APOD: NGC 7331 Close Up (2022 Sep 22)

[APOD Robot]

NGC 7331 Close Up

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. This Hubble Space Telescope close-up spans some 40,000 light-years. The galaxy's magnificent spiral arms feature dark obscuring dust lanes, bright bluish clusters of massive young stars, and the telltale reddish glow of active star forming regions. The bright yellowish central regions harbor populations of older, cooler stars. Like the Milky Way, a supermassive black hole lies at the core of spiral galaxy NGC 7331.

<< Previous APOD

This Day in APOD

Next APOD >>

[Ronald Jensen]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

[Chris Peterson]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

[Ann]

NGC 7331 close up. Image Credit & License: ESA/Hubble & NASA/D. Milisavljevic (Purdue University).

---

That's a very nice portrait of the central parts of NGC 7331!

I can't help it though, my favorite portrait of NGC 7331 is this picture by Vicent Peris:

NGC 7331 and the Deer Lick Group of galaxies. Photo: Vicent Peris.

---

The dancing feet of NGC 7331!

The orientation of Vicent Peris' image is correct. NGC 7331 is actually "standing up". Not only that, but it is "dancing"! To my knowledge, NGC 7331 is the best example of a spiral galaxy where you can see the "end" of spiral arms so clearly.

Vicent Peris' image also gives us a splendid portrait of the Deer Lick Group of galaxies. Starting with the achingly lovely barred spiral galaxy below center, the members of the Deer Lick Group are, clockwise, NGC 7337, NGC 7340, NGC 7336 and NGC 7335.

NGC 7331 is located close to Stephan's Quintet as seen from our perspective:

NGC 7331 is in the upper right corner and Stephan's Quintet in the lower left corner in this ultraviolet view. Photo: NASA/JPL-Caltech/SSC.

---

The blue foreground member of Stephan's Quintet, NGC 7320, is believed to possibly be a dwarf member of the NGC 7331 sphere of influence. Photo: Jschulman555.

---

Ann

[AVAO]

...
I can't help it though, my favorite portrait of NGC 7331 is this picture by Vicent Peris:
...
Ann

... and my favorite NGC 7331 in IR (Spitzer ST): ... and a nice target for JWST


https://www.spitzer.caltech.edu/image/s ... laxys-twin
Whitney Clavin (818) 354-4673Jet Propulsion Laboratory, Pasadena, Calif.

[VictorBorun]

...
I can't help it though, my favorite portrait of NGC 7331 is this picture by Vicent Peris:
...
Ann

... and my favorite NGC 7331 in IR (Spitzer ST): ... and a nice target for JWST

Click to view full size image 1 or image 2

In IR the galaxy kind of loses the feeling of foreground/background edges given with all the dust

[orin stepanek]

A beautiful galaxy; heavily laden with stars!

[MarkBour]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

--- 1 ---
The effect you're asking about, could be detected, if we had supernatural knowledge that we don't have from our human vantage point. And maybe it can be observed with supermassive black hole jets. Imagine a perfectly straight barbell 100,00 light years long. If it were rotating, it would appear bowed to us, sitting a ways off in one direction; the near end would appear to have rotated more and the far end to have rotated less than in "reality". (I think Einstein would take objection with my construction here of a "reality" outside of what is observable.)

Capture0.png (7.4 KiB) Viewed 2635 times

--- 2 ---
I even drew the far sphere on the end of the barbell as slightly smaller than the near one. But both the size differences and the bowing would be imperceptible in practice. The bowing imperceptible because the rotation is too slow. The size differences imperceptible because 100,00 out of 50 million is also very small. Things like individual stars in a distant galaxy can't be resolved to a detectable diameter anyway, so there would definitely not be any size difference to measure for stars at the near and far arms of a galaxy.

Chris pointed out that this bowing would be too tiny to make much difference in the shapes we would observe on a spiral galaxy, with only about a 0.1% deformation in his typical galaxy estimation. So the bowing would be measurable with high precision instruments, if we knew what to measure, but we don't have anything special to measure, and any distortion of the overall appearance would be imperceptible.

