Messier 101 – The Pinwheel Galaxy



M101, or the Pinwheel Galaxy, is a spiral galaxy residing in the constellation Ursa Major (the Great Bear). It has a magnitude (relative brightness) of around 7.8. Located near the handle of the Big Dipper, the best time to view this spectacular galaxy is in April, though it’s visible between the months of September and October.

If you know the location of the Big Dipper, M101 is fairly easy to find. Focus in on the two stars at the end of the Big Dipper’s handle: Alkaid and Mizar. It’s about 5.5° northeast of Alkaid, the third most luminous star in Ursa Major. Alternatively, you can find the double star, Mizar & Alcor, and head about 5° West. If you run into the tip of the Boötes constellation, you’ve gone a bit too far.


Since M101 is nearly an eighth magnitude galaxy, it’s visible to telescopes and binoculars, given the right conditions of course. Though the resolution won’t be great, the Pinwheel Galaxy is easily locatable using only a small telescope. However, with a small telescope or binoculars, only the central region of the galaxy will be viewable—none of the intricacies of the spiral arms will be revealed. In order to view the more complex details, such as the young stars spattered across the arms, you’ll need a larger telescope and access to a location with little to no light pollution.

Ideally, you’ll want to use a telescope with a focal length of around 1300mm, which is pretty large. The galaxy is still viewable with smaller telescopes, such as an 80 mm refracting telescope, but it’ll appear much smaller. With a focal length of 1300mm or more, and a perfectly dark location, the individual stars and spiral arms are much easier to distinguish. If at all possible, we highly recommend viewing it with at least a moderately a powerful telescope. M101 is a truly glorious and magnificent galaxy to behold. It’s entirely worth it!


Messier 101 is located about 25 million light-years away from Earth and is 170,000 light-years in diameter. With over 1 trillion stars residing within, the Pinwheel Galaxy has ten times as many stars as the Milky Way.

Aside from the illustrious arms spiraling out of its core, M101 has a few distinct features residing within. Of particular note, it has a very high population of H II regions. H II regions are basically large clouds of partially-ionized gas which give birth to new stars. M101 has over 1264 H II regions and many of the stars within there are even capable of creating superbubbles. As with most spiral galaxies, the central core contains the majority of the older stars. The outskirts of the galaxy, especially the arms, are filled with lovely and bright blue stars recently birthed from H II regions.

Galaxies across the universe are stuffed full of fascinating quandaries and M101 is certainly no exception. In 2001, the Chandra X-ray Observatory discovered that deep within M101 is an ultra-luminous x-ray source, properly named ULX-1. Later in 2005, observations by Hubble and XMM-Newton revealed that ULX-1 is most likely an x-ray binary star, which are fairly rare. X-ray binary stars emit x-rays, an exceptionally high-energy form of electromagnetic radiation, and are accompanied by a collapsed star such as a neutron star or a black hole.


M101 was discovered in 1781 by Messier’s colleague, Pierre Méchain. M101 was one of the final entries in the Messier Catalogue. Méchain described it as “a nebula without star, very obscure and pretty large, 6 to 7 arcminutes in diameter, between the left hand of Boötes and the tail of the Great Bear.”

Just three years later, William Herschel noted that a larger reflector would obviously help to make out the details of the nebula. “In my 7, 10, and 20” reflectors shewed a mottled kind of nebulosity…I expect my present telescope will, perhaps, render the stars visible.”

It wasn’t until the 19th century that anyone was able to make out and describe the spiral structures of the galaxy. In 1861, Lord Rosse made 8 observations and 3 individual sketches, describing M101 as “Large, spiral, faintish; several arms and knots. 14’ diameter at least.”

M101 still remains a source of fascinating study for astronomers today. Five supernovae have been detected in the galaxy since 1909, with the most recent discovery taking place in 2015. In 2006, the Hubble Telescope released a detailed image of the galaxy composed from 51 exposures taken over the course of 10 years. It is the single most detailed image of the galaxy today and is still the largest image Hubble has ever captured of a spiral galaxy.

Image Credits: Dave Jurasevich


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