[rhlopez]

Ricardo Hernandez  asked the Naked Scientists:
   
Hi,

Let's say there is a sphere made of dense metal (Osmium?) of radius N which is capable of sustaining an atmosphere of hydrogen; the hydrogen is so condensed by the gravitational force of the metal sphere that starts to produce atomic fusion.

Could this even exist?
What is the radius of such a metal sphere?

Well, maybe this is not possible, but it would be interesting to know the answer.


Thanks again!

Ricardo

What do you think?

[graham.d]

I can't think of any reason why, given a sufficient gravitational field, that significant fusion of an "atmosphere" of hydrogen would not occur. The density of the core of the sun is about 5x more dense than Osmium, even though the average density of the sun is not very high. What would happen to the Osmium under such pressure and temperatures, I am not sure.

Fusion occurs at even low pressures because there are always a probability of some atoms with sufficiently high energy to fuse. The problem with a fusion reactor is to get enough to get usable energy out - I assume this is the intent. You probably need quite a large mass of Osmium to do this - something like 10^28 kg - based on a rule of thumb that you need at least a mass similar to Jupiter to create fusion even if collapsed to a red dwarf density.

[LeeE]

If you're thinking about generating fusion energy via gravitation then you also need to be thinking of a way of transporting that energy away too.

As graham.d implies, gravitational fusion will only occur on a very large scale because you'll need a correspondingly large mass to achieve the degree of gravitation required.  As a consequence, you'll need to be able to transport correspondingly large amounts of energy away from the collecting apparatus before it overheats and is damaged/destroyed.

Alternatively, microscopic Black Holes could be used, if you had a way of maintaining and managing them, but I certainly wouldn't one in my back yard.  For safety reasons, I think they'd have to be located in solar orbit, and preferably around a different solar system, in which case you've still got a problem, albeit a different one, with transporting the generated energy to where it's required.

As it happens though, the smaller a BH is, the hotter it is, so perhaps at some point we'll be able to generate BHs that are small enough to guarantee their rapid decay via Hawking radiation whilst harvesting heat energy from them.  Once again though, it's probably something best done in distant outer space for safety reasons, for should an accident occur the micro BH could become large enough that it wouldn't evaporate in a short time span and would thus become a permanent hazard.

[Soul Surfer]

The sustaining of fusion due to gravitational attraction is well known and you only need a lot of hydrogen to achieve this.  The nearest object demonstrating this effect is of course the sun and you need at least one tenth of the mass of the sun or about one hundred times the mass of jupiter to achieve the required density.  There is no point in requiring dense materials because the pressure of the overlying material makes the density of the material  very large anyway.

You can put the required density in a lump about the size of the earth if you use a neutron star but the overall mass  is then greater than that of the sun so you need a lot more material and some very violent compressive forces using a supernova to achieve this.  Feeding hydrogen on to a neutron star will produce fusion energy but will also release a great deal of gravitational energy.

The problem is that gravitational confinement is such a weak force that you need very large objects containing vast masses of material to enable fusion.  It is far better to use the stronger and more controllable electromagnetic forces or just the energy density itself As in laser fusion to enable the material to be confined compressed and heated to fuse the nuclei.