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

--- 4 ---
"What does this galaxy look like right now?" is a question sometimes posed by those who, relativity aside, imagine a notion of an absolute reality beyond the observations. If NGC 7331 is currently there, 50 million light years away from us, and we could jump there "now" instantly. And imagining that it has a rotation rate like our own galaxy, rotating once every 200 million years or so, we would find that it has rotated about 90^o compared to the way it looks to us right now in our telescopes. It seems that professionals, when asked about this kind of thing, mostly say "So what? It hardly matters."

--- * ---
I struggle sometimes with my current view of the existence of an absolute reality, while having a basic understanding of the ideas behind relativity theory, though I have not managed to study it in detail, the way a trained astronomer or physicist would have.

[johnnydeep]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

...snipped...

--- * ---
I struggle sometimes with my current view of the existence of an absolute reality, while having a basic understanding of the ideas behind relativity theory, though I have not managed to study it in detail, the way a trained astronomer or physicist would have.

You and me both!

[Chris Peterson]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

--- 1 ---
The effect you're asking about, could be detected, if we had supernatural knowledge that we don't have from our human vantage point.

Another way of looking at it is that the effect creates a visually different galaxy than we'd have if the speed of light were infinite, or if the galaxy didn't rotate. Very, very slightly different, but presumably measurable. But absent any reference like your hypothetical bar, that's pretty meaningless.

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

Redshift is pretty useless for the near and far parts of the galaxy, because both have the same Doppler shift, since the movement is lateral. That can be used for the sides to tell which way the galaxy is rotating, and how fast. Brightness might be used to distinguish the near and far sides, but nothing related to rotation.

[johnnydeep]

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

--- 1 ---
The effect you're asking about, could be detected, if we had supernatural knowledge that we don't have from our human vantage point.

Another way of looking at it is that the effect creates a visually different galaxy than we'd have if the speed of light were infinite, or if the galaxy didn't rotate. Very, very slightly different, but presumably measurable. But absent any reference like your hypothetical bar, that's pretty meaningless.

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

Redshift is pretty useless for the near and far parts of the galaxy, because both have the same Doppler shift, since the movement is lateral. That can be used for the sides to tell which way the galaxy is rotating, and how fast. Brightness might be used to distinguish the near and far sides, but nothing related to rotation.

Doesn't the image below depict how red shift can be used to determine rotation direction? Are you just making a distinction between doppler shift and red shift? Doppler shift being due to the expansion of space and red shift being due to the relative velocity of different parts added or subtracted to the overall Doppler shift?

Click to view full size image

[Chris Peterson]

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

--- 1 ---
The effect you're asking about, could be detected, if we had supernatural knowledge that we don't have from our human vantage point.

Another way of looking at it is that the effect creates a visually different galaxy than we'd have if the speed of light were infinite, or if the galaxy didn't rotate. Very, very slightly different, but presumably measurable. But absent any reference like your hypothetical bar, that's pretty meaningless.

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

Redshift is pretty useless for the near and far parts of the galaxy, because both have the same Doppler shift, since the movement is lateral. That can be used for the sides to tell which way the galaxy is rotating, and how fast. Brightness might be used to distinguish the near and far sides, but nothing related to rotation.

Doesn't the image below depict how red shift can be used to determine rotation direction? Are you just making a distinction between doppler shift and red shift? Doppler shift being due to the expansion of space and red shift being due to the relative velocity of different parts added or subtracted to the overall Doppler shift?

Click to view full size image

Well, yeah. That's what I said. You can use it at the sides to determine direction and speed, but it provides no information about the front and back as both have the same Doppler shift- zero with respect to each other.

Doppler shift is a product of relative motion. Cosmological redshift if a product of the expansion of space.

[Ann]

One thing I've often wondered about these pictures of distant galaxies, especially ones that are from an angle like this one. These galaxies are tens of thousands of light years across, often more than a hundred thousand light years across. This means that our view of the far side of the galaxy is from several tens of thousands of light years farther away than our view of the nearer side. Because the light from the far side therefore takes tens of thousands of years longer to reach us than the light from the near side, there must be some significant and visible distortion in the image. Wouldn't that kind of "warp" the shape of the galaxy when it is seen from an angle? -- Ron

Not significantly. Galaxies like this have rotation periods on the order of 100 million years or more. So the nearest and farthest parts will be out of "sync" with each other by less than 0.1%, which isn't going to result in any visible distortion.

I totally accept the answer Chris gave here. But you pose an interesting question.
I think today, I'll put down my thoughts about it and see if anyone feels I have gone astray at one or more points.

