actually two separate clouds that overlap
along our line of sight. Each cloud is nearly 1.4
light-years (50 arcseconds) along its longest dimension,
and collectively, they contain enough material to
make more than 15 stars like our Sun. IC 2944, the
surrounding HII region, is filled with gas and dust
that is illuminated and heated by a loose cluster
of O-type stars. These stars are much hotter and
more massive than our Sun. IC 2944 is relatively
close by, located only 5900 light-years (1800 parsecs)
away in the constellation Centaurus.
Thanks to the remarkable resolution
offered by the Hubble Space Telescope, astronomers
can, for the first time, study the intricate structure
of these globules. The globules appear to be fragmented,
as if in the process of being torn apart. When radio
astronomers observed the faint hiss from molecules
within the globules, they realized that the globules
are actually in constant, churning motion, moving
supersonically among each other. This chaotic motion
may be caused by the powerful ultraviolet radiation
from the luminous, massive stars. These stars also
heat the glowing hydrogen gas, causing it to expand
against the globules, leading to their destruction.
Despite their serene appearance, the globules may
actually be likened to clumps of butter put onto
a red-hot pan.
It is likely that the globules are
dense clumps of gas and dust that existed before
the massive O-stars were born. But once these luminous
stars began to irradiate and destroy their surroundings,
the clumps became visible when their less dense
surroundings were eroded away, thus exposing them
to the full brunt of the ultraviolet radiation and
the expanding HII region. The new images catch a
glimpse of the process of destruction. Had the appearance
of the luminous O-stars been a bit delayed, it is
likely that the clumps would actually have collapsed
to form several more low-mass stars like the Sun.
Instead they are now being toasted and torn apart.
The hydrogen-emission image that
clearly shows the outline of the dark globules was
taken in February 1999 with Hubble's Wide Field
Planetary Camera 2 (WFPC2) by Bo Reipurth (University
of Hawaii) and collaborators. Additional broadband
images that helped to establish the true color of
the stars in the field were taken by the Hubble
Heritage Team in February 2001. The composite result
is a four-color image of the red, green, blue and
Credit: NASA and The Hubble
Heritage Team (STScI/AURA)
Acknowledgment: Bo Reipurth (University of Hawaii)