Brown dwarfs are the smallest class of star ranging from 13 to 80 times the mass of Jupiter, 0.01-0.08 solar masses; it is thought that they are stars only because fusion reactions involving hydrogen's heavy isotopes, deuterium and tritium, are possible at much lower temperatures and pressures than the proton-proton fusion of their heavier cousins the Red dwarfs. Brown Dwarfs are consequently thought to be quite short lived, burning through their heavy isotopes in a 100 million years or less before going cold.

Deuterium and Tritium can form through neutron capture but Hydrogen and Deuterium both have small neutron capture cross sections making the event unlikely under what might be considered normal circumstances.

Now the question, could hotter true stars in multi-star systems with low mass dwarf candidates produce enough neutron flux, and capture, to make a noticeable difference to the lifespan of their Brown Dwarf companions?

I'd like answers to take into account the temperature/flux range of known stars and assume a tight orbit similar to that of 51 Pegasi b for the Brown Dwarf candidate.

  • $\begingroup$ How many neutrons leave the star? They are electrically neutral, what is the mechanism of their escape? High enough thermal speed? $\endgroup$ Sep 3, 2021 at 6:07
  • $\begingroup$ @RadovanGarabík That would rather depend on the size and temperature of the star in question. All stars have a neutron flux but I'm not sure of the exact magnitudes or mechanisms involved. $\endgroup$
    – Ash
    Sep 3, 2021 at 23:56
  • $\begingroup$ Brown dwarfs stay warm, they are just powered by gravity not fusion. Gravity can keep a brown dwarf warm enough to glow for a long time, and releases a lot more energy than deuterium fusion. $\endgroup$
    – James K
    Sep 4, 2021 at 19:00

2 Answers 2


What's the odds that the two stars are close enough for a significant fraction of any neutrons to cross the gap? Remember that a free neutron has a rather short half-life of 10 minutes, and that the target star will span a small part of the arc of the sky, the extreme majority of particles would simply miss.

What sort of star gives significant levels of neutrons in the first place? The star would need to have fusion or fission right at the surface, in practice this means localized infrequent events like large solar flares.

So the Brown Dwarf receives a tiny fraction of the neutrons created by the larger star,
and those neutrons are only created by a tiny fraction of reactions on the parent star,
and many of those neutrons will decay while crossing the space between the stars.

The Brown dwarf will indeed receive an energy boost in fusables from the neutron flux of the bigger star, but it is a minuscule fraction of a minuscule fraction of a fraction of the total energy output of the bigger star.

The direct photonic illumination will be many magnitudes more relevant.

  • $\begingroup$ I have set a distance parameter for the two bodies, 0.052AU per 51 Pegasi. "The direct photonic illumination will be many magnitudes more relevant." meaning the direct photon radiation can/will heat the maybe a brown dwarf more than it will heat itself? $\endgroup$
    – Ash
    Sep 5, 2021 at 0:51
  • 1
    $\begingroup$ @Ash More heat than the brown dwarf itself produces... maybe. That depends on many factors. More than heating in the brown dwarf caused by neutron enrichment of its hydrogen and helium, ab-so-lutely. $\endgroup$
    – PcMan
    Sep 5, 2021 at 4:06

Seems unlikely because whatever capture is going on would likely be high in the brown dwarf's atmosphere in relation to the comparatively tiny fusion region.

  • $\begingroup$ Given the way that the atomic density of the star goes up with depth I'm not sure that this actually follows. The odds of capture are very small and increase with depth. $\endgroup$
    – Ash
    Sep 4, 2021 at 0:00
  • $\begingroup$ @Ash this one is correct one. Neutron energy will define depth, but it will be way before reactive zone. In general enrichment with heavy isotopes helps(if it happens) as dwarfs have convective processes, which heavier stars do not have. Look at (maybe) wiki/Muon-catalyzed_fusion generation of muons by some star, in suficient quantity etc, may be the biggest q in that case, but it not necessarly has to penetrate too deep $\endgroup$
    – MolbOrg
    Sep 4, 2021 at 8:16
  • $\begingroup$ Brown Dwarfs are also fully convective so anything in the upper atmosphere will get to fusion depth. $\endgroup$
    – Ash
    Sep 5, 2021 at 1:06
  • $\begingroup$ @Ash Not just brown dwarfs--all small stars are fully convective. $\endgroup$ Sep 5, 2021 at 3:18
  • $\begingroup$ @LorenPechtel If by small you mean Brown and Red dwarf stars then yes but Sol is a yellow dwarf, tiny by comparison to many stars in our galaxy and it's non-convective. $\endgroup$
    – Ash
    Sep 5, 2021 at 5:07

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