--- 1 ---
The effect you're asking about, could be detected, if we had supernatural knowledge that we don't have from our human vantage point. And maybe it can be observed with supermassive black hole jets. Imagine a perfectly straight barbell 100,00 light years long. If it were rotating, it would appear bowed to us, sitting a ways off in one direction; the near end would appear to have rotated more and the far end to have rotated less than in "reality". (I think Einstein would take objection with my construction here of a "reality" outside of what is observable.)

Click to view full size image

--- 2 ---
I even drew the far sphere on the end of the barbell as slightly smaller than the near one. But both the size differences and the bowing would be imperceptible in practice. The bowing imperceptible because the rotation is too slow. The size differences imperceptible because 100,00 out of 50 million is also very small. Things like individual stars in a distant galaxy can't be resolved to a detectable diameter anyway, so there would definitely not be any size difference to measure for stars at the near and far arms of a galaxy.

Chris pointed out that this bowing would be too tiny to make much difference in the shapes we would observe on a spiral galaxy, with only about a 0.1% deformation in his typical galaxy estimation. So the bowing would be measurable with high precision instruments, if we knew what to measure, but we don't have anything special to measure, and any distortion of the overall appearance would be imperceptible.

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

--- 4 ---
"What does this galaxy look like right now?" is a question sometimes posed by those who, relativity aside, imagine a notion of an absolute reality beyond the observations. If NGC 7331 is currently there, 50 million light years away from us, and we could jump there "now" instantly. And imagining that it has a rotation rate like our own galaxy, rotating once every 200 million years or so, we would find that it has rotated about 90^o compared to the way it looks to us right now in our telescopes. It seems that professionals, when asked about this kind of thing, mostly say "So what? It hardly matters."

--- * ---
I struggle sometimes with my current view of the existence of an absolute reality, while having a basic understanding of the ideas behind relativity theory, though I have not managed to study it in detail, the way a trained astronomer or physicist would have.

I'm impressed, Mark. It's very good to have people like you here at Starship Asterisk*. You are extremely knowledgeable and great at math - well, you haven't managed to study general relativity in detail, oh well - and you are so willing to share your insights and thoughts with the rest of us.

You gave me a suggestion as to how I could identify individual stars in star trail images, and I didn't even reply. Don't think I'm not grateful, but what you suggested was not for me.

But please keep up the things you are doing here. I love you image of the bowed barbell, which represents a galactic bar. Thank you!

Ann

[johnnydeep]

Another way of looking at it is that the effect creates a visually different galaxy than we'd have if the speed of light were infinite, or if the galaxy didn't rotate. Very, very slightly different, but presumably measurable. But absent any reference like your hypothetical bar, that's pretty meaningless.


Redshift is pretty useless for the near and far parts of the galaxy, because both have the same Doppler shift, since the movement is lateral. That can be used for the sides to tell which way the galaxy is rotating, and how fast. Brightness might be used to distinguish the near and far sides, but nothing related to rotation.

Doesn't the image below depict how red shift can be used to determine rotation direction? Are you just making a distinction between doppler shift and red shift? Doppler shift being due to the expansion of space and red shift being due to the relative velocity of different parts added or subtracted to the overall Doppler shift?

Click to view full size image

Well, yeah. That's what I said. You can use it at the sides to determine direction and speed, but it provides no information about the front and back as both have the same Doppler shift- zero with respect to each other.

Doppler shift is a product of relative motion. Cosmological redshift if a product of the expansion of space.

Ok, right. I missed your distinction between the lateral and front and back parts of a galaxy.

[MarkBour]

Thanks for the encouragement Ann !
I certainly appreciate your postings, too, they add a lot of value to APOD.

[MarkBour]

Good points, johnnydeep and Chris. Just to clarify what I was talking about in my point 3,

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

I was wondering about cosmological redshift. Chris is clearly correct that the galactic rotation would produce only lateral motion at the front and back of the galaxy, so that motion would not produce different doppler shifts.

But using what little I know about cosmological expansion, if it were a consistent linear effect, the same through a galaxy as between galaxies, then for a nearly edge-on galaxy that had a diameter of 100,000 light years and was 50 million light years away, one would expect the cosmological redshift to the stars at the farthest parts of said galaxy to be a comparison of numbers like 50,000,000 versus 50,100,000. So only a 0.2% difference, the same kind of ratio we keep getting here.

