The Pinwheel Galaxy (M101)
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| M101 | Spiral Galaxy | 14h 03m 13s | +54° 20′ 56″ | Ursa Major | +7.8 | 24×23 arcmins |
| If you enjoy this guide and want another fifty like this, PLUS new ones every month, CLICK HERE to join the Virtual Astronomy Club |
Overview
In photos like the one below, M101 is a large face-on galaxy with beautiful splayed arms rising out of its center.
As all of us know, however, that’s not the view we’re going to get at the eyepiece. This magnitude 8 galaxy is larger and its light is spread over a that surface area, making its surface brightness quite faint. In short, you have to work at this one under dark skies to enjoy its splendor, which is, in part, what makes it such a fun object to hunt for.
Although it sits in Messier’s catalog, the galaxy was actually discovered by Pierre Mechain in 1781. Messier added it to his catalog just before it was published, making this one of the last entries in it.
M101 is an Sc-type galaxy, which means it has a bright central core and well-defined spiral arms emerging from it. As you can see from the image, the galaxy is asymmetrical and its core is not at the center of its disc.
Altogether, there are estimated to be over one trillion stars in this galaxy, which spans 170,000 light years from edge to edge. At least 100 billion of those stars are believed to be like our Sun, so goodness knows how many planets that equates to. These numbers are truly mind-boggling, as is the fact that this Pinwheel Galaxy is actually considered to be ‘close’ at just 23 million light-years away.
The brighter areas in the spiral arms have their own NGC designations and many of them are known to be hydrogen cloud nebula that are home to dramatic star formation.
Finder Charts
The Pinwheel galaxy is an easy find, located as it is off the end of the Big Dipper‘s ‘handle’. Ursa major, which is the constellation where we find the Big Dipper, is a circumpolar constellation. This means that it never sets for most of the northern hemisphere, whic also means that M101 is always above the horizon.
Late spring and early summer is the best time to see it in the evening, when, as the first image below shows, it is high towards the zenith at 10pm. The first image is for the middle of June and you can see M101 high above the northwest horizon near the bright star at the end of the Big Dipper, called Alkaid.
(All sky maps on this page are courtesy of SkySafari 6 and can be clicked on for a full-screen version, or downloaded as a pdf at the bottom of the page).
Zooming in, and showing stars to magnitude 7.0, we can see that we can find M101 by following the curved line from Megrez to Mizar, via Alioth. In the final finder chart, we’ll take a closer look at that line of brighter stars off the end of Mizar which we’ll use to star-hop our way to M101.
This final finder chart for the Pinwheel Galaxy shows how we can use the line of magnitude six stars which strings out east from the famous double star Mizar to reach our target.
The first image is in color and shows 2° field of view hops (numbered 1-5). The second image is identical, but presented in inverse monochrome to use at your telescope. Don’t forget that you can print the PDF version of this (and all the star maps in this guide) using the buttons at the bottom of the page.
For the first hop, place Mizar on the western edge of your view and you’ll have 81 Ursa Majoris (UMa) at the eastern edge. Keep 81 UMa in view and move eastwards until 83 UMa comes into view (step 2). Carry on this fashion, bringing 84 UMa (step 3) and then 86 UMa (step 5) into your eyepiece. When 86 UMa is in place, you need move that once more so it’s at the western edge of your view and M101 will move into the field.
We’re now ready to examine this galaxy, after we’ve presented views that might be more useful for your telescope type.
Individual Telescope Views
The following views will help you find M101 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Each image can be clicked on for a full-screen version.
Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope
Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal
Mirrored View – Refractors and Cassegrain models with a star diagonal show this view
Observation
Although its surface brightness is low, you’ll be able to pick up M101 under a dark sky with a modest telescope and a low magnification. You’ll see that it covers in the region of quarter of a degree of sky and you’ll also be able to see that its core is brighter than the surrounding structure. However, it’ll be very tough to make out any of that structure.
More magnification will show extra detail. In a smaller scope, you’re going to see the galaxy’s nucleus and you may be able to discern the brighter arms spiralling out from that center.
A larger scope reveals more detail, especially at stronger magnifications. In excess of 100x, you can tease out the multiple arm segments and spy for yourself that the bright core is offset from the center. Can you see the brighter star forming regions mentioned earlier?? Trace an arm from the center outwards and see if it dims but then brightens – this happens when you encounter the nebulous regions of star birth.
This is a tantalising object to view. Try and study it over several nights to get the most information from it. Tracing its faint arms often requires switching between higher and lower powers because they are disjointed – it’s easy to lose the thread of which one you’re staring at as the image dances in your peripheral vision.
The Pinwheel is a real treat, one that deserves repeated visits!
| If you’ve enjoyed this guide and want another fifty like this, PLUS new ones every month, CLICK HERE to join the Virtual Astronomy Club |
PDFs for Printing
Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M101-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.