Starship Asterisk*

Spiral Galaxy NGC 6744

Explanation: Beautiful spiral galaxy NGC 6744 is nearly 175,000 light-years across, larger than our own Milky Way. It lies some 30 million light-years distant in the southern constellation Pavo but appears as only a faint, extended object in small telescopes. We see the disk of the nearby island universe tilted towards our line of sight in this remarkably detailed galaxy portrait, a telescopic view that spans an area about the angular size of a full moon. In it, the giant galaxy's elongated yellowish core is dominated by the light from old, cool stars. Beyond the core, grand spiral arms are filled with young blue star clusters and speckled with pinkish star forming regions. An extended arm sweeps past smaller satellite galaxy NGC 6744A at the lower right. NGC 6744's galactic companion is reminiscent of the Milky Way's satellite galaxy the Large Magellanic Cloud.

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Spiral galaxies come in very different shapes. NGC 6744 and spiral galaxy M101, the Pinwheel galaxy, have at least two things in common: they are spiral galaxies, and they are large. Definitely large. And they are quite similar in size, too. According to the caption, NGC 6744 is 175,000 light-years in diameter, which is really a lot. And according to Wikipedia, M101 is some 170,000 light-years in diameter. Both galaxies are much bigger than the Milky Way.

But while the arms of M101 are characterized by individual emission nebulas nebulas so bright that they have received their own NGC designations, the "blue knots" (nebulas and clusters) of NGC 6744 are, to borrow the description by James D Wray, "numerous but not prominent". You said it, James D Wray. Note how the inner spiral system of NGC 6744 displays a yellowish tint, while the outer spiral system almost fades into a faint bluish haze without much structure.

NGC 6744 is not a grand design galaxy, which is a type of galaxy that is characterized by two long dominant arms. Let's compare NGC 6744 with a galaxy that really is a grand design galaxy, M81:


NGC 6744 clearly has two outer arms, but its inner structure is more of a mess. The two-armed structure of M81 is much clearer. (An interesting difference between NGC 6744 and M81 is that M81 has a much larger yellow non-ultraviolet bulge than NGC 6744. The bulge of NGC 6744 appears smallish for such a large galaxy. The same thing is true for M101.)

Let's take a look at a galaxy that is an even more striking grand design galaxy than M81, NGC 1566:


Actually the difference in spiral structure between NGC 6744 and NGC 6744 (and M81) is less obvious in ultraviolet light than at optical wavelengths. That is because it is the older population of stars that forms "the backbone" of galaxies and trace out their overall structure, whereas the bright emission nebulas and brilliant star clusters are mostly short-lived foam on the waves. And the ultraviolet images detect the foam more than the underlying structures.


NGC 936 is almost certainly an ex-spiral galaxy that used up all its gas and burned out all its hot stars and drove itself into a state of exhaustion, so that it is now "red and dead". All its stars are old and yellow. But we can still very clearly see the galactic bar and the bar-end enhancements. These structures live on long after the flimsy, glittering spiral arms have disintegrated.

Ann

Again, really bad job at denoising , so much noise in the core , and i think this is data captured with Telescope live , so basically this is just a reprocess of a dataset everyone can get. If only the author would at least post the specifications on detail ?

I wonder if we do know if Milky Way is smaller than NGC 6744 of this APOD.
Sun is in the disk, right? So we have to look at any Milky Way's outskirts through our dusty neighbourhood in the disk.
Some hot things shine through the dust, in radio and X-rays. I mean all the SN remnants and of course Sgr A*.
We know some things are just 3000 ly away because we can see stars' parallaxes.
We can model the 27000 ly to Sgr A* with stars' Doppler radial velocities.
But can we really see the far outskirts of Milky Way? And can we tell if the things are 10 000 or 50 000 ly away?
What if there are NGC 6744-like parts we are blind to? What if there are NGC 6744-like parts we take for closer things?

NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…


Eric

Ahh! Another beautiful Galaxy!

