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 14 August 2018