NGC 6888 The Crescent Nebula
Distance: 5,000 Light Years Telescope: Takahashi TOA-130
Magnitude: 7.4 Camera: QSI 683
Size: 18×12 Arc-minutes Mount: AP 900
Age: 250-400K Years Exposures: L 15×900 Bin 1, Ha+OIII 7×900 Bin 1
NGC 6888, the Crescent Nebula, is an emission nebula in the constellation Cygnus. The illuminating source is the Wolf Rayet star WR 136. In general a Wolf Rayet star is a very large hot star that is rapidly shedding mass in the form of ultraviolet radiation. WR 136 is estimated to be 250,000 times brighter than our sun, 15 times more massive, and with a temperature of 70,000 Kelvin. With WR 136, the ejected matter is leaving at a speed of 6.1 million kilometers per hour. This high speed matter hits the ambient dust and gas and gives the Crescent nebula its shape. Part of the surrounding gas and dust is previously ejected material from WR 136 when it was a red giant about 250,000 years ago.
NGC 6888 is a windblown bubble formed by a Wolf-Rayet star that is shedding very hot material in to the surrounding inter stellar medium. It is the gas and dust in the surrounding medium that is compressed in to a thin shell giving the nebula its shape. There are actually two waves of shocked gas, one corresponding to the shocked stellar wind and the other to shocked interstellar gas. The hot interior of this bubble is where the energy is stored and subsequently used for driving the entire structure.
The top image is composed of 900 second Ha and OIII images. The entire area around the nebula is rich in OIII while the interior area is stronger in Ha. This is the first time I tried imaging the Crescent nebula in narrow band and was a bit surprised to see the large OIII ejection area outside of the arc. I did a bit of research on this and a 2014 paper [arXiv:1310.2801v2] said the following:
“The hot gas in NGC 6888 is distributed inside the optical shell delineated by OIII emission. The spatial distribution of the X-ray emission shows enhancements towards the caps and a blowout present in the NW region of NGC 6888. This blowout, not discussed in previous studies, has no Ha counterpart, but an outer skin of OIII is detected. The X-ray emitting gas is, thus, traced by Ha clumps inside the nebular shell and by the blowout. No clear evidence of limb brightening is detected.”
The image below is an LRGB combination captured from my house in an orange zone. The exposures are L 900 seconds bin 1 + RGB 450 seconds bin 2. I was pleasantly surprised that I could get 15 minutes at F7.7 without any difficult gradients.