XgeoX wrote: ↑Thu Jun 23, 2022 11:05 am NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…

All I see is a circular central bulge seen at a tilt.

XgeoX wrote: ↑Thu Jun 23, 2022 11:05 am NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…


Eric

Wikipedia wrote:

NGC 6744 (also known as Caldwell 101) is an intermediate spiral galaxy about 30 million light-years away[2] in the constellation Pavo (Peacock).

An intermediate spiral galaxy is a spiral galaxy that "intermediate" between being barred and non-barred.

Ann

VictorBorun wrote: ↑Thu Jun 23, 2022 10:53 am I wonder if we do know if Milky Way is smaller than NGC 6744 of this APOD.
Sun is in the disk, right? So we have to look at any Milky Way's outskirts through our dusty neighbourhood in the disk.
Some hot things shine through the dust, in radio and X-rays. I mean all the SN remnants and of course Sgr A*.
We know some things are just 3000 ly away because we can see stars' parallaxes.
We can model the 27000 ly to Sgr A* with stars' Doppler radial velocities.
But can we really see the far outskirts of Milky Way? And can we tell if the things are 10 000 or 50 000 ly away?
What if there are NGC 6744-like parts we are blind to? What if there are NGC 6744-like parts we take for closer things?

When I read about a galaxy, I look for information on how big the galaxy is compared to the Milky Way. My impression is that most galaxies are smaller than the Milky Way. Or, to put it differently, their diameters are smaller than 100,000 light-years.

My point is that if the diameter of the optical disk of NGC 6744 is really 175,000 light-years, then it is really big as galaxies go. Or, to put it differently, it is NGC 6744 that is remarkable, not the Milky Way. (Even though the Milky Way is "kind of big" too, since most galaxies appear to be smaller than our celestial star city. At least most spiral galaxies appear to be smaller than the Milky Way.)

How do astronomers know the distance to NGC 6744?

Miranda Yew et al. wrote:

Tully et al. (2013) estimated a distance to NGC 6744 to be 9.2 ± 0.4 Mpc, using the ‘tip of the red giant branch’ (TRGB) method (see Lee et al. 1993).

The tip of the red giant branch (or the tip of the asymtotic giant branch?) is the brightest, reddest and final stage of stars similar to the Sun, before they shut down fusion, shed their outer layers and turn into white dwarfs. Look at the diagram by Lithopsian at left to see the evolutionary track of a star like the Sun. The tip of the asymtotic giant branch is at top right in the diagram, and the top of the red giant branch is a little below it.

Now take a look at dwarf galaxy NGC 4214. You can see a big bright blue cluster. Intermingled with the brilliant blue stars are bright red stars, which are red supergiants like Antares and Betelgeuse. Yes, but look at the profusion of red stars at lower right and upper left in the picture. These red stars are far away from the really bright blue stars, and they are unlikely to be supergiant stars themselves. Instead, they are bright red giants, similar to, I think, Arcturus and Aldebaran. If astronomers can see a sufficient number of them in another galaxy clearly enough to estimate their brightness, they can also figure out the true distance to that other galaxy, since the true brightness of stars at the tip os the red giant branch is known.

Okay, so... I'm not absolutely sure about whether astronomers look for stars at the tip of the red giant branch, or stars at the top of the asymtotic giant branch. I'm also not sure if astronomers are looking for bright K-type stars like Arcturus or Aldebaran of if they are looking for M-type giants like Gamma Crucis or Beta Pegasi. But in any case, we are talking about a class of stars whose average maximum brightness is reasonably well understood, and these stars can be used as standard candles.

And if astronomers can figure out the true distance to a galaxy, they can also figure out the overall luminosity and the size of that galaxy.

Ann

In the June 23, 2022 APOD of the Spiral Galaxy NGC 6744, there is a blue dot along the ~11:00 line moving away from the galaxy. Quite a few of the stars have a light blue color, but this star(?) has a distinctly different color (a darker blue) from all the others. What is it? and what gives it this deeper blue color?

