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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.astro.expert
Subject: Re: black holes and comets
Message-ID: <458@utastro.UUCP>
Date: Mon, 3-Mar-86 11:49:48 EST
Article-I.D.: utastro.458
Posted: Mon Mar  3 11:49:48 1986
Date-Received: Wed, 5-Mar-86 05:46:02 EST
References: <1457@decwrl.DEC.COM> <457@utastro.UUCP>
Organization: U. Texas, Astronomy, Austin, TX
Lines: 25
Summary: black holes are worse at eating

In article <457@utastro.UUCP>, nather@utastro.UUCP (Ed Nather) writes:
> 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.
> 
In fact, it is arguable that black holes are worse at consuming passing
objects since the cross section for a direct hit is miniscule.  A black
hole the size of the sun would have a radius of a few kilometers.  On
the other hand, the radius for tidal disruption would be the same.
-- 
"Ma, I've been to another      Ethan Vishniac
 planet!"                      {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               ethan@astro.UTEXAS.EDU
                               Department of Astronomy
                               University of Texas