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Messier 8, Messier 20 and IC 4685 in Hydrogen-Alpha Light

This wide field image was photographed with a hydrogen alpha filter which highlights the emission nebulosity Messier 8 and the surrounding area. The large bright area just below the center of the image is Messier 8. Messier 20 can be seen at the top of the image and another large area of emission nebulosity, which includes IC 4685, IC 1274, IC 1275 and NGC 6559 is at the left edge of the image.

Messier 8 can be found in the constellation of Sagittarius and is a wonderful summertime viewing object. Also known as “The Lagoon Nebula” and by its New General Catalogue designation of NGC 6523, Messier 8 is one of only two emission nebula which can be seen with the unaided eye under sufficiently dark skies in the northern latitudes. The other is Messier 42 in the constellation of Orion. Messier 8 can easily be seen with small telescopes or binoculars under moderately dark skies.

Messier 8 is a large area of ionized atomic hydrogen which is very bright in the hydrogen-alpha emission line spectra. Astronomers have determined that the main source of the ionization of much of the visible nebulosity of Messier 8 is the large binary star system 9 Sagittarii. The 9 Sagittarii system is comprised of two giant main sequence class O stars. Each of the stars is several hundred thousand times the brightness of our own Sun. Massive amounts of ultraviolet radiation emitted from the 9 Sagittarii system ionizes the surrounding hydrogen gas clouds which in turn release photons which include strong hydrogen-alpha emission line emissions. Those emissions fall within the “red” area of the visual spectrum which accounts for the predominantly red or pink appearance of Messier 8 in color photographs. The 9 Sagittarii system can be seen in the images below as the bright star just to the left of the brightest area of the Messier 8 nebulosity.

The open cluster NGC 6530 can be seen at the center of the Messier 8 nebulosity.

Astronomers have estimated that the size of the Messier 8 nebulosity is about 110 x 50 light years and is somewhere between 4,000 and 6,000 light years distant. Messier 8 is believed to be a massive star forming region.

Messier 8 was first described by Giovanni Hodierna in 1654. It was latter added to Charles Messier’s famous catalogue as object number 8 1764.

Also captured at the top of the image is Messier 20, which has its own emission nebula component. Messier 20 has both emission nebula and reflection nebula components and is a spectacular red and blue object in color photographs. However, since this image was taken with a narrow bandwidth filter which only passes light near the wavelength of hydrogen alpha emissions [656 nanometers] the blue light from the reflection nebula component is blocked. The emission portion of Messier 20 is also believed to be an area where stars are presently being formed. Messier 20 has several areas of very prominent dark nebulosity.

IC 4685 is a portion of the area of bright nebulosity on the eastern side [left side of this image] of the Messier 8 area. A prominent feature of IC 4685 are the areas of dark nebulosity.

In addition to the wide field view, I have also posted 100% resolution crops of Messier 8, Messier 20 and IC 4685 below which nicely highlight the bright and dark areas of each object.

Exposure Information – 50 x 270 second exposures

Image Date – May 20, 2018

Equipment – Stellarvue SV70T with dedicated SFFR70 reducer/field flattener, ZWO ASI 1600MM-C camera, Baader 7.5nm hydrogen-alpha filter.

Processing – All image calibration, alignment and image stacking [integration] was performed with PixInsight. The LocalNormalization and the DrizzleIntegration tools were also implemented in the image integration process. The stacked image was stretched with the PixInsight HistogramTransformation tool and then saves as a 16 bit TIFF file for export to Photoshop. No noise reduction or sharpening processes were used in PixInsight.

I used a three-layer masking technique to help visualize the bright central portion of the nebula which would otherwise have been mostly featureless and pure white. I took an unstretched linear copy of the stacked image and layered it onto a moderately stretched copy using layer masking to allow visualization of the core area and most of the central area of the nebula. I then took that layered composite image and layered it onto a third stretched version [again using the layer masking technique] which had the core of M8 burned out but nicely showed the surrounding dim nebulosity. This final version is the composite of those three differently stretched versions. Lastly, I brought out more of the core contrast by using a high-pass filter set at 30 pixels which was used with a luminance mask allowing the high-pass filter to operate only on the bright central core area.

The images below are higher resolution versions. Click on each to see a larger image.



Posted in Bright Nebula, Processing Notes

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