M33 is a spiral galaxy high in the nothern sky in the autumn, into the winter, about 2.7 million light years from us (very close, in cosmic terms); it is among the most distant celestial objects that can be seen with the naked eye (only in very dark skies). It is thought to be gravitationally-bound to Messier 31, the Andromeda galaxy, the largest galaxy in our local group of galaxies. The pink regions are ionized hydrogen gas, indicating star-forming regions; perhaps the intense star formation is related to gravitational interaction between M33 and M31. The largest pink region, toward the right center of the image, is NGC604, a huge (more than 1500 light years across) star-forming region, reminiscent of (but even larger than) the Tarantula Nebula, in one of our satellite galaxies (the Large Magellanic Cloud). The amount (and sizes) of the star-forming regions is unusual in my experience of imaging galaxies; I have included a pure-Ha version of the image to show how much Ha emissions (indicating intense star formation) are in that galaxy; click on the image to toggle between the two.

M33 has a diameter of about 60,000 light years (a good-sized galaxy, but about half that of our Milky Way). It is thought to contain about 40 billion stars, compared to the 400 billion stars in our Milky Way (and 1,000 billion stars--one trillion--in the Andromeda galaxy).

The galaxy is tilted 54 degrees from us, so that it looks more elliptical than it is.

The entire field of the photo is about the same width as a full moon.

I last imaged this striking galaxy 19 years earlier. To see what a combination of better equipment, better skies, and better skills does, click here

Technical Information:

(HaL)(HaR)GB: 600:660:180:180:240 (a total of 31 hours of exposures); the luminance layer consists of blend of 32 fifteen-minute images using a luminance filter, 36 five-minute images using a luminance filter, and 20 thirty-minute images using an Ha filter; the R channel is a blend of 12 fifteen-minute images taken through a red filter and the Ha data; G consists of 12 fifteen-minute images taken through a green filter, while B is the combination of 12 twenty-minute images taken through a blue filter.

Equipment: RC Optical Systems 14.5 inch Ritchey-Chrétien carbon fiber truss telescope, with ion-milled optics and RCOS field flattener, at about f/9, and an SBIG STX-16803 with internal filter wheel (SBIG filter set), guided by an SBIG AO-X/STX Guider, all riding on a Bisque Paramount ME German Equatorial Mount.

Image Acquisition/Camera Control: Maxim DL, controlled with ACP Expert/Scheduler, working in concert with TheSky X.

Processing: All images calibrated (darks, bias and sky flats), aligned, and combined in Pixinsight. Color combine in Pixinsight. Some finish work (background neutralization, color calibration, deconvolution, gradient removal, Multiscale Linear Transform for noise reduction and sharpening, and blending the Ha data with the broadband data) done in Pixinsight; some finish work (LRGB combination, contrast and saturation adjustment) was done in Photoshop CC.

Location: Data acquired remotely from Sierra Remote Observatories, Auberry, California, USA.

Date: Images taken on many nights during November and December of 2021. Image posted December 26, 2021.

Date: Image scale of full-resolution image: 0.56 arcseconds per pixel.

Seeing: Generally very good, with individual luminance images having FWHM varying from 1.7 to 2.6 arcseconds; the luminance master was deconvolved to 1.5 arcsecond FWHM.

CCD Chip temperature: -25C

Copyright 2021 Mark de Regt

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