APOD: NGC 7331 Close-Up (2018 Feb 07)

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APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby APOD Robot » Wed Feb 07, 2018 5:10 am

Image 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. In this Hubble Space Telescope close-up, 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 of spiral galaxy NGC 7331.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Ann » Wed Feb 07, 2018 5:53 am

Nice picture. It's good to see a close-up of a galaxy that doesn't often get its high-magnification mugshot taken.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby sillyworm2 » Wed Feb 07, 2018 2:08 pm

Pictures of Galaxies! My favorite subject here.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby ta152h0 » Wed Feb 07, 2018 4:20 pm

those black holes are the cosmic muscle cars
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Visual_Astronomer » Wed Feb 07, 2018 5:45 pm

Great close-up and nice detail, but I like the wide-field view of 7331 and its neighbors - the "Deer Lick Group" - as shown here:
https://apod.nasa.gov/apod/ap080712.html

That's more like the view I get at the eyepiece.

Zuben L. Genubi

Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Zuben L. Genubi » Wed Feb 07, 2018 5:50 pm

What would this galaxy have looked like to Messier through the best telescope available at the time? I'm guessing like a fuzzy little blob,but stationary and hence not a comet.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Chris Peterson » Wed Feb 07, 2018 7:36 pm

Zuben L. Genubi wrote:What would this galaxy have looked like to Messier through the best telescope available at the time? I'm guessing like a fuzzy little blob,but stationary and hence not a comet.

That pretty much describes what it looks like through the best telescopes of our own time!
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby bystander » Wed Feb 07, 2018 8:11 pm

Visual_Astronomer wrote:Great close-up and nice detail, but I like the wide-field view of 7331 and its neighbors - the "Deer Lick Group" - as shown here: https://apod.nasa.gov/apod/ap080712.html

Im1[1].jpg


Another wide field image of NGC 7331 and the Deer Lick Group (top, left of center) along with Stephan's Quintet (bottom, right of center) from CFHT's MegaCam.

viewtopic.php?t=38012
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby neufer » Wed Feb 07, 2018 8:22 pm

Chris Peterson wrote:
Zuben L. Genubi wrote:
What would this galaxy have looked like to Messier through the best telescope available at the time?
I'm guessing like a fuzzy little blob,but stationary and hence not a comet.

That pretty much describes what it looks like through the best telescopes of our own time!

    The faintest extended objects observed visually through a telescope appear to be magnitude 13 planetary nebulas:
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Spicy Meatball » Wed Feb 07, 2018 9:10 pm

Really shows us what are galaxy could look like from the side.
From this, we can get a perspective of how small we are in this universe.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby MarkBour » Wed Feb 07, 2018 10:36 pm

I realize that images like this are processed, and sometimes what I am unaware of leads me to terribly wrong conclusions. Because of this, I'll ask about something that looks rather obvious in this and many other grand galaxy portraits: the closer you get to the center of the galaxy, the brighter the visual light output. Correct? Especially around central black holes, which one might guess would be places of relative darkness, there the light output of the galaxy tends to shine quite intensely compared to the outer regions. Certainly this is true for face-on galaxies, but I think it even holds for edge-on images, provided you remove the effects of obscuring dust lanes.

So, is that a correct observation? If so, why is it the case? Why would the center of a galaxy be its greatest powerhouse of light output? And is this only true of the human-visible range of the spectrum? I'm thinking it holds pretty much across the electromagnetic spectrum.
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Chris Peterson » Thu Feb 08, 2018 12:53 am

MarkBour wrote:I realize that images like this are processed, and sometimes what I am unaware of leads me to terribly wrong conclusions. Because of this, I'll ask about something that looks rather obvious in this and many other grand galaxy portraits: the closer you get to the center of the galaxy, the brighter the visual light output. Correct?

Yes. And the difference is much greater than most images show. That's because when we process galaxy images we normally compress the dynamic range to fit within what our displays are capable of. That means we make the centers dimmer and the edges brighter.

