What is the smallest amount of hydrogen needed for fusion

[Dragonfall]

What is the smallest amount of hydrogen needed so that fusion occurs automatically under its own gravity? How long would it last?

Not homework.

 

[Matterwave]

For proton proton chain fusion to occur, you have to be in the ~.08 solar mass range. Below that, you can still fuse deuterium and be a brown dwarf down to the ~13 Jupiter mass range (or ~.013 solar mass range).

 

[sophiecentaur]

This something that the popular press never seem to consider. The 'endless supply' of fuel that's available for fusion, from the sea is actually in very low concentration (about 160ppm). Any fusion reactor that we could make would need to use deuterium rather than Hydrogen. We can't afford to wait for the p+p reactions to take place. It's another reason why the sea won't suddenly 'explode' as people fear.

 

[dschlink]

I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.

 

[Matterwave]

Red dwarfs live for hundreds of billions to trillions of years because they burn very slowly. I'm not sure how fast brown dwarfs typically burn through their supply of deuterium.

 

[sophiecentaur]

I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.

I don't know the statistics. Do we really have a lot of boring? I guess it could well be 'cheap' at any price.

 

[Drakkith]

I don't know the statistics. Do we really have a lot of boring? I guess it could well be 'cheap' at any price.

I can't find the reference, but I remember reading somewhere that the supplies of Boron in seawater was enough to fuel us for like a million + years or something. For Deuteium it was way more.

 

[JeffKoch]

I consider it more likely that the proton-boron reaction will produce the first commercially successful fusion reactor (see Polywell) and both elements are abundant and accessible.

http://en.wikipedia.org/wiki/Aneutronic_fusion

I hope you're kidding, I don't know of any way to produce temperatures of 600 keV (!) in any amount of material worthy of the word "temperature".