Quote:
On 2001-11-18 00:41, lpetrich wrote:
* Thermal emissions. This requires a body with a significant amount of internal heat, which would have to be at least as massive as Jupiter. However, Jupiter is close to the largest "cold" object with its composition, "cold" meaning not hot enough for temperature to contribute a significant amount of pressure. Meaning that a several-Jupiter-mass brown dwarf would be smaller than Jupiter.
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In general the minimum brown dwarf mass is about 11 times the mass of Jupiter (according to the models). Above this the core temperature and pressure is enough for a period of deuterium fusion into helium. This period is shorter than the lifetime of full-fledged stars, so older brown dwarfs can get "cold" too. Below the 11-Jupiter-mass cutoff no fusion occurs, so gravitational collapse is the most important source of heat.
Brown dwarfs, if defined as objects than ever fused deuterium, would range from 11 Jupiter masses to 65-85 Jupiter masses. This upper limit is more dependent on composition, and is the lower limit for fusing regular hydrogen into helium, i.e., a real star.
For sub-stellar objects, age is as important a factor as size in determining internal heat.
The fact that objects of several Jupiter masses could be smaller than Jupiter is a matter of phase transitions (transitions of matter under pressure to a more compact state) and is not necessarily related to whether or not fusion occurs.
As far as Jupiter being close to the largest possible "cold" object, bear in mind that it's shy by a factor of ten. That's close in terms of the scale of the universe, but Jupiter has only 9% of the mass needed to "burn" deuterium and only 1% of the mass needed to be a star.
Bob Johnston
http://www.johnstonsarchive.net