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Cosmic Challenge: M51's spiral arms

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Cosmic Challenge:

M51's spiral arms


May 2019


Phil Harrington


This month's suggested aperture range

6-inch (15 cm) to 9.25-inch (23 cm) telescopes









M51 spiral arms

Galaxy structure

13h 29.9m

+47° 11.8'

Canes Venatici




Before we conquer this month's Cosmic Challenge, I'd like to pause for a moment and reflect on a terrible loss that befell the Cloudy Nights family the weekend of April 27th. That day, we lost not just a member of CN, we lost our patriarch, the center of the CN universe, Tom Trusock. I recall reading about it later that weekend, and sat in front of my computer monitor in stunned silence as I tried to process this unimaginable news. "Not Tom!," I thought.

I know that most of us realize the pivotal role Tom has played here pretty much since Cloudy Nights was founded in 2000 by Allister St. Claire. Later, after Astronomics purchased the site, Tom was the central figure here.

Beyond that, Tom wrote extensively for Astronomy magazine, and in fact, has an article on collimation in the May issue.

But I think what moved me most about all this are the postings on his Facebook wall since word of his death broke. Tom was a math teacher (as well as astronomy and computer science) at Bad Axe High School (Bad Axe, MI) for years. Hundreds of his former students have posted their reactions to this terrible news, all attesting to the positive impact he had on their lives. Many are quite poignant.

Thank you, Tom, for all you did in life -- an outstanding example for all who knew you. My deepest sympathies are with his family, especially his wife and two daughters.

Mike Bieler from Astronomics has set up a GoFundMe page to help raise money for Tom's family. I hope that the worldwide community of amateur astronomers who knew Tom can band together and show our support his family at this most tragic time. Click here for more information and to donate. And thank you!

Of the thousands of spiral galaxies visible through backyard telescopes, one stands above the rest in terms of visual interest: M51, the famous Whirlpool Galaxy in Canes Venatici.  Everything adds up in M51's favor.  We are seeing it very nearly face-on, its spiral arm halo is bright and peppered with star clouds and vast regions of nebulosity, and it brings with it a friend in the form of a smaller companion galaxy that can even be seen through giant binoculars.


Above: Spring star map. Credit: Map adapted from Star Watch by Phil Harrington

Above: Finder chart for this month's Cosmic Challenge.
Click on the chart to open a printable PDF version in a new window


Charles Messier was first to lay eyes on the Whirlpool when he accidentally bumped into it on October 13, 1773.  His notes recall a "very faint nebula without stars."  The fact that he referred to it in the singular indicates that he saw only the bright core of M51 itself, and not its smaller companion, NGC 5195.  The discovery of the latter is credited to Messier's friend and contemporary, Pierre Méchain, who noted a double core on March 21, 1781.

The first hint that there was more to see within M51 than just a pair of nebulous blobs came from an observation by John Herschel on April 26, 1830.  Herschel recorded "a very bright round nucleus surrounded at a distance by a nebulous ring" through his 18.7-inch telescope.  A later drawing by him recorded a large, bright core centered perfectly in a fainter surrounding ring.  The companion, NGC 5195, is also shown as round, but smaller than M51's core and positioned outside of the mysterious ring.  Herschel later mused:

Supposing it to consist of stars, the appearance it would present to a spectator placed on a planet attendant on one of them eccentrically situated towards the north preceding quarter of the central mass, would be exactly similar to that of our Milky Way, traversing in a manner precisely analogous the firmament of large stars, into which the central cluster would be seen projected, and (owing to its distance) appearing, like it, to consist of stars much smaller than those in other parts of the heavens. Can it, then, be that we have here a brother-system bearing a real physical resemblance and strong analogy of structure to our own?

Fifteen years later, in the spring of 1845, Herschel's puzzling nebulous ring was resolved into a pinwheel structure by Lord Rosse at Birr Castle in Ireland.  Aiming toward it with his newly completed 72-inch "Leviathan" reflector, the largest telescope in the world at the time, Lord Rosse saw "spiral convolutions; with successive increase of optical power, the structure has become more complicated.  The connection of the companion with the greater nebula is not to be doubted; the most conspicuous of the spiral class."  Later, in 1861, Lord Rosse noted that "the outer nucleus unquestionably spiral with a twist to the left."

Left: John Herschel's rendition of M51 through his 18.7-inch reflector. 
Right: Lord Rosse's view of M51 through his 72-inch "Leviathan."

For an historical overview of the discovery of spiral structure in M51, I recommend reading
The First Drawing of a Spiral Nebula by Michael Hoskin.  His 1982 paper appeared in the Journal for the History of Astronomy, volume 13.

While discovering the spiral structure took a 72-inch aperture, knowing it's there gives you and me a distinct advantage.  In fact, hints of M51's pinwheel construction have been reported through telescopes as small as 4 inches across, given extraordinary sky conditions and an exceptionally sharp-eyed observer.  I find those reports absolutely amazing, since spotting the spiral structure through my own 8-inch reflector, even given dark skies, is a rare treat.  The sketch below captures one of those moments.

Before we discuss strategy, let's first zero in on M51.  At 8th magnitude, M51 is bright enough to be visible through small binoculars even from suburbia.  Start at Alkaid [Eta (
η) Ursae Majoris], the end star in the handle of the Big Dipper.  Hop to 24 Canum Venaticorum, a 4th-magnitude point just 2° to the west-southwest, then slide another 2° to the southwest to a trapezoid of faint stars.  M51 lies inside the trapezoid's northeast corner.


