Canes Venatici
M51, “The Ring Nebula in the Canes Venatici”
RA: 13 29 52.7
DEC: +47 11 43
Magnitude: 8.4 (v)
Size: 11.2’ x 6.9’
M51 Herschel_negative_rotated.jpg M51HST-GendlerM_rotated_grayscale_smaller.jpg
(John Herschel's 1830 sketch of M51) (HST image of M51)
For a moment, let’s pretend the year is 1844. William Parsons hasn’t finished his 72-inch reflector yet, and the accepted appearance of M51, then called the “the ring nebula in Canes Venatici”, is John Herschel’s drawing from 1830.
Herschel described it as “a vB neb 1’ in diameter of a resolvable kind with a double ring or rather 1 ½ ring like an armillary sphere” using his 18 ¼-inch reflector. There are no hints of spiral structure. Although he prophetically speculated that M51 could be “like our Milky Way & perhaps this is our Brother System”, the main expectation of the time was that the core of M51 would be fully resolved into stars with the advent of larger telescopes.
A thought experiment:
1. You and your 2020 telescope and eyepieces are magically transported back to 1844.
2. You don’t know about M51’s spiral arms but are fully aware of Herschel’s Ring.
3. Your main observing goal for M51 is to resolve the core of NGC 5194 into stars.
Would you have seen the spiral arms?
There’s no way to know, and I swing between thinking most of us would see the spiral arms or none of us would. Could we look past our Herschel Ring bias to see the spiral arms, or would we focus on resolving the core of NGC 5194 and continue to see Herschel’s Ring surrounding it?
Herschel was using his 18 ¼-inch scope, and before Parsons finished the 72-inch he made a successful 36-inch reflector – both plenty large enough to clearly show the spiral arms - and they didn’t see them. If we were back in their time, and knew what they did, why would any of us see M51’s spiral arms?
While you’re chewing on that, let's discuss seeing Herschel’s Ring in the 21st century. Under what conditions and with what telescopes have you seen it?
I’ve seen it three times in three different telescopes. It takes a mental shift with my 28-inch scope but none with an 8-inch and only a little with a 13-inch. In fact, the first time I saw Herschel’s Ring was with the 13-inch, and I was caught completely by surprise by how evident it was. I most recently saw it in February 2020 with my 28-inch from my backyard under a 20.35 SQM sky. The moderately dark sky hid M51’s distinctive spiral structure just enough that I easily saw the ring. It’s a delight to see M51 as Herschel did.
HerschelsRing_8inch_small.JPG HerschelsRing_28inch.JPG
(8-inch sketch of M51) (28-inch sketch of M51)
Herschel’s Ring consists of the brightest parts of the spiral arms. The beginning of the arms in the core, and the north and south tidal arms aren't bright enough to be part of the ring. That leaves the core of NGC 5194 as a fuzzy bullseye separated from the ring, and the core of 5195 as separate object outside the ring. It's similar to the appearance of Hoag's Ring (PGC 54559) in Jimi's 48-inch scope, but with a bright companion.
Seeing Herschel’s Ring is somewhat like the crater illusion – craters shifting between looking concave or convex – but Herschel’s Ring points toward something more subtle, and potentially troublesome. How do we know what we see is real? How do we separate what we expect to see from what we think we see? Or want to see?
Our biases are most powerful when we aren’t aware of them, and although awareness helps a great deal, like a clever optical illusion they can still be hard to see past even knowing the trick.
I contend that M51 is a prime example of photographic bias – we see spiral arms because we know they’re there and what they look like from photographs. Observer’s saw Herschel’s Ring between 1830 and 1845 because that’s what he saw, and being the pre-eminent observer of his day, his observation set the expectation – the bias. It took the massive 72-inch to transform Herschel’s Ring into spiral arms.
M51 is an interesting case in another way too – the visibility of its tidal bridge between NGC 5194 and NGC 5195. It’s easy enough to find online reports and sketches of M51 with small telescopes clearly showing the bridge, but I’ve always seen it as one of the fainter features of M51. I see no trace of it with my 8-inch even under excellent conditions.
We are subjective creatures by nature, and objectivity is difficult to attain simply because of how our brains are wired. For an extensively photographed and widely observed object like M51 it may be difficult to look past its well-known spiral arms and see Herschel’s Ring. However, the effort is worthwhile not only to see M51 as Herschel’s did, but to demonstrate how elastic our perceptions can be.
Give it go and let us know!
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