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 nearby 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 still known about their origin and nature, except that they
are generally associated with large hydrogen-emitting star-formation
regions, called “HII regions” due to their glowing light of hydrogen
gas.
The largest of the globules in this image is actually two separate
clouds that gently 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 equal over 15 solar
masses. 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 much 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 heavily
fractured, as if major forces were tearing them apart. When radio
astronomers observed the faint hiss of molecules within the globules,
they realized that the globules are actually in constant, churning
motion, moving supersonically among each other. This may be caused
by the powerful ultraviolet radiation from the luminous, massive
stars, which also heat up the gas in the HII region, causing it to
expand and stream 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 H-alpha filters.
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Thackeray’s Globules in IC 2944: Strangely glowing dark clouds float serenely in this remarkable and beautiful image taken with the Hubble Space Telescope. These dense, opaque dust clouds — known as “globules” — are silhouetted against nearby bright stars in the busy star-forming region, IC 2944. View this image: – medium – movie Date Published: Thursday, January 03, 2002 |
