Too Long; Didn't Read
The sun is a vast fusion furnace 866,000 miles across, but it is a controlled one. The sun compresses itself, through its equally enormous gravitational field, into huge densities and temperatures at its center. To achieve fusion ignition, therefore, temperatures must be considerably higher than those at the center of the sun. There is, as far as scientists know, no conceivable way of concentrating a high gravitational field in the absence of the required mass, and the creation of controlled fusion on earth must therefore be done without the aid of gravity.