NGC 6995 The Bat Nebula clearly visible expansion over 68 years

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Menno555
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NGC 6995 The Bat Nebula clearly visible expansion over 68 years

#1

Post by Menno555 »

NGC 6995 (The Bat Nebula) is a part of the Veil Nebula.
Capture and video showing a very clear expansion of the nebula over 68 years.

I captured it for 2 hours last night and like usual I did compare my stacked capture in Aladin but also with a capture from https://archive.stsci.edu/cgi-bin/dss_form and the the POSS1 Blue capture from 1953 made with the 18 inch Mount Palomar Schmidt telescope.
When layering my capture and the Palomar capture in Photoshop, there were some proper motion stars as expected but the movement of the nebula was the big surprise.

I first though that was because I captured in dual narrowband and the Palomar one was done with a broadband blue filter. But the movement was too big for that.
Did some searching and learned that this expansion first was observed by the Hubble telescope which took images between 1997 and 2015, an 18 years period. Also, this expansion happens at a velocity of about 1.5 million kilometers (932057 miles) per hour.
Ergo: the movement I did see is real :)

So of course I had to make it as video. Besides the Hubble one, I couldn't find anything similar, so I think it's a kind of first? :)
Video is best observed in full. Note: it looks like everything is moving but that's an optical illusion: the stars stay on one place and the nebula moves.

Here the video:

https://youtu.be/a9BFnjciq90

And of course also the colored capture itself below.

Bortle 6/7
Meade LX200 8" f/10 ACF OTA
Ioptron CEM25EC montering (no guiding)
Optolong L-eXtreme filter
Zwo ASI071MC Pro camera

Captured with SharpCap Pro @ -10 Celsius / White balance R50 B50
12 x 600 sec / Gain 90 / Offset 20)
20 x darks, 50x flats en 50x darkflats

Stacked with DeepSkyStacker

Processed in SiriL and Photoshop
Siril: Crop and Histogram Transformation
Photoshop: Levels, Curves, Camera Raw Filter (blacks, clarity, sharpening, color, nose reduction) en nog wat bewerkingen. Resize 5000 to 2000px

NGC 6995sc.jpg
NGC 6995sc.jpg (571.57 KiB) Viewed 959 times
timh
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Re: NGC 6995 The Bat Nebula clearly visible expansion over 68 years

#2

Post by timh »

Interesting post again Menno!

When I saw the title I first thought of the other 'bat' nebula in Cepheus but I can see a few bats in the Eastern veil as well and why it is also called that :-)

Of course now you have set yourself up for doing the same on the Western veil as well :-) - although maybe no suitable archival material? The expansion movement that you demonstrate appears all in one direction on the Eastern side - presumably away from the locus of the original supernova explosion - so it must be going in the opposite direction on the Western veil side?

I also casually wondered whether the visible movement you see in 68 years (10 arcsec? don't know image scale just guessing) ?- about works for estimating an upper limit for how long ago the original supernova happened? So, for example, if the veil centre is say 60 arc minutes away then the explosion must have happened within the last 25,000 years? The official estimates say about 10,000 years but I guess that the rate of expansion is gradually slowing.

Anyway great food for thought again..thanks

Tim
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Menno555
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Re: NGC 6995 The Bat Nebula clearly visible expansion over 68 years

#3

Post by Menno555 »

Thanks Tim :)
The Western Veil is available and I already have it planned to do the same :)
It should indeed move in the the other way since it all has a central point with the expansion.

The image scale. This is what SiriL is giving as solution:
Resolution: 0.486 arcsec/px
Focal: 2027.91 mm
Pixel size: 4.78 µm
Field of view: 40' 3.72" x 26' 36.64"

Apparent size is around 95% of the moon.

I checked in Photoshop and the average movement is around 5 px, so that would mean a visible movement of around 2 to 3 arcsec?
There is some difference is distance though: the outside is around 6 px, while the inside is 4 px. Plus there is some swirling of the clouds, so it's hard to determine.

Menno
timh
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Re: NGC 6995 The Bat Nebula clearly visible expansion over 68 years

#4

Post by timh »

Thanks for the analysis Menno,

Well it’s fun to try and calculate. If you are visually estimating expansion at say 3-arcsec in 68 years then that’s really not bad in terms of agreement with professional estimates of 1.5 M km/ hour. i.e. perhaps about half the expected maximum visible rate? .. Also, aside from variation due to orientation versus line of sight there is perhaps no reason to expect that the expansion rate should remain the same in every direction.

i.e. If you assume that it is 1500 ly away then after 68 years of expansion at 1.5M km/ h the diameter would be expected to increase by ~ 0.1 ly- or ~ 0.05 ly in radius. At a distance of 1500 light years that radial increase then works out at about ~ 6.5 arcsec (I think)?

It also says that - if the current estimate that the Veil is 8000 years old is correct - then the rate of expansion must have slowed down dramatically over time. i.e. whereas the Veil is about 90 ly across (i.e. subtending a radius of 1.5 degrees or so at 1500 ly distant) constant expansion for 8000 years at the current rate would have created a nebula that was only ~ 12 light years across.

So the expansion rate must have slowed down by at least 8 fold or so. Makes sense really because I guess all those collisions with interstellar material that help to keep it hot and glowing also slow it down? It must have been a hotter and brighter object a few thousand years ago!

https://asd.gsfc.nasa.gov/blueshift/ind ... mnant-age/



Tim
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