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From: nather@utastro.UUCP (Ed Nather)
Newsgroups: net.astro.expert
Subject: Re: neutron stars, black holes, etc. (less long)
Message-ID: <303@utastro.UUCP>
Date: Fri, 18-May-84 22:45:33 EDT
Article-I.D.: utastro.303
Posted: Fri May 18 22:45:33 1984
Date-Received: Sun, 20-May-84 00:44:35 EDT
References: <333@kpno.UUCP>
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 25

[]
>From: allan@kpno.UUCP
>
>The evidence that neutron stars exist is very good. The best evidence comes
>from observations of pulsars, since it is virtually impossible to come up
>with a mechanism that gives pulses with the constant pulse rate that pulsars
>have without invoking rotation of the source. 

This is a commonly-held belief; it is also completely wrong.  The pulsating
white dwarf star G117-B15A, one of many very stable pulsators located in the
ZZ Ceti instability strip, has a primary pulsational period now known to be
constant with 1 part in ten to the 14th power.  Most pulsars are far poorer
clocks than this.  The pulsation mechanism (hydrogen ionization zone driving)
is well understood, and does not involve rotation.  (The slight rotation of
the object causes modal splitting of the pulsations, by making the star
slightly oblate, and is measurable.)  What *does* seem to be required for
such phenomenal stability involves a lot of mass, at very high density --
but rotation is not the only mechanism that can evoke it.


-- 

                                       Ed Nather
                                       ihnp4!{ut-sally,kpno}!utastro!nather
                                       Astronomy Dept., U. of Texas, Austin