APOD: NGC 7331 and Beyond (2011 Aug 12)

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APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by APOD Robot » Fri Aug 12, 2011 4:06 am

Image NGC 7331 and Beyond

Explanation: Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way. About 50 million light-years distant in the northern constellation Pegasus, NGC 7331 was recognized early on as a spiral nebula and is actually one of the brighter galaxies not included in Charles Messier's famous 18th century catalog. Since the galaxy's disk is inclined to our line-of-sight, long telescopic exposures often result in an image that evokes a strong sense of depth. The effect is further enhanced in this sharp image by galaxies that lie beyond the gorgeous island universe. The background galaxies are about one tenth the apparent size of NGC 7331 and so lie roughly ten times farther away. Their close alignment on the sky with NGC 7331 occurs just by chance. Seen here through faint foreground dust clouds lingering above the plane of Milky Way, this visual grouping of galaxies is also known as the Deer Lick Group.

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by garry » Fri Aug 12, 2011 4:07 am

Great picture! Why would the central core of the galaxy be out of parallel with the main body of the galaxy?

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by Beyond » Fri Aug 12, 2011 4:27 am

WOW! I'm in an APOD. Even though i wasn't specifically pointed out, i don't mind giving way to such a nice looking Galaxy. :mrgreen:
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by alter-ego » Fri Aug 12, 2011 4:28 am

garry wrote:Great picture! Why would the central core of the galaxy be out of parallel with the main body of the galaxy?
See this APOD
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by dworden78947 » Fri Aug 12, 2011 8:50 am

Is it just me or am I just stupid? I live on a ranch in Lexington TX. I have beautiful dark skies all the time. I also have an 8" Meade telescope :)

I looked at this photo and counted hundreds of galaxies. The author must be nuts saying there is only 7 galaxies!

magicman

Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by magicman » Fri Aug 12, 2011 11:28 am

Using the inverse of the square of distance, isn't the statement about 1/10 size are ten times further away incorrect?

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by casubelli » Fri Aug 12, 2011 12:50 pm

Me, the trouble maker again.
This picture makes me want to say: sorry folks, the universe is bigger than 14 billion light years across.
If NGC 7331 is 50 mil l-yrs away, then the one a tenth its size straight above its core is 500 million. Then, looking below the left edge of the galaxy, at about 7 o'clock, are even tinier spirals about a tenth of the size of the second galaxy, so they're about, what, say 5 billion light years away? THEN...further out.... red-shifted dots or mere specks that look another tenth of these last...?so??...?50 billion???
Oh, I know, save your breath..."The Big Bang is the best theory we currently have that explains the majority of data..."

casubelli

Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by casubelli » Fri Aug 12, 2011 12:55 pm

Didn't mean by "across" a diameter. Should have said distant or whatever, to mean radius. 14 billion light years old would mean a radius of 14 billion l-yrs in BB theory. [As if dimensions or time would be uniform (not) during such a reverse apocalyse]

casubelli

Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by casubelli » Fri Aug 12, 2011 12:58 pm

Dang, I should have waited till my coffee hit my brain. No, the universe, if it expanded, didn't expand at the speed of light. Not that there is speed where there are no dimensions, in whatever the BB took place in, since there wasn't anything...
I better just shut up.

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by Sam » Fri Aug 12, 2011 1:10 pm

magicman wrote:Using the inverse of the square of distance, isn't the statement about 1/10 size are ten times further away incorrect?
You're thinking of gravity.
Angular diameter varies with the arctangent of the distance
(unless there happens to be a lot of gravity in between).
http://en.wikipedia.org/wiki/Angular_diameter wrote: The angular diameter of an object can be calculated using the formula:

Image
in which δ is the angular diameter, and d and D are the visual diameter of and the distance to the object, expressed in the same units.

