Crescent Nebula NGC 6888

 

By Gary J. Becker, MD, former ABR Executive Director

Recently, I was asked to provide a little more detail about how the remote acquisition of astroimages at my observatory in Benson actually occurs under control of my MacBook Pro here at home in Tucson. I run all of the acquisition software programs (listed below for techno-geeks) on a desktop PC at the observatory in Benson, and I simply access the desktop PC from my MacBook Pro using TeamViewer software. None of the astronomy software is actually running on my Macbook.  Other access to the observatory that I use routinely includes a Digital Logger power controller, which enables me to separately power on/off the camera, telescope, focusing motor, room lights, flat light, telescope heater, and infrared webcam. The latter is important for confirming scope position/clearance before closing the roof.  I also have a separate roof power switch that enables me to open/close the roof under direct observation via the Webcam.

The image above, depicting emission nebula NGC 6888, was acquired at my remote observatory in Benson, AZ, in July 2018. This is the first time I’ve shared images of this object since I began working with my new camera, and the results are superior to any I’ve been able to obtain in the past. NGC 6888 is found in the direction of the constellation Cygnus in the summer night skies of the northern hemisphere, at a distance of approximately 4,700 light-years (27.6 quadrillion miles) from Earth. It is approximately 25 light-years (147 trillion miles) across.

The unusual rippled appearance of this nebula is attributed to the star at its center, a special kind of star known as a Wolf-Rayet star (in this case, WR-136). WR stars, named for Charles Wolf and Georges Rayet who discovered this object type at the Paris Observatory in 1867, have masses many times that of our Sun, larger size, and are brighter and hotter than our Sun as well. Comparing WR-136 with our Sun, the former is 15 x more massive, 3.3 x the size, 250,000 times the brightness, and has a surface temperature of approximately 70,000 degrees Kelvin. From an astronomical science point of view, the feature that distinguishes WR stars as rare is the broad lines in their emission light spectra indicating the presence of helium, nitrogen, carbon, silicon, oxygen, and nitrogen, but very little hydrogen.

What is actually happening here (i.e., what does the image depict)? WR-136 is only about 4.7 million years old (0.1% of the Sun’s age), but is nearing the end of its stellar life.  A more massive star to begin with, it rapidly used up the hydrogen fuel in its core in nuclear fusion, and blew off an outer shell of gases from its upper atmosphere some 200,000 years ago when it became a red giant star. Now it is rapidly moving through fusion of helium and heavier elements identified in its emission spectrum.  In the process, WR-136 continues to shed its outer envelope at a rate equivalent to one solar mass every 10,000 years (a very rapid rate).  Only now, the stellar wind ejected from WR-136 at about 3.8 million MPH, is catching up with and impacting the previously ejected material to shape the shell that is NGC 6888.  Highly energetic UV rays emitted from WR-136 cause the gas in the outer shell to glow.  A few hundred thousand years from now, when WR-136 is attempting to fuse iron in its core, it will explode as a supernova.

For the techno-geeks, following describes the image planning, acquisition, and processing:

Hardware:

Celestron CPC 1100 telescope fork-mounted with wedge onto a pier and polar-aligned for simple equatorial mount

Hyperstar Optics

QSI 683 CCD Camera

F 2.0

Focal length: 561mm

Pixel size: 1.99” per pixel

FOV: 110.5’ x 82.9’

Astrodon filter set; each filter (Red, Green, Blue, Hydrogen-alpha) utilized separately

Microtouch Automatic Focusing motor

Starlight Express Lodestar X-2 Autoguider

Software:

Planning Software: CCD Navigator, Megastar, ACP Planner

Acquisition Software:  TheSkyX Pro, Maxim-DL6, FocusMax 4, ACP Observatory Control

Remote: TeamViewer for access to observatory desktop

Image Processing Software: Pixinsight and Adobe Photoshop CC 2017

Acquisition:

Total imaging acquisition time: 3 hours, 28 minutes; all RGB images unbinned; H-alpha images binned 2×2; individual frames each 480 sec

RA Center: 20:12:0.0

Dec: 38:21:0.0