aman is right. However, technology has progressed and shaped charges allow smaller amounts of fissile material to gain critical mass.
Using plutonium requires the least mass, but is the most difficult. The easiest is just banging two large lumps of uranium together, but that's going to be ineffective unless you've enriched it with a larger ratio of 235 to 238.
4. Astrophysics. A point in space-time at which gravitational forces cause matter to have infinite density and infinitesimal volume, and space and time to become infinitely distorted.
5. Mathematics. A point at which the derivative does not exist for a given function but every neighborhood of which contains points for which the derivative exists. Also called singular point.
We don't prove that a sun is needed, but it is one factor which we are CERTAIN can cause life, rather than speculating on possible other types of life, which may not even exist, and trying to find objects to match those conditions.
A singularity in this case means a point of infinite space time curvature; it is a mathematical term.
See: 4 and 5 on this link
http://dictionary.reference.com/search?q=singularity
A black hole is an n dimensional entity, where n is the number of dimensions that exist, however many that is now.
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