APOD: Blue Straggler Stars in Globular M53... (2021 Feb 07)

[APOD Robot]

Blue Straggler Stars in Globular Cluster M53

Explanation: If our Sun were part of this star cluster, the night sky would glow like a jewel box of bright stars. This cluster, known as M53 and cataloged as NGC 5024, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M53 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. These young stars might contradict the hypothesis that all the stars in M53 formed at nearly the same time. These unusual stars are known as blue stragglers and are unusually common in M53. After much debate, blue stragglers are now thought to be stars rejuvenated by fresh matter falling in from a binary star companion. By analyzing pictures of globular clusters like the featured image taken by the Hubble Space Telescope, astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster and hence a limit on the age of the universe. M53, visible with a binoculars towards the constellation of Bernice's Hair (Coma Berenices), contains over 250,000 stars and is one of the furthest globulars from the center of our Galaxy.

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[AZSteve]

Besides a cluster being part of a galaxy, and therefore orbiting around the galaxy's center, do the stars in a globular cluster orbit around a common center in the cluster, so that they don't just collapse together?

[Ann]

Blue Straggler Stars in Globular Cluster M53
Image Credit: ESA/Hubble, NASA

Blue Straggler Stars in Globular Cluster NGC 6397.
NASA/ESA/Francesco Ferraro (Bologna Astronomical Observatory)

Today's APOD is a fine portrait of globular cluster M53. As a portrait of blue stragglers, however, it is pretty useless.

"Spot the difference", said ESA/Hubble about the image that is today's APOD, referring to the blue stragglers of M53. But it is hard to spot any but the brightest of the individual stars in that picture, due to overcrowding. And anyone who wants to single out blue or bluish stars should avoid the choice of filters that were used for this image: The picture was made with an orange (606 nm) and an infrared (814 nm) filter.


That's why I recommend the picture at right, an old Hubble image from 2002. Do you remember the days when the Hubble pictures had that black "cutout" in the upper left corner?

Blue stragglers in NGC 6397.

The picture from 2002 was made with six filters!!

ESA/Hubble wrote:

This ESA/NASA Hubble Space Telescope Wide Field Planetary Camera 2 image shows the globular cluster NGC 6397 in the constellation Ara (the Altar). It is composed of six exposures through different filters: violet/indigo (Strvmgren u, 345 nm, 15080 seconds), blue (B, 418 nm, 8580 seconds), dark green (V, 515 nm, 978 seconds), light green (H-alpha, 656 nm, 24180 seconds), red-orange (R, 678 nm, 1538 seconds), and red (I, 839 nm, 978 seconds).

Because so many filters were used, it's no wonder you can spot many different types of stars in NGC 6397.

And indeed you can spot blue stragglers in this image! I have marked them in the picture at right. As you can see, the blue stragglers are fainter and typically less blue than the highly evolved metal-poor blue horizontal branch stars, which left the main sequence long ago.

Found a better picture of NGC 6397. Here you go!

---

The blue horizontal stars are comparable in mass to the cosmologically important RR Lyrae stars, which, according to Wikipedia, are only about half as massive as the Sun. The blue stragglers, by contrast, have gained mass from a companion star, and they are often more massive than almost all other stars in a globular cluster. That's why they often sink to the center of the globular.

But because the blue stragglers are still on the main sequence, they are not as bright as the blue horizontal branch stars, which have used up their core hydrogen and can be thought of as "giants".

So take a look at NGC 6397 if you want to see blue straggler stars!

Ann

[Ann]

Besides a cluster being part of a galaxy, and therefore orbiting around the galaxy's center, do the stars in a globular cluster orbit around a common center in the cluster, so that they don't just collapse together?

Click to play embedded YouTube video.

Check out this simulation of stellar orbits inside a cluster. The cluster in the video is an open cluster, but the principle is the same.

Ann

[VictorBorun]

But because the blue stragglers are still on the main sequence, they are not as bright as the blue horizontal branch stars, which have used up their core hydrogen and can be thought of as "giants".

I wonder why can there be comparable number of bright outlier stars of two sorts: quick burning massive blue giants (in a gasless place like globular stellar cluster they must come from binary mergers) and collapsing phase of an old smallish star that finally burnt out the hydrogen at its core?

[orin stepanek]

Wow; what looks like a white center is actually a large field of stars clomped close together!

[Chris Peterson]

Besides a cluster being part of a galaxy, and therefore orbiting around the galaxy's center, do the stars in a globular cluster orbit around a common center in the cluster, so that they don't just collapse together?