Above.  M51 showing its spiral structure, as sketched through the author's 8-inch (20.3cm) reflector

M51 may be visible easily from suburban skies through 6- to 9.25-inch scopes, but sky darkness and transparency are the overriding factors when looking for its spiral structure.  My first view of the spiral arms came in 1974 while observing through my venerable 8-inch f/7 Criterion RV-8 at the Stellafane amateur telescope makers convention in Springfield, Vermont.  The sky was especially dark that year, with M33 visible without optical aid.  With the same equipment under lesser conditions, however, I see no hint whatsoever.  Indeed, from my suburban backyard, it takes my 18-inch reflector to make out any suggestion of the arms.

If this is your first time looking for the spiral arms, strategy is everything.  First, choose the right magnification.  The best views seem to come with eyepieces producing an exit pupil of between 2mm and 3mm.  That narrow range seems to offer a good compromise between image size and contrast.

Next, you need to know how to look for the arms.  Take a look at the glow surrounding the core of M51.  It may look uniform at first, but careful study with averted vision will reveal some irregularities.  One arm starts to the south of M51's core and hooks to the northeast, with the brightest portion lying halfway between the core and NGC 5195. 

A second spiral arm begins just west of the core, curves to its south, and then spirals around toward the northeast.  It fades from view as it extends toward NGC 5195.  Using averted vision, can you also detect the faint nebulous bridge that reaches outward for the companion galaxy?  Some observers report better success seeing the arms by focusing their attention on the dark gaps between them rather than looking for the bright arms themselves.

By waiting for an especially dark, clear spring night and letting your gaze sweep across the faint glow of the Whirlpool's halo, perhaps tapping the telescope gently to vibrate the image, Lord Rosse's elusive "spiral convolutions" should be discernible.

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, including Cosmic Challenge: The Ultimate Observing List for Amateurs.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2019 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.


  • CollinofAlabama, Scott in NC, okiestarman56 and 7 others like this


May 01 2019 02:05 AM

I can attest that the arms really are visible in a 4", at least a 4" high-quality refractor. This happened to me recently on a very clear night. I didn't expect it at all and caught me completely by surprise. I'm not quite sure of the NELM, as I never checked, but M13 was dimly visible to the naked eye. The scope was my 4" f/11 ED and the magnification was a fairly high 90x with a 9mm ES100 and an (approximately) 0.7x compressor. I can confirm that the dark bands became visible first and then the brightest parts of each arm. In 2012 I made a drawing of M51, using a 6" f/8 achromat and it shows essentially what I saw with the 4":




The overall shape of the galaxy - including the bridge - can be seen in a much smaller instrument. Here's a drawing I made in 2011 with my 63mm Zeiss:





Clear skies!

Thomas, Denmark

    • ArizonaScott, PhilH, Scott in NC and 7 others like this

The arms are easy direct vision features through a 12" scope, even with M51 rising to barely 30 degrees as it does here.  The bridge to NGC 5195 requires averted vision, except on the most transparent nights.  

    • PhilH likes this
John O'Hara
May 01 2019 06:39 PM

I've found sky darkness and seeing are important in viewing spiral structure in M51.  I've seen decent structure in my old 6" f/8 A-P at Cherry Springs State Park, Pennsylvania.  My best view was in Florida at Shell Mound Park near the Gulf Coast hamlet of Cedar Key.  The sky there was about Bortle 2 at the time, and the seeing rock steady, possibly the best seeing I've ever experienced.  The view was through my 12.5" f/5 Teeter Dobsonian at 227x and was spellbinding.  (However, that's a larger scope than the range for this challenge.)


Thomas' experience with a 4" has my wanting to take out my 100 SW ED.  The night would have to be a good one!  I could try it in my 2.4" Swift refractor, but I'm sure it will fall short of Thomas' Zeiss (I'm sure his skill is a factor as well!).

    • PhilH, Astrojensen and clusterbuster like this
Astro Canuck
May 02 2019 02:50 PM

This was what m51 looked like as I saw though my open 8th floor apartment window

in light polluted Halifax last year.   I may try again this year when it is better placed.

    • PhilH likes this
May 03 2019 06:43 PM

M51, along with M101, is one of the few galaxies where you can actually detect the spiral structure visually.  I have seen hints of the spiral structure several times through my 8-inch SCT from my backyard here in NE Minnesota, Bortle 3 skies.  I would guess that they would be even more obvious in the Bortle 2 and 1 conditions an hour to the north.

    • PhilH likes this
John O'Hara
May 06 2019 04:14 PM

Very sad news, indeed concerning Tom.  He was too young and will definitely be missed.  We've been loosing to many lately in our ranks.  We're such a small community really.

Very sad news, indeed concerning Tom.  He was too young and will definitely be missed.  We've been loosing to many lately in our ranks.  We're such a small community really.

True, John.  Sadly, true.

Phillip Creed
May 10 2019 11:43 AM

I used a pair of 4" scopes in the form of a set of 25x100 binoculars from Big Bend National Park.  Under that Bortle Class 1 sky, it wasn't that difficult to see the spiral arms in M51.  Like others have mentioned, sky conditions are CRITICAL.

Best view was throw Bill Prewitt's 20" dob from Spruce Knob, WV.  That was AMAZING.

Clear Skies,


    • PhilH and John O'Hara like this

I used my 6" f/4.3 last year at Stellafane and was able to see the arms relatively easily. At home (Bortle 5 or 6) the arms can just be seen with my 12" or a 10".

    • PhilH and John O'Hara like this

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