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Comment by cambalache | Hacker News Reader

I suspected so. But all those objects are in this solar system.Brown dwarfs are sub-stellar bodies.

Touche, you are technically correct.

Though some might argue that in the changing nature of fusion reactions and body mass, this sorting of sub-stellar bodies is only true for as long as our star is a red dwarf.

Once that's over (thinking in the larger timescale of things) and it becomes a brown dwarf, what then? Wasn't it a solar system at one point in time?

(I'm not an astronomer, my understanding is not absolute, please correct if I'm wrong) I believe our Sun is a type G, yellow dwarf. A red dwarf is usually much smaller, if considering "red dwarf" being a type M, then usually no more than 0.6 solar masses, usually much smaller, and a tiiiiny fraction of the luminosity of Sol. Brown dwarfs are not massive enough to fuse hydrogen, where red dwarfs generally are. According to https://en.wikipedia.org/wiki/Red_dwarf, above 0.25 solar masses, they should evolve into red giants, otherwise will shrink into a white dwarf once hydrogen fusion is ceased. The universe isn't old enough to have any red dwarf of 0.25 solar masses to do so, however as their lifespans are so long.

> Once that's over (thinking in the larger timescale of things) and it becomes a brown dwarf,

Our sun will never become a brown dwarf. Stars have their life-cycle and nomenclature. You cannot made your own willy-nilly.

I find it kind of weird to have a different word for the same object depending on where it is. As I understand it, Jupiter would be a brown dwarf except that the presence of the Sun disqualifies it.

Jupiter is orders of magnitude too small to be a brown dwarf. The "brown" implies IR radiation generated by gravitic collapse. If Jupiter was massive enough to generate its own radiance it's unlikely Earth would be around to observe it.

The simplistic solar systems and textbooks in grade school are not just misleading when it comes to scale, but they're really really misleading. Few people really comprehend the scale of things in the universe.

Scale isn't the right thing to complain about here. Compare https://en.wikipedia.org/wiki/Brown_dwarf :

> most brown dwarfs are slightly smaller than Jupiter (15–20%)

> Brown dwarfs are all roughly the same radius as Jupiter.

The suggestion that Jupiter is "orders of magnitude too small" to be a brown dwarf is obviously wrong in terms of size, but it's also wrong in terms of mass:

> at the low end of the range (10 [Jupiter masses]), their volume is governed primarily by Coulomb pressure

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