APOD: NGC 891 vs Abell 347 (2017 Nov 02)

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APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby APOD Robot » Thu Nov 02, 2017 4:06 am

Image NGC 891 vs Abell 347

Explanation: Distant galaxies lie beyond a foreground of spiky Milky Way stars in this telescopic field of view. Centered on yellowish star HD 14771, the scene spans about 1 degree on the sky toward the northern constellation Andromeda. At top right is large spiral galaxy NGC 891, 100 thousand light-years across and seen almost exactly edge-on. About 30 million light-years distant, NGC 891 looks a lot like our own Milky Way with a flattened, thin, galactic disk. Its disk and central bulge are cut along the middle by dark, obscuring dust clouds. Scattered toward the lower left are members of galaxy cluster Abell 347. Nearly 240 million light-years away, Abell 347 shows off its own large galaxies in the sharp image. They are similar to NGC 891 in physical size but located almost 8 times farther away, so Abell 347 galaxies have roughly one eighth the apparent size of NGC 891.

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RocketRon

Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby RocketRon » Thu Nov 02, 2017 4:56 am

Edge on, square on or an oblique angle - is there any pattern to viewing other galaxies in our universe ?
Or are they completely random.

What does that tell us about galaxy formation, or how they travel.
Or, is this answer 'nothing'.

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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby Boomer12k » Thu Nov 02, 2017 5:52 am

RocketRon wrote:Edge on, square on or an oblique angle - is there any pattern to viewing other galaxies in our universe ?
Or are they completely random.

What does that tell us about galaxy formation, or how they travel.
Or, is this answer 'nothing'.


After looking at other galaxy group clusters, I would say "Random"...but maybe just my opinion and observation.

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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby Boomer12k » Thu Nov 02, 2017 5:54 am

Nice juxtaposition, and comparison of sizes and shapes...

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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby De58te » Thu Nov 02, 2017 10:19 am

So if nearby Andromeda Galaxy would be in the picture, what would Andromeda's apparent size be? M31 is twice the diameter of NGC 891, and it is about 12 times closer. Would that mean Andromeda would appear 14 times bigger than NGC 891, or 24 times bigger?

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Chris Peterson
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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby Chris Peterson » Thu Nov 02, 2017 1:27 pm

RocketRon wrote:Edge on, square on or an oblique angle - is there any pattern to viewing other galaxies in our universe ?
Or are they completely random.

When looking at the Universe over cosmological distances, some subtle but statistically significant patterns in orientation have been noted. But at the scale of individual clusters or of the sorts of fields we're going to see in images like this, random is pretty much correct.
Chris

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Chris Peterson
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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby Chris Peterson » Thu Nov 02, 2017 1:30 pm

De58te wrote:So if nearby Andromeda Galaxy would be in the picture, what would Andromeda's apparent size be? M31 is twice the diameter of NGC 891, and it is about 12 times closer. Would that mean Andromeda would appear 14 times bigger than NGC 891, or 24 times bigger?

The FOV of this image is about 1°, which is the size of M31 from top to bottom. So if Andromeda were in this frame, we'd be seeing just its central bulge, with about two-thirds of its entire length off the edges.
Chris

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Fred the Cat
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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby Fred the Cat » Thu Nov 02, 2017 3:55 pm

Happy Thor’s day in our curiously flat universe. It makes sense if the origin of the universe created dimensionality from precursors that started from a point expanding with time until ending up Thursday. :wink:
Feynman's Felicity "Only ascertain as a cat box survivor"

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MarkBour
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Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby MarkBour » Thu Nov 02, 2017 5:09 pm

De58te wrote:So if nearby Andromeda Galaxy would be in the picture, what would Andromeda's apparent size be? M31 is twice the diameter of NGC 891, and it is about 12 times closer. Would that mean Andromeda would appear 14 times bigger than NGC 891, or 24 times bigger?

To really answer your question, ""what would Andromeda's apparent size be?", I guess one should set aside your estimates and just say (from Wikipedia) that Andromeda subtends an angle of about 3.167° along its long axis (angular diameter). And NGC891 is also listed by Wikipedia at 13.5' of angular diameter.

The hypothetical question, "How would the visual angle change if a small, distant object were doubled in size and brought 12 times closer?" would be answered as follows. The tangent of the angle it subtends would be 24 times what it was. And the tangent is a non-linear function, but at sufficiently small angles, it is close to linear. So it is not a bad approximation to say that based on your estimates, you would expect Andromeda to appear about 24 times as large as NGC891.

Going in the reverse, if you cut the size of Andromeda in half and moved it 12 times as far away, it would subtend an angle that is pretty close to (3.167°/24) = 0.132°, or about 8' . You could do the math more carefully, converting to a tangent, dividing by 24, then converting back to an arctangent, and you get almost the same result (I get 7.9' using Excel).
Mark Goldfain

sillyworm2

Re: APOD: NGC 891 vs Abell 347 (2017 Nov 02)

Postby sillyworm2 » Thu Nov 02, 2017 7:46 pm

I am humbled as I enjoy all the marvels attained by modern technology. Calculations aside...I simply enjoy the grand mystery of The Universe.I am super excited about the upcoming James Webb Space Telescope. I'm 62 ....hoping for many more years of wonders revealed!


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