Jump to content


- - - - -

Cosmic Challenge: Dissecting M101

Discuss this article in our forums

Cosmic Challenge:

Dissecting M101

July 2018

Phil Harrington

This month's suggested aperture range:

Large scopes


10- to 14-inch (25cm-36cm)










M101's nebulae

Emission nebulae

14 03.2

+54 20.9

Ursa Major




Just spotting the gigantic Pinwheel Galaxy, M101, can sometimes be challenging enough. Its low surface brightness can drive suburban observers crazy, especially when we see photographs that show it so big and bright, or that it is listed as 8th magnitude. It all comes down to surface brightness, or more accurately, lack of surface brightness. Seeing the dim glow of the galaxy's small core, or the even dimmer glimmer of the surrounding spiral arms, can take a concerted effort. But with time and patience, M101 is visible, with difficulty, through 50-mm binoculars even given a suburban sky with a naked-eye limiting magnitude of perhaps 4.5.


Above: Summer star map showing the location of this month's Cosmic Challenge.


Credit: Map adapted from Star Watch by Phil Harrington


Above: Finder chart for this month's Cosmic Challenge.


Credit: Chart adapted from Cosmic Challenge by Phil Harrington
Click on the chart to open a printable PDF version in a new window



For double-digit apertures, the test presented by M101 is not only seeing the galaxy, but also finding latent structural details hidden within. Pierre Méchain may have discovered the Pinwheel in 1781, but it took the trained eye of William Herschel gazing through his 18.5-inch telescope to begin to crack M101's galactic vault and find the first hidden gems, three of the galaxy's interior clouds.

That was a good beginning, but by no means was the end of the story. The next chapter opened in 1845 when William Parsons, the third Earl of Rosse, first examined the galaxy through his monstrous 72-inch reflector at Birr Castle in Parsonstown, Ireland. Lord Rosse was the first to spot nine knots scattered throughout the galaxy's amazing spiral arms.

Rosse's discoveries were added to John Herschel's General Catalog in 1864 and subsequently, to John Dreyer's 1888 New General Catalog. Today, the M101 family of hydrogen-II regions holds eleven entries in the NGC, more than any other single object. Each of these clouds is a huge expanse of ionized hydrogen surrounding embedded stars, like the Orion Nebula (M42) and the Lagoon Nebula (M8), among others.

Use a wide-field eyepiece producing no more than 75x to find M101 initially and to trace out the full breadth of its spiral-arm disk. Can you repeat Lord Rosse's historic observation by spotting the subtle serpentine arms curving away from the galactic core? One arm branches off the southern tip of the core, curling around the core and twisting toward the west and south. The second major arm curves away from the northern edge of the core, hooks to the west, and then comes around the other side, where it divides.

There has been long-standing confusion over the exact locations of many of the NGC targets within M101 dating back to a drawing that Lord Rosse made in 1861. John Herschel subsequently used that drawing to determine the positions of those objects for inclusion in his General Catalog, which ultimately resulted in errors that have been carried over to the present day. After more than a century, these galactic boo-boos were finally corrected by Harold G. Corwin, Jr. of the California Institute of Technology. The positions and labels listed in the table below as well as plotted on the chart above are based on Corwin's research.


Table: Nebulae within M101







NGC 5450

14 02.5

+54 16.2



NGC 5447

14 02.5

+54 16.8



NGC 5449

14 02.5

+54 19.8



NGC 5451

14 02.6

+54 21.8



NGC 5453

14 02.9

+54 18.5



NGC 5455

14 03.0

+54 14.5



NGC 5458

14 03.2

+54 17.9



NGC 5461

14 03.7

+54 19.1



NGC 5462

14 03.9

+54 21.9



NGC 5471

14 04.5

+54 23.8




Our first stop is NGC 5471 at the far end of the eastern arm, 11.5' northeast of the core. Heinrich Louis d'Arrest was the first to spot it in 1863. Its isolation so far from the heart of M101 led many 20th-century observers to conclude that NGC 5471 was actually a separate galaxy, and in fact, its appearance through my 10-inch at 254x mimics a small elliptical galaxy perfectly, with an amorphous glow surrounding a brighter central core. Today, there is no longer any question as to its true nature. Photos taken with the Hubble Space Telescope reveal a glowing area some 200 times as vast as the Orion Nebula with several brighter regions embedded within. Detection of extremely strong X-ray emissions emanating from with have led researchers to conclude that NGC 5471 is home to no fewer than three supernova remnants.