When D is much larger than d, δ may be approximated by the formula δ = d / D, in which case the result is in radians.
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by orin stepanek » Fri Aug 12, 2011 1:27 pm

APOD Robot wrote:Image NGC 7331 and Beyond

Explanation: Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way.
Nice galaxy! 8-) Any galaxy that is similar to the Milky Way has to be nice. 8-)
Orin

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by Chris Peterson » Fri Aug 12, 2011 1:53 pm

Sam wrote:You're thinking of gravity.
Angular diameter varies with the arctangent of the distance.
Angular diameter varies linearly with distance (or an inverse linear relationship, depending on how you look at it). Double the distance, the angular extent is reduced by two. Ten times the distance, it is reduced by ten. When you're simply looking at how the angle varies with distance, the trig terms drop out.
Chris

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by Chris Peterson » Fri Aug 12, 2011 1:59 pm

casubelli wrote:This picture makes me want to say: sorry folks, the universe is bigger than 14 billion light years across.
The observable universe is about 93 billion light years across, so the most distant objects we can see are about 46 billion light years away. However, this image doesn't come close to going deep enough to see any objects more than a fraction of that distance.
Chris

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casubelli

hubble

Post by casubelli » Fri Aug 12, 2011 3:09 pm

Thanks, Chris.
So, are you saying the universe then is at least 46 billion years old?

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Re: hubble

Post by Chris Peterson » Fri Aug 12, 2011 3:25 pm

casubelli wrote:Thanks, Chris.
So, are you saying the universe then is at least 46 billion years old?
No, the Universe is 13.7 billion years old. But when you consider the size of the Universe, you need to consider that it has been expanding that entire time. So the most commonly given size for the observable Universe, based on comoving distance is 46 billion light years in radius. In other words, a photon that was emitted 13.7 billion years ago and is just reaching us came from a source which is now 46 billion light years away.
Chris

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by saturn2 » Fri Aug 12, 2011 3:57 pm

Constellation Pegasus.
Distance from Earth to NGC 7331, 50 million light- years.
NGC 7331 and Our Milky Way are twin galaxies.
Two galaxies very beautiful, indeed.
Messier didn´t "saw" spiral galaxy NGC 7331.

tECH hIPPY

Re: hubble

Post by tECH hIPPY » Fri Aug 12, 2011 4:21 pm

Chris Peterson wrote:
casubelli wrote:Thanks, Chris.
So, are you saying the universe then is at least 46 billion years old?
No, the Universe is 13.7 billion years old. But when you consider the size of the Universe, you need to consider that it has been expanding that entire time. So the most commonly given size for the observable Universe, based on comoving distance is 46 billion light years in radius. In other words, a photon that was emitted 13.7 billion years ago and is just reaching us came from a source which is now 46 billion light years away.
I don't fully understand - if an object emitted light 13 billion years ago (which is just reaching us now), how could that object have gotten so far away from the origin point of the Big Bang (billions of lightyears) to the point at which we observe it (which is closer to us than its actual current position) without moving faster than light? Or is that actually what the Big Bang was (FTL expansion) and therefore isn't it likely the universe is considerably larger than anything we could possibly measure?

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Re: hubble

Post by Chris Peterson » Fri Aug 12, 2011 4:27 pm

tECH hIPPY wrote:I don't fully understand - if an object emitted light 13 billion years ago (which is just reaching us now), how could that object have gotten so far away from the origin point of the Big Bang (billions of lightyears) to the point at which we observe it (which is closer to us than its actual current position) without moving faster than light?
With respect to the physical point where the photon was emitted, that photon is moving faster than light. That just means that point is outside the observable Universe, and that we are causally disconnected from it. No rules are being broken here.

The speed of light (or more properly, c) does not constrain the rate at which two points in the Universe can move with respect to each other due to the cosmological expansion of the intervening space.
Or is that actually what the Big Bang was (FTL expansion) and therefore isn't it likely the universe is considerably larger than anything we could possibly measure?
In all likelihood most of the Universe is outside the observable Universe, and therefore unmeasurable. But that has nothing to do with this discussion of the size of the observable Universe.
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by tECH hIPPY » Fri Aug 12, 2011 4:34 pm

Thanks Chris. I think I'm beginning to see the photon!