I've seen statements that, for whatever reason, one just doesn't see cosmological expansion within galaxies, only between them. I don't know enough to know if a 0.2% difference would be detectable by our instruments for a mostly-edge-on galactic disk, even if cosmological expansion does happen within galaxies. If such a thing were real, then Andromeda would be the best place to look for it. Since it is much closer, the ratio of near-point to far-point distance compared to galactic distance would be much greater. Dang it, though, Andromeda is heading towards us, so it seems that it's just not applicable at all in this case.

But certainly, Ronald Jensen's post brought up some interesting related questions.
I remember a discussion that contributor Ironwood started back in the forum discussion:
http://asterisk.apod.com/viewtopic.php? ... al#p281085
In that discussion, several contributors, including Ann, Chris, and neufer helped sort out some of those questions.

The doppler information that johnnydeep and Chris posted in this current discussion is similar to (and is consistent with) some of the notes posted there.

[Chris Peterson]

Good points, johnnydeep and Chris. Just to clarify what I was talking about in my point 3,

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

But using what little I know about cosmological expansion, if it were a consistent linear effect, the same through a galaxy as between galaxies, then for a nearly edge-on galaxy that had a diameter of 100,000 light years and was 50 million light years away, one would expect the cosmological redshift to the stars at the farthest parts of said galaxy to be a comparison of numbers like 50,000,000 versus 50,100,000. So only a 0.2% difference, the same kind of ratio we keep getting here.

I've seen statements that, for whatever reason, one just doesn't see cosmological expansion within galaxies, only between them. I don't know enough to know if a 0.2% difference would be detectable by our instruments for a mostly-edge-on galactic disk, even if cosmological expansion does happen within galaxies. If such a thing were real, then Andromeda would be the best place to look for it. Since it is much closer, the ratio of near-point to far-point distance compared to galactic distance would be much greater. Dang it, though, Andromeda is heading towards us, so it seems that it's just not applicable at all in this case.

Indeed, there is no cosmological redshift across a galaxy, because there is no expansion of space across a galaxy. The force of gravity is much greater than the "force" of expansion. Expansion does not even occur in galaxy clusters. Galaxies and clusters remain the same size even as the Universe expands.

[johnnydeep]

Good points, johnnydeep and Chris. Just to clarify what I was talking about in my point 3,

--- 3 ---
What about red shifts or average brightness of the stellar populations? Are any differences measurable for the nearest versus the farthest arms? I have no idea. I wonder if effects from the dust in and around the galaxy would complicate any detection of tiny effects of distance differences.

But using what little I know about cosmological expansion, if it were a consistent linear effect, the same through a galaxy as between galaxies, then for a nearly edge-on galaxy that had a diameter of 100,000 light years and was 50 million light years away, one would expect the cosmological redshift to the stars at the farthest parts of said galaxy to be a comparison of numbers like 50,000,000 versus 50,100,000. So only a 0.2% difference, the same kind of ratio we keep getting here.

I've seen statements that, for whatever reason, one just doesn't see cosmological expansion within galaxies, only between them. I don't know enough to know if a 0.2% difference would be detectable by our instruments for a mostly-edge-on galactic disk, even if cosmological expansion does happen within galaxies. If such a thing were real, then Andromeda would be the best place to look for it. Since it is much closer, the ratio of near-point to far-point distance compared to galactic distance would be much greater. Dang it, though, Andromeda is heading towards us, so it seems that it's just not applicable at all in this case.

Indeed, there is no cosmological redshift across a galaxy, because there is no expansion of space across a galaxy. The force of gravity is much greater than the "force" of expansion. Expansion does not even occur in galaxy clusters. Galaxies and clusters remain the same size even as the Universe expands.

About that. Cosmological expansion of space is due to "dark energy", correct? And I thought dark energy was in fact operating all all scales, from the very small to the very large, suffusing all space everywhere, including between galaxies, within galaxies, within solar systems, and even here on Earth, between me and my computer screen. So, is it just that over short distances, dark energy doesn't exert enough "force" to counteract gravity? But if so, in theory, we might still be able to notice how much the force of gravity is diminished depending on distance: the more space between objects, the more the gravity between them is counteracted!