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Re: "giant molecular clouds"

Subject: Re: "giant molecular clouds"
From: Steve Pope
Date: Mon, 13 Mar 2006 09:43:57 +0000 UTC
Newsgroups: sci.astro
Joseph Lazio  <[email protected]> wrote:

> Steve Pope writes,

>SP> I found the statement quoted in my first posting that a GMC passes
>SP> by every 500 million years or so interesting.  This raises some
>SP> immediate questions, such as do we know when in geologic history
>SP> these things passed by, and did they leave a record -- was there
>SP> altered climate, orbital changes, more comets?  It seems they are
>SP> quite massive so likely there would be an effect.

>Actually, it is probably not the mass but the density that would be
>most important.  The heliosphere or the region of the Sun's influence
>is determined by pressure balance between the outflowing solar wind
>and the surrounding medium.  If the density of the surrounding medium
>increases, its pressure increases, and the heliosphere shrinks.  That
>could result in more cosmic rays getting to the Earth, which could
>result in more clouds forming, in turn affecting the Earth's albedo.

>Priscilla Frisch has done a fair amount of work in this area, e.g.,
> >.

Thanks.  I'll take a look.  I'm particularly interested in
whether there specific evidence that a GMC has passed by on
average every 500 million years, or if this is an extrapolation
of some sort.

Also, what "molecules" would be in a "molecular cloud" other
than hydrogen?


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