h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Excellent Article!!

One thing though - What you refer to as the 'critical circumference' was later termed the 'Schwarzschild Radius' - a star collapsing enough to form a black hole.

Paul Davies put it nicely in his book 'About Time' - If you compress the earth into a ball the size of a large marble (~20mm diameter), it'll become a singularity.

This is obviously a very crude and impractical example, but informative nonetheless.

Peace

If any macroscopic object is squeezed small enough, it could become a singularity. the only problem is the squeezing. Only these supermassive stars have the sufficient gravity in the first place to get that small.

Hawking suggested that the compressional forces of the big bang itself may have compressed masses smaller than the Chandrasekhar
limit into black holes.

This provides one of the possible explanations for galactic halo gamma ray bursters (out where the dark matter is supposed to be), gravitational decay of Hawking's primordial black holes.