Peacock Globular NGC 6752

[Placidus]

New moon, but clouds about. What can you do in a single very dark but rather dubious night? A globular cluster!

NGC 6752 is according to APOD (https://apod.nasa.gov/apod/ap200123.html) blessed with an unusually high number of Blue Stragglers - hot young blue stars formed from the merger of much cooler and more geriatric binary stars.

Our mission here was to try to strike a balance between showing the intensely saturated blue colour in the Blue Stragglers, and showing that they are very bright. This is very tricky with only 8 bits of JPEG to play with.

We've just recently started processing images in HSV space rather than RGB space. We are learning many things.

Out of the camera, the Blue Stragglers are seriously blue, but preserving their saturated blue colour makes them look implausibly dim compared with their more red neighbours, and even dimmer compared with the abundant hyper-luminous red giants in the image.

We looked at a very fine APOD, where perhaps they've gone too far in the other direction, of showing the core of the globular as a creamy white.

Here, we've actually desaturated the core a little bit, to find a middle ground.

A hunt around the original image (https://photos.smugmug.com/Category/Clusters/i-PgRRgzs/0/926ee840/O/Pavo%20Globular%20RGB%206hrs48%20to tal.jpg)reveals something of the order of 15 very faint and very distant background galaxies. At least half a dozen should be apparent here.

FLI PL16803 on 20 inch PlaneWave. 81 x 5 minute subs. Field approx 40 min arc, north up. No moon, but about 20% of frames lost to cloud. Seeing was ok rather than brilliant, at about 2.2 sec arc. All robotics hardware, firmware, and software, and processing software as usual in done in house by us.

Very best,
Mike and Trish

[Placidus]

Here's the same data (https://photos.smugmug.com/Category/Clusters/i-ZGVhm8L/0/db5c2143/O/Pavo%20Globular%20Core%20RGB%206hrs 48%20total.jpg)with far less arcsinh stretch.

The faint stars of the outer halo (and of course the distant galaxies) are now almost invisible, but the unique features of this particular globular cluster are much more obvious.

The badly named "blue stragglers" are also much more obvious.

[marc4darkskies]

I'm not a glob man but this is lovely M&T! The stretched version has my vote. I've always wondered why they call them stragglers? :question:

[Andy01]

Nice shooting Tex! Maybe a combination of both versions would yield the best of both worlds? :D

[Placidus]

Thanks Marcus!

Blue stragglers: If a cluster forms out of fresh new gas then at that moment of creation (ten billion years in the past) there will be a main sequence of stars, with a very small number of brilliant hyper-luminous blue O, a small number of B, etc etc, and lots and lots of F, G, K, and M.

A million years later, the O's will be all gone, as supernovas. A bit later still, the B's will go, then the A's will run out of hydrogen in the core, swell up, becoming hyperluminous but cooler, as they move into the giant branch.

With the passage of billions of years, some of the F's and G's will also run out of hydrogen in the core, just as our sun will in another 5 billion years, and swell up, becoming hyperluminous, as red giants. There are indeed quite a lot of red giants in the image!

But the key thing is that with the passage of time, more and more stars, in OBAFGKM order, peel off the main sequence, swelling up, becoming more luminous, and entering the giant branch. The "kink point", or point of peel-off, can be easily picked, and clever people can say, "Oh, the kink point is at F or G, so the cluster is 5 or 10 billion years old", etc, etc, etc.

BUT

In the actual image, there are all these hyperluminous OB stars that just should not be there. They are still on the main sequence, ten billion years after that should be impossible. They should have left ten billion years ago. That is seriously late for work. They are stragglers.

The mystery was solved when it was realized that they are new main sequence stars produced by mergers.

[Ryderscope]

A wonderful globular and informative discussion. Good for me too as I have almost finished collecting data on this very object. Provides a good benchmark for processing. Thanks for sharing MnT.

[Placidus]

Scopes from the Central West Observe Alike!


We look forward very much to seeing your version Rodney.

[vlazg]

A wonderful glob M&T and thanks for the notes on its structure, very interesting.

[PeterSEllis]

Hi Mike and Trish
That is a lovely image, sharp, great resolution and excellent color. I must admit, I resort to clusters when conditions are not favorable for photographing galaxies (my favorite). When the moon is up I play with collimation, I don't like to waste a clear night, assuming the conditions are up too it. I do envy your site up there, 2.5 arc/sec is about normal for us in Melbourne (2.2 arc/sec is a good night) and that is not mentioning the light pollution problem.
Ahhh we live what clear skies we can get. :D

Cheers
Peter

[Paul Haese]

Huge resolution Mike. Wicked colour too. I think I like the first image the most. Fantastic resolution which only comes with a big scope. couple of distant galaxies in the background too. Nice image.

[Atmos]

Very nice MnT, such great colour! You’re right, the blue stragglers definitely pop more in the lower stretched version but the original is more impactful overall :thumbsup:

Blue Stragglers are not quite though mergers however. They’ve also been called Vampire Stars as it happens when you have a fairly close binary star system. One of the stellar pair will exit the main sequence (MS) first as it will have a larger initial mass. As it moves off the MS it becomes a red giant and swells up to hundreds of times its size (think of our sun swallowing Mercury and Venus). If this material is within the gravitational well of its smaller binary star it’ll strip the newly closer hydrogen from the larger star.

As the smaller binary strips more and more hydrogen from its larger companion it becomes hotter and hotter. The once larger red giant now becomes smaller and cooler.

This process is also the process that causes repeating nova where the companion binary is a white dwarf.

[Placidus]

Thank you George!





Thanks very much Peter. You have reminded us that next time the moon is bright we should review the collimation. We should probably clean the primary mirror too. :sadeyes:





Thanks Paul! The consensus is now definite that the more stretched image wins.





Thanks Colin for both the encouragement and the detailed and very clear explanation.



Very best wishes,
Mike and Trish