Re: Observing - More Milestones...m42

From: Bill Ferris (billferris_at_aol.comic)
Date: 07/31/04 

Date: 31 Jul 2004 17:48:31 GMT

>obviously. but only after telescopic observation began.
>before that, even naked-eye nebulousity had never
>been mentioned. in any case, [M42 has] certainly
>expanded alot since then.

If you compare Messier's sketch of M42
(http://www.seds.org/messier/Pics/History/m42m.jpg ) with modern observations,
it's not at all obvious how the nebula has changed in the last 250 years.
Edward Holden, in his 1882 "Monograph of the Central Parts of the Nebula of
Orion," reviewed 250-years of visual observations, concluding that the nebula's
form had remained, for the most part, unchanged. The multiple star group, Theta
Orionis, is the source of the ultraviolet radiation that causes the surrounding
cloud of gas to fluoresce. Theta C is the primary of this group. Its age is
estimated at less than one million years, with the nebula estimated to be
between 10,000 and 100,000 years in age.

In the absence of physical evidence indicating the size and brightness of M42
changed dramatically in the last two millennia, the lack of detailed visual
observations prior to the 1700s are simply an insufficient foundation upon
which to conclude that the nebula changed significantly in size and brightness
around that time.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
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