Why Only In M13?

[Mr Q]

I'm referring to the 3-bladed propeller appearance of stars in the globular. At different powers, they seem to "come and go" with both visual and photographic observing. Another peculiar trait of these "propellers" is that they always appear with one blade straight up - leading me to believe this propeller effect is only illusionary (even when looking at photos).

So even IF this "illusion" is just that, why ONLY in M13???

I have asked if any observers have seen this effect and several observers have verified that they have, in past posts both here and on other forums, so it is not just an imaginary effect of any one observer.

I have done many web searches on this subject and keep coming up empty. Anyone have any sources as to this strange phenomena? I can believe it's an illusion but why only with this globular

[Tim]

I don't think it is an illusion?

On all of the pics I have taken of it the propeller appearance is exactly the same, and is characterised by one of the 'blades' being slightly darker in appearance than the other three.

In this picture it is visible as a distinct Y shape, the upright facing blade towards the bottom, not straight up? Of course in this picture the size of the globular and the number of stars far exceeds what you would ever hope to see visually.

[Tim]

Ringed version for those who are not familiar with the propeller. When I have seen it visually it has been towards the edge of the cluster as seen in the eyepiece. But this picture was with very long exposures revealing the full extent of the fainter stars around the edge, and putting the propeller more centrally that it would appear at the eyepiece.

I think it is the absence of a definite source point of light at the centre of the propeller that makes it easier for the brain to make sense of the shape and 'see' the propeller in the cluster, but I still dont think that makes it an illusion? It's just the way the stars there are laid out.

[Mike73]

Thanks Tim, I could never spot it before but now that you have pointed it out its quite obvious!

[MikeD]

It always shows up the same in my images too and at a push there are multiple propellers.

Mike.

Click the image below for outline overlay.

[Mike73]

hmmm so in Mikes image its now a five bladed propellor?!

I must have a poor imagination as I always struggle to see shapes and asterism's in open and globular clusters. From a good dark site with my 16" M13 is almost photographic in appearance but like I said before I still havent seen the prop!

[MikeD]

It is also a five bladed propeller in Tim's image.

The point is the human brain is wired to find patterns/symmetry in chaos.

Mike.

[Tim]

Ah, now I only ever see three blades, and much smaller than yours Mike.

[acey]

The Propeller was first seen in the 19th century and was thought to be a physical feature, it's now known to be a chance effect because of the particular pattern of stars and the brain's propensity for finding regular shapes. Similar effects can be found in other globular clusters if you look for them.

The 19th century observers also noted the long curved chains of stars, which we can easily see in M13 and other globs. These were interpreted at the time as being spiral structure analogous to what was seen in other "nebulae" (as they were all called then) such as M51. Even M42 was thought to have spiral structure. Look hard enough and you can see all sorts of shapes in DSOs.

There was a long thread on the Propeller a couple of years ago to which I contributed my own observing report. It's easy once you know what you're looking for.

[Mr Q]

Acey - Strange that I only see these patterns in this GC and have tried to see other unusual patterns in other GCs but only detect curved arcs of stars So I have to assume that this weird pattern does occur in other objects and perhaps the patterns depend on each of our brains' interpretation? If so, I would think that there would be a wide range of patterns equally reported but with most reports of 3-bladed propellers...well, that's even more bizzar

[Beulah]

Here's an article for the observers:

http://www.lvastronomy.com/observing-challenge/41-june-2009-m-13-and-the-propeller

[Tim]

There is a definite dark blob thing in the foreground of one of the three blades of the propeller as I see it, and that serves as a solid visual reference point.

Sam, that link mentions Lord Rosse observing a propeller shape. I didn't know they had propellers in the 18th Century

[Mr Q]

Thanks for the link Beulah. I have seen it before and it does show that mag. power has a lot to do with seeing the phenomena. When I see these propellers (sometimes only one or two and at other times several smaller ones), i often look for them in other GCs but so far have come up empty.

So my question of 'what causes this phenomena?' still is not answered.

When in New Mexico observing the awesome Omega GC, I was able to scan its whole body (it was so large) with my 10" newt under different powers and never saw any stand-out star formations other than some curves in lines of stars, which I assume was my imagination.

