APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

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APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by APOD Robot » Thu Feb 08, 2024 5:07 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, 47 Tuc lies about 13,000 light-years away. It can be spotted with the 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 (2024 Feb 08)

Post by Ann » Thu Feb 08, 2024 8:04 am

Ah, 47 Tuc! I'm not copying today's APOD, because it's too big. Instead I've going to ask you, can you find 47 Tuc in the picture below?


Can't find it? It's the very bright "star" next to the Small Magellanic Cloud!


In the picture above, you can see how bright and concentrated (and white) 47 Tuc is, and how faint and "thin" (and bluish) the Small Magellanic Cloud is. But wait! There is also another globular cluster in the picture. That's NGC 362, at 12 o'clock in the picture.


Imagine living inside 47 Tuc! No, I don't think it would be possible. If life is dependent on a planet in a stable orbit around its sun, you are not likely to find that for a very long time inside a dense globular cluster, where there is just so much gravitational jostling going on all the time. But let's say, for the sake of the argument, that it was possible. What would your night sky look like?


How can we know that the picture above shows an inside view of 47 Tuc? There are a couple of reasons. Note that there are two concentrations of light in the picture. The brightest one, at 4 o'clock, is the core of 47 Tuc. The core of this globular is very dense and bright. The other, much fainter and yellower light, at 11 o'clock, is that other globular cluster, NGC 362. Because, yes, these two globulars really are physically close to one another.

You might wonder what the Small Magellanic Cloud looks like from inside 47 Tuc. Actually the SMC wouldn't be readily visible at all from inside 47 Tuc, because this magnificent cluster is not actually close to the SMC. It is just seen in the same part of the sky as the SMC from our perspective, and compared with the SMC it is a foreground object.

But what is that large reddish and dust lane-riddled something that can be seen in the left part of the 47 Tuc picture? Why, that's the Milky Way! Because 47 Tuc really is close to the Milky Way - indeed, it belongs to our galaxy!

Let's look at a picture of 47 Tuc as seen from outside:


Is it really surprising that there are no blue stars in 47 Tuc? This globular cluster is at least 10 billion years old (could be 12 billion years old for all I know), and isn't it true that blue stars die young, and there has been no star formation in the Milky Way globulars for billions of years? So how can there be blue stars in any billion-year-old globular clusters?

The answer is that because the stars in globular clusters are so old, they were made from very pristine gas, which contained very few other elements than hydrogen and helium. Such stars are called metal-poor. When these stars have used up the hydrogen in their cores, they first swell up and become huge red giants, and then they shrink and become "smallish blue giants". Yes, really! And after that they swell up once more, become red giants once more, until they shrug off their outer layers and become white dwarfs.

This is what a color-magnitude diagram of a typical metal-poor globular cluster looks like:

47 Tuc No blue arc.png
47 Tuc has no blue arc.

47 Tuc has a pitifully short "horizontal branch", as it is called. The reason why its horizontal branch is so short and contains no blue stars is likely that the stars of 47 Tuc are comparatively metal-rich for their age.

But 47 Tuc not only lacks blue horizontal branch stars, it also lacks RR Lyrae stars. And they are important!


RR Lyrae stars are metal-poor variable stars of spectral classes A to F. Like Cepheids, they pulsate, and the period of their pulsations is intricately linked to their true brightness. Therefore, if you find an RR Lyrae star in a globular cluster and find out how fast it pulsates, then you know how bright it is, and from this knowledge of the star's true brightness you can find out how far away the entire globular cluster is. Since 47 Tuc has none, you can't use RR Lyrae stars to determine the distance to this cluster. (There are other ways... I'll talk about them some other time... maybe...)


RR Lyrae stars are standard candles:


On the whole, RR Lyrae stars are not as useful as standard candles as Cepheids, because Cepheids are considerably brighter. Cepheids have been used to determine the distances to nearby galaxies up to and including the Virgo Cluster. Farther away, other standard candles are more useful, such as type Ia supernovas.


Without standard candles we would be lost (beyond our capabilities of parallax measurements), and deep space would be more of a two-dimensional canvas than a three-dimensional space of endless(?) depth.

Ann
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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Christian G. » Thu Feb 08, 2024 1:50 pm

Is it fair to say that seeing a globular cluster is like seeing a tiny spherical galaxy within our own?

Roy

Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Roy » Thu Feb 08, 2024 2:54 pm

I followed the link about a star orbiting a “black hole” to the article, designated X9. Given the supposed components and parameters, I got a chance to play with the calculator, and came up with the supposed white dwarf orbiting the supposed black hole at an orbital velocity of 3596 km/sec, which is 0.012 C. I don’t think it is possible for a star to fall into a circular stable orbit this tight at this velocity about any mass. The politest thing I can say about their conjectured explanation is “fanciful”.
Yet, there it is, varying every 28 minutes, in a tight, calm cluster.

