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Image of the Week, September 2, 2016


Gamma Cygni Nebula, by André van der Hoeven


Sometimes you decide to make an image of a region of the sky and it just comes out exactly as you had hoped for. This was one of these moments for me. I decided a while back to image the Gamma Cygni Nebula around the star Sadr in the constellation of Cygnus. I knew this was a very interesting region in the sky and I started with gathering H-alpha data for a monochrome 3-panel mosaic as weather in the Netherlands doesn’t permit extended imaging projects most of the time. But somehow we had a very good weather period and I was able to image this region during 6 nights in a 9 day period which is quite unique.

When I noticed the weather forecasts I decided that I wanted to go for a color version of the image in a narrowband Hubble palette. So I just imaged one object during these six nights, but I’m so happy I did that. The combination came out very nicely and really showed me the details that I wanted to get out of the image.

With an apparent visual magnitude of +2.23, Sadr (Gamma Cyg) is among the brighter stars visible in the night sky. Parallax measurements give a distance estimate of 1,800 light years (560 parsecs), with a 15% margin of error. The stellar classification of this star is F8 Iab, indicating that it has reached the supergiant stage of its stellar evolution. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.

Compared to the Sun this is an enormous star, with 12 times the Sun’s mass and about 150 times the Sun’s radius. It is emitting over 33,000 times as much energy as the Sun, at an effective temperature of 6,100 K in its outer envelope. This temperature is what gives the star the characteristic yellow-white hue of an F-type star. Massive stars such as this consume their nuclear fuel much more rapidly than the Sun, so the estimated age of this star is only about 12 million years old.

The spectrum of this star shows some unusual dynamic features, including variations in radial velocity of up to 2 km/s, occurring on a time scale of 100 days or more. Indeed, on the Hertzsprung–Russell diagram, Gamma Cygni lies close to the instability strip and its spectrum is markedly like that of a Cepheid variable. This star is surrounded by a diffuse nebula called IC1318, or the Gamma Cygni region. [source: Wikipedia]

The field of view spans over 3 degrees (six full moons) on the sky and includes emission nebula IC 1318 and open star cluster NGC 6910. Right of Gamma Cyg and shaped like two glowing cosmic wings divided by a long dark dust lane, IC 1318’s popular name is understandably the Butterfly Nebula. Below and right of Gamma Cyg, are the young, still tightly grouped stars of NGC 6910. Some distance estimates for Gamma Cyg place it at around 1,800 light-years while estimates for IC 1318 and NGC 6910 range from 2,000 to 5,000 light-years. [source: APOD]

This image shows the region taken with my TMB92SS and QSI583ws camera with a resolution of 4.35 arcsec/pixel.

Exposure info:

H-alpha: 56x900s (14h)
OIII: 27x900s (7h)
SII: 24x900s (6h)

Adding up to 27 hours of imaging data in total.

Image center:
RA: 20 21 22.492
Dec: +40 09 39.20

About the photographer

André van der Hoeven
Location: Netherlands


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