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Posted (edited)

Hi all, some very basic or hypothetical questions here I can't seem to find online at the moment.

I was just looking at an excellent image of M101 Pinwheel Galaxy here on SGL, and noticed numerous bright areas dotted throughout the spiral arms.

Does anyone know if these areas are massive emissions/reflection nebulae or newly formed stars? Or a mix of both? Sort of like our Milky Way?

I see some HII regions internally in M101 listed on Stellarium, but they don't seem to correspond. What is the difference between an HII region and a nebula?

Can you have a smaller galaxy form inside a 'host galaxy'?

Sorry for all the questions but thought I'd go ahead and put them in one post.

If anyone knows of a good detailed text on galaxies please let me know. A fascinating topic that I've not delved into a great deal.

Cheers all.

(photo credit: Dr George Jacoby, NOAO, Kitt Peak)

https://www.noao.edu/image_gallery/html/im0512.html

M101 HR crop.jpg

Edited by Ships and Stars

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31 minutes ago, Ships and Stars said:

Does anyone know if these areas are massive emissions/reflection nebulae or newly formed stars? Or a mix of both?

Probably emission nebulae / young star clusters.

Take for example Orion molecular cloud complex here in Milky way. It has size of about 5000 ly (give or take). M101 has diameter of 170,000 ly.

image.png.bfc48bf9f6b9ba62ede195b0d55178ca.png

If we take one such feature in the image and measure length of it and measure size of galaxy on the image (or if you want to be more precise - plate solve to get arc seconds per pixel and get estimated distance to M101 and solve for size) - we get following:

37.7 ly/px and 130 px = 4901 ly (within error of rough measurement of course).

This just shows that feature above in the image is the size of feature that we know in our own galaxy - Orion molecular cloud complex. Both "glow" with knots and features:

Orion_Head_to_Toe.jpg

credit: Rogelio Bernal Andreo, source Wiki page on Orion Molecular Cloud Complex: https://en.wikipedia.org/wiki/Orion_Molecular_Cloud_Complex

42 minutes ago, Ships and Stars said:

I see some HII regions internally in M101 listed on Stellarium, but they don't seem to correspond. What is the difference between an HII region and a nebula?

Nebula is any kind of nebulosity - can be dark nebula (gas/dust that is not illuminated and obstructs view of background stars / objects), reflection negula (gas / dust being illuminated by nearby star or stars), planetary nebula (exploding star remains), emission nebula (hot gas emitting light - similar to neon light - excite it with electricity and it will glow, but gas in interstellar space is excited by temperature/atomic collisions or stellar wind) and special case of emission nebula where dominant component of gas is HII - HII region (often birth place of stars since young stars are formed from Hydrogen gas collapsing under gravity).

47 minutes ago, Ships and Stars said:

Can you have a smaller galaxy form inside a 'host galaxy'?

No, not really - closest thing to small galaxy in host galaxy would be globular cluster, and I think one of hypothesis for their formation was - core of small galaxy that was absorbed into host galaxy.

HTH

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49 minutes ago, Ships and Stars said:

If anyone knows of a good detailed text on galaxies please let me know. A fascinating topic that I've not delved into a great deal.

I find wiki to be very good starting point. For example:

https://en.wikipedia.org/wiki/Galaxy

then follow up with:

https://en.wikipedia.org/wiki/Galaxy_formation_and_evolution

and of course - if at any time you want to go deeper into particular subject - there is list of references that you can follow.

If you want to play a bit with dissecting our nearest neighbor - here is excellent resource:

https://www.spacetelescope.org/images/heic1502a/zoomable/

I just love the way you can zoom in on a "bright star" to learn it is in fact globular cluster :D

Here is feature "in a distant galaxy not really resolved by our telescope"

image.png.85a8846b61d35c897d9a8b6f663c9e2a.png

Same feature a bit better resolved:

image.png.c1108bfc5ff646c2674f5c83aedb5610.png

But what are those? A star and a cloud?

image.png.40bfecfc3eb1f3e689f99a57a7919949.png

Young open cluster in a bit of gas and globular cluster! Imagine that! :D

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Thanks Vlaiv and Stu! Helps clarify some basics I've not fully appreciated when viewing galaxies as faint fuzzies, even through my big scope, it's only when looking at the excellent photos on here that I begin to think more about these things. I may consider imaging some day soon. I'll dig around online for more articles, Wikipedia is usually the first port of call but didn't see the link for galaxy formation before, shall check that out at home. Out for my daily walk if I don't get blown away, pretty breezy at the moment!

Thanks again!

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PS if there are any academic texts on galaxies that anyone knows of, that would be great. I've downloaded the astronomy text from Rice University, the galaxy section is a good starting point.

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PS the galaxy evolution article on Wikipedia is quite interesting, plus this was in the further reading section:

Mo, Houjun; van den Bosch, Frank; White, Simon (June 2010), Galaxy Formation and Evolution (1 ed.), Cambridge University Press, ISBN 978-0521857932

Thanks again Vliav & Stu.

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the bright spots in the galaxy they are many they only must be emmissions nebulae.😁

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