Brown Dwarfs

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Why wouldn't a brown dwarf grow in radius if it started to gain mass?
 

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  • #2
phinds
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Why wouldn't a brown dwarf grow in radius if it started to gain mass?
Why do you think it wouldn't? Here on PF it is bad form to just make a bald statement like that without citing where you are getting your informaiton. Are there particular conditions under which it wouldn't and some under which it would?
 
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Chronos
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Adding mass to a degenerate body would cause it to shrink only so far as nothing else changes. For a brown dwarf, that something else would be hydrogen fusion. Once that occurs, the shrinkage would cease and it would expand due to the heat. A brown dwarf can remain degenerate, but initiate tritum fusion,at around 60 Jupiter masses [Mj]. At a mass around 13 Mj deuterium fusion is believed possible. Brown dwarfs in the 60-90 Mj] qualify as L spectral class, hot enough to initiate hydrogen fusion, but enough to trigger lithium fusion. Brown dwarfs in the 13-60 Mj range are capable of deuterium fusion, but, not hydrogen or even lithium fusion. They are considered spectral class T or Y, depending upon temperature. Once the temperature falls below about 1000K it is no longer considered a brown dwarf, just an ordinary planet.
 
  • #5
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If you add mass to an asteroid, its density is not changed, because at the low pressure inside an asteroid, rocks are practically incompressible. An asteroid with 200 % mass of another has 200 % the volume.
However, as the body gets larger, interior pressure increases and increasingly compresses the contents.
Jupiter has 330 % the mass of Saturn, but only 170 % the volume.
If you keep adding mass and pressure, then after a body gets only slightly bigger than Jupiter, the pressure in the interior has reached such a level that further addition of mass causes decrease of volume, as the interior is compressed more than the added mass takes up.
That's the range characteristic of brown dwarfs.
Adding mass to a degenerate body would cause it to shrink only so far as nothing else changes. For a brown dwarf, that something else would be hydrogen fusion. Once that occurs, the shrinkage would cease and it would expand due to the heat. A brown dwarf can remain degenerate, but initiate tritum fusion,at around 60 Jupiter masses [Mj].
Tritium is short-lived and rare in nature.
At a mass around 13 Mj deuterium fusion is believed possible. Brown dwarfs in the 60-90 Mj] qualify as L spectral class, hot enough to initiate hydrogen fusion, but enough to trigger lithium fusion.
So which do they and do they not initiate?
Brown dwarfs in the 13-60 Mj range are capable of deuterium fusion, but, not hydrogen or even lithium fusion. They are considered spectral class T or Y, depending upon temperature. Once the temperature falls below about 1000K it is no longer considered a brown dwarf, just an ordinary planet.
That's probably an arguable question of classification. Some white dwarfs are known to have cooled to 4000 K, yet they are called white dwarfs rather than red dwarfs.
 
  • #6
Chronos
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1. I i meant lithium, not tritium, my mistake. The heaviest brown dwars can achieve lithium fusion, which is la little below the temperature needed for hydrogen fusion.

2. Perhaps you misread the word 'hot' for 'not'.

3 I fail to see the relevance of white dwarf temperatures. These links may be of interest
https://arxiv.org/abs/1008.5150 The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets
https://arxiv.org/abs/1008.515 Outstanding Issues in Our Understanding of L, T, and Y Dwarfs
 

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