M33: The Farthest Thing You Can See
The tiny northern constellation Triangulum contains the gorgeous face-on spiral galaxy M33, also called the Pinwheel Galaxy. At 3 million light-years away, M33 is a next-door neighbor of the Milky Way. Under dark skies it’s visible without telescope or binoculars. This makes M33 the most distant object you can see with your unaided eye.
The Basics
• M33 is listed as magnitude 5.7, so you may think it’s bright enough to see easily in a telescope. Not so. Its brightness is spread over an area 4x larger than the full moon, so M33 is notoriously hard to find in light-polluted or moonlit sky.
• Some observers use M33 as a sky test: if you can see M33 with your naked eye, you have extremely dark and clear sky (and pretty good vision, too).
• To find this fine galaxy, slowly sweep the region just west of the modest star alpha Trianglulum. Use low power. Moving your scope, or tapping on the side may help you see this faint galaxy. Once you find it, try higher power but don’t reduce the field of view too much or you won’t see the faint spiral arms. Binoculars give fine views in dark sky.
• Try to find the massive nebula NGC 604 (see Good To Know below), using higher magnification and try a UHC or OIII filter.
Deeper Look
• The spiral arms of M33 are loosely bound; the galaxy is a type-Sc spiral using Hubble’s classification system. The galaxy spans a diameter of 50,000 light years, about half the size of the Milky Way.
• M33 belongs to the Local Group of Galaxies, which includes the Andromeda Galaxy, the Magellanic Clouds, and our own Milky Way. M33 may itself be a large satellite of the much larger Andromeda galaxy, M31.
• Astronomers use the Cepheid variable stars in M33 to establish the distance scale of the Universe. But that’s a subject for a future issue.
Good To Know
• The spiral arms of M33 are festooned with the pink glow of star-forming nebula. The largest, NGC 604, is some 100x the size of the Orion nebula and hosts more than 200 massive stars at its center. You can see this extra-galactic nebula with a small scope in dark skies just 0.2 degrees northeast of the center of M33.
Personal View
I haven’t seen this object in 20 years. It’s a tough object in the city, and dark clear winter sky means bone-chilling cold! But M33 is on my list for this winter. I want to see it with a wide-field eyepiece and 4-inch refractor under dark sky.