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by tamarshall » Fri Aug 12, 2011 4:38 pm

Would it help the discussion about the size and age of the universe to say that the expansion of the universe is not constrained by the constant (speed of light) so the universe is bigger than it is older? That is the size of the universe is not the same as the age of the universe, i.e. it is bigger than approx 14 billion l.y.

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by Ann » Fri Aug 12, 2011 4:43 pm

orin stepanek wrote:
APOD Robot wrote:Image NGC 7331 and Beyond

Explanation: Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way.
Nice galaxy! 8-) Any galaxy that is similar to the Milky Way has to be nice. 8-)
Ooohh!! Congratulations, beyond! :D :D :D

It's a very nice APOD. I particularly like all the background galaxies. There is one that I can't remember having seen before, the faint bluish one at two o'clock. It is probably a dwarf galaxy made up of an intermediate population, or else of old metal-poor stars.

The Deer Lick Group looks very nice. Note NGC 7337, the pretty barred galaxy with thin, beautiful, well-formed arms.

As for NGC 7331 itself, note how we can clearly see that its arms seem to "end" or "open" at lower left. That is why we can be sure that it rotates clockwise, at least from this point of view.

In the more or less opposite direction from NGC 7331, you can find the famous Stephan's Quintet group. See here.

Fascinatingly, Stephan's Quintet and the Deer Lick Group have similar redshifts. These galaxies all recede from us at between 6,000 and 7,000 kilometers per second. (Well, except the biggest-looking member of Stephan's Quintet, which is a foreground dwarf galaxy.)

It is fascinating to consider that the Deer Lick Group and Stephan's Quintet may be at a similar distance from us. The galaxies appear to be similar in size, too. Comparing the Deer Lick Group with Stephan's Quintet really brings home how close together the Stephan's Quintet members really are.

What about NGC 7320, the interloper in Stephan's Quintet? Would you believe that it appears to be at about the same distance from us as NGC 7331?

So there are a whole lot of galaxies in this part of the sky, but maybe the Deer Lick Group and Stephan's Quintet form a kind of association, and maybe NGC 7320 is associated with NGC 7331!

Ann
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by alter-ego » Fri Aug 12, 2011 4:47 pm

Chris Peterson wrote:
Sam wrote:You're thinking of gravity.
Angular diameter varies with the arctangent of the distance.
Angular diameter varies linearly with distance (or an inverse linear relationship, depending on how you look at it). Double the distance, the angular extent is reduced by two. Ten times the distance, it is reduced by ten. When you're simply looking at how the angle varies with distance, the trig terms drop out.
Sam is technically correct here. However, subtended angle is nearly linear with distance for almost all astronomical objects. For subtended full angles <20º, the deviation from linearity is <1%.

Of course, the linearity principal for astronomical objects is not perfect either, GR affects this too.
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by ExplorerAtHeart » Fri Aug 12, 2011 8:53 pm

I dont see a bar.

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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by neufer » Fri Aug 12, 2011 9:10 pm

ExplorerAtHeart wrote:
I dont see a bar.
I think that Ann may have been posting from a bar.
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Re: APOD: NGC 7331 and Beyond (2011 Aug 12)

Post by bystander » Fri Aug 12, 2011 11:42 pm

ExplorerAtHeart wrote:I dont see a bar.
neufer wrote:I think that Ann may have been posting from a bar.
[attachment=0]NGC7331 © Ken Crawford zoom.png[/attachment][/i]

That may be, but I think Ann was referring to the small
image just above and left of NGC 7331, see the zoom.

Also check out the description in R. Jay Gabany's
portrayal of NGC 7331: The Deer Lick Group.
The easily identifiable galaxies in this group are (left to right, from the top) NGC7340, NGC7337, NGC7335, NGC7336, NGC7331 (the largest), NGC3727 (the upper fuzzy component of the red-blue "double star" below the right edge of the largest) & NGC 7326.

NGC7337, NGC7335, NGC7336 are ten times farther away and would dwarf the largest in this scene if their distances were equalized.
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