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-Z- (z@gundam.com)
Wed, 15 Nov 2000 19:12:04 -0800
> -----Original Message-----
> From: owner-gundam@1u.aeug.org [mailto:owner-gundam@1u.aeug.org]On
> Behalf Of Lim Jyue
> Sent: Tuesday, November 14, 2000 23:26
> To: gundam@aeug.org
> Subject: Re: [gundam] What about Minovski Drives? (Re: Long!] Minovsky
> Applications Theories)
>
> Sorry -- forgot my Astro courses. You'll need quite a number of Sol
> masses to get a black hole (Cue for -Z- to step it here.. =) But I do think
> their presence would be indeed be observable via gravitational means.
OK -- you asked for it. (^_^)
There are two different ways to describe how big something is: how much mass it
has or how much space it takes up. First, the mass.
There is no limit in principle to how much or how little mass a black hole can
have. Any amount of mass at all can in principle be made to form a black hole
if you compress it to a high enough density. Theory holds that most of the
black holes that are actually out there were produced in the deaths of massive
stars, and so those black holes should be about as massive as a massive star. A
typical mass for such a stellar black hole would be about 10 times the mass of
the Sun, or about 10^31 kilograms. (10,000,000,000,000,000,000,000,000,000,000.)
Astronomers also suspect that many galaxies harbor extremely massive black holes
at their centers. These are thought to weigh about a million times as much as
the Sun, or 10^36 kilograms.
The more massive a black hole is, the more space it takes up. In fact, the
Schwarzschild radius (radius of the horizon) and the mass are directly
proportional to one another: if one black hole weighs ten times as much as
another, its radius is ten times as large. A black hole with a mass equal to
that of the Sun would have a radius of three kilometers. So a typical
10-Solar-mass black hole would have a radius of 30 kilometers, and a
million-Solar-mass black hole at the center of a galaxy would have a radius of
three million kilometers. Three million kilometers may sound like a lot, but
it's actually not so big by astronomical standards. The Sun, for example, has a
radius of about 700,000 kilometers and so that supermassive black hole has a
radius only about four times greater than the Sun.
Incidentally, the term "black hole" was invented by John Archibald Wheeler
(b.1911, Princeton physics professor 1938-1976, co-author with Niels Bohr of
"The Mechanism of Nuclear Fission" in 1939, co-developer of the H-bomb
1949-1951) and seems to have stuck because it was much catchier than previous
terms, such as "frozen star" -- so called because, as the material that is to
form the black hole collapses, an outside observer would see it get smaller and
smaller, approaching but never quite reaching its Schwarzschild radius, seeming
to "freeze" at a size just slightly bigger than the Schwarzschild radius.
-Z-
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