Object of the Week October 27, 2019 - The Pinwheel Treasures
Constellation: Triangulum
H II Regions
A66RA: 01h 33.8m DEC: +30° 45’
A71RA: 01h 34.1m DEC: +30° 48’
IC 135, A100RA: 01h 34.3m DEC: +30° 37’
IC 136, A101RA: 01h 34.3m DEC: +30° 34’
IC 137, A12RA: 01h 33.6m DEC: +30° 31’
IC 139, A4RA: 01h 34.0m DEC: +30° 34’
IC 140, A5RA: 01h 34.0m DEC: +30° 33’
IC 142, A67RA: 01h 33.9m DEC: +30° 45’
IC 143, A75RA: 01h 34.1m DEC: +30° 47’
NGC 588, A27RA: 01h 32.8m DEC: +30° 39’
NGC 592, A59RA: 01h 33.0m DEC: +30° 35’
NGC 595, A62RA: 01h 33.8m DEC: +30° 42’
NGC 604, A84RA: 01h 34.5m DEC: +30° 47’
Globular Cluster
C39RA: 01h 34.8m DEC: +30° 22’
Triple Star
NGC 603RA: 01h 34.8m DEC: +30° 22’
My first telescope many, many years ago was a 6” dobsonian. I fondly recall first light on that scope, shared with David Levy, the well-known comet hunter and astronomy author, at the Hidden Hollow Star Party. That night and the next was absolutely pure magic for me, as I was guided from one deep sky jewel after another in the Autumn sky. I distinctly recall my first three galaxy observations. First was the magnificent Andromeda galaxy (M31). The Andromeda galaxy was clearly visible to my naked eye in the semi-rural skies outside of Mansfield, Ohio. In my scope it was…wow! It seemed to go on forever and I could easily discern brightening toward the core and…double wow…that fuzzy patch to one side I was told was another galaxy called NGC 205. Next up was the Pinwheel Galaxy (M33) in the constellation Triangulum. Unlike the Andromeda Galaxy it was not visible to my naked eye nor in my Telrad (zero-power) finder. So David star-hopped for me to M33. At first, I didn’t see anything in the eyepiece. I looked again and again. Finally with some averted vision and perhaps some creative imagination, I thought I saw some haze. What a contrast between M31 and M33 for a novice observer!
I later learned my experience was far from unusual. Inexperienced observers cite M33 as one of the hardest objects in the Messier catalog. Why is this? Its large size, low surface brightness, and lack of a bright central disk make it a challenging object for small telescopes. The problem is the observer, expecting to see something much smaller and brighter, fails to spot its dim glow which covers an area larger than that of the full Moon. Sky conditions are a critical factor; the least bit of haze can render it invisible. However when viewed near the zenith under clear, dark skies with a large aperture telescope (20” or greater) your first view of M33 may shock you!
This week’s OOTW is not about the Pinwheel Galaxy (M33) - it is about some remarkable deep sky treasures inside the galaxy that are visible under dark, transparent skies with a large aperture telescope at high power.
A pioneering guide to these objects in M33 appeared in Observing Handbook and Catalogue of Deep-Sky Objects by Christian B. Luginbuhl and Brian A. Skiff (Cambridge University Press, 1990). They started the tradition of naming knots within M33 based on a paper published in the November 1980 Astrophysical Journal Supplement Series by Roberta M. Humphreys (University of Minnesota) and Allan R. Sandage (Carnegie Observatories). The names consist of the letter A followed by a number.
More than two-dozen individual clusters, stellar associations, and H II regions are visible as distinct objects in amateur-sized scopes, but many do not reveal themselves without effort. And since moderate to high powers are necessary to detect many of the objects, the observing conditions must allow for the higher magnifications required.
The largest and easiest H II region to spot is NGC 604 (A84) which lies near the tip of the galaxy’s main northern arm. A novice observer may mistake it for an unrelated object resembling an elliptical galaxy. OOTW contributor Howard Banich documented NGC 604 in his OOTW for December 6, 2013. Click here to read it.
The following is a list of visible objects within M33 compiled by Sky and Telescope contributing editor, Ted Forte. He observed these from the dark skies outside Sierra Vista, AZ with an 18” telescope at 197x and 262x.
- A66 Large, bright patch in the northern arm
- A71 Bright spot adjacent to IC 143
- C39 Tiny, faint, very challenging; M33’s largest and brightest globular cluster
Conspicuous in their relative isolation, two nebulous stellar associations like to the east of the southern arm. IC 135 and IC 136 glow with the combined light of their component stars and surrounding gas. Both are a little elongated and nearly resolvable.
- IC 135 (A100) Bright, elongated, isolated knot
- IC 136 (A101) Larger, elongated, mottled knot
- IC 137 (A12) Large, formless glow in the southern spiral arm
- IC 139 (A4) Together with IC 140, bright knot in the combined glow
- IC 140 (A5) Together with IC 139, bright knot in the combined glow
- IC 142 (A67) Stellar knot in the northern arm
- IC 143 (A75) Elongated know in the northern arm
- NGC 588 (A27) Round, less obvious than other NGC knots. On the outskirts of the southern arm. Larger than NGC 592 but fainter. Appears round.
- NGC 592 (A59) Smallish, but easy to spot. Smaller and more star-like, it is easier to spot, lying in a blank area west of the core and inside the curve of the southern spiral arm.
- NGC 595 (A62) Large and bright oval masked by nearby core. Brightest and largest H II region after NGC 604. May be overlooked because it lies just 4’ northwest of the galaxy’s core.
- NGC 603 Triple star that Lord Rosse noted as a nebulous object, lies 28’ southeast of M33’s core
M33_treasure_map.jpg
So much for my nearly invisible galaxy. I can’t wait for my next dark sky outing to give these objects a try with my 15” scope.
Now it is your turn.
Give it a go and let us know!
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