Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!gamma!epsilon!zeta!sabre!petrus!bellcore!decvax!genrad!panda!talcott!harvard!seismo!ut-sally!utastro!nather
From: nather@utastro.UUCP (Ed Nather)
Newsgroups: net.astro.expert
Subject: Re: black holes and comets
Message-ID: <457@utastro.UUCP>
Date: Mon, 3-Mar-86 11:17:08 EST
Article-I.D.: utastro.457
Posted: Mon Mar  3 11:17:08 1986
Date-Received: Wed, 5-Mar-86 05:45:48 EST
References: <1457@decwrl.DEC.COM>
Organization: U. Texas, Astronomy, Austin, TX
Lines: 28

In article <1457@decwrl.DEC.COM>, goutal@dec-parrot.UUCP writes:
> The question she asks is, why don't the comets get gobbled up by
> black holes out beyond the planets?  
> 
> Can anyone shed any light on this for us?

Hidden in the question is (what appears to be) an assumption that is
incorrect: that black holes are better at eating distant things than
an equivalent amount of "luminous mass" would be.  Not so: their
ability to attract and eat things is proportional to their mass and
inversely proportional to the SQUARE of the distance to the "eatee"
just like anybody else.  A massive star that becomes a black hole is no
better at cosmic gastronomy as a black hole than it was as a star.

I guess the basic reason cosmic things don't eat each other very often
is the enormous distances between them.  Despite their appearance on
photographs as near-solid objects, a galaxy of stars is mostly empty
space.  If you shot through one, even edge-on, the chance of hitting
a star (getting eaten) is very small; you'd get bored long before you'd
get eaten.



-- 
Ed Nather
Astronomy Dept, U of Texas @ Austin
{allegra,ihnp4}!{noao,ut-sally}!utastro!nather
nather@astro.UTEXAS.EDU