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So, let’s face it, brown dwarfs are dull. They’re too big for gas giants to want anything to do with them, and too weeny to undergo fusion and become a proper sun. However, if this is a matter of mass, what if two brown dwarfs were merged, forming a single object massive enough to fuse and form a habitable sun. Is this possible? (I don’t mean on a technological front, obviously).

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It would likely take more than two brown dwarves (of usual masses) to add up to enough mass to get even the smallest M9 red dwarf -- but three or four might well do it.

Unfortunately, there's more to it than just being able to move three (or five or six) almost-stars so they collide -- because the impact energy of them falling into their mutual gravity well would produce much more than an equilibrium level of fusion conditions. There would be a short transient (by stellar lifetime standards -- this might last a century or more, in human terms) during which far too much hydrogen would fuse. There's a real risk of the coalescence transient actually blowing the nascent star apart as fusion rates suitable for, perhaps, an F or G star occur with only the mass of a M to hold things together.

If your Kardashev 2 (?) civilization wants to make stars out of brown dwarves, then, they'd need to be careful to bring them together gradually, using decaying orbits rather than direct collision, and that means they'd have to be patient enough to wait millennia for the dwarves to merge completely enough to light their combined fusion furnace.

There's also the issue of spin; it would likely be necessary to merge two or three dwarves, wait for them to fully coalesce, then bring in another one or two in retrograde orbits to soak up some of the angular momentum -- and again, this would have to be done very gradually, to avoid fusion in shock waves where the envelopes begin to merge at orbital velocities.

So, tl;dr this is theoretically possible, but it's nothing like as simple as just dropping enough stuff into a mold and getting what you want out.

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    $\begingroup$ Do you mean there is a gap in the size between brown dwarves and stars? $\endgroup$ Nov 22, 2022 at 0:36
  • $\begingroup$ @PaŭloEbermann Not in physical dimensions (brown dwarves are about as large as the smallest red dwarf stars), but in terms of mass, yes; brown dwarves are around 13-78 times Jupiter's mass, while stars are above 78 times that. $\endgroup$
    – KEY_ABRADE
    Nov 22, 2022 at 5:06
  • $\begingroup$ @PaŭloEbermann No, I mean that brown dwarves near the upper mass limit (the "hydrogen burning limit") are surely less common than smaller ones (in the 10-30 Jupiter mass range). $\endgroup$
    – Zeiss Ikon
    Nov 22, 2022 at 12:06
  • $\begingroup$ Thanks, I suggested an edit to make this clearer. $\endgroup$ Nov 22, 2022 at 22:07
  • $\begingroup$ Could you combine a 75 Jupiter brown dwarf with a much smaller 10 Jupiter brown dwarf to get over the threshold while minimising the problems you've outlined? $\endgroup$
    – user86462
    Nov 24, 2022 at 11:08
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It's exactly what the Hertzsprung-Russel diagram shows

enter image description here

As you can see, the brown dwarfs transition into red dwarfs, subdwarfs and main sequences stars. The more mass you add, the better the hydrogen fusion can happen until the star engine can start.

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The answer is Yes.

Brown dwarves and hydrogen-burning stars exist on a continuum - there is no dividing line that prevents essentially any sufficiently large collection of gas from forming a planet or star. The largest brown dwarves have nearly the same mass as the smallest stars. In fact, there is likely an overlap because the mass threshold for hydrogen-burning also depends upon the composition (metallicity) of the mass.

This threshold is also known as the hydrogen burning limit, which is estimated as 0.07-0.077 solar masses. This corresponds to an upper limit of about 80 MJup for a brown dwarf. Our sun is about 1047 MJup.

Combining two brown dwarves each massing 50 MJup would result in a red dwarf. The lowest known mass of a hydrogen burning star is that of 2MASS J0523-1403 which is estimated at 67.54±12.79 MJup - note that this is actually below the estimated 0.07 MSol limit for hydrogen burning. Not everyone agrees that this star is main sequence.

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