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 H-alpha filters.

Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: Bo Reipurth (University of Hawaii)