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.
47 Tuc with the Small Magellanic Cloud in the background.
Note the tiny globular cluster belonging to the SMC (there is another one
to the right of 47 Tuc). Photo: Eder Ivan.
Closeup of 47 Tuc. Image: ESA/Hubble.
47 Tuc is a densely packed ball of, if I remember correctly, about a million stars. It is hanging photogenically in front of one of our galaxy's satellite galaxies, the Small Magellanic Cloud.
Note in Eder Ivan's image two small globular clusters belonging to the SMC, in contrast to large-looking 47 Tuc, which is the second-largest globular cluster of the Milky Way.
47 Tuc is slightly more metal-rich than most Milky Way globulars, and it lacks the blue horizontal branch stars which are so prominent in, for example, the great northern globular M13. Blue horizontal branch stars are fainter than the red giant stars, but brighter than the myriad of yellow and white stars of globular clusters.
But 47 Tuc does have some blue straggler stars. These are stars that have gained mass through interactions with another star, and the extra helping of hydrogen has made them brighter and bluer, and allowed them to stay on the main sequence (that is, allowed them to fuse hydrogen to helium in their core) at a time when all non-blue straggler stars have used up the hydrogen in their cores and turned into giants).
A possible blue straggler star can be seen at about 2 o'clock in the ESA/Hubble image. It is quite faint, and it may be similar to an A-type star like, say, Altair in Aquila. (There is also a fairly bright blue star seen at about 4 o'clock, but this must be another sort of beast altogether. It is either a foreground star or a really weird oddball of 47 Tuc.)
The caption ends with a link to a NASA page containing the following illustration (Credits: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss, reduced size).
It shows the white dwarf to the left and the black hole and its accretion disk to the right. But what could the star-like object in the middle be? It is not mentioned in the NASA caption. Is it just one of the background stars in 47 Tucanae? But then, should the sky not be littered with bright background stars?
Holger Nielsen wrote: ↑Thu Mar 10, 2022 9:03 am
The caption ends with a link to a NASA page containing the following illustration (Credits: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss, reduced size).
It shows the white dwarf to the left and the black hole and its accretion disk to the right. But what could the star-like object in the middle be? It is not mentioned in the NASA caption. Is it just one of the background stars in 47 Tucanae? But then, should the sky not be littered with bright background stars?
I guess that's the hotspot as the stream of matter from the white dwarf hits the accretion disk of the black hole.
Possible blue straggler in 47 Tuc. Photo: ESA/Hubble.
Possible blue straggler in 47 Tuc, annotated.
Blue stragglers are stars that have gained mass through interactions with other stars, and the extra helping of hydrogen has made them brighter and bluer, allowing them to stay on the main sequence (that is, allowing them to fuse hydrogen to helium in their cores) at a time when all non-blue straggler stars have used up the hydrogen in their cores and turned into giants).
The possible blue straggler star that I have annotated may be similar to an A-type star like, say, Altair in Aquila.
ddorn777 wrote: ↑Thu Mar 10, 2022 7:10 pm
One wonders what the night sky would look like from an earth-like planet circling a star in one of those densely populated clusters...
Bright. But I doubt a planet can survive around one of those stars long enough for complex life to develop.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Possible blue straggler in 47 Tuc. Photo: ESA/Hubble.
Possible blue straggler in 47 Tuc ESA Hubble annotated.png
Possible blue straggler in 47 Tuc, annotated.
Blue stragglers are stars that have gained mass through interactions with other stars, and the extra helping of hydrogen has made them brighter and bluer, allowing them to stay on the main sequence (that is, allowing them to fuse hydrogen to helium in their cores) at a time when all non-blue straggler stars have used up the hydrogen in their cores and turned into giants).
The possible blue straggler star that I have annotated may be similar to an A-type star like, say, Altair in Aquila.
Ann
I don't know. I seem to see many other similar looking stars in this field, which I've indicated with (thin) red lines. Are they all blue stragglers?
More blue stragglers?