Traveling inward along the same spiral arm, we next come to NGC 5462, one of Herschel's trio of discoveries. Unlike NGC 5471, which appears nearly circular, NGC 5462 looks quite distended, oriented northeast-southwest. It is slightly dimmer than NGC 5471, but should still be apparent in a 10-inch telescope. NGC 5462 shows little improvement with a narrowband or O-III nebula filter.

Closer in along the same spiral arm, we next come to NGC 5461, another of Herschel's finds. NGC 5461 is about 5' south-southeast of the galaxy's nucleus and looks like a faint, slightly fuzzy star through my 10-inch. My 18-inch at 411x begins to hint at some of the cloud's subtle structure, including what appears to be a stellar brightening at its northeastern edge. Again, a narrowband filter offers only a modicum of help.

Finally, NGC 5458 is situated along the same spiral arm, just prior to where it wraps into M101's nucleus. Look for a very small, very faint glow measuring less than 30" across set 5' directly south of the core.

M101's western arm also offers a variety of H II regions. Working out from the galactic nucleus, we first come to NGC 5451, found about 5' to its west. This is a tough catch. Unless your skies and optics are close to perfect, the low surface brightness of this nebulous tuft will probably escape unnoticed. A pair of faint field stars is only 1' to the cloud's west, so use them as a guide. But unless you can see those stars and the nebula, odds are good that you are only seeing the stars. NGC 5449, about 2' further south along the arm, is also a difficult target. Use high power for both.

A close-set pair of nebulous knots, NGC 5447 and NGC 5450, is found toward the southern tip of the western arm. Less than ideal seeing conditions will merge these into a single, elongated blur, but under steady skies, each can be resolved as a separate glow just south of a 14th-magnitude Milky Way star. NGC 5447 is a huge association of hot O- and B-type stars, while NGC 5450 is an H II region that may eventually evolve to resemble its neighbor.

Following a fork in the western arm that hooks back toward the galactic center brings us to NGC 5453. Look for its tiny presence about 2' west-northwest of NGC 5458.

NGC 5455 is found nearly half a degree south of M101's core, near the outskirts of the galaxy's vast spiral arm halo. Curiously, some computer software programs plot NGC 5455 as just another field star, failing to recognize its true extragalactic nature. It forms the southern point in an equilateral triangle with two 14th-magnitude field stars, one to its northeast and the other to its northwest.



Above: M101 as sketched through the author's 18-inch (46cm) reflector.


Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my web site or post to this month's discussion forum.


Until next month, remember that half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2018 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • Bill_H, okiestarman56 and Tyson M like this


Jul 03 2018 09:38 PM

But with time and patience, M101 is visible, with difficulty, through 50-mm binoculars even given a suburban sky with a naked-eye limiting magnitude of perhaps 4.5.

Hi, Phil! What Bortle number would you estimate those kinds of skies at? I live in the city under hard Bortle 9 skies, but reach limiting magnitudes of ~5.0-5.5.


I set up my 15x70 binoculars + parallelogram mount last night to search for M101, but got no convincing hints. I was able to barely, with effort, see M51, and in a brief moment of clarity saw a mag 11.5 star pop a time or two. The transparency was not the best, but good for summer (I live in Houston so there's tons of gulf moisture). The skyglow was bright enough that I was able to see plainly into the deepest shadows under the trees 50' away, and the sky was the color of a planetary nebula overhead and in the direction of Ursa Major, simultaneously blue and green. Towards the horizon or my many light domes it turned tan. I will keep trying for M101 under hopefully better conditions.


