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Date: 16 Aug 2007 21:41:16
From: Andrew Smallshaw
Subject: ASTRO: Comet-like trail of Mira
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This story concerning the trail left by Mira has been widely reported
in the last day or two: amongst others, it's at http://tinyurl.com/32k3ag
if you're unfamiliar with the story.
Whilst the jury is still out the precise mechanisms causing the
trail even among the professional scientific community, one thing
is puzzling me. That is why there would be any tendency to leave
a trail: the articles I've read simply put it down to the star's
proper motion but surely all the material shedded from the star
would have similar momemtum to the star itself.
The only possibility I can think of is resistance caused by the
interstellar medium, but I would have thought that would be too
tenuous to cause this kind of effect, and even then it wouldn't
form such a neatly defined trail. Anyone have any thoughts on
this?
--
Andrew Smallshaw
andrews@sdf.lonestar.org
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Date: 17 Aug 2007 06:13:18
From: Bill Hudson
Subject: Re: ASTRO: Comet-like trail of Mira
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On Thu, 16 Aug 2007 21:41:16 +0000, Andrew Smallshaw wrote:
> This story concerning the trail left by Mira has been widely reported in
> the last day or two: amongst others, it's at http://tinyurl.com/32k3ag
> if you're unfamiliar with the story.
>
> Whilst the jury is still out the precise mechanisms causing the trail
> even among the professional scientific community, one thing is puzzling
> me. That is why there would be any tendency to leave a trail: the
> articles I've read simply put it down to the star's proper motion but
> surely all the material shedded from the star would have similar
> momemtum to the star itself.
>
> The only possibility I can think of is resistance caused by the
> interstellar medium, but I would have thought that would be too tenuous
> to cause this kind of effect, and even then it wouldn't form such a
> neatly defined trail. Anyone have any thoughts on this?
From what I gather, Mira is moving through the inter-stellar medium at
quite a clip, and the ejecta is interacting with the ISM, which is what
is causing the ejecta to trail off behind Mira, as well as causing the
stuff to light up in UV like that.
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Date: 17 Aug 2007 03:38:35
From: Sam Wormley
Subject: Re: ASTRO: Comet-like trail of Mira
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Andrew Smallshaw wrote:
> This story concerning the trail left by Mira has been widely reported
> in the last day or two: amongst others, it's at http://tinyurl.com/32k3ag
> if you're unfamiliar with the story.
>
> Whilst the jury is still out the precise mechanisms causing the
> trail even among the professional scientific community, one thing
> is puzzling me. That is why there would be any tendency to leave
> a trail: the articles I've read simply put it down to the star's
> proper motion but surely all the material shedded from the star
> would have similar momemtum to the star itself.
Maybe the optical system used, suffers from coma! :-o
>
> The only possibility I can think of is resistance caused by the
> interstellar medium, but I would have thought that would be too
> tenuous to cause this kind of effect, and even then it wouldn't
> form such a neatly defined trail. Anyone have any thoughts on
> this?
>
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Date: 17 Aug 2007 03:37:02
From: Ed Holden
Subject: Re: ASTRO: Comet-like trail of Mira
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"Andrew Smallshaw" <andrews@sdf.lonestar.org > wrote:
> The only possibility I can think of is resistance caused by the
> interstellar medium, but I would have thought that would be too
> tenuous to cause this kind of effect, and even then it wouldn't
> form such a neatly defined trail. Anyone have any thoughts on
> this?
Yep, I have lots of thoughts on this, but time permits me to post
only two of those. Yes, the ISM is tenuous, but even if we only
assume a pessimistic 1 atom per cm3, the sheer angular size of
Mira would therefore allow it encounter a lot of atoms at its
velocity of 130,000 m/sec.
Another idea is that the star's wind ejection speed is faster than
it's forward motion, and is therefore ramming into its own gas.
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