APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

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APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by APOD Robot » Fri Jan 01, 2021 5:06 am

Image Galaxies and the South Celestial Pole

Explanation: The South Celestial Pole is easy to spot in star trail images of the southern sky. The extension of Earth's axis of rotation to the south, it's at the center of all the southern star trail arcs. In this starry panorama streching about 60 degrees across deep southern skies the South Celestial Pole is somewhere near the middle though, flanked by bright galaxies and southern celestial gems. Across the top of the frame are the stars and nebulae along the plane of our own Milky Way Galaxy. Gamma Crucis, a yellowish giant star heads the Southern Cross near top center, with the dark expanse of the Coalsack nebula tucked under the cross arm on the left. Eta Carinae and the reddish glow of the Great Carina Nebula shine along the galactic plane near the right edge. At the bottom are the Large and Small Magellanic clouds, external galaxies in their own right and satellites of the mighty Milky Way. A line from Gamma Crucis through the blue star at the bottom of the southern cross, Alpha Crucis, points toward the South Celestial Pole, but where exactly is it? Just look for south pole star Sigma Octantis. Analog to Polaris the north pole star, Sigma Octantis is little over one degree fom the the South Celestial pole.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by alter-ego » Fri Jan 01, 2021 5:37 am

σ Oct.jpg
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Fri Jan 01, 2021 9:10 am

2020_12_16_Kujal_Jizni_Pol_1500px-3[1].jpg
Magellanic Clouds annotated Petr Horalek.png

What a stunningly glorious and amazing APOD!!!! 😍

Where blue is blue and yellow is yellow and pink is pink!!! And everything in between is in between! And where the stars are as plentiful as grains of sand on the beach! ⛱


The most amazing thing about today's APOD is the portrait of the Magellanic Clouds, and particularly the Large Magellanic Cloud.


First of all, the wide-angle APOD does a wonderful job of picking out the emission nebulas of the LMC, not least the slightly isolated but second biggest one, N11, which can be seen at the "bottom" of the LMC in the APOD. Check out N11 here in an image by C. Aguilera, C. Smith and S. Points/NOAO/AURA/NSF.

This is the meaning of the numbers in the image that I annotated at right:

1) The Tarantula Nebula.

2) The N11 nebula.

3) The large "halo" surrounding LMC. The halo seems to envelop the entire galaxy.

4) General "fluff" in the vicinity of LMC.

Does the fluff belong to the Milky Way, i.e., is it part of our galaxy galactic cirrus? Or is it produced by the interactions of the Large and Small Magellanic Clouds, or possibly by the interactions of the Milky Way and the LMC?

The latter alternative seems unlikely to me. I would assume that is is galactic cirrus from the Milky Way... but... the way it seems to surround the LMC????


And finally, Sigma Octantis shines bright in today's APOD! ⭐️ (Well, 40 times brighter than the Sun, though from a distance of 280 light-years.) At right it can be found in the brilliant APOD from June 28, 2018, by Frank Sackenheim, but I can't possibly find it in today's APOD. 🕵️🔎 Does anyone else have any luck?


Ann
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by DL MARTIN » Fri Jan 01, 2021 10:26 am

HAPPY NEW YEAR to the APOD editors and the regular contributors who do so much to enliven my day.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Holger Nielsen » Fri Jan 01, 2021 12:01 pm

A stunning picture indeed. I reminds me of two rules-of-thumb for locating the position of the celestial south pole when looking at the sky:

2) Together with the LMC and the SMC it forms approximately an equilateral spherical triangle. This can be seen on alter-ego's image, although projection deformations might suggest otherwise.

1) It is better to use the intersection of two great circles, one going through the "vertical" axis of the Southern Cross (seen at the top) and the other being the center normal for the two bright stars Alpha and Beta Centauri. Sadly only the latter is visible at the upper left, Alpha Cen being just outside of the image.

