The Faint End of the Main Sequence

New infrared observations of the two faintest known, late M dwarfs, Wolf 359 and + (= VB 10) provide accurate luminosities and moderately well-determined temperatures (2500 and 2250 K, respectively). The photometric observations are fitted to a blackbody energy distribution on the assumption that line and band blocking affect most of the spectrum below I the temperature structure is taken as that of a gray body. The resulting data, together with Johnson's observations for dM4 and dM5 stars, which have been reanalyzed, calibrate the faint end of the main sequence, with results given in a table and a figure. The bolometric corrections are very large and increase steeply to 6 mag, so that the faintest known stars are, in fact, not very faint; Wolf 359 has L = 13 X t0- L0, and VE 10 has L = 5 X 10- L0. A statistical discussion of Luyten's faint red stars of large proper motion gives L = 4 X 10- L0. With a conventional mass-luminosity relation, > 0.09 , for stars of known luminosity. Stars of still lower mass, such as L726-8, are difficult to interpret.