Barnard 68Edit
Barnard 68 (B68) is one of the closest and most studied dark nebulae in the night sky. Classified as a Bok globule—a compact cloud of gas and dust that blocks background starlight—it presents a near-perfect silhouette against the dense stellar backdrop of the Milky Way. The cloud lies in the constellation Ophiuchus and is a prominent member of Barnard's catalog of dark nebulae, a compilation long used as a reference for the structure and evolution of small, star-forming clouds. With a round, almost textbook-like appearance, B68 has become a natural laboratory for exploring how stars begin to form in environments that have not yet produced an infant star.
B68 is relatively nearby on a galactic scale. Estimates place it at roughly 140 parsecs from the Sun, making it accessible to a range of observational techniques that probe its interior structure. Its small size, combined with a high central density, yields a contrast that makes it ideal for testing theories of prestellar evolution. The cloud’s outer layers are diffuse compared with its dense core, and the overall mass is a few solar masses. The evident darkness is caused by dust grains that absorb and scatter visible light, while the interior remains shielded and cold, typically at temperatures around 10 kelvin. These characteristics are why B68 is frequently described as a nearly quiescent, pre-stellar core that has not yet formed a protostar.
Overview
Location and distance
B68 sits in the sky near the Ophiuchus star-forming region, a complex where several dense clouds and young stellar objects are found. Its distance places it well within the local arm of the Milky Way, which has allowed astronomers to apply high-resolution techniques to map its internal structure. The proximity also makes B68 a convenient reference object when calibrating methods that measure extinction and dense-gas properties in more distant clouds. For background context, see Ophiuchus and interstellar extinction.
Structure and composition
The cloud is remarkably round, a shape that has driven interest in its internal physics. Extinction maps show a steep gradient from a low-density envelope to a dense interior, consistent with a core that is nearly in hydrostatic balance. The material is predominantly molecular gas entwined with dust grains, the latter responsible for the cloud’s dark appearance by absorbing visible light. Observations across the spectrum—from near-infrared to submillimeter wavelengths—reveal a cold interior with densities reaching well into the 10^5 cm^-3 regime in the core region. Submillimeter continuum emission traces the densest zones and complements extinction-based measurements, providing a multi-faceted view of the mass distribution. See dust and interstellar extinction for related concepts, and SCUBA and Herschel Space Observatory for instruments that have probed B68’s dust emission.
Observational history
B68’s standout profile has made it a focal point for contemporary star-formation studies. In the early 2000s, a landmark analysis proposed that B68 resembles a near-critical Bonnor–Ebert sphere, a theoretical model describing a self-gravitating, isothermal gas sphere confined by external pressure. This interpretation offered a concrete link between observations and a classical stability analysis, illustrating how a cloud can hover on the brink between stability and collapse. The idea gained traction because the observed density profile and lack of an internal luminous object fit a BE-sphere framework quite well. See Bonnor–Ebert sphere for the theoretical model and Alves et al. for the observational application to B68.
The Bonnor–Ebert sphere interpretation and debates
The notion that B68 is near the critical point of a Bonnor–Ebert configuration has been influential, but it is not without controversy. Proponents argue that the cloud’s density structure and lack of embedded sources imply a precise balance between self-gravity and external pressure, with magnetic fields and a modest level of turbulence playing subsidiary roles. Critics, however, point out uncertainties in the exact external pressure exerted by the surrounding cloud and the possible influence of magnetic support, ambipolar diffusion, or subtle dynamical motions. In this debate, measurements of the magnetic field via polarization and line-of-sight techniques, as well as kinematic studies of molecular tracers, have been invoked to argue for different degrees of support against collapse. The dialogue around B68 thus highlights broader questions in star formation theory: how prestellar cores achieve and maintain equilibrium, and under what conditions a quiescent core might tip into collapse. See Interstellar magnetic field and Prestellar core for related concepts.
Implications for star formation
Because B68 appears devoid of an internal protostar, it serves as a benchmark for understanding the earliest stages of star formation. Its apparent simplicity—spherical symmetry, cold interior, and absence of a central heat source—helps constrain models of core evolution, the role of external pressure, and the timescales involved in the transition from a quiescent cloud to a star-forming core. The object thus informs discussions about how common such near-equilibrium states are in the Galaxy and what triggers might precipitate collapse in otherwise quiet environments. See Star formation and Prestellar core for broader context.
Related observations and modern perspectives
Advances in instrumentation have allowed researchers to map B68 with greater fidelity. Infrared extinction mapping, millimeter and submillimeter continuum surveys, and high-resolution spectroscopic observations have all contributed to a more nuanced picture of B68’s internal structure. Scientists continue to compare the observed density and temperature profiles with evolving theoretical models, including refinements to the BE-sphere framework and the incorporation of magnetic-field effects. The ongoing study of B68 sits at the intersection of observational refinement and theoretical development in the science of star formation. See ALMA for a facility often used to probe similar cores, and Herschel Space Observatory for historical context on far-infrared to submillimeter data.