Origin of name and discoverer
The Pinwheel Galaxy, also known as M101 and NGC 5457, is a Messier object, discovered in 1781 by the French astronomers Charles Messier and Pierre Méchain. It is the 101st object in the Messier catalog: a list of 110 non-comet diffuse astronomical objects that Charles Messier created in 1774 to help astronauts interested in locating comets. His purpose was to distinguish comets from nebulae to avoid confusion of which Messier himself was a victim. Although he called the objects of his catalog nebulae and star clusters, it also included galaxies among which the Pinwheel Galaxy which owes its nickname to its spiral pinwheel-like shape.
The Pinwheel Galaxy is located at 14h 03m 12.6s right ascension and +54°20’57” declination. It forms a triangle with the double star Mizar-Alcor and the Alkaid star that we can easily find right above the handle of the Big Dipper constellation. M101 is about 25 million light-years from our home planet. It is only visible from the Northern hemisphere. For the Southern hemisphere observers, another galaxy, M83, is considered the southern Pinwheel, because of their similarities.
With a magnitude of 7.9, the Pinwheel Galaxy (M101) is observable through binoculars under excellent conditions. But it will look like a hazy patch of light. With a small telescope, we can observe the galaxy’s white central bulge, but not the spiral arms. The bigger the telescope, the more details appear.
The galaxy is about 170,000 light-years across, which is 70% larger than the Milky Way (100,000 light-years across). It occupies about 28.8 by 26.9 arc minutes of the sky, which is approximately the angular size of the full Moon.
M101 is a spiral galaxy. Spiral galaxies are a type of galaxies characterized by a central bulge of old stars surrounded by spiral arms of young stars. The Pinwheel Galaxy appears face-on, clearly showing its beautiful spiral arms.
The bulge at the center of the galaxy is composed essentially of old red stars. It is a region where there is little star formation compared to the surrounding spiral arms region.
It holds about one trillion stars, which is a thousand times more than the number of stars in our Galaxy. The combination of visible, X-ray and infrared images showed that these stars are evenly distributed along the galaxy’s arms.
One of the most prominent features of the Pinwheel Galaxy is that it has an unusually high number of star formation regions, characterized by H II emission, detected using H-alpha filters, like the one we used in this image, obtained using the telescope Skywatcher ED80, and a ZWO ASI 183 MM Pro Mono Cooled camera. These H II regions appear in red. We also used an LRGB and a Baader 1.25″ filters to show the amazing details in colors. Bright clumps that host active star formation are presented in red on this image, and in blue on this one, due to the different filters used. The center appears yellowish because of the old stars it hosts, which are usually red. The dark lanes are cold and dense interstellar clouds of gas and dust that may collapse to form new stars.
M101 is not alone. It has at least eight companions, all are gravitationally bonded. The whole forms the M101 Group.
In the last century, astronomers observed three supernovae in the Pinwheel Galaxy (SN 1909A, SN 1951H, and SN 1970G). In 2011, they detected another Ia supernova, SN 2011fe. It is a kind of supernova that happens in binary systems where a dwarf star, orbiting around another star or a black hole, explodes.
The Pinwheel Galaxy is a whole world of wonders. It shows us supernovae, in addition to X-ray binaries, spiral arms holding star formation that we can witness. It is like the Milky Way’s big sister. The more we learn about it, the more we learn about our home Galaxy.