Blue "stars" in image of NGC 55

Processing Deep Sky NGC55 NGC Objects Galaxy
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George Hatfield avatar
George Hatfield avatar
I've been working on an image of NGC55 from The Astro Imaging Channel (YouTube) 2023 Workshop.  It includes LRGB and Ha from Terry Robison.   The attached image is an LRGB...  I have not added the Ha.  At first, I thought the blue specks were stars, but then I realized this galaxy is 6.5 million light-years away, and there is no way I could see individual stars.   Are these an artifact of processing?  I checked the original luminance, and they are there even without processing.  So what are these blue specks?  Individual clouds of blue stars?

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Matthew Proulx avatar
Matthew Proulx avatar
You might not see individual stars but you will see clusters. Add the Ha and see what color they change to. I've imaged many a galaxy where the nebulosity was blue/teal in just the LRGB.
Brian Puhl avatar
Brian Puhl avatar
Those definitely don't look like artifacts.   Probably Oiii structure and star clusters.  (just a guess, I'm not an expert)
andrea tasselli avatar
andrea tasselli avatar
As Matthew said above, they are clusters of OB giant stars, usually with associated Ha emission which should shown in a LRGB composition such as this (note that I blew the luminance in few locations with 600s integrations):

https://www.astrobin.com/full/tc8rmh/0/
George Hatfield avatar
George Hatfield avatar
Andrea…  What did you do in processing to bring the Ha regions out in the LRGB?  Thanks in advance.

George
andrea tasselli avatar
andrea tasselli avatar
George Hatfield:
Andrea...  What did you do in processing to bring the Ha regions out in the LRGB?  Thanks in advance.

George

Nothing special as I recall. Just PCC for color calibration but CC would work just as well see the raw RGB here below (simple Arcsinh followed with HT):



This with MaskedStretch and Arcsinh:


Selective saturation may have followed in the LRGB image.
Tim Hawkes avatar
Tim Hawkes avatar
On the HA.  I must say - in agreement I think with the comment above from  https://www.astrobin.com/users/battleriverobservatory/ -   it looks like a normally processed  LRGB image of an active star-forming galaxy but before much or any of any additional HA data has been added into the red channel.  How much HA is added and therefore how red some of the blue regions become is rather arbitrary and a matter of choice.
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