Bok globule

Bok globule. A Bok globule is a small, isolated, and very dense cloud of interstellar dust and molecular gas that appears as a dark silhouette against a brighter background. These compact, opaque objects are among the coldest known regions in the Milky Way and are significant as potential sites for the formation of low-mass stars. Named for astronomer Bart Bok, who first brought widespread attention to their potential importance, they are typically observed within larger H II regions or bright emission nebulae.

Discovery and naming

The distinct, small, and ink-black patches against luminous nebulae were noted by early observers like E. E. Barnard, who cataloged many dark nebulae. However, it was the work of astronomer Bart Bok and his wife, Priscilla Fairfield Bok, in the 1940s that systematically highlighted these objects as potentially crucial to understanding stellar origins. In a seminal 1947 paper published in the Astrophysical Journal, Bok postulated that these dense, dark clouds might be "cocoons" for nascent stars, an idea initially met with skepticism. The term "Bok globule" was later coined by the astronomical community in his honor. Subsequent infrared and radio observations, particularly following the launch of the Infrared Astronomical Satellite and studies with instruments like the James Clerk Maxwell Telescope, confirmed Bok's hypothesis by revealing embedded protostars.

Physical characteristics

Bok globules are characterized by their high density and low temperature, typically containing masses ranging from a few to several hundred times that of the Sun within a region often less than one light-year in diameter. Their densities can exceed 10,000 molecules per cubic centimeter, and temperatures hover around 10 Kelvin, just above absolute zero. This extreme cold and density make them opaque to visible light due to dust grains that efficiently absorb and scatter background starlight, as seen in iconic images from the Hubble Space Telescope. Internally, they are primarily composed of molecular hydrogen, with traces of carbon monoxide, water, and silicate dust, which can be studied via their submillimeter and radio wave emissions.

Formation and evolution

Bok globules are thought to form through the fragmentation and gravitational collapse of larger molecular clouds, often triggered by external pressures from nearby supernova explosions or the intense radiation and stellar winds from massive O-type stars in adjacent H II regions. As a globule condenses under its own gravity, it may develop a dense core, a process that can be influenced by internal turbulence and weak magnetic fields. Over time, if the core surpasses a critical density threshold described by the Jeans instability, it begins to collapse irreversibly. This evolution from a quiescent cloud to a protostellar core is a key phase in the star formation sequence, often preceding the formation of a T Tauri star.

Role in star formation

Bok globules are considered one of the simplest and most isolated environments in which low-mass star formation occurs, providing a relatively "clean" laboratory compared to chaotic giant molecular clouds like the Orion Nebula. Their isolation minimizes disruptive external influences, allowing astronomers to study the initial conditions of collapse in detail. Observations from facilities like the Atacama Large Millimeter Array have revealed that many globules contain compact infrared sources or bipolar Herbig–Haro objects, direct evidence of ongoing accretion and outflow from embedded protostars. The study of these objects has been instrumental in testing theories of accretion disk formation and the role of angular momentum in early stellar systems.

Notable examples

Among the most famous and well-studied Bok globules is Barnard 68, located in the constellation Ophiuchus, which appears as a nearly symmetrical dark patch and has been extensively analyzed by the European Southern Observatory. Another prominent example is the globule within the Carina Nebula, a region of intense massive star formation studied by the Chandra X-ray Observatory. The Coalsack Nebula, a vast dark cloud near the Southern Cross, contains numerous smaller globules. In the NGC 281 region, also known as the Pacman Nebula, several Bok globules are being eroded by radiation from a nearby star cluster. These objects, along with others cataloged in the Lynds Dark Nebula catalogue, continue to be primary targets for observatories like the Spitzer Space Telescope to understand the earliest stages of stellar life.

Category:Interstellar clouds Category:Star formation Category:Nebulae