So my question of what causes this phenomena, mostly in M13, still stands unanswered

Tim - You forgot about windmill propellers that go back further than the 1700s!

[earth titan]

Could it be ti do with the blind spot in the eye. Our eyes naturally make up what happens in this region from details of the surrounding data. This may explain widefield views.

One of the reasons the 'blinking nebula' works.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[Beulah]

There is a definite dark blob thing in the foreground of one of the three blades of the propeller as I see it, and that serves as a solid visual reference point.

Sam, that link mentions Lord Rosse observing a propeller shape. I didn't know they had propellers in the 18th Century

They had the precursor to the wind turbine back in ye olde dayes..

[acey]

Sam, that link mentions Lord Rosse observing a propeller shape. I didn't know they had propellers in the 18th Century

William Parsons, 3rd Earl of Rosse, was a 19th-century observer. His son Charles invented the steam turbine. His grandson Lawrence (4th Earl of Rosse) continued observations with the 72-inch Leviathan that William built. The first paper on the propeller was by Markham in 1886 where it is referred to as "three dark rifts", seen with 6" and 12" telescopes. The nickname "propeller" seems to be modern.

[acey]

I said Markham above but the 1886 paper was written by his co-observer Harrington. The paper (with drawings) can be seen here:

http://archive.org/s.../search/markham

What they found (and I can confirm from my own observations with a 12") is that you need to use high power, but not too high. They used 500-600, I used less.

[Tim]

Thanks for the links all.

MrQ, i would say that the answer is that m13 actually has an apparent 3 sided dark shape within it, that shows up with images of varying focal lengths and of varying duration, but never varying in size, shape or orientation. What is observed at the eyepiece is too tuned to the individual involved I guess, but cameras can't lie

[acey]

When in New Mexico observing the awesome Omega GC, I was able to scan its whole body (it was so large) with my 10" newt under different powers and never saw any stand-out star formations other than some curves in lines of stars, which I assume was my imagination.

So my question of what causes this phenomena, mostly in M13, still stands unanswered

In the picture of Omega Centauri below, I can see a shape like an M (or an anchor) above and slightly left of centre. Would that ever be visible in the eyepiece? I have no idea. These random patterns depend on the relative brightness of the stars (which are themselves randomly scattered). This explains why they show up in some images and not others, and why they show up in some telescopes and not others. In the latter case, varying the magnification will affect the number and relative brightnesses of the stars in the field of view. It just happens with M13 that if you get the magnification right then you are able to see three straight lines where there are few bright stars, set against a mass of bright stars.

The propeller in M13 is no doubt a unique feature - you would never expect to come across another random pattern like that (nor would you expect to find another Coathanger). But a random pattern is all that it is.

[Beulah]

Thanks for that information regarding propellers, Acey. You are a treasure trove of interesting knowledge!

[Mr Q]

Thanks for all your inputs to this unique feature of M13. For those who want to try seeing these propellers (yes, sometimes you can see several in one view), they appear as dark lanes and their size/numbers depends on the scope's EP used - usually a medium to high power and (I'm assuming here) the scope's size. Most people who report seeing the propellers were using scopes of 6 inches or larger - scopes that can give a clear view of the GCs core.

Here's a thought I never saw mentioned - could the propellers be due to the colors of the stars playing tricks with our view, even if the colors are barely observed? This would explain them being seen in photos as well as direct eyesight? I can't recall if the propellers are seen on B&W photos. If so, the mystery deepens

In regards to the size scope needed to detect these propellers, what is the smallest size limit in aperture that the propellers have been detected? I'm guessing an aperture big enough to start detecting the colors of the stars

[Bart]

I'm afraid I must be missing out as I can barely see it in the images so I'm guessing, visually, I'm snookered. Still love M13 though, despite it witholding its propeller from me..

Barry

[Tim]

Maybe this helps Bart

[Mike73]

I still can't see it!!

But I can see a squirrel holding a pint of lager (I think

[LukeSuppalot]

I still can't see it!!

But I can see a squirrel holding a pint of lager (I think ).

I think you'll find it's Bitter not lager.