Especially around central black holes, which one might guess would be places of relative darkness, there the light output of the galaxy tends to shine quite intensely compared to the outer regions.

The central black hole is far too tiny to impact the appearance of a galaxy. We're talking about a body smaller than a solar system. In an image, just a fraction of a pixel.

Why would the center of a galaxy be its greatest powerhouse of light output? And is this only true of the human-visible range of the spectrum? I'm thinking it holds pretty much across the electromagnetic spectrum.

Yes. Because the central section has more stars, closer together. And in the middle, you're looking through a somewhat spherical bulge, as opposed to a thin disc as you move outward.
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Ann » Thu Feb 08, 2018 2:33 am

bystander wrote:
Visual_Astronomer wrote:Great close-up and nice detail, but I like the wide-field view of 7331 and its neighbors - the "Deer Lick Group" - as shown here: https://apod.nasa.gov/apod/ap080712.html

Im1[1].jpg


Another wide field image of NGC 7331 and the Deer Lick Group (top, left of center) along with Stephan's Quintet (bottom, right of center) from CFHT's MegaCam.

viewtopic.php?t=38012


I have a question regarding that image from CFHT, more precisely the details it reveals in Stephan's Quintet.

My question is here. I apologize if one of the pictures I have posted in it is too large. I have a new computer, and I have somehow lost the ability to determine the number of pixels in a picture.

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Boomer12k » Thu Feb 08, 2018 4:23 am

I think it has more Dust than us...and hence we would have a better view of them, than they would have of us....

Great image... love the red regions...some must be HUGE over there...

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby MarkBour » Fri Feb 09, 2018 1:04 am

Chris Peterson wrote:
MarkBour wrote:Why would the center of a galaxy be its greatest powerhouse of light output? And is this only true of the human-visible range of the spectrum? I'm thinking it holds pretty much across the electromagnetic spectrum.

Yes. Because the central section has more stars, closer together. And in the middle, you're looking through a somewhat spherical bulge, as opposed to a thin disc as you move outward.

Thanks for the answers, Chris.

I find it an interesting fact about galaxies, then. I've only read just a bit about the study of galaxy evolution. That seems to be a field that has lots of open questions. As a starting point, I have no reason to expect that galaxies would form with a matter density distribution that follows a curve such as the below
galaxymass1.jpg

rather than that many other options would be common as well, for example:
galaxymass2.jpg

And it seems philosophically interesting that the energy output curve is about the same as the mass density curve, or so it seems. That would mean that all of that energy output, which would seem to be a steadfast fact for billions of years, does not dissipate the mass at all, nor does it seem to "burn out" in the center, the way a forest fire would.

Of course, everyone take this with a grain or two of salt. I'm just trying to grasp it, so I may have it wrong. Counter-statements are welcome.
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby neufer » Fri Feb 09, 2018 6:39 pm

Chris Peterson wrote:
MarkBour wrote:
I realize that images like this are processed, and sometimes what I am unaware of leads me to terribly wrong conclusions. Because of this, I'll ask about something that looks rather obvious in this and many other grand galaxy portraits: the closer you get to the center of the galaxy, the brighter the visual light output. Correct?

Especially around central black holes, which one might guess would be places of relative darkness, there the light output of the galaxy tends to shine quite intensely compared to the outer regions.

The central black hole is far too tiny to impact the appearance of a galaxy.
We're talking about a body smaller than a solar system. In an image, just a fraction of a pixel.

    The accretion disks & jets of an active galactic nuclei are not smaller than a solar system.
https://en.wikipedia.org/wiki/Active_galactic_nucleus wrote:
<<An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much higher than normal luminosity over at least some portion—and possibly all—of the electromagnetic spectrum, with characteristics indicating that the excess luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. AGN are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects.

