Pinwheel Galaxy M33

UA 0.4m image of Pinwheel Galaxy M33

This image of the nearby spiral galaxy M33 (NGC 598) in Triangulum is from a 30-minute V-band exposure obtained with the UA 0.4m telescope, spanning a region almost 0.3 by 0.5 degree. This logarithmically-scaled image shows numerous star-forming regions, especially NGC 604 in the upper left corner. NGC 604 is among the brightest such regions in the entire Local Group, comparable in output to the Tarantula Nebula and perhaps our own NGC 3603.

For historical continuity, here is teh first image posted here some years ago: The nuclear region of the nearby spiral M33 (NGC 598), imaged in the V band using the Lowell Observatory 1.1-meter telescope. This field, 4.7 arcminutes square, shows numerous bright clusters around the nucleus itself, as well as some of the intricate dust-lane structure. The monochrome image has been mapped into a color scale for clarity.

This is one of two bright spirals often called the Pinwheel Galaxy (the other being M101). M33 is bright enough to be a binocular object under reasonably good skies, and occasionally I've picked it up with the naked eye from fairly dark locations. It lies a bit farther away than the Andromeda Galaxy, about 2.7 million light-years away. Observers in these two galaxies would have spectacular views of each other... M33 is the home of one of the two brightest star-forming regions among all Local Group Galaxies, NGC 604 (well seen in this spectacular HST image.

M33 is the nearest spiral with enough star formation to be a prominent ultraviolet source. This allows many kinds of detailed study. As one example, the role of dust is graphically shown in this overlay of the ultraviolet image from the shuttle-based UIT instrument and a visible-light image, where the UV light doesn't just miss the dust lanes, it avoids their whole vicinities. This effect, in which UV extinction mostly acts in an all-or-nothing manner compared to the range of reddening and extinction seen in the optical, has been called the picket-fence effect. This shows that the extended dust still blocks enough UV to cause biases - such as the nucleus looking off-center as well as inconspicuous in the UV (it's there but none too obvious). This kind of thing serves as a cautionary tale for interpreting galaxies at high redshifts (like may in the Hubble Deep Fields) from images in the emitted ultraviolet alone.

M33 (and its brightest star-forming region NGC 604) are so bright in the far-ultraviolet that both their structure and spectra were observed by Voyager 2 before it passed the orbit of Mars, all the more telling because the Voyager UV spectrographs didn't actually use telescopes, just metal boxes to limit their fields of view to slits on the sky. This trace shows the E-W slice Voyager 2 scanned across M33 over a 2-day period, with a separate peak marking NGC 604.

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