“New Cutoff for Star Sizes”
Sky and Telescope, (Apr, 2014)
John Bochanski

A main-sequence star is an astronomical object massive enough to support thermonuclear fusion, that is, a sphere of ionized gas whose mass is sufficient to provide temperature and pressure in its interior sufficiently high to allow hydrogen to fuse into helium.

Objects less massive may still look like stars, and glow by releasing radiation derived from its gravitational collapse. These range from “brown dwarfs” all the way down to gas-giant planets like our own Jupiter. Jupiter is cold, its gasses un-ionized, and shines mostly by reflected sunlight, but it does emit radiation at the radio and microwave regions of the spectrum. The ancient gravitational potential energy of the outer layers is radiated away at the surface and the interior temperature and pressure supports the weight of those layers and keeps the objects from gravitationally collapsing to a point.

Most massive stars wind up this way at the end of their lives, as white dwarfs that have consumed all their nuclear fuel and are only kept in stellar configuration by the relict gravitational potential energy of their outer layers. They no longer produce energy, they only lose the old gravitational energy of the nebula where they were born.

Brown dwarfs produce no energy either, although they may still be bright enough to look like faint stars. We know the distinguishing criterion between the brightest brown dwarfs and faintest red dwarfs is whether or not fusion occurs in their cores. The determining parameter is mass. Presumably, the Mass Function (the histogram of stellar masses from Main Sequence stars through brown dwarfs down to gas giant planets) is a smooth curve, and it is poorly understood, although we do know the least massive objects are the most numerous. Dynamical stability considerations prevent stars being formed of more than a hundred or so solar masses. Jupiter is about 1/1000 of a solar mass.

Recent research by Sergio Dieterich and Todd Henry at Georgia State University has published detailed studies of nearby low-mass stars and brown dwarfs and established the boundary between the brown and red dwarfs.

We can now point to a temperature of (2100 K), radius (8.7% that of our Sun), and luminosity (1/8000th of the Sun) and say,’The main sequence ends there,’

The Sun has a surface temperature of 5800 K, or about 5500 degrees C. The Kelvin and Celsius scales are identical, although the zero point of the former is at absolute zero (-273 C).