PS: I am not sure, if "central normal" is the correct English term; what I mean is the line (or great circle) going through the midpoint of two points A and B and being orthogonal to AB.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Fri Jan 01, 2021 12:23 pm

Goodness me, alter-ego, you pointed it out for us and I missed it! :slaps face:

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by orin stepanek » Fri Jan 01, 2021 1:14 pm

2020_12_16_Kujal_Jizni_Pol_1500px-3.jpg

:mrgreen: ☺️

Today's APOD is lovely as usual!

FairbairnCROSSTOCARINA.jpg

I can see the Southern Cross being called that; but it reminds me of a kite!🙄
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Sa Ji Tario » Fri Jan 01, 2021 3:05 pm

I join Orin

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Post by neufer » Fri Jan 01, 2021 5:03 pm

https://en.wikipedia.org/wiki/Sigma_Octantis wrote: <<Sigma Octantis (also written as σ Octantis; abbreviated as Sig Oct), officially named Polaris Australis, is the current South Star. This is a solitary star in the southern circumpolar constellation of Octans.

Located approximately 281 light-years from Earth, it is classified as a giant star with a spectral type of F0 III. Sigma Octantis is a 1.59 M Delta Scuti variable, with its average magnitude of 5.47 varying by about 0.03 magnitudes every 2.33 hours.

[Polaris Aa is a 5.4 M F7 yellow supergiant low-amplitude Population I classical Cepheid variable varying in magnitude 1.86–2.13 every 4 days or so.]


Sigma Octantis is barely visible to the naked eye, making it unusable for navigation. Because of this, the constellation Crux is often preferred for determining the position of the South Celestial Pole.>>
https://en.wikipedia.org/wiki/Delta_Scuti_variable wrote:
<<A Delta Scuti variable (sometimes termed dwarf cepheid when the V-band amplitude is larger than 0.3 mag.) is a subclass of young pulsating star. These variables as well as classical cepheids are important standard candles and have been used to establish the distance to the Large Magellanic Cloud, globular clusters, open clusters, and the Galactic Center. SX Phoenicis variables are generally considered to be a subclass of Delta Scuti variables that contain old stars, and can be found in globular clusters.

The OGLE and MACHO surveys have detected nearly 3000 Delta Scuti variables in the Large Magellanic Cloud. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the fluctuations can vary greatly. The stars are usually A0 to F5 type giant or main sequence stars.

Delta Scuti stars exhibit both radial and non-radial luminosity pulsations. The stars have a helium rich atmosphere. As helium is heated it becomes more ionised, which is more opaque. So at the dimmest part in the cycle the star has highly ionised opaque helium in its atmosphere blocking part of the light from escaping. The energy from this “blocked light” causes the helium to heat up, expand, ionise, become more transparent and therefore allow more light through. As more light is let through the star appears brighter and, with the expansion, the helium begins to cool down. Hence the helium contracts and heats up again and the cyclical process continues. Throughout their lifetime Delta Scuti stars exhibit pulsation when they are situated on the classical Cepheid instability strip. They then move across from the main sequence into the giant branch.

The prototype of these sorts of variable stars is Delta Scuti (δ Sct), which exhibits brightness fluctuations from +4.60 to +4.79 in apparent magnitude with a period of 4.65 hours. Other well known Delta Scuti variables include Altair, Denebola (β Leonis) and β Cassiopeiae. Vega (α Lyrae) is a suspected Delta Scuti variable, but this remains unconfirmed.>>
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by johnnydeep » Fri Jan 01, 2021 8:22 pm

Ann wrote:
Fri Jan 01, 2021 9:10 am
And finally, Sigma Octantis shines bright in today's APOD! ⭐️ (Well, 40 times brighter than the Sun, though from a distance of 280 light-years.) At right it can be found in the brilliant APOD from June 28, 2018, by Frank Sackenheim, but I can't possibly find it in today's APOD. 🕵️🔎 Does anyone else have any luck?
Yes, I was able to find it without help by comparison with the old APOD, though it took me a good 5 minutes of flipping between the two. I think I must be getting more skilled :ssmile:
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Sun Jan 03, 2021 1:37 am

Ann wrote:
Fri Jan 01, 2021 12:23 pm
Ann
I wonder if LMC is an ordinary flat gas/dust disk spiral galaxy with a bar at its core.

