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From: ecphssrw@bob.csun.edu (Stephen Walton)
Newsgroups: sci.astro,sci.space,sci.space.shuttle
Subject: Re: Extinctions: Asteroids and Dinosaurs
Message-ID: <738@solaria.csun.edu>
Date: 29 May 89 18:09:28 GMT
References: <4792@tekigm2.MEN.TEK.COM>
Sender: ecphssrw@solaria.csun.edu
Reply-To: ecphssrw@bob.csun.edu (Stephen Walton)
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Organization: California State Univ., Northridge
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In-reply-to: timothym@tekigm2.MEN.TEK.COM (Timothy D Margeson)

In article <4792@tekigm2.MEN.TEK.COM>, timothym@tekigm2 (Timothy D Margeson) writes:

>The current issue of Scientific American has an excellant article on the
>extintion matter.

Which I admit that I've not read yet.  However:

>The article also points out that the 26 million year periodicity meets 
>roughly with the rate at which the solar system moves through the arm of our
>galaxy and some correlation is plausible.

As the social scientists say, "Correlation is not cause and effect."
It is difficult to think of anything which would happen *every* time
the solar system went through a spiral arm which could cause
extinctions on Earth.  Contrary to popular belief, the steller density
(stars per cubic parsec) in spiral arms is the same as between the
arms; it is just that most of the new, hot, and therefore bright stars
are in the arms, causing them to stand out.  Yes, they have a higher
concentration of dust and gas too, but it is more like 1 atom/cc
instead of 0.1 atom/cc in the solar neighborhood.  Not enough to make
a difference.  A dense molecular cloud *might* cause enough absorption
to make the earth colder for a long period, but such clouds fill a
small proportion of the volume of even a spiral arm.
--
Stephen Walton, Dept. of Physics & Astronomy, Cal State Univ. Northridge
RCKG01M@CALSTATE.BITNET       ecphssrw@afws.csun.edu
swalton@solar.stanford.edu    ...!csun!afws.csun.edu!ecphssrw