APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

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APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by APOD Robot » Sat Oct 24, 2020 4:05 am

Image Globular Star Cluster 47 Tuc

Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with some 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye close on the sky to the Small Magellanic Cloud in the constellation of the Toucan. The dense cluster is made up of hundreds of thousands of stars in a volume only about 120 light-years across. Red giant stars on the outskirts of the cluster are easy to pick out as yellowish stars in this sharp telescopic portrait. Tightly packed globular cluster 47 Tuc is also home to a star with the closest known orbit around a black hole.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Ann » Sat Oct 24, 2020 6:37 am

47 Tucanae is a very interesting globular cluster, and I'm going to need many pictures to show it to you!

Let's start by comparing 47 Tuc, which is the Milky Way's second largest globular, with Omega Centauri, which is the largest globular of our galaxy:





















Can you spot a difference? You can, can't you? 47 Tuc has an incredibly compact and concentrated center. Omega Centauri doesn't. It shows no central concentration at all.

Why the difference? Personally I don't find 47 Tuc's central concentration so surprising. It is a known fact that the most massive stars tend to sink to the center of a globular cluster, whereas the most lightweight stars run a not negligible risk of being ejected altogether. The most massive stars of 47 Tuc have all died, but their ghostly and still moderately massive remnants still inhabit the center of this magnificent globular in the form of pulsars and even a black hole.

It is actually stranger that Omega Centauri is so "fluffy" and "unconcentrated". But Omega Centauri is believed to be the center of a dwarf galaxy that once collided with the Milky Way, leaving the center of the galaxy intact but shredding its outer regions. Note that small galaxies often have fluffy centers. Even the third largest galaxy in the Local Group, M33, may have such a loose center that it even lacks a central supermassive black hole. The "naked" center of the dwarf galaxy that once collided with the Milky Way is now in orbit around around our galaxy as globular cluster Omega Centauri.

But 47 Tuc is strange in other ways. Let's compare it with the great northern globular cluster, M13.


























Can you tell the difference between the colors of these two clusters? You can, can't you?

There is a large population of blue stars in M13. In 47 Tuc, such stars are completely absent.

The two images are not directly comparable, because different filters were used for them. For the 47 Tuc image, NASA used an ultraviolet filter but not a blue one, while for M13 NASA used a blue filter but not an ultraviolet one. This difference enhances the visible concentration of "moderately blue" stars (say, A- to F-type stars) in M13 and suppresses it in 47 Tuc. But really, there is very little to suppress in 47 Tuc, because this globular completely lacks the class of blue stars that is so eminently visible in M13: the blue horizontal branch stars.
























Compare the color-magnitude diagram of 47 Tuc at left with the illustration of the evolution of a one solar mass star at right. First you must identify the main sequence, which is the diagonal track running from lower right to upper left. In the illustration at right, this track is extremely short. In the color-magnitude diagram at left, it is moderately long. Stars that are on the main sequence fuse hydrogen to helium in their cores.

Then the star reaches the turnoff point, when it has (or is about to) exhaust the hydrogen in its core. The turnoff point is located right across from the letter "V" in the color-magnitude diagram at left. In the illustration at right, there are only the words "Subgiant branch".

After the star has completely exhausted the hydrogen in its core, the star's core shrinks, which in itself releases a lot of energy. In response, the stars grows very much bigger and redder. It is now on the red giant branch.

When the core of the star has grown sufficiently hot, it initiates helium fusion. Helium is fused to carbon and oxygen. As this happens, the star shrinks considerably and its outer layers also become somewhat hotter. This is the red clump stage. You can find it in the illustration at right. This is a relatively stable evolutionary stage, and many (if not most) K-type stars in the sky belong to the red clump.

After the star has exhausted the helium in its core, its core shrinks again, the star rises again on the so called asymtotic giant branch, becomes larger and redder again, until it starts pulsating, sheds its outer layers and becomes a white dwarf.

Anyway. Here is my point. Can you see the red clump in the color-magnitude diagram of 47 Tuc? It is right across the number 14 on the Y-axis.

Now let's take a look at the color-magnitude diagram of a more "normal" globular cluster:


























The main difference between the color magnitude diagrams of 47 Tuc and most other globulars is that most other globulars have a long, mostly blue horizontal branch reaching to the left, whereas 47 Tuc only has a small red clump of stars.

And that is exactly why we could see no blue stars in 47 Tuc.

The reason why 47 Tuc lacks a long blue horizontal branch is that this globular is too metal-rich. That is, its stars were made from a nebula that contained a too high concentration of elements heavier than hydrogen and helium. Most other globulars do contain blue horizontal stars, because their stars were made from more metal-poor gas.

The fact that 47 Tuc lacks a long blue horizontal branch also means that it lacks RR Lyrae stars. These pulsating stars can be found in the middle of the horizontal branch, on the so called instability strip. The pulsations of RR Lyrae stars reflect the absolute magnitude of them, in a manner reminiscent of Cepheid stars, and they can be used to infer the distance to the globulars that contain RR Lyrae stars. But 47 Tuc hasn't got them, so its distance can't be constrained that way.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Iksarfighter » Sat Oct 24, 2020 8:08 am

Wonderful picture.

heehaw

Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by heehaw » Sat Oct 24, 2020 9:09 am

What a night sky planets must have!