What is a bar in a barred galaxy anyway?
Is it a binary system of cores in the moment of merge?
Is it a disk with another plane that we get to see edge-on?

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Sun Jan 03, 2021 5:55 am

VictorBorun wrote:
Sun Jan 03, 2021 1:37 am

I wonder if LMC is an ordinary flat gas/dust disk spiral galaxy with a bar at its core.

What is a bar in a barred galaxy anyway?
Is it a binary system of cores in the moment of merge?
Is it a disk with another plane that we get to see edge-on?
1) Yes, the Large Magellanic Cloud is a spiral galaxy, although it is not an "ordinary" spiral galaxy. It is a Magellanic spiral galaxy.
Wikipedia wrote:

Magellanic spiral galaxies are (usually) dwarf galaxies which are classified as the type Sm (and SAm, SBm, SABm). They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies.

The APOD showed the one spiral arm of the LMC very clearly, but it also showed a faint and mostly circular disk or possibly ring outside "main body" of the LMC. Perhaps the arm connects to that outer faint ring. The spiral arm of the LMC is broad and blue, whereas the outer ring is faint and mostly non-blue.



You asked about bars. A galactic bar is an elongated feature running through the center of a spiral galaxy. The spiral arms of a barred galaxy does not start at the center of that galaxy, but start at the ends of the bar.



At right I have posted pictures of NGC 1313, a barred galaxy, and M51, an unbarred galaxy. If you look closely, you can see a tiny elongated feature running through the center of M51, but it doesn't affect the overall appearance of the galaxy. I have also posted a picture of what the Milky Way is believed to look like. Astronomers are certain that the Milky Way has a bar.



Bars are not made of two cores of a galaxy, and they are not a disk seen edge on.
Wikipedia wrote:
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars.
...
The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure.
...
Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts.

Check out this video, which shows the formation of a spiral galaxy. Note how the galaxy gradually evolves a bar.


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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Mon Jan 04, 2021 3:11 am

Ann wrote:
Sun Jan 03, 2021 5:55 am
Check out this video, which shows the formation of a spiral galaxy. Note how the galaxy gradually evolves a bar.
Ann
Video is great, thanks.
But now I don't understand how come that the core, though low-gas and populated mostly with long-living stars, is solid enough to cunduct a density wave.
A galaxy arm, with many shining blue nebules, is one thing. Blue supergiants manage to form and discharge while an arm is propagating across the media.

But the core? The bar? Are there this many stars forming and discharging as the bar's ends move across the disk?

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Mon Jan 04, 2021 4:10 am

VictorBorun wrote:
Mon Jan 04, 2021 3:11 am
Ann wrote:
Sun Jan 03, 2021 5:55 am
Check out this video, which shows the formation of a spiral galaxy. Note how the galaxy gradually evolves a bar.
Ann
Video is great, thanks.
But now I don't understand how come that the core, though low-gas and populated mostly with long-living stars, is solid enough to cunduct a density wave.
A galaxy arm, with many shining blue nebules, is one thing. Blue supergiants manage to form and discharge while an arm is propagating across the media.

But the core? The bar? Are there this many stars forming and discharging as the bar's ends move across the disk?
The bar structure is not solid, but it is dense as stellar structures go. That is to say, it contains a very large number of stars.

Also remember that almost all bars are yellow. Galactic features that are bright and yellow always contain a very high number of stars. That is because the individual stars that make up bright yellow features in galaxies are not bright themselves, and in fact, most of them are fainter - or indeed, a lot fainter - than the Sun. That is because there is almost never star formation in the bar, and the vast majority of long-lived stars are little red dwarfs.

But although the red dwarfs are faint, they are surprisingly massive. They are less massive than the Sun, of course, but they are not that much less massive.

Consider Lacaille 8760 (or AX Microscopii), the brightest red dwarf in the sky. From a distance of a little less than 13 light-years, it is still too faint to be seen with the naked eye. Its mass is 60% solar, which is not so bad. Two of these stars would weight more than the Sun.

