Starship Asterisk*

Spiral Galaxy NGC 6744

Explanation: Big, 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 smudge in the eyepiece of a small telescope. We see the disk of the nearby island universe tilted towards our line of sight in this remarkably deep and detailed galaxy portrait, a telescopic image 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 upper left. NGC 6744's galactic companion is reminiscent of the Milky Way's satellite galaxy the Large Magellanic Cloud.

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Nice APOD!


But as a Color Commentator, I scratch my head over this image - and over NGC 6744 in general. I mean, doesn't it look blue? ███ But according to the standard method of determining the overall color of a galaxy (or, for that matter, a star), the B-V index, NGC 6744 is "on the yellow side" of galaxies! ███

Let me explain. The B-V index measures the flux of B (blue) light versus the flux of V (yellow-green) light. The smaller the value, the bluer is the star or the galaxy, and the larger the value, the yellower (or, in the case of some stars, the redder) the object is.

Galaxies have pale colors, because they are made up of large numbers of stars of different colors. Elliptical galaxies with no star formation are typically made up of old stars only. Most stars in an old population of stars will be yellow or red, but some will still be "white" like the Sun. (Yes, the Sun is white, or possibly yellow-white.) And an extremely small fraction of the stars will actually be blue even in an old population, because they will be blue straggler stars or extremely old and metal-poor blue horzontal branch stars.

However, the typical color of elliptical or lenticular galaxies, whose populations are typically "all old", is around 0.90 to 0.99. We may "translate" these B-V numbers to a color between the hue of the bright star Capella, and the hue of the bright star Pollux, though it is closer to Pollux.


A yellow elliptical galaxy full of old stars is giant elliptical galaxy M87:


As for "The Eyes", this is a better portrait of them. You can see that the thin distorted arms of NGC 4438 are a very pale shade of blue:


Spiral starforming galaxies contain different populations of stars of different colors. Young populations, typically found in spiral arms, get their color from very bright blue stars, although there are almost always also a few bright evolved red stars among them. Old populations are found in the spiral galaxies' centers, and they are yellow, just like the yellow populations of elliptical galaxies.

Photographing spiral galaxies containing both young and old populations is difficult when it comes to showing their "true colors". Here are two pictures of the well-known Whirlpool Galaxy, M51:


Anyway, back to NGC 6744. Let's look at the APOD again, and compare it with an ultraviolet picture from GALEX, which highlights the hot blue stars of the galaxy:


As you can see from the GALEX image, NGC 6744 is absolutely crammed full of ultraviolet light. This suggests that NGC 6744 is a quite blue galaxy. And yet, its B-V is as high as 0.86!!! That's higher than the B-V of NGC 4438 with its gossamer thin blue arms! That's crazy!!


I think there are two explanations for the crazily red B-V index for blue-looking NGC 6744. First, the B-V value for NGC 6744 is not its "total" but its "effective" B-V, and those values may well be different, for all I know. The second explanation is that its yellow center must be bright, and its blue arms and armlets must be dim. I think that is the most reasonable explanation. The ultraviolet stars are most certainly there in NGC 6744, scattered all over the place, but they are nowhere near as numerous, and their combined light is nowhere near as bright, as the APOD (and GALEX) lead us to believe.

So why does NGC 6744 look as blue as it does in the APOD? I'd say, filters and processing. Because it is so, so, so hard to produce a "true" color image of a galaxy!!

Finally, how bright and large is NGC 6744? Who knows. This is what Wikipedia said about the distance to NGC 6744:

Wikipedia wrote:

NGC 6744 (also known as Caldwell 101 or the Pavo Galaxy) is an intermediate spiral galaxy in the constellation Pavo (Peacock). Its velocity with respect to the cosmic microwave background is 802 ± 3 km/s, which corresponds to a Hubble distance of 38.6 ± 2.7 Mly (11.82 ± 0.83 Mpc). However, 21 non redshift measurements give a distance of 23.63 ± 1.68 Mly (7.244 ± 0.514 Mpc).

So the distance to NGC 6744 is either ~39 million light-years, or ~24 million light-years. If it is 39 million light-years away, then NGC 6744 is sure big and impressive! But if it is "only" 24 million light-years away, then it becomes much less remarkable.

Take your pick. They say that NGC 6744 is a Milky Way twin. I have absolutely nothing to say about that!!

Ann

What?! No cat or dog links?!?! I want a refund

Ann wrote: ↑Fri Nov 01, 2024 8:12 am
I think there are two explanations for the crazily red B-V index for blue-looking NGC 6744. First, the B-V value for NGC 6744 is not its "total" but its "effective" B-V, and those values may well be different, for all I know. The second explanation is that its yellow center must be bright, and its blue arms and armlets must be thin, short and pale. I think that is the most reasonable explanation.

Influencing the first is how the data was taken. Photometric measurements typically involve defining an "aperture" around some part of the data and adding up the value of all the pixels in it. Easy for a star, but for an extended object how that aperture is defined could make a big difference.

But that aside, I agree that your second point is the most important. Galaxies are a lot brighter in their cores than in their disks, but we don't normally see that in images because we process to flatten out that intensity range which our monitors can't show. We push brights down, and bring dim areas up. Here is a simple luminance image of M51, processed in this non-linear way and one processed purely linearly and giving a pretty accurate view of the true intensity relationships of the different parts of the galaxy. Pretty clearly, the photometric color of the core will dominate.
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Very nice 20 hr deep image (from the info on Astrobin)! Here it is on the left with the Hubble closeup of the center's elongated yellowish core on the right, slightly enlarged:


AVAO could do much nicer job at a mouse over or zoom-in of some sort. (Hint, hint.)

I wonder why no one demands to align disk galaxy images, when no regard to any coordinate system is being paid,
similar to a plate on a desk before an observer at that desk
rather than a lampshade on a ceiling before an observer under that ceiling

Dust lanes in front of the core in this APOD make it clear that it hangs the disk under the ceiling of our field of view

Chris Peterson wrote: ↑Fri Nov 01, 2024 6:28 pm

Ann wrote: ↑Fri Nov 01, 2024 8:12 am
I think there are two explanations for the crazily red B-V index for blue-looking NGC 6744. First, the B-V value for NGC 6744 is not its "total" but its "effective" B-V, and those values may well be different, for all I know. The second explanation is that its yellow center must be bright, and its blue arms and armlets must be thin, short and pale. I think that is the most reasonable explanation.

Influencing the first is how the data was taken. Photometric measurements typically involve defining an "aperture" around some part of the data and adding up the value of all the pixels in it. Easy for a star, but for an extended object how that aperture is defined could make a big difference.

But that aside, I agree that your second point is the most important. Galaxies are a lot brighter in their cores than in their disks, but we don't normally see that in images because we process to flatten out that intensity range which our monitors can't show. We push brights down, and bring dim areas up. Here is a simple luminance image of M51, processed in this non-linear way and one processed purely linearly and giving a pretty accurate view of the true intensity relationships of the different parts of the galaxy. Pretty clearly, the photometric color of the core will dominate.
_
Screenshot 2024-11-01 121935.jpg

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