NGC 6357 - Emission Nebula in Scorpius

                    
 

John Herschel discovered this nebula in 1837 from the Cape of Good Hope, and cataloged it as h 3682 in his 1847 catalog. It became GC 4297 in his General Catalogue of 1864, and NGC 6357 in J.L.E. Dreyer's NGC.

A perfect line-up of four 7th magnitude stars point towards the heart of this faint emission nebula. It is a difficult visual object because of its very low surface brightness and large extension: about the size of two full moons. Just like neighbouring NGC 6334, it is heavily obscured by galactic dust: 10 magnitudes of extinction in the green and much more in the blue.

Because of this, this star-forming nebula shows no evidence of a blue component in its colour - nor is there any obvious sign of the bright blue stars normally found in these objects. It is an excellent example of the phenomenon known as 'interstellar reddening', the selective removal of blue light by minute particles of dust in the line of sight. This accounts for both the ruddy hue and apparent absence of blue stars. The hot stars are present but only some of the red part of their light is seen so they are not conspicuous.

NGC 6357 is quite nearby (5500 light years) but located close to the Galactic plane and are buried in the dust of the Milky Way. Careful measurement of the colour of stars associated with the nebulae indicate that they are dimmed by a factor of about 10 in the green part of the spectrum, much more in the blue, but relatively little at red wavelengths. .

Text taken from www.seds.org and www.aao.gov.au

Here is the same nebula shot in narrowband.  Using the same equipment as below, this image took several weeks to capture for a total of 26 hours exposure.

     Med Size
 (2865 x 1941)

     Full Size
(3820 x 2588)                              

Equipment

Telescope:                Takahashi FS128 @ f6
Mount:                      Paramount ME by Software Bisque
Instrument:               STL11000M CCD Camera by SBIG with Astrodon filters

Exposure:                  This is an (HA+R)RGB Image of 390:140:140:140 mins
Capture Software:       CCDSOFT V5 and CCD AutoPilot
Processing:                Maxim DL/CCD, Photoshop CS2

Narrowband Image:     Using Astrodon NarrowBand filters, the individual    
                                exposures are:
                                                    HA - 6.5 hours
                                                    OIII - 7.5 hours
                                                    SII - 7 hours

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