Free Astronomy Magazine July-August 2024

51 JULY-AUGUST 2024 ASTRO PUBLISHING Cometary globules are a subclass of the dark nebulae known as Bok globules — isolated clouds of dense cosmic gas and dust surrounded by hot, ionized material. When these clouds exhibit stripping of material that results in an extended tail, they are referred to as cometary globules because of their vague resemblance to a comet, though they have noth- ing in common. The features that classify CG 4 as a cometary globule are hard to miss in this image cap- tured with the Department of En- ergy-fabricated Dark Energy Camera (DECam) mounted on the U.S. Na- tional Science Foundation Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF NOIRLab. Its dusty head, which has a diameter of 1.5 light-years, and its long, faint tail, which is about eight light-years long, make CG 4 a comparatively small Bok globule, a general charac- teristic of cometary globules. First recognized in 1976 from pic- tures taken with the UK Schmidt Telescope in Australia, cometary globules went undetected by as- tronomers for a long time because they are so faint. Their tails, shrouded in dark stellar dust, block most light from passing through. But with its special Hydrogen-alpha filter, DE- Cam can pick up the faint red glow of ionized hydrogen present within CG 4’s head and around its outer rim. This light is produced when hy- drogen becomes excited after be- ing bombarded by radiation from nearby hot, massive stars. The intense radiation generated by these neighboring massive stars, however, is gradually destroying the head of the globule and sweeping away the tiny particles that scatter the starlight. Still, the dusty cloud of CG 4 con- tains enough gas to feed the active formation of several new, Sun-sized stars. While astronomers have ob-