Southern Pinwheel M83

[AusGuy]

Last night was a rare event. No Moon all night. No clouds. Very little light pollution. And no dew. My theoretical Bortle 4 skies don't often perform to specification but last night they certainly did. And it's M83 season! It rises just before dark, transits just after midnight and sets just after dawn. And when it transits it's just 3 ½ degrees away from zenith. What more could one want?

For EAA work M83 is big, bright and splashy. At a public event it's best to use it to end one's viewing session. It is such a showstopper that most everything else that might follow would be a bit of a come-down. So what makes it so easy to view? The answer is primarily that it is very close – well at least as far as galaxies go. Just shy of 15 million light years. It's almost face-on to us so it's easy to make out its central bar and its spiral arms. And it is undergoing a massive amount of starburst activity which makes for an interesting juxtaposition of its many bright HII knots and clusters with the blue of the myriad stars in the body of the arms. This starburst activity is clearly located mainly along the leading edges of the arms. Add in the yellow glow from the core and you have got quite a colourful display.

Of course there are many articles about, and amazing images of, M83 on the internet and they are well worth looking at. M83 is about half the size of the Milky Way. Apparently it is at the centre of a group of galaxies and it is possible that tidal interaction with its near neighbour NGC 5253 has caused the starburst activity that we now see. That it is a turbulent region is confirmed by our having detected six supernovae there in the last 100 years.

It seems that M83 was first discovered by Nicolas Louis de Lacaille in February 1752 when in South Africa. With our experience using our fancy modern equipment we no doubt blithely assume that he saw something like what we now can so easily see. But that was not the case at all. His description was “Small nebula, shapeless”. His discovery must have encouraged Messier to look for it from Paris in February 1781 even 'though it was so low in his sky – it is the southernmost galaxy in his catalogue. Messier's description was “Nebula without star ... it appears as a faint & even glow, but it is difficult to see in the telescope, as the least light to illuminate the micrometer wires makes it disappear. One is only able with the greatest concentration to see it at all”. Even James Dunlop in 1827 had difficulty in determining was he was looking at from Australia. He described it as “a very beautiful round nebula, with an exceedingly bright well-defined planetary disk or nucleus, about 7" or 8" diameter, surrounded by a luminous atmosphere or chevelure, about 6' diameter. The nebulous matter is rather a little brighter towards the edge of the planetary disk, but very slightly so. I can see several extremely minute points or stars in the chevelure, but I do not consider them as indications of its being resolvable, although I have no doubt it is composed of stars”.

Vixen R200SS; SW AZ-EQ6 Pro; ZWO ASI294MC Pro; Baader MPCC; Bortle 4.

SharpCap livestack 20 x 60 seconds exposures at 400 gain. Minor adjustments in Photoshop.

 

Edited April 14, 2021 by AusGuy

[Mike JW]

Superb.

Mike