PoppaPaul wrote: ↑Thu Jun 23, 2022 4:45 pm In the June 23, 2022 APOD of the Spiral Galaxy NGC 6744, there is a blue dot along the ~11:00 line moving away from the galaxy. Quite a few of the stars have a light blue color, but this star(?) has a distinctly different color (a darker blue) from all the others. What is it? and what gives it this deeper blue color?

That's interesting. It's a nice blue color. However in virtually the same picture from https://apod.nasa.gov/apod/ap191205.html , that blue star is much fainter and harder to find. Although I did find it.

So what does it mean? Perhaps in the two and half years between the photographs, that that star has brightened in color perhaps going nova. Or maybe because the older picture in general has duller colors it could just be a processing glitch. (I don't claim to be an expert.)

De58te wrote: ↑Thu Jun 23, 2022 5:35 pm

PoppaPaul wrote: ↑Thu Jun 23, 2022 4:45 pm In the June 23, 2022 APOD of the Spiral Galaxy NGC 6744, there is a blue dot along the ~11:00 line moving away from the galaxy. Quite a few of the stars have a light blue color, but this star(?) has a distinctly different color (a darker blue) from all the others. What is it? and what gives it this deeper blue color?

That's interesting. It's a nice blue color. However in virtually the same picture from https://apod.nasa.gov/apod/ap191205.html , that blue star is much fainter and harder to find. Although I did find it.

So what does it mean? Perhaps in the two and half years between the photographs, that that star has brightened in color perhaps going nova. Or maybe because the older picture in general has duller colors it could just be a processing glitch. (I don't claim to be an expert.)

It could be a tight cluster of young blue stars, perhaps similar to NGC 3293.


Ann

PoppaPaul wrote: ↑Thu Jun 23, 2022 4:45 pm In the June 23, 2022 APOD of the Spiral Galaxy NGC 6744, there is a blue dot along the ~11:00 line moving away from the galaxy. Quite a few of the stars have a light blue color, but this star(?) has a distinctly different color (a darker blue) from all the others. What is it? and what gives it this deeper blue color?

A similar darker blue star can be found at the exact opposide side of NCC 6744, around the 4:00 line near the edge of the image…

Locutus76 wrote: ↑Thu Jun 23, 2022 6:13 pm

PoppaPaul wrote: ↑Thu Jun 23, 2022 4:45 pm In the June 23, 2022 APOD of the Spiral Galaxy NGC 6744, there is a blue dot along the ~11:00 line moving away from the galaxy. Quite a few of the stars have a light blue color, but this star(?) has a distinctly different color (a darker blue) from all the others. What is it? and what gives it this deeper blue color?

A similar darker blue star can be found at the exact opposide side of NCC 6744, around the 4:00 line near the edge of the image…

There are several little blue dots scattered in the disk. They are almost certainly clusters.

In fact, they are clusters. Because NGC 6744 is a very big galaxy with quite small clusters.

Ann

The text says this galaxy is "tilted toward" us. To me that means the top (in this photo anyway) is closer than the bottom. Is that true here? It looks like the exact opposite to me. But maybe it's completely irrelevant since whether the "top" or "bottom" of a galaxy is closer to us depends entirely on the orientation of the picture, and in fact, it might be that the "left" or "right" side is the portion that is closer?

johnnydeep wrote: ↑Thu Jun 23, 2022 8:44 pm The text says this galaxy is "tilted toward" us. To me that means the top (in this photo anyway) is closer than the bottom. Is that true here? It looks like the exact opposite to me. But maybe it's completely irrelevant since whether the "top" or "bottom" of a galaxy is closer to us depends entirely on the orientation of the picture, and in fact, it might be that the "left" or "right" side is the portion that is closer?

looking at the dust backlighted with the core we surely can see you are right: the top must be farther away from us to account for fewer dust lanes visible there

