M33 Triangulum Galaxy

Enhanced Hydrogen Comparison

Spiral Galaxy in Triangulum

Uploaded 10/26/06

Normal 3 hours of RGB Exposure (G2V) Select an image size for a larger view: 1024 x 768 1600 x 1200
3h RGB + 4h of Hydrogen Alpha Select an image size for a larger view: 1024 x 768 1600 x 1200

This pair of images should satiate both camps: Those purists that insist on genuine G2V calibrated colors (Left), and those who would like to see a more complete picture of the stellar dynamics in process (Right). First, let's give a brief description of the galaxy in the images above, and then we will go into some of the details of the pair of shots.

M33 - Local Group Face on Spiral

Known also as NGC598, this 6th magnitude object is spread over a huge 70 x 40 arcminute field which we are looking at about half of it here in these images. Classed as a SA(s)cd spiral, it is inclined only 4 degrees to our line of sight. Its fairly neutral in color overall, with a B-V index of .55, which makes it somewhat "tawny". The core is very unconcentrated, and in fact we can see the bare nucleus near the center, surrounded by a dim yellowish disk with dark brownish dust lanes traceable all the way into the core. This galaxy has more free hydrogen than any other Local Group galaxy in the northern sky, and produces some very enlightening comparisons when additional exposures are added that enhances the hydrogen lines.

Comparing the images

Images of Local Group galaxies tend to share a common characteristic with CCD images and my 12.5" - They almost always appear very granular or grainy looking. This is because we are resolving the galaxies individual stars, and even on the largest images here, the galaxy will show tiny blue and yellow stars down to the magnitude limit in huge numbers. There is not a piece of this image that does not show this granular appearance in fact. On the left standard RGB image, the yellow core is surrounded by blue highlights in the arms. These are clusters of star clusters, known also as OB associations. They consist of dominantly blue supergiant stars with a maximum brightness of magnitude 17.5 and down to the images estimated limit of 22.5 magnitude. Only hinted at here is the amazingly complex interaction between the hydrogen and the OB associations. For this, we turn to the right image, that has been Halpha/Hbeta hydrogen enhanced.

The right image has been produced in a special way which can combine every shred of hydrogen alpha nebulosity in a deep (4h) dedicated set of images of the same field as the RGB frame. The entire galaxy is filled with an enormous amount of pink glowing hydrogen. From concentrations along the edges of the arms, to small linear wisps all the way into the very core. Also huge numbers of very dim red filaments are in the spaces between the major regions in the arms, yielding a soft general hydrogen glow to the entire disk. If you look at the larger image, you can see the fine hair like wisps everywhere. Now lets take a look more closely at some of the details that can only be seen by this type of image.

Below are some blowups of small areas of the original frame to illustrate the unique processes in this galaxy that can be visualized by hydrogen enhancement:

The top row shows how the OB associations with their hot fierce stellar winds will blow bubbles in the hydrogen, forming dark areas around the O/B stars in the cores. These three bubbles show this process very well and are all but invisible in the standard RGB image.

Some additional closeups of OB associations. (O and B are hot blue spectral types that emit large amounts of ultraviolet radiation) You can still see the areas immediately adjacent to the stars has been cleared out, and some of the fantastic shapes and structures of nebulosities are seen here. The huge NGC604 on the right is blown out a bit in its core, but is the largest bright nebula in M33.


I must admit to you that I belong to both camps - I enjoy the purity of the precise G2V calibrated RGB images very much, because it transports my mind to that galaxy and I can see it as the eye might if I could make it big and bright enough. On the other hand, I want all the information and details across the entire spectrum to view and contemplate. I love to see the amazing intricate interactions between nebulosity and stellar groupings. Mix in swirls of dust, a black hole or two in the core, and I'm happy!


A standard RGB non binned image with 10 minute subframes totaling 3h total was produced in the normal way: Calibrate, curves, and cosmic ray cleanup. On two other nights 4h of Ha data was taken, combined with Gralaks Sigma Reject using standard deviation masking to minimize artifacts, and cosmic ray hits. The two were combined in Photoshop CS2 by first adjusting the Ha image to range from complete black to nearly pure white - but NOT white clipped anywhere. The RGB data was split again into its components and the Hydrogen data was projected onto first the R channel at 100% using "Screen" and then onto the B channel at 15% the same way to fill in the Hbeta information. The results were recombined into RGB again. The final result is the correct pink coloration, with the stars and the remainder of the galaxy very close to G2V in coloration.

Instrument: 12.5" f/5 Home made Newtonian Platform: Astrophysics 1200 QMD CCD Camera: SBIG 10XME NABG with Enhanced Water Cooling Guider: SBIG ST4 Exposure: Ha+LRGB = 240:60:60:60 RGB Combine Ratio: 1: 1.05: 1.11 Filters: AstroDon RGB Tricolor Location: Payson, Arizona Elevation: 5150 ft. Sky: Seeing FWHM = 5 arcsec (Maxim DL - 10min subframe), Transparency 9/10 Outside Temperature: 40F CCD Temperature: -20 C Processing Tools: Maxim DL, Gralaks Sigma, Photoshop, PixInsight, Starizona Debloomer. HOME GALAXIES EMISSION NEBS REFLECTION NEBS COMETS GLOBULARS OPEN CLUST PLANETARIES LINKS


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