Yes, but the luminosity of Lacaille 8760 is only 7% solar. And now we are talking bolometric luminosity, which means all the energy that Lacaille 8760 emits at all wavelengths. The visual luminosity of Lacaille 8760 is only 3% solar. So a star which is 60% of the Sun's mass generates only 3% of its visual light.

The average star in a galactic bar is likely to contain less mass than Lacaille 8760, and they are therefore going to be very much less luminous than Lacaille 8760. Or to put it differently: A galactic bar is likely to be made up of stars whose average mass is (perhaps) 40-50% of the Sun's mass, but whose average luminosity is going to be far less than 1% of the Sun's luminosity.

So in order to be as bright as bars often are, they have to be very full of small stars and therefore really massive. Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.

Not all barred galaxies have central rings, but many do. Sometimes these rings are really bright and full of young stars.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Thu Jan 07, 2021 5:22 am

wow
a real galactic jewelry
An amberish bar with blue ends and a blue-ringed pearl at the center.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Thu Jan 07, 2021 5:33 am

Ann wrote:
Mon Jan 04, 2021 4:10 am
Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.
Ann
So the bar does not propagate through a media, it rotates as a heavy and solid body.
To stay stick-like, it must have one and the same rotation period T, and so acceleration toward the center must be (4π²/T²)R ~ R, and so there must be constant density dark matter halo to account for gravity ~R. Or not?

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Thu Jan 07, 2021 6:08 am

VictorBorun wrote:
Thu Jan 07, 2021 5:33 am

So the bar does not propagate through a media, it rotates as a heavy and solid body.
To stay stick-like, it must have one and the same rotation period T, and so acceleration toward the center must be (4π²/T²)R ~ R, and so there must be constant density dark matter halo to account for gravity ~R. Or not?
Victor, I wish I knew, but I am a complete non-mathematician.

You'll have to ask one of the mathematicians of this site, like Chris or Neufer. I think that Markbour and alter-ego are mathematicians too, and probably johnnydeep. There are others, too, I'm sure.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by neufer » Thu Jan 07, 2021 2:27 pm

Ann wrote:
Mon Jan 04, 2021 4:10 am
Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.
VictorBorun wrote:
Thu Jan 07, 2021 5:33 am

So the bar does not propagate through a media, it rotates as a heavy and solid body.

To stay stick-like, it must have one and the same rotation period T, and so acceleration toward the center must be (4π²/T²)R ~ R, and so there must be constant density dark matter halo to account for gravity ~R. Or not?
Beyond the central black hole, the first ~20% of most disc galaxies shows a quasi-linear velocity rotation curve indicative of a (harmonic) constant rotation period T due to a constant density (mostly light) spherical matter halo.

Now... whether the bars themselves are semi-permanent objects that rotate at this very same period T remains to be seen...but it seems like a not unreasonable assumption.
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Thu Jan 07, 2021 7:11 pm

neufer wrote:
Thu Jan 07, 2021 2:27 pm
Beyond the central black hole, the first ~20% of most disc galaxies shows a quasi-linear velocity rotation curve indicative of a (harmonic) constant rotation period T due to a constant density (mostly light) spherical matter halo.

Now... whether the bars themselves are semi-permanent objects that rotate at this very same period T remains to be seen...but it seems like a not unreasonable assumption.
Aha! It is possible!
But I was wrong to assign all the density to dark halo. Galactic cores are special in this aspect.
I wonder if the gravitational sorting lets large black holes sink to the center, long-living stars sink to the core, gas disk stay where it is, and the dark halo soar to outskirts — and all this happening without loss or gain of total galactic angular momentum.

By the way I have never known that non-dark matter is called light matter. I like it.
We are made of light matter then and so is our enviroment.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Thu Jan 07, 2021 7:25 pm

Come to think of it, if we have a rotating globular cloud of constant density, its equatorial plane cross-section must be moving like a solid body.
There can be strings or bars that don't get smeared by the difference in orbiting periods.
Sadly real globular clouds are not dense enough and self-gravitating enough to rotate fast and manage to do multiple round-abouts. No chance to see bars inside globular gas clouds.