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
I found an article about the X9 black hole system indicating the white dwarf star velocity is 12 million kilometers/hour. That converts to about 2000 miles/second.
Ann wrote: ↑Thu Mar 10, 2022 7:51 am
Can't keep this from you...
. . . Lots of info on 47 Tuc and the SMC . . .
Ann
Thanks, Ann, very interesting stuff.
I couldn't find your "2 o'clock" blue straggler until you marked it in your second post. Thanks!
I would have guessed that your very bright "4 o'clock blue straggler" is actually a foreground star, but I don't know. HD 1707 and HD 2466 are foreground stars adjacent to 47 Tuc in the sky. Wikipedia mentions one post-asymptotic blue giant, spectral class B8III, simply known as the "Bright Star". I sure don't know if that's it or not. (See https://phys.org/news/2021-08-brightest ... canae.html.)
You estimated from memory that 47 Tuc, a.k.a. NGC 104 has about a million stars. Its Wikipedia article is harmonious with that, saying: "containing millions of stars".
Possible blue straggler in 47 Tuc. Photo: ESA/Hubble.
Possible blue straggler in 47 Tuc ESA Hubble annotated.png
Possible blue straggler in 47 Tuc, annotated.
Blue stragglers are stars that have gained mass through interactions with other stars, and the extra helping of hydrogen has made them brighter and bluer, allowing them to stay on the main sequence (that is, allowing them to fuse hydrogen to helium in their cores) at a time when all non-blue straggler stars have used up the hydrogen in their cores and turned into giants).
The possible blue straggler star that I have annotated may be similar to an A-type star like, say, Altair in Aquila.
Ann
I don't know. I seem to see many other similar looking stars in this field, which I've indicated with (thin) red lines. Are they all blue stragglers?
nam888id wrote: ↑Thu Mar 10, 2022 1:18 pm
I find the structure of globular clusters kind of stunning and inspirationally incandescent.
I wanted to know more of what was going on.
The cluster in question may be a rich open cluster or a globular cluster, or even two merging clusters. Never mind. Note the following aspects:
1) At first the cluster is dominated by bright blue-white stars.
2) Note how more and more of the bright blue-white stars go orange and then pop. The bright blue stars turn into red giants, and then they either explode as supernovas or shed their outer layers more gently and turn into tiny white dwarfs.
3) Note that stars disappear from the cluster by moving out of it. Some stars in the outskirts of the cluster just move quietly away, as they have slipped the gravitational grip of the cluster. A close encounter between two stars near the center of the cluster sends both stars flying out of the cluster in opposite directions.
4) Over time, the cluster therefore becomes fainter and poorer in stars. This means that today's globular clusters, which are impressive enough, were unimaginably magnificent when they first formed.
How and why did all those globular clusters form? Wikipedia doesn't have an answer for us:
According to the NASA illustration above, the Universe is 13.77 billion years old. The first stars formed when the Universe was 400 million years old. According to Wikipedia, 47 Tuc is estimated to be 13.06 billion years old, which means that it formed when the Universe was 700 million years old. What were things like in the Universe back then?
Well... First of all the Universe was full of gas! Imagine that the cars in these images all represent a chunk of gas. Compare the amount of gas in the Universe then and now:
Why was there so much more gas in the Universe back then than there is now? The answer is - little red dwarf stars. (And the expansion of the Universe, because the available gas is spread inside a larger volume as the Universe expands).
But let's talk about the little red dwarfs. Consider cluster NGC 6397. According to a rather old caption by ESO, the age of NGC 6397 is 13,400 ± 800 million years. Well, if the Universe is 13.77 billion years old, and the first stars appeared when the Universe was 400 million years old, then an age of 13.4 billion years would make NGC 6397 precocious indeed. It is probably just a little younger.
Let's look at a closeup of NGC 6397. (And while we are at it, let's look at a massive and mass-losing young Wolf-Rayet star, too.)
Please note that the bright stars in NGC 6397 are not as blue as they look, because an orange and an infrared filter were used to create this image. But never mind. Let's focus on the small orange stars in NGC 6397.