There is no single observational signature of an AGN. The list below covers some of the features that have allowed systems to be identified as AGN.

    Nuclear optical continuum emission. This is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.

    Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related emission.

    Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.

    Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines.

    Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation.

    X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present.

    X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.
>>
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Chris Peterson » Fri Feb 09, 2018 10:33 pm

neufer wrote:
    The accretion disks & jets of an active galactic nuclei are not smaller than a solar system.

Nor did I suggest otherwise. The discussion seemed to be about the central black hole somehow depleting the galactic core or otherwise making it darker in images. That isn't the case.

That said, I don't think any AGN accretion disc has been resolved, and the way their intensity varies places an upper limit on their size of around a light day. So the accretion discs actually are smaller than solar systems (ours, anyway). Still much smaller than a single pixel in an image of a galaxy. Jets, of course, can be very long.
Chris

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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby MarkBour » Fri Feb 09, 2018 11:28 pm

neufer wrote:... info about active galactic nuclei ...

Thanks for the additional enlightenment, Art. Your comments also led me to another web page: http://astronomy.swin.edu.au/cosmos/G/Galactic+Nuclei, which is a short but helpful article about the subject. (Not really new, though, it coincides with what has been said in this thread by you and Chris.)

I guess what I'm talking about, which is more "meditation" than science, is something I find interesting.

And now that I think about it, in this aspect, the center of a galaxy has a lot in common with the center of a solar system.
  • In both cases, you have a body at the system's center of mass that is gravitationally huge. Indeed, a very dangerous place, that humans would not enjoy. Something quite awful would happen to us even before we could actually get there.
  • But when we look at this point, and indeed our eyes are drawn to this point, instead of seeing a place where there is just a darkness and a crushing into oblivion, in both cases these are places of intense luminosity. In both cases mind-boggling amounts of energy stream out.
  • And, unlike a fire burning on earth, they last and last, able to radiate out this energy for billions of years. Maybe this last point is not so strange. It is thanks to gravity, in both cases, having been the quiet collector, working across vast regions, to prepare the fuel for the furnaces of the cosmos.
So, in a sense, a galactic nucleus is much like our Sun (in effect, not in mechanism), but in a far bigger way. And, apparently, across more of the electromagnetic spectrum than a single star.

One of the differences, I guess, is that our galactic nucleus is growing in mass through this process (well, the central black hole must be growing, I'm not sure if the galactic nucleus is or not), whereas our Sun does not. I have seen the term "feeding", for active galactic nuclei. But another thing I am unsure of here is whether or not a galactic nucleus actually has to consume matter to give off bursts of radiation. Perhaps the overall process need only capture a small portion of the mass it is "feeding on" when it is active.
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Re: APOD: NGC 7331 Close-Up (2018 Feb 07)

Postby Chris Peterson » Fri Feb 09, 2018 11:38 pm

MarkBour wrote:And now that I think about it, in this aspect, the center of a galaxy has a lot in common with the center of a solar system.[list][*]In both cases, you have a body at the system's center of mass that is gravitationally huge.

In a stellar system, you have most of the mass of the entire system located at the very center. In a galaxy, you have a distributed mass, and the black hole at the very center represents only a very small fraction of the total galactic mass. You could remove it and only have a tiny impact on the orbits of the billions of stars in the galaxy.

Indeed, a very dangerous place, that humans would not enjoy.

Of course, if the central black hole was in an active phase, you'd want to stay a few light years away, and out of any jets. But otherwise, it's not a bad place. Supermassive black holes are much more comfortable to be around than stellar mass black holes!

But another thing I am unsure of here is whether or not a galactic nucleus actually has to consume matter to give off bursts of radiation. Perhaps the overall process need only capture a small portion of the mass it is "feeding on" when it is active.

The only detectable radiation coming from the region around a black hole (not the black hole itself) is from material falling into it. That's something that probably goes in cycles as the region is cleared of material and then new material gets captured.
Chris

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