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Chris Peterson » Thu Jan 07, 2021 10:46 pm

VictorBorun wrote:
Thu Jan 07, 2021 7:25 pm
Come to think of it, if we have a rotating globular cloud of constant density, its equatorial plane cross-section must be moving like a solid body.
There can be strings or bars that don't get smeared by the difference in orbiting periods.
Sadly real globular clouds are not dense enough and self-gravitating enough to rotate fast and manage to do multiple round-abouts. No chance to see bars inside globular gas clouds.
There is no fixed equatorial plane in a GC. All of the stars in a GC are orbiting at different inclinations.
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Fri Jan 08, 2021 12:00 am

But the bar is a stick rotating in galactic disk plane, isn't it?

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Chris Peterson » Fri Jan 08, 2021 12:02 am

VictorBorun wrote:
Fri Jan 08, 2021 12:00 am
But the bar is a stick rotating in galactic disk plane, isn't it?
A "stick" is a rigid body. A galactic bar is not. Any dynamics are going to be very different.
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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by VictorBorun » Fri Jan 08, 2021 6:25 pm

Image
ok, then a galactic bar is such a stick-shaped region of dense stellar population that happens to stand still in a rotating reference system.
It's a rotating diameter within a hole in the center of the galactic gas-dust-stars disk.
Or rather it's a pair of radii from the little central globe.

Anyway we need a dark matter globe for bar phenomen. Or do we?

Let's look at a synestia 1 kiloyear after pre-Moon merged with pre-Earth.

The gas is so hot and its pressure is so high that the cloud is a thorus, not a disk. Its gravity is diffuse, and the differential rotation of orbiting matter is less differential than usual.

Can there be a bar?

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Re: APOD: Galaxies and the South Celestial Pole (2021 Jan 01)

Post by Ann » Sat Jan 09, 2021 7:39 am

Ann wrote:
Mon Jan 04, 2021 4:10 am

The bar structure is not solid, but it is dense as stellar structures go. That is to say, it contains a very large number of stars.

Also remember that almost all bars are yellow. Galactic features that are bright and yellow always contain a very high number of stars. That is because the individual stars that make up bright yellow features in galaxies are not bright themselves, and in fact, most of them are fainter - or indeed, a lot fainter - than the Sun. That is because there is almost never star formation in the bar, and the vast majority of long-lived stars are little red dwarfs.

So in order to be as bright as bars often are, they have to be very full of small stars and therefore really massive. Through density waves, the bars rotate as solid bodies, even though they are not solid. As they rotate, they sweep up gas and channel it into two places: At the ends of the bar, which often see increased star formation, and into a small central ring surrounding the nucleus.

Ann
It's embarrassing to quote myself, but I have to correct something I said in the post I quoted.

I said, in the image caption, that the ring of NGC 1300 is not bright. Well, correction.

I just checked my trusted old Color Atlas of Galaxies by James D Wray. Two things are great about that atlas: 1) it features a large number of galaxies in UBV, which gives the reader a really good idea about the overall stellar populations of these galaxies, and 2) it faithfully records bright parts of galaxies as bright and faint parts of galaxies as faint.

So guess what NGC 1300 looks like in Wray's atlas? The picture is small and blurry, as is often the case in his atlas, but we can clearly see that the center of NGC 1300 is bright. It's impossible to see whether there is a ring or not, because we just see a roundish blob of yellow brightness.

What else do we see? We see some faint blue stuff at the right end of the bar, where the arm on that side begins. We see an even fainter and mostly non-blue hint of an arm at left. And we can just make out, just barely, the faintest, faintest hint of the bar itself. In short, the bar of NGC 1300 is exceedingly faint.

I don't think James D Wray is lying to us. I think that the ring around the nucleus of NGC 1300 is at least "kind of bright", whereas the bar is extremely faint.

However, if the ring around the nucleus of NGC 1300 is bright - and I think it is - it is probably non-blue. That is to say, there is probably little or no ongoing star formation in that ring. And the last episode of star formation in that ring probably happened perhaps a billion years ago, so that the massive blue stars that were formed in that last burst now all have evolved off the main sequence and either exploded as supernovas or at least turned into red giants.

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