In January 2014, the European Southern Observatory’s VLT recently gave us some fantastic images of a very small star of low luminosity, WISE J104915.57-531906.1B, now known as Luhman 16B.  It is a brown dwarf, a class of small stars with very cool surface temperatures. Luhman 16B and its binary partner are the closest brown dwarfs yet discovered at around 6 light years from earth. They blur the boundary between Jupiter-like substellar objects and stars, sustaining only minimal fusion in their cores and exhibiting surface weather that is similar to our gas giant planets.  Above about 13 Jupiter Masses, deuterium fusion can occur.  Above about 65 Jupiter Masses, fusion of lithium can occur as well.  To be technically considered a star, an object should be fusing hydrogen, and this occurs above 75-80 Jupiter masses.  In any case, these stars are very difficult to detect in the visual wavelengths, since they emit most of their energy in the infrared.

Like other dwarf stars of low luminosity, they can become incredibly dense.  Since there is very little pressure from fusion to sustain an expanded gas envelope, the object is supported by the thermal energy left over from initial gravitational collapse.  As the star ages, it consistently shrinks, supported at first by coulomb pressure and later by electron degeneracy pressure.  Of particular interest is their usual radii.  For a brown-dwarf star that has had time to settle, it can contain perhaps 20 times the mass of Jupiter in as little as 1.5 Jupiter Radii.

Many brown dwarfs exhibit extreme weather.  They tend to rotate rapidly and sustain convection throughout, which in turn creates strong and variable magnetic fields.  Brown Dwarfs have been identified as X-ray sources, likely the result of flares due to magnetic activity.

 

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1. Tables VII, VIII, Empirical bolometric corrections for the main-sequence, G. M. H. J. Habets and J. R. W. Heinze, Astronomy and Astrophysics Supplement Series 46 (November 1981), pp. 193–237, Bibcode: 1981A&AS…46..193H. Luminosities are derived from M_bol figures, using M_bol(☉)=4.75.
2. T. J. Dupuy & A. L. Kraus; Kraus (2013). “Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects”. Science. published online 5 September 2013 (6153): 1492–1495. arXiv:1309.1422. Bibcode:2013arXiv1309.1422D. doi:10.1126/science.1241917.
3. First weather map of a brown Dwarf”           http://www.eso.org/public/news/eso1404/  “
4. “X-rays from a Brown Dwarf’s Corona”. April 14, 2003.