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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Chris Peterson » Thu Feb 08, 2024 3:12 pm

Roy wrote: Thu Feb 08, 2024 2:54 pm I followed the link about a star orbiting a “black hole” to the article, designated X9. Given the supposed components and parameters, I got a chance to play with the calculator, and came up with the supposed white dwarf orbiting the supposed black hole at an orbital velocity of 3596 km/sec, which is 0.012 C. I don’t think it is possible for a star to fall into a circular stable orbit this tight at this velocity about any mass. The politest thing I can say about their conjectured explanation is “fanciful”.
Yet, there it is, varying every 28 minutes, in a tight, calm cluster.
Why do you think that would be impossible? It's a simple two-body Newtonian (or Keplerian) system. There are no limitations on orbital speed. And orbital speed has nothing to do with orbital stability.
Chris

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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Ann » Thu Feb 08, 2024 3:19 pm

Roy wrote: Thu Feb 08, 2024 2:54 pm I followed the link about a star orbiting a “black hole” to the article, designated X9. Given the supposed components and parameters, I got a chance to play with the calculator, and came up with the supposed white dwarf orbiting the supposed black hole at an orbital velocity of 3596 km/sec, which is 0.012 C. I don’t think it is possible for a star to fall into a circular stable orbit this tight at this velocity about any mass. The politest thing I can say about their conjectured explanation is “fanciful”.
Yet, there it is, varying every 28 minutes, in a tight, calm cluster.
Just maybe such an orbital speed is at least possible.
VOA News wrote:

Researchers recently described an unusual binary system in the science publication Nature. The two stars orbit each other every 51 minutes in one of the fastest orbital periods known for a rare kind of binary system...

The unusual system includes a star similar to our sun orbiting very closely to what is called a white dwarf. A white dwarf is the hot and dense center of a burned out star...

Massachusetts Institute of Technology space scientist Kevin Burdge was lead writer of the study. He said, “Imagine if the moon zipped across the sky 10 times a night. That's the kind of speed we are talking about.”...

There are more than a thousand known cataclysmic variables. Only 12 or so have orbital periods below 75 minutes. While the binary system’s 51 minutes is quick, it is not the record when compared to other kinds of binaries.

The fastest known orbital period among binary stars is just five minutes and 21 seconds.
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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Chris Peterson » Thu Feb 08, 2024 3:23 pm

Christian G. wrote: Thu Feb 08, 2024 1:50 pm Is it fair to say that seeing a globular cluster is like seeing a tiny spherical galaxy within our own?
Only in the most superficial way, given that the formation processes of the two are likely quite different and the distribution and evolution of the contained stars is quite different.
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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Ann » Thu Feb 08, 2024 4:08 pm

Christian G. wrote: Thu Feb 08, 2024 1:50 pm Is it fair to say that seeing a globular cluster is like seeing a tiny spherical galaxy within our own?
I think Chris would object. He says, or so I have understood it, that globular clusters are a thing unto themselves and not like anything else.

If you ask me what sets globular clusters apart from galaxies, I would say that almost all galaxies have undergone several episodes of star formation, where the earliest and the latest episode are typically separated by at least a couple of billion years. Globular clusters are typically either one-generation collections of stars, or else they have undergone two or at most three episodes of star formation that took place relatively close together in time.

Another difference is that globular clusters are typically very metal-poor, whereas galaxies typically contain many more elements heavier than hydrogen and helium. It is of course ironic that 47 Tuc, which is relatively metal-rich as globular clusters go, is seen right next to the Small Magellanic Cloud, which is quite metal-poor for a galaxy.

Another obvious difference is that globular clusters lack gas, whereas most spiral, some lenticular and even some elliptical galaxies do contain gas.

Space.com wrote:

A striking new image from the Hubble Space Telescope captures an extremely detailed view of the galaxy NGC 4696 and the tangled, thread-like filaments stretching from its bright galactic core...

NGC 4696 is an elliptical galaxy, which means it boasts a round, elongated shape and lacks the distinctive swirling arms of spiral galaxies...

The unique structure of NGC 4696 likely resulted from an active supermassive black hole that lies at the galaxy's core. Energy generated by the black hole heats gas within the galaxy's inner regions and propels that material outward, creating the marbling effect seen in the Hubble telescope photo...
Yet another obvious difference is that many galaxies are disk-shaped, but globular clusters are spherical. Of course, elliptical galaxies are indeed spherical (or ovoid).

Another difference is that all large galaxies and indeed also some small galaxies contain globular clusters themselves. But to my knowledge, no globular cluster sports a satellite globular cluster of its own.

m87aat[1].jpg
Giant elliptical galaxy M87 sports some 12,000 globular clusters.
Most of the small white dots in this image are globulars.
Credit: David Malin/Anglo-Australian Observatory.
Ann
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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Christian G. » Thu Feb 08, 2024 4:55 pm

Thank you for these answers, Ann and Chris. You make a great team!

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Re: APOD: Globular Star Cluster 47 Tuc (2024 Feb 08)

Post by Fred the Cat » Thu Feb 08, 2024 6:38 pm

It’s like asking the question, “What came first the chicken or the egg?”

It seems like galaxies may be coops with very old chickens and all their eggs are either fried, hard-boiled, or scrambled. :wink:
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