Ann wrote: ↑Thu Jun 23, 2022 4:32 pm

VictorBorun wrote: ↑Thu Jun 23, 2022 10:53 am I wonder if we do know if Milky Way is smaller than NGC 6744 of this APOD.
Sun is in the disk, right? So we have to look at any Milky Way's outskirts through our dusty neighbourhood in the disk.
Some hot things shine through the dust, in radio and X-rays. I mean all the SN remnants and of course Sgr A*.
We know some things are just 3000 ly away because we can see stars' parallaxes.
We can model the 27000 ly to Sgr A* with stars' Doppler radial velocities.
But can we really see the far outskirts of Milky Way? And can we tell if the things are 10 000 or 50 000 ly away?
What if there are NGC 6744-like parts we are blind to? What if there are NGC 6744-like parts we take for closer things?

stars can be used as standard candles.
And if astronomers can figure out the true distance to a galaxy, they can also figure out the overall luminosity and the size of that galaxy.
Ann

You mean NGC 6744 galaxy — we can see the whole galaxy and judge it.
I mean the outskirts of the disk of Milky Way galaxy — we try to look through the dust and mostly can see in radio and X-rays
What if Milky Way galaxy is in fact of the same size?
Here I mark where Sun fits at 27 kly from SgrA*

VictorBorun wrote: ↑Thu Jun 23, 2022 10:32 pm

Ann wrote: ↑Thu Jun 23, 2022 4:32 pm

VictorBorun wrote: ↑Thu Jun 23, 2022 10:53 am I wonder if we do know if Milky Way is smaller than NGC 6744 of this APOD.
Sun is in the disk, right? So we have to look at any Milky Way's outskirts through our dusty neighbourhood in the disk.
Some hot things shine through the dust, in radio and X-rays. I mean all the SN remnants and of course Sgr A*.
We know some things are just 3000 ly away because we can see stars' parallaxes.
We can model the 27000 ly to Sgr A* with stars' Doppler radial velocities.
But can we really see the far outskirts of Milky Way? And can we tell if the things are 10 000 or 50 000 ly away?
What if there are NGC 6744-like parts we are blind to? What if there are NGC 6744-like parts we take for closer things?

stars can be used as standard candles.
And if astronomers can figure out the true distance to a galaxy, they can also figure out the overall luminosity and the size of that galaxy.
Ann

You mean NGC 6744 galaxy — we can see the whole galaxy and judge it.
I mean the outskirts of the disk of Milky Way galaxy — we try to look through the dust and mostly can see in radio and X-rays
What if Milky Way galaxy is in fact of the same size?
Here I mark where Sun fits at 27 kly from SgrA*

Click to view full size image

It is really very hard to be sure about the full size of the Milky Way, although I must point out that Gaia is in the process of measuring the distance to a billion objects in our Galaxy, and Gaia will give us a better idea of the size, likely shape and properties of the Milky Way than we have ever had before.

What I'm saying is this. We can be sure that NGC 6744 is really very big. It's not the biggest spiral galaxy that humanity has detected, far from it, but it is really unusually big. For that reason alone, we expect the Milky Way to be smaller. And our best measurements of our Galaxy agrees with that assessment.

Ann

VictorBorun wrote: ↑Thu Jun 23, 2022 10:14 pm

johnnydeep wrote: ↑Thu Jun 23, 2022 8:44 pm The text says this galaxy is "tilted toward" us. To me that means the top (in this photo anyway) is closer than the bottom. Is that true here? It looks like the exact opposite to me. But maybe it's completely irrelevant since whether the "top" or "bottom" of a galaxy is closer to us depends entirely on the orientation of the picture, and in fact, it might be that the "left" or "right" side is the portion that is closer?

looking at the dust backlighted with the core we surely can see you are right: the top must be farther away from us to account for fewer dust lanes visible thereNGC6744_chakrabarti2048R-.jpg

I'm not convinced there are more dust lanes visible in the "top" than the bottom of the picture. Looking at the full size image, the top and bottom appear much the same to me. Also, your 180 degree rotated image also appears to be tilted away from us to me!

