Quote:
Originally Posted by ngcles
Hi Alexander & All,
Yes I agree, it is not a particularly easy object from suburbia, though a UHC or OIII filter improves it somewhat and it is certainly visible from suburban Engadine in my old 25cm at low-moderate magnifications with a filter.
It is not really surprising that Messier saw it -- it was almost at Zenith in his sky which was somewhat darker overall that the suburban skies of today. He also saw it while observing a comet that passed almost straight over the nebula! Messier didn't discover it though -- it was found by Bevis about 25 years earlier.
We also have to remember that modern values show the whole nebula is now about 13ly across -- a size it has taken 1000-odd years to achieve. Back 250 years ago it was probably more like 8ly across and accordingly would have had higher surface-brightness than now (the integrated brightness is certainly no more than it was 250 years ago and is probably a bit less). In fact, the surface-brightness probably would have been about double then compared to now.
As for nowadays, the red-coloured tendrils that were once the outer envelope of the red-supergiant progenitor star (mainly ionised Hydrogen & Helium) are not really visually observable unless you have a giant telescope. They record well on film and CCD that is much more sensitive to H-Alpha emission.
The part that is more readily visible is the slightly smaller-sized, diffuse, blue emission which is by-and-large powered by synchrotron radiation. Synchrotron radiation is produced by electrons being accelerated within strong magnetic fields (pulsars have extreme magnetic fields) to velocities close to c.
Best,
Les D
P.S 990 -- nearly there!!
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Fascinating too to read about objects that have changed over the last 100-200 years, especially "faint fuzzies" like the Crab neb. I enjoyed this info too thanks Les.