APOD: NGC 7331 and Beyond (2011 Aug 12)

[APOD Robot]

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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[garry]

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

[Beyond]

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.

[alter-ego]

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

See this APOD

[dworden78947]

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]

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

[casubelli]

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]

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]

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.

[Sam]

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).

Diagram for the formula of the angular diameter

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The angular diameter of an object can be calculated using the formula:


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.

---
Sam

[orin stepanek]

NGC 7331 and Beyond

Explanation: Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way.

Nice galaxy! Any galaxy that is similar to the Milky Way has to be nice.

[Chris Peterson]

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 Peterson]

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.

[casubelli]

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

[Chris Peterson]

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.

[saturn2]

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]

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?

[Chris Peterson]

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.

[tECH hIPPY]

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

[tamarshall]

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.

[Ann]

NGC 7331 and Beyond

Explanation: Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way.

Nice galaxy! Any galaxy that is similar to the Milky Way has to be nice.

Ooohh!! Congratulations, beyond!

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

[alter-ego]

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.

[ExplorerAtHeart]

I dont see a bar.

[neufer]

I dont see a bar.

I think that Ann may have been posting from a bar.

[bystander]

I dont see a bar.

I think that Ann may have been posting from a bar.

[attachment=0]NGC7331 © Ken Crawford zoom.png[/attachment][/i]

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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.