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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 11 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

I'm really looking forward to heading out to my dark sky site in the next couple weeks; just picked up a 10mm Hyperion eyepiece which I'm hoping is going to give me better views of M51 than the stock 26mm Meade that came with the scope + the 2X 2" barlow

    • PhilH likes this

I had my first look at M51 this season 3 nights ago, as it peaked at just over 30o above the northern horizon.  It was very dark as usual, but transparency down low was below par.  Disconcertingly, I could not see the spiral arms no matter how hard I tried, at any power.  The best I got was a ring around it, that did not seem to attach to the central core.


(note: I could easily make myself "see" the spiral arms if I was careless, especially at 109x, the power at which I have most often observed this galaxy.  This is how averted imagination works, and it is good practice to learn to control it.  I kept conjuring up the "image" of the arms, and of course, every time I did so they attached to the core at a different position, etc., giving away their false nature).


Two nights ago I was out again, and this time transparency was up to standards for this time of year.  The spiral arms were clear and detailed, and with averted vision I could see the bridge.  M101 was looking very good too, even lower in the sky.


From this I had the lesson reinforced that transparency is the key to observing detail in galaxies - perhaps more so than even light pollution.  Haze certainly makes any LP much worse.

    • PhilH likes this

M51 is one of my favourite objects. I have a small house, 160 km off Mumbai, India, which is fully dedicated for my hobby. With my 18", F4.5, the view of M51 is stunning. I normally begin with my Televue 21mm ethos and then switch to 11mm Delite. Delite gives much better details. However, I have found that using 21mm earlier and then using 11mm, the eyes get adjusted better and arms are visible better.

    • Araguaia likes this
Jun 18 2019 05:33 PM

This is timely because my wife and I just bought our first telescope (8" Dob) and I'm trying to learn how to find objects. I've looked for M51 a couple of times from the back yard and failed, but last weekend there were high clouds that were making even a few bright objects a little hard to see. I know an 8" reflector doesn't technically qualify for the challenge, but it's a good exercise for me and if I can find it with the telescope maybe I'll take a crack at seeing it with my binoculars or photographing it too.


Paul, with 8", you would definitely see M51 as fuzzy spot and NGC 5194 smaller fuzzy spot. The spiral arms may not be seen possibly. But I am sure you will have a satisfying view.

Dave Mitsky
Jun 20 2019 09:00 PM

Here's the section on M51 from my post at https://www.cloudyni...mers/?p=4592919


M51 (the Whirlpool Galaxy)


M51 (NGC 5194, integrated magnitude=8.4, surface brightness=12.6 magnitudes per square arcminute), a type SA(s)bc pec face-on spiral galaxy, and its irregular companion M51b or NGC 5195 (magnitude 9.6, surface brightness=13.1 magnitudes per square arcminute) are perhaps the most prominent example of an interacting pair of galaxies. They are best seen in the spring.


Due to the work of Lord Rosse In 1845, M51 was the first galaxy to be recognized as having a spiral shape. Of course, M51 was merely a "nebula" at that time, which was long before galaxies were determined to be objects external to the Milky Way.


M51 and NGC 5195 may be part of the M101 group of galaxies. Three supernovae have occurred in M51: SN 1994I, SN 2005cs, and SN 2011dh.


To star-hop to the Whirlpool Galaxy, proceed southwest from Alkaid (Eta Ursae Majoris), the final star in the Big Dipper's handle, to the fifth-magnitude star 24 Canum Venaticorum. Continue southwest to an isosceles triangle of seventh-magnitude stars. M51 and NGC 5195 lie just to the south of the triangle, approximately 3.5 degrees from Alkaid and 1/4 of the way to Cor Caroli (Alpha Canum Venaticorum).


Star-hops to M51 can be found at the following sites: 








Telrad finder charts for M51 are available at the following sites: 


http://www.custerobs...cs/messier2.pdf (map 5)




For further information on the Whirlpool Galaxy, consult these sites:




















Dave Mitsky

    • antariksha likes this

Last night M51 was looking good!  I could see both spiral arms at all powers, from 51x to 277x.


The bridge between the galaxies, however, was only visible in AV.  It is a lot fainter than the main arms.

Jun 23 2019 05:02 PM

I tried and failed to find M51 again last night. I had some new-to-me eyepieces to try out, as well as a Dioptrx so I can try viewing without my glasses. I suspect the sweet lens to initially find it will be a Nagler Type 4 22mm that got its first trial last night. I also bought a Nagler Type6 11mm but from what I've read I think I should start with the 22mm (I welcome correction if that's wrong). I was viewing from my back yard and the skies were clear but it is on the East bench of Salt Lake City so there's a bit of light pollution. I'm pretty sure it's me and not the 'scope that's failing. I suspect that once I see it and know what I'm looking for that I'll be able to make it out with conditions like we had last night. I think the Dioptrx is going to help too because I was having a lot of trouble with the kit eyepieces and my prescription glasses. My astigmatism is bad enough (-3.25 in both eyes) that I can't even really focus the telescope without correcting for it. 

I love the resources posted above... I'll see if they help me nail it down. I'm looking forward to my next opportunity to head to South or West out of the city now that I've got better eyepieces and a little better understanding of how to find my way around. 

I tried and failed to find M51 again last night.


For some reason M51 is also a problem for me to find.  I have seen it many times and had difficulty finding it many times.  I think, at least for me, it is because it is so far North and directions like East and West become less defined the closer you get to the pole.  Or maybe I am just overthinking it, I always seem to go the wrong way from Alkaid at first, then I eventually get my bearings and find it.  It is so close to a prominent star that you would think it would be easier.

There are two stars of about magnitude 5 just to the west of Alkaid. 


If you imagine that there was a third star, forming a right triangle with the other two, so that the right angle corner points to Alkaid, and the leg to the imaginary star is just a bit shorter than the leg between the two real mag 5 stars, M51 is at the position of this "imaginary star".


If you look at a star chart you will know what I mean.

Dave Mitsky
Jun 25 2019 03:37 PM

I removed the dead links from post #16 and added ones that work.


Dave Mitsky

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