NGC 6888
The Crescent Nebula - NGC 6888
The Crescent Nebula (also NGC 6888 or the Euro Sign Nebula) is an emission nebula located about 5000 light years away in the direction of the constellation Cygnus the swan. This nebula formed when very fast wind generated by its parent star collided with slower moving wind previously ejected by the star in an earlier part of its life. This star is designated a Wolf-Rayet (WR) star after Charles Wolf and Georges Rayet who discrovered this new class of star in 1867.  The WR star of the Crescent Nebula is visible as the brightest star in the center of the image at 7.5 magnitude. Note the entire nebula is only magnitude 7.4 and about the same brightness as if the star were distributed across its entire 25 light year width (or 150 trillion miles). All WR stars exhibit the strong afore mentioned wind which was first discovered by Wolf and Rayet as broad emission lines in the spectra of the stars. Stellar spectra are formed and studied when a prism or grating is used to separate light into its individual wavelengths as is seen in a rainbow here on earth. All stars have absorption lines in their spectra seen as narrow dark lines caused by various elements absorbing light at very specific frequencies. What is unique about WR stars, however, is that in addition to absorption lines, their spectra also show emission lines or bright lines instead of dark. Equally as important, these bright lines are wide, not narrow like the absorbtion lines. The emission lines had only before been seen naturally in the sun resulting in the discovery of the element helium; this element and all elements when excited enough emit their own light at a very specific and unique frequencies and is how astronomers know the composition of objects in the universe. Equally as important is the width of the lines of WR stars. This width is not visible in the sun, but is caused in WR stars by the doppler effect of the wind moving both toward the observer and away from the observer simultaneously, causing the same emission line to shift a bit toward the blue end of the spectrum and a bit toward the red end of the spectrum. Carefully measuring the width of these line indicates the speed at which the emitted light is moving toward and away from us. Hovering your mouse over the image shows only the luminance image (no color) enhanced to show more of the size and density of the nebula. The stats for NGC 6888 are Bright / Emission Nebula, RA 20h 12m 01s, Dec +38° 23' 00", Mag 7.4, Size: 18.0'x8.0'.
 
Optics: RC Optical System 20" F/8.2 (4165.6 mm Focal Length) Date: July-August, 2020
Camera: SBIG STXL-11000 with AO-X Adaptive Optics Location: Columbus, Texas
Exposure: LRGB = 510:80:80:90 minutes Imager: Kent E. Biggs