johnnydeep wrote: ↑Fri Jun 24, 2022 2:22 pm

VictorBorun wrote: ↑Thu Jun 23, 2022 10:14 pm

johnnydeep wrote: ↑Thu Jun 23, 2022 8:44 pm The text says this galaxy is "tilted toward" us. To me that means the top (in this photo anyway) is closer than the bottom. Is that true here? It looks like the exact opposite to me. But maybe it's completely irrelevant since whether the "top" or "bottom" of a galaxy is closer to us depends entirely on the orientation of the picture, and in fact, it might be that the "left" or "right" side is the portion that is closer?

looking at the dust backlighted with the core we surely can see you are right: the top must be farther away from us to account for fewer dust lanes visible thereNGC6744_chakrabarti2048R-.jpg

I'm not convinced there are more dust lanes visible in the "top" than the bottom of the picture. Looking at the full size image, the top and bottom appear much the same to me. Also, your 180 degree rotated image also appears to be tilted away from us to me!

Let's try to darken the red channel of RGB to highlight the difference.
I mean the core region which must be a sphere and thus to make some difference; the planar disk is harder to make out when you can not really make out the thickness of the arms pattern

VictorBorun wrote: ↑Fri Jun 24, 2022 8:34 pm

johnnydeep wrote: ↑Fri Jun 24, 2022 2:22 pm

VictorBorun wrote: ↑Thu Jun 23, 2022 10:14 pm

looking at the dust backlighted with the core we surely can see you are right: the top must be farther away from us to account for fewer dust lanes visible thereNGC6744_chakrabarti2048R-.jpg

I'm not convinced there are more dust lanes visible in the "top" than the bottom of the picture. Looking at the full size image, the top and bottom appear much the same to me. Also, your 180 degree rotated image also appears to be tilted away from us to me!

Let's try to darken the red channel of RGB to highlight the difference.
I mean the core region which must be a sphere and thus to make some difference; the planar disk is harder to make out when you can not really make out the thickness of the arms patternNGC6744_chakrabarti2048R-2.jpg

Rationally, I can understand how we could be - or even are likely to be - looking through more dust in order to see the far side versus the near side, but I just don't see that here.

Chris Peterson wrote: ↑Thu Jun 23, 2022 3:19 pm

XgeoX wrote: ↑Thu Jun 23, 2022 11:05 am NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…

All I see is a circular central bulge seen at a tilt.

Looks much more oval than circular to me, just saying.

Ann wrote: ↑Thu Jun 23, 2022 3:30 pm

XgeoX wrote: ↑Thu Jun 23, 2022 11:05 am NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…


Eric

Wikipedia wrote:

NGC 6744 (also known as Caldwell 101) is an intermediate spiral galaxy about 30 million light-years away[2] in the constellation Pavo (Peacock).

An intermediate spiral galaxy is a spiral galaxy that "intermediate" between being barred and non-barred.

Ann

I was just noting how close it appeared, to me and only me, to being a full fledged bar.

XgeoX wrote: ↑Sat Jun 25, 2022 6:04 pm

Chris Peterson wrote: ↑Thu Jun 23, 2022 3:19 pm

XgeoX wrote: ↑Thu Jun 23, 2022 11:05 am NGC 6744, depending on the exposure and filters, looks to be right on the precipice of being a barred spiral…

All I see is a circular central bulge seen at a tilt.

Looks much more oval than circular to me, just saying.

It appears to me to have the same eccentricity as the galaxy as a whole. As the galaxy is a disc, that would imply that the core is, as well.

Chris Peterson wrote: ↑Sat Jun 25, 2022 6:14 pm

XgeoX wrote: ↑Sat Jun 25, 2022 6:04 pm

Chris Peterson wrote: ↑Thu Jun 23, 2022 3:19 pm

All I see is a circular central bulge seen at a tilt.

Looks much more oval than circular to me, just saying.

It appears to me to have the same eccentricity as the galaxy as a whole. As the galaxy is a disc, that would imply that the core is, as well.

Hmm, never looked at it that way…