Caldwell 11 (North Up, East Left)
Caldwell 11, also known as NGC 7635 and the “Bubble Nebula”, is an emission nebula in the constellation of Cassiopeia.  The nebula was formed when the 8.4 magnitude Wolf-Rayet star SAO20575 (the brightest star inside the central bubble) expelled a shell of dust and gas. The exact classification of the star is uncertain, with a number of spectral peculiarities and inconsistencies between the appearance of the star itself and the effects on the nearby nebulosity, but it is undoubtedly a highly luminous hot massive star. Direct spectroscopy yields a spectral class of O6.5 and an effective temperature around 37,500K. It is a member of the Cassiopeia OB2 stellar association in the Perseus Arm of the galaxy at about 8,500 light-years distance. Although the star is around two million years old, the surrounding nebula is apparently only about 40,000 years old. The bubble is expected to be formed as a shock front where the stellar wind meets interstellar material at supersonic speeds. The wind from the star is travelling outwards at 1,800 - 2,500 km/s, causing the star to lose over a millionth of the mass of the sun every year. The bright blue star to the East SouthEast of SAO20575 is 6.9 magnitude SAO 20562. Thie first image comprises 25 x Luminance (15 seconds each) and 8 x Red, Green and Blue (30, 31 and 30 seconds each), 0.5m f/2.9 ASA Astrograph with FLI ML3200 camera at an altitude of 55 degrees on 31 October 2016.  The field of view is 32’ x 21’. The second and third images comprises 10 x Ha, 8 x OIII and 10 x SII (300 seconds each), 0.5m f/2.9 ASA Astrograph with FLI ML3200 camera at an altitude of 55 degrees over two nights, 15 and 18 July 2017.  The images were processed using the CFHT convention, Ha-OIII-SII mapped to RGB.  The fields of view are 34’ x 23’ and 18’ x 12’.
Hills Observatory: 1 January 2013 to 8 October 2018
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