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Ann » Sat Oct 24, 2020 9:25 am

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by sillyworm 2 » Sat Oct 24, 2020 1:12 pm

Thanks Ann! Always appreciated.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by bystander » Sat Oct 24, 2020 3:17 pm

Ann wrote:
Sat Oct 24, 2020 6:37 am
47 Tucanae is a very interesting globular cluster, and I'm going to need many pictures to show it to you!

Let's start by comparing 47 Tuc, which is the Milky Way's second largest globular, with Omega Centauri, which is the largest globular of our galaxy:

Can you spot a difference? You can, can't you? 47 Tuc has an incredibly compact and concentrated center. Omega Centauri doesn't. It shows no central concentration at all. ...

The image you show of Omega Centauri is just the central region. A wider field shows the central concentration.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by VictorBorun » Sat Oct 24, 2020 3:40 pm

Ann wrote:
Sat Oct 24, 2020 9:25 am
Well... I guess the night sky of 47 Tuc might look something like the picture at right.
Ann
I wonder why does that starry sky look like a view from a flattening distance.
I would expect some close stars to shine as bright as Moon and be visible through daytime skyshine.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Ann » Sat Oct 24, 2020 4:54 pm

bystander wrote:
Sat Oct 24, 2020 3:17 pm

The image you show of Omega Centauri is just the central region. A wider field shows the central concentration.

Britannica wrote:

Omega Centauri is relatively nearby, at a distance of 17,000 light-years, and it lacks a sharp nucleus. The cluster designated 47 (NGC 104), with an absolute visual magnitude of −9.42 at a similar distance of 14,700 light-years, has a different appearance with strong central concentration.
So according to Britannica, 47 Tuc has a a strong central concentration of stars, but Omega Centauri lacks a concentrated core.

Also, according to Sky Catalogue 2000.0, Volume 2, globulars are classified according to their concentration from 1 to 12, with 1 being the most concentrated and 12 the loosest. And also according to Sky Catalogue 2000.0, Volume 2, 47 Tuc belongs to class 3 (quite highly concentrated) and Omega Centauri to class 8 (relatively loose).

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Ann » Sat Oct 24, 2020 7:40 pm

Bystander, thank you for your link to the article about a 2008 study implying the existence of a 40,000 solar mass black hole in Omega Centauri. More recent work, however, does not confirm the existence of an intermediate-mass black hole in this great globular:
Wikipedia wrote:

However, more recent work has challenged these conclusions, in particular disputing the proposed location of the cluster center. Calculations using a revised location for the center found that the velocity of core stars does not vary with distance, as would be expected if an intermediate-mass black hole were present. The same studies also found that starlight does not increase toward the center but instead remains relatively constant. The authors noted that their results do not entirely rule out the black hole proposed by Noyola and colleagues, but they do not confirm it, and they limit its maximum mass to 12,000 solar masses.
I find it particularly interesting that starlight does not increase toward the center of Omega Centauri.

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by Chris Peterson » Sun Oct 25, 2020 4:50 am

Ann wrote:
Sat Oct 24, 2020 7:40 pm
Bystander, thank you for your link to the article about a 2008 study implying the existence of a 40,000 solar mass black hole in Omega Centauri. More recent work, however, does not confirm the existence of an intermediate-mass black hole in this great globular:
Wikipedia wrote:

However, more recent work has challenged these conclusions, in particular disputing the proposed location of the cluster center. Calculations using a revised location for the center found that the velocity of core stars does not vary with distance, as would be expected if an intermediate-mass black hole were present. The same studies also found that starlight does not increase toward the center but instead remains relatively constant. The authors noted that their results do not entirely rule out the black hole proposed by Noyola and colleagues, but they do not confirm it, and they limit its maximum mass to 12,000 solar masses.
I find it particularly interesting that starlight does not increase toward the center of Omega Centauri.
It's not clear why globular clusters would be denser in their centers. I'd expect them to be fairly uniform in density. So I think the more interesting ones are those that do show such structure. Perhaps it means they are further along in the process of evaporating.
Chris

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Re: APOD: Globular Star Cluster 47 Tuc (2020 Oct 24)

Post by alter-ego » Tue Oct 27, 2020 5:16 am

Ann wrote:
Sat Oct 24, 2020 7:40 pm
...
I find it particularly interesting that starlight does not increase toward the center of Omega Centauri.
Just to clarify perspective, the full view of Omega Centauri is dominated by central brightening. In van der Marel's paper, the flattening exists within the central 1 to 2 arcminutes (5 to 10ly) of the cluster. Even for a uniform stellar density within a sphere, the apparent cluster brightness increases towards the cluster center. The increase is an artifact of more stars along a line of sight nearer the center, and not due to an actual increase in star density. However, a black hole will tend to increase the observed brightness and density. The Noyola et al paper shows the brightness increase in their profile from which they estimated a 40,000 M☉ black hole. From the degree of noise in, and low number of, data points near the center, you can see the difficulty in extracting evidence for a "small" black hole at the center of Omega Centauri.
 
Omega Centauri Profile - Hubble Images.jpg
Omega Centauri Line-of-Sight Profile_van der Marel.jpg
Profile with Evidence for Black-Hole_ E. Noyola et al.jpg
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