Brown Dwarfs Home

Brown dwarfs are best thought of as failed stars, straddling the realms of low-mass stars (typically a tenth of a solar mass) and planets (Jupiter-like objects). A star such as the Sun generates energy from thermonuclear reactions in the hot (several millions of degrees) and ultra-dense core. When a star has a mass below 8% of the mass of the Sun, its central temperature is not hot enough to convert hydrogen into helium by the proton-proton cycle of nuclear reactions. As a consequence, those objects, termed brown dwarfs, will never reach the main-sequence where stars spend most of their lifetime. In a nutshell, brown dwarfs have masses between 0.075 Msun and 0.013 Msun (between 23964 and 4327 Earths), a radius similar to Jupiter (1 Rjup = 71492 km) and fade inexorably with time to very faint luminosities. The first genuine brown dwarfs were discovered in 1995 in the field and in the Pleiades, the same year as the first extrasolar planet around a solar-type star (the image above is of G229B, the first brown dwarf to be discovered). The number of brown dwarfs has increased dramatically with the advent of large-scale surveys and the development of sensitive and larger detectors at optical (0.5-1.0 microns) and near-infrared (1-2.5 microns) wavelengths. Several hundreds of brown dwarfs have now been discovered in the field, as companions to stars, in open clusters, and in star-forming regions. However, their formation mechanism remains currently unsettled: do they form from the collapse of a molecular cloud like low-mass stars, or within a circumstellar disk as planets do? The work of the Leicester group deals with the search for nearby and old field brown dwarfs, and young and more distant ones in open clusters, including the Pleiades, Alpha Per, Hyades, Collinder 359, and Coma Ber. We are currently highly involved in the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey and Galactic Cluster Survey to shed light on the origin of brown dwarfs.