Clear Skies!

Lauren Herrington

    • Augustus likes this
Jul 04 2018 09:32 PM

I’m sure under pristine unpolluted skies that M101 is visible with instruments under 100mm.  I was checking my observer’s log from last year around this time and viewed the sketch with my 8” at about 70x.  It was a barely discernible irregular grey patch only marginally lighter than the background. More magnification did not produce meaningful detail and certainly nothing like the structure Phil sketched with his 18”.  I live in a Bortle 5 zone and took an SQM reading of the area which yielded a reading of 20.35.  I typically have good-excellent transparency.

    • Tyson M and TieDyeAstronomer like this
Jul 05 2018 07:48 PM

Hey Chesterguy1! Thanks for sharing your report!


I’m sure under pristine unpolluted skies that M101 is visible with instruments under 100mm.


Definitely! I've been able to see it quite "easily" (for a deep sky object) in my 15x70 binoculars from a Bortle 3-4 site, and one where the big light dome is to the North too, so not the best for M101. But having such a low surface brightness, it sure does get wiped out fast. I bet it would be a bigger struggle from my club's Bortle 4.5 dark site.


Clear Skies!

Lauren Herrington

Jul 06 2018 07:58 AM

Hi Tie:


For me, it's always a better target this time of year:  high in the sky, more north/northwest and away from the small light dome of my city, which for me is NE.  It's still a tough target and even with my 15" it reveals little structural detail from my admittedly suburban zone.  It is, of course, much easier to find with the 15" (although I would never classify it as easy from my locale).  What I can see readily--but is more elusive in the 8" are the nearby NGC galaxies, 5473 and 5474.  I cannot see 5477.  Thus, the galaxies Phil sees within the structure of M101 are simply not possible unless I go to a much darker site.



    • PhilH likes this
Dave Mitsky
Jul 07 2018 02:27 PM

I can sweep up M101 fairly easily with my Celestron 8x42s from a really good dark site.


Dave Mitsky

Dave Mitsky
Jul 07 2018 02:32 PM

M101 and its HII regions can be quite spectacular through a large aperture from a pristine site.






Here's an image that I captured years ago using the BRT.


M101 (The Pinwheel Galaxy)
14" f/11 Celestron C14 SCT working at f/5.3
FLI MicroLine CCD camera
100 seconds
Bradford Robotic Telescope
Tenerife, the Canary Islands


Dave Mitsky

Attached Thumbnails

  • Attached Image: M101 2-22-2013 Reprocessed CN 640.jpg
    • PhilH and John O'Hara like this

Great article, Phil!

    • Dave Mitsky and PhilH like this
Jul 28 2018 03:02 PM
I had the wonderful experience of viewing M-101 from an elevation
of 8,000 feet on the North Rim of the Grand canyon during new
moon in June. I observed it through a 20" f/4.5 reflector owned by
Steve Dodder of the Saguaro Astronomy Club, who host a week
long star party at the North Rim each year as an outreach event
in conjunction with the North Rim Lodge and the Park Service.
Through the 20-inch using a 14mm 82 degree eyepiece (150X)
M-101 looked exactly like Dave Mitsky's astrophoto, above.
ALL of the H-II and star-forming knots in the spiral arms
were clearly visible. With averted vision, even the faintest
knots shown in Dave's photo were visible. This was the finest
view of M-101 I have ever had, and will remain one of my
greatest moments at the eyepiece of a telescope. The skies
at 8,000 feet were incredibly dark, with so many stars that the\
constellations were hard to pick out! M-101 was AWESOME!!

Ron Abbott
    • Dave Mitsky and PhilH like this

I never knew these NGC regions existed inside M101. I just discovered the pinwheel spiral arm structure for the first time this year with my 8" dob from a dark site. I know now to bump up the magnification for closer scrutiny!  


Also I found this galaxy easy to sweep up with the 10 x 50 binos from my dark site.


Thanks for the article

    • PhilH likes this

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics