APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4903
Joined: Fri Dec 04, 2009 3:27 am

APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by APOD Robot » Wed Oct 19, 2022 4:05 am

Image A Galaxy Beyond Stars, Gas, Dust

Explanation: Do we dare believe our eyes? When we look at images of space, we often wonder whether they are "real", and just as often the best answer varies. In this case, the scene appears much as our eyes would see it, because it was obtained using RGB (Red, Green, Blue) filters like the cone cells in our eyes, except collecting light for 19 hours, not a fraction of a second. The featured image was captured over six nights, using a 24-inch diameter telescope in the Sierra Nevada Mountains, in California, USA. The bright spiral galaxy at the center (NGC 7497) looks like it is being grasped by an eerie tendril of a space ghost, and therein lies the trick. The galaxy is actually 59 million light years away, while the nebulosity is MBM 54, less than one thousand light years away, making it one of the nearest cool clouds of gas and dust -- galactic cirrus -- within our own Milky Way Galaxy. Both are in the constellation of Pegasus, which can be seen high overhead from northern latitudes in the autumn.

<< Previous APOD This Day in APOD Next APOD >>

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by VictorBorun » Wed Oct 19, 2022 5:18 am

were NGC 7497 not seen at an angle, the circular view of the disk would not impress us that much
NGC 7497..jpg
You do not have the required permissions to view the files attached to this post.

User avatar
Ann
4725 Å
Posts: 12477
Joined: Sat May 29, 2010 5:33 am

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Ann » Wed Oct 19, 2022 6:12 am

This is a weird-looking thing.

APOD 18 October 2022 NGC 7497 and cirrus Howard Trottier .png
Andromeda Dieter Beer and Patrick Hochleitner.png
Andromeda by Dieter Beer and Patrick Hochleitner.

Note a couple of things here. First, note how the large Milky Way cirrus cloud changes color close to NGC 7497. Much of the dust is reddish, but it is slightly bluish close to NGC 7497, as if it was scattering the galaxy's blue light our way. Well, that is clearly impossible, since NGC 7497 is a distant background object.

Note, nevertheless, how the dust appears to interact with NGC 7497. This is particularly obvious at the edge of the galaxy's disk at 5 o'clock, where one streamer of dusty gas appears to rise "up", while another curlicue appears to flow off to the right. It looks as if the galaxy had been stirring the gas.

Note the similarity in shape between NGC 7497 and Andromeda. But NGC 7497 is clearly a much bluer object than Andromeda, with much brighter and larger pink emission nebulas and many bright blue clusters.

Even though NGC 7497 is clearly a quite blue object, its color indices suggest otherwise. It's B-V index is 0.720, and its U-B index is 0.150. I find the measly U-B index particularly surprising, in view of all the pink emission nebulas that are visible in NGC 7497.

I find the far infrared magnitude of NGC 7497 surprising, too. The far infrared magnitude is 1.3 magnitudes brighter than the B magnitude of this galaxy. This suggests to me that NGC 7497 is dustier than it looks, unless the lower dark edge of the galaxy's disk is indicative of a huge amount of dust.

But I'm wondering if the foreground Milky Way dust perhaps significantly reddens this galaxy. Are its color indices redder and its far infrared magnitude brighter because of foreground Milky Way dust?

Ann
You do not have the required permissions to view the files attached to this post.
Color Commentator

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by VictorBorun » Wed Oct 19, 2022 11:01 am

Ann wrote: Wed Oct 19, 2022 6:12 am at the edge of the galaxy's disk at 5 o'clock, where one streamer of dusty gas appears to rise "up", while another curlicue appears to flow off to the right. It looks as if the galaxy had been stirring the gas.
Ann
just guessing:
NGC 7497 2.jpg
You do not have the required permissions to view the files attached to this post.

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Chris Peterson » Wed Oct 19, 2022 1:25 pm

Ann wrote: Wed Oct 19, 2022 6:12 am Note a couple of things here. First, note how the large Milky Way cirrus cloud changes color close to NGC 7497. Much of the dust is reddish, but it is slightly bluish close to NGC 7497, as if it was scattering the galaxy's blue light our way. Well, that is clearly impossible, since NGC 7497 is a distant background object.
That is precisely how forward scatter works! A dust cloud like that with a distant light source directly behind it is an ideal scattering medium.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
orin stepanek
Plutopian
Posts: 7887
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by orin stepanek » Wed Oct 19, 2022 1:29 pm

Ngc7497Cirrus_Trottier_960.jpg
The ghost and the galaxy! In time for Halloween! The galaxy seem to
have a glow! 8-)
You do not have the required permissions to view the files attached to this post.
Orin

Smile today; tomorrow's another day!

GWoop

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by GWoop » Wed Oct 19, 2022 3:03 pm

Many of the small, dim stars in this image appear to be equally spaced along lines with eachother. This does not look random to my eyes. Could they be the same object in relative motion compared to the galaxy and dust cloud?

User avatar
Ann
4725 Å
Posts: 12477
Joined: Sat May 29, 2010 5:33 am

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Ann » Wed Oct 19, 2022 3:17 pm

Chris Peterson wrote: Wed Oct 19, 2022 1:25 pm
Ann wrote: Wed Oct 19, 2022 6:12 am Note a couple of things here. First, note how the large Milky Way cirrus cloud changes color close to NGC 7497. Much of the dust is reddish, but it is slightly bluish close to NGC 7497, as if it was scattering the galaxy's blue light our way. Well, that is clearly impossible, since NGC 7497 is a distant background object.
That is precisely how forward scatter works! A dust cloud like that with a distant light source directly behind it is an ideal scattering medium.
Okay, Chris. I thought it had to be the other way round, with the dust behind the object whose light it scattered forward. Apparently it works just as well with the light source being behind the dust.

Thanks.

Ann
Color Commentator

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Chris Peterson » Wed Oct 19, 2022 3:20 pm

Ann wrote: Wed Oct 19, 2022 3:17 pm
Chris Peterson wrote: Wed Oct 19, 2022 1:25 pm
Ann wrote: Wed Oct 19, 2022 6:12 am Note a couple of things here. First, note how the large Milky Way cirrus cloud changes color close to NGC 7497. Much of the dust is reddish, but it is slightly bluish close to NGC 7497, as if it was scattering the galaxy's blue light our way. Well, that is clearly impossible, since NGC 7497 is a distant background object.
That is precisely how forward scatter works! A dust cloud like that with a distant light source directly behind it is an ideal scattering medium.
Okay, Chris. I thought it had to be the other way round, with the dust behind the object whose light it scattered forward. Apparently it works just as well with the light source being behind the dust.
If the dust were behind the object, it's more likely you'd see reflected light, not scattered light. There is such a thing as back scatter, but it is usually less efficient than forward scatter. You've probably noticed this before with the Sun behind tall grass as compared with the Sun behind you while looking at such grass. The backlit grass is usually much brighter, almost mirror-like sometimes because of the strength of forward scattering.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Chris Peterson » Wed Oct 19, 2022 3:22 pm

GWoop wrote: Wed Oct 19, 2022 3:03 pm Many of the small, dim stars in this image appear to be equally spaced along lines with eachother. This does not look random to my eyes. Could they be the same object in relative motion compared to the galaxy and dust cloud?
No. That's just an indicator of how hard our brains try to find patterns. And we find them... even when they aren't physically meaningful. We quickly pick out both lines and arcs in star fields.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Ann
4725 Å
Posts: 12477
Joined: Sat May 29, 2010 5:33 am

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Ann » Wed Oct 19, 2022 4:06 pm

Chris Peterson wrote: Wed Oct 19, 2022 3:20 pm
Ann wrote: Wed Oct 19, 2022 3:17 pm
Chris Peterson wrote: Wed Oct 19, 2022 1:25 pm

That is precisely how forward scatter works! A dust cloud like that with a distant light source directly behind it is an ideal scattering medium.
Okay, Chris. I thought it had to be the other way round, with the dust behind the object whose light it scattered forward. Apparently it works just as well with the light source being behind the dust.
If the dust were behind the object, it's more likely you'd see reflected light, not scattered light. There is such a thing as back scatter, but it is usually less efficient than forward scatter. You've probably noticed this before with the Sun behind tall grass as compared with the Sun behind you while looking at such grass. The backlit grass is usually much brighter, almost mirror-like sometimes because of the strength of forward scattering.
kings-trail_-_84[1].jpeg
King's trail in Sweden. Photo: Anders Norén.

Not sure I get you there, Chris. Obviously grass is brilliantly bright when the Sun seems to shine right through it. But then the grass is between you and the Sun and the Sun is in front of you and behind the grass. Isn't the grass in front of you less bright when the Sun is behind you?

Ann
You do not have the required permissions to view the files attached to this post.
Color Commentator

User avatar
Chris Peterson
Abominable Snowman
Posts: 17155
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Chris Peterson » Wed Oct 19, 2022 5:12 pm

Ann wrote: Wed Oct 19, 2022 4:06 pm
Chris Peterson wrote: Wed Oct 19, 2022 3:20 pm
Ann wrote: Wed Oct 19, 2022 3:17 pm

Okay, Chris. I thought it had to be the other way round, with the dust behind the object whose light it scattered forward. Apparently it works just as well with the light source being behind the dust.
If the dust were behind the object, it's more likely you'd see reflected light, not scattered light. There is such a thing as back scatter, but it is usually less efficient than forward scatter. You've probably noticed this before with the Sun behind tall grass as compared with the Sun behind you while looking at such grass. The backlit grass is usually much brighter, almost mirror-like sometimes because of the strength of forward scattering.
Not sure I get you there, Chris. Obviously grass is brilliantly bright when the Sun seems to shine right through it. But then the grass is between you and the Sun and the Sun is in front of you and behind the grass. Isn't the grass in front of you less bright when the Sun is behind you?
When the Sun is behind you, the light coming from the grass is mainly reflected, and only a small part of the total visible spectrum comes back. When the Sun is almost behind the grass, a lot of light is scattered, and that represents the entire spectrum. This is why grass and leaves are often much brighter when they are backlit than when they are front lit. Usually, the light that comes through the leaves themselves isn't a major component of the total intensity.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by VictorBorun » Wed Oct 19, 2022 7:27 pm

GWoop wrote: Wed Oct 19, 2022 3:03 pm Many of the small, dim stars in this image appear to be equally spaced along lines with eachother. This does not look random to my eyes. Could they be the same object in relative motion compared to the galaxy and dust cloud?
Myself, too, saw some unexpected patterns at first:
NGC 7497 3.jpg
I wonder though if that black tower is a real dust pillar and those lights at the top are newly born in the same rotating planar structure
You do not have the required permissions to view the files attached to this post.

User avatar
VictorBorun
Commander
Posts: 731
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by VictorBorun » Wed Oct 19, 2022 7:47 pm

Chris Peterson wrote: Wed Oct 19, 2022 5:12 pm
Ann wrote: Wed Oct 19, 2022 4:06 pm
Chris Peterson wrote: Wed Oct 19, 2022 3:20 pm

If the dust were behind the object, it's more likely you'd see reflected light, not scattered light. There is such a thing as back scatter, but it is usually less efficient than forward scatter. You've probably noticed this before with the Sun behind tall grass as compared with the Sun behind you while looking at such grass. The backlit grass is usually much brighter, almost mirror-like sometimes because of the strength of forward scattering.
Not sure I get you there, Chris. Obviously grass is brilliantly bright when the Sun seems to shine right through it. But then the grass is between you and the Sun and the Sun is in front of you and behind the grass. Isn't the grass in front of you less bright when the Sun is behind you?
When the Sun is behind you, the light coming from the grass is mainly reflected, and only a small part of the total visible spectrum comes back. When the Sun is almost behind the grass, a lot of light is scattered, and that represents the entire spectrum. This is why grass and leaves are often much brighter when they are backlit than when they are front lit. Usually, the light that comes through the leaves themselves isn't a major component of the total intensity.
with 25% colour saturation boost we can clearly see two orange dust regions right and left of the galaxy
NGC 7497 5.jpg
and I can't see bright stars shining through the right orange dust monster to account for all of that orange.

with 75% colour saturation boost we can clearly see the galaxy's 2 symmetric arms:
NGC 7497 4.jpg
You do not have the required permissions to view the files attached to this post.

User avatar
johnnydeep
Captain
Posts: 1725
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by johnnydeep » Wed Oct 19, 2022 9:45 pm

Chris Peterson wrote: Wed Oct 19, 2022 5:12 pm
Ann wrote: Wed Oct 19, 2022 4:06 pm
Chris Peterson wrote: Wed Oct 19, 2022 3:20 pm

If the dust were behind the object, it's more likely you'd see reflected light, not scattered light. There is such a thing as back scatter, but it is usually less efficient than forward scatter. You've probably noticed this before with the Sun behind tall grass as compared with the Sun behind you while looking at such grass. The backlit grass is usually much brighter, almost mirror-like sometimes because of the strength of forward scattering.
Not sure I get you there, Chris. Obviously grass is brilliantly bright when the Sun seems to shine right through it. But then the grass is between you and the Sun and the Sun is in front of you and behind the grass. Isn't the grass in front of you less bright when the Sun is behind you?
When the Sun is behind you, the light coming from the grass is mainly reflected, and only a small part of the total visible spectrum comes back. When the Sun is almost behind the grass, a lot of light is scattered, and that represents the entire spectrum. This is why grass and leaves are often much brighter when they are backlit than when they are front lit. Usually, the light that comes through the leaves themselves isn't a major component of the total intensity.
You've blown my mind once again, Chris! The images shown here seem to show the effect you're talking about:http://spacecraftkits.com/scatter/
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Ann
4725 Å
Posts: 12477
Joined: Sat May 29, 2010 5:33 am

Re: APOD: A Galaxy Beyond Stars, Gas, Dust (2022 Oct 19)

Post by Ann » Thu Oct 20, 2022 3:57 am

johnnydeep wrote: Wed Oct 19, 2022 9:45 pm
Chris Peterson wrote: Wed Oct 19, 2022 5:12 pm
Ann wrote: Wed Oct 19, 2022 4:06 pm
Not sure I get you there, Chris. Obviously grass is brilliantly bright when the Sun seems to shine right through it. But then the grass is between you and the Sun and the Sun is in front of you and behind the grass. Isn't the grass in front of you less bright when the Sun is behind you?
When the Sun is behind you, the light coming from the grass is mainly reflected, and only a small part of the total visible spectrum comes back. When the Sun is almost behind the grass, a lot of light is scattered, and that represents the entire spectrum. This is why grass and leaves are often much brighter when they are backlit than when they are front lit. Usually, the light that comes through the leaves themselves isn't a major component of the total intensity.
You've blown my mind once again, Chris! The images shown here seem to show the effect you're talking about:http://spacecraftkits.com/scatter/
Thanks, Johnny, that's a great illustration! We might want to look at backlit Saturn, too:


It is interesting that the far side of Saturn is not dark, because it is being lit up by its own rings. But it is only the "upper part" of Saturn that is lit up, because that is where the rings are passing in front of it. "The other side of the rings" are "on the other side of Saturn". The contour of Saturn, its upper atmosphere, is brilliantly lit by the Sun.

It is interesting to see the colors of the brilliantly lit rings. The inner, bright rings are very yellow. The outer E ring is very blue. However, it is bluest "above" and "below" the disk of Saturn, where it is also brightest. Off to the right, it isn't very blue. Why does its hue vary? Or maybe the hue stays the same, and it is just the saturation that varies?

There is another ring between the E ring and the bright inner ring that is reasonably blue, too. And there is one ring, separated from the bright A ring by a narrow gap, that is brilliantly white.

Well, I guess it all has to do with the size of the particles in the rings and the way these particles scatter light.

Ann



Ann
Color Commentator