Most stars formed in any starburst are going to be small red dwarf stars. These stars look tiny, but they are surprisingly massive. The average mass of a red dwarf star may be half the mass of the Sun. That is not so little. And because they are so incredibly numerous - a lot more numerous than the closeup of NGC 6397 suggests, because a lot of small red stars have undoubtedly drifted away from NGC 6397 since it first formed - the red dwarf stars contribute mightily to the combined mass of all the baryonic ("ordinary") mass of the Universe.
What happens to the gas that ends up inside a red dwarf star? Answer: It stays there, and for a much longer time than the current age of the Universe. Little red dwarf stars are gas-traps, sweeping up the gas of the Universe.
Massive stars, by contrast, give back much of their gas to the Universe. The picture of massive Wolf-Rayet star WR 124 dramatically demonstrates the violent mass loss during the late stage of their lives. Of course, any star that explodes as a supernova is going to give back most of its original gas to the Universe. And any star that sheds its outer layers to end up as a white dwarf is going to give back much of its gas to the Universe. I believe that the Sun, which is expected to end up as a white dwarf perhaps 5 billion years from now, is going to give back 50-60% of its original gas to the Universe.
But all those little red dwarfs are stingy hoarders that hang on to their gas for a much longer time than the Universe has existed. And when they finally release some of it, the Universe will have grown so incredibly large because of the acceleration caused by dark energy, that it will be very hard for massive gas clouds to form and to give birth to new stars.
So, to summarize. Most globular clusters are very old, 11-13 billion years old. Back then the Universe was quite literally smaller (because you must remember that it has been expanding ever since), and it was incredibly full of gas, because few stars had yet formed. Huge gas clouds must have collided and sparked incredible bursts of star formation. Think of it as an incredible fireworks show!
Take a look at the pictures of NGC 3603, a very massive and very young cluster in the Milky Way, and the double Cluster in Perseus. The age estimates of NGC 3603 vary from 300,000 years to 1.5 million years, and the age estimate of the Double Cluster in Perseus is ~12 million years.
You can see that there is still a lot of gas in the vicinity of NGC 3603, but the mighty cluster itself has cleared a cavity in the gas. As for the Double Cluster in Perseus, we see no sign of any gas cloud or any nebula whatsoever. The strong stellar winds must have blown away all the gas that remained after the Double Cluster themselves had formed.
Consider galaxy M82. Its gas, its starforming material, is rushing out of it because of a violent starburst in its center. M82 has already lost its ability to form stars in its disk. Just because it is forming stars (in its center) so violently, its starforming days will soon be over.
That's how globular cluster once formed, and that's probably why they generally stopped forming.
(And Chris will tell you that the gas that went into forming the globular clusters was quite different than the gas that is available in the Universe today, because the current gas is so heavily mixed with heavier elements forged in massive stars that not even a massive young metal-rich cluster like NGC 3603 can ever turn into a "true" globular cluster. I'll leave it to Chris to elaborate on that.)
I should mention, too, that some globulars are "stolen" cores of small galaxies. It looks something like this:
These stellar streams are the hapless remnants of dwarf galaxies captured by the Milky Way. The disks of the galaxies have spread out into long, long streamers, but you can see a "thickening", an almost spherical object, in one stellar stream. That would be the core of the captured dwarf galaxy. The largest globular cluster of the Milky Way, Omega Centauri, is believed to be the core of a captured dwarf galaxy.
Ann
Last edited by Ann on Fri Mar 11, 2022 9:44 pm, edited 1 time in total.
Thanks Ann for your response. The approach was incandescent for me the way it broke things down physically and chronologically and so clearly and succinctly expressed. And I sense the approach may be useful in other areas, like my Life.
nam888id wrote: ↑Sun Mar 13, 2022 4:00 pm
Thanks Ann for your response. The approach was incandescent for me the way it broke things down physically and chronologically and so clearly and succinctly expressed. And I sense the approach may be useful in other areas, like my Life.
You're very welcome! Glad you liked it and found it useful!
Globular Star Cluster 47 Tuc