Yes. Every star is in orbit around a center that is defined by the mutual gravity of all the other stars in the cluster. Stars orbit at random inclinations, which is why the cluster is globular, and not, say, disc-like. And because the stars are so close together, they are constantly being perturbed by near passes, so the orbits themselves are not nice stable ellipses. That also means that stars exchange orbital angular momentum, with the consequence that some are shifted into smaller orbits, and some are ejected from the cluster completely (which is called evaporation, and sets a maximum lifetime for GCs).

[bystander]

And indeed you can spot blue stragglers in this image! I have marked them in the picture at right. As you can see, the blue stragglers are fainter and typically less blue than the highly evolved metal-poor blue horizontal branch stars, which left the main sequence long ago.

A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster.

[Ann]

But because the blue stragglers are still on the main sequence, they are not as bright as the blue horizontal branch stars, which have used up their core hydrogen and can be thought of as "giants".

I wonder why can there be comparable number of bright outlier stars of two sorts: quick burning massive blue giants (in a gasless place like globular stellar cluster they must come from binary mergers) and collapsing phase of an old smallish star that finally burnt out the hydrogen at its core?

Stellar evolutionary tracks off the main sequence. Source: Australian Telescope National Facility.

---

Take a look at the chart at right that tracks the evolution of stars of different masses.

The gray diagonal line is the main sequence. Here you find all stars that shine by fusing hydrogen to helium in their cores. When a star is on the main sequence, it is brighter and bluer the more massive it is, and fainter and redder the more light-weight it is.

The chart tells you happens when a star has used up the hydrogen in its core and evolves off the main sequence. "Metal-rich solar-mass" stars like the Sun always become redder and brighter when they evolve off the main sequence. (Well, until they become white dwarfs, that is.) More massive stars than the Sun also become redder when they turn into giants.

But for metal-poor stars, like the ones we often find in globular clusters, the story is a bit different.

Color-magnitude diagram of metal-poor globular cluster M92. Source: 'BVI Photometry and the Luminosity Functions of the Globular Cluster M92', by Nathaniel E. Q. Paust et. al.

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Color-magnitude diagram of more metal-rich globular cluster 47 Tuc. Source: Chegg Study Textbook solutions.

---

I want you to pay attention to the difference in the horizontal branch of these two globulars. In metal-poor cluster M92, the stars first rise in brightness on the red giant branch when they have used up the hydrogen in its core. At the top of the red giant branch, the stars undergo a "helium flash" and start fusing helium to carbon and oxygen. As they do so, they become considerably fainter, but still brighter than they were when they were on the main sequence. (Please note that this is true for low-and medium-mass stars.)

In a metal-poor cluster like M92, the stars move to the horizontal branch after they have undergone the helium flash. Here the stars become quite hot and blue. Can you see the horizontal branch of globular cluster M92? It is the curving line sloping downwards at around magnitude 16. Note the B-V color of this horizontal branch. Its B-V value is around 0.0, but the lower part of it has a negative B-V value. That means it's very blue.

Metal-rich clusters like 47 Tuc are different. They have "short and red" horizontal branches, so their stars never become blue after undergoing a helium flash. Can you see the horizontal branch of 47 Tuc? It is the short horizontal line at about magnitude 14. Note the B-V color of it. It is between +0.6 and +0.8, which means the stars here are the color of the Sun or yellower.

Metal-poor globular cluster M5. Note the blue horizontal branch stars in it. Photo: ESA, NASA and Hubble.

---

In short: The "medium-bright" blue stars that you can see in many globular clusters are horizontal branch stars. They are highly evolved, quite low in mass (because they were never very massive to begin with, and they have lost a lot of mass during their evolution) and they are quite hot and blue. Their presence means that the cluster is metal-poor.

So if you see a color picture of a globular cluster and note that it contains a relatively rich scattering of "medium-bright" blue stars, then you are seeing the horizontal branch stars of a metal-poor globular cluster.

By contrast, you will probably not be able to spot any blue stragglers in a typical photo of a globular cluster. The blue stragglers are much fainter and often less blue than the blue horizontal branch stars.

Ann

[JohnD]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

[shaileshs]

>> If our Sun were part of this star cluster, the night sky would glow like a jewel box of bright stars.
I'm wondering Is there a simulation video or even a photo of artist's imagination of this ? What a sight it'd be. What's the closest 2 stars are near the core of this cluster ? I'm guessing closer than our Sun and Alpha Centauri ? Maybe within light hours ? Wow!

[Chris Peterson]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

To be clear, GCs may or may not show a degree of common rotation, but their stability has nothing to do with rotation. They are (mostly) stable because each of their member stars exists in its own orbit. That was understood from as early as GCs were observed and understood to be collections of stars.

[johnnydeep]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

The article was too detailed for me. Does it actually say that GC rotation is what prevents them from collapsing sooner? And does it say how much sooner collapse would happen without rotation? And how fast do GCs rotate on average?

[johnnydeep]

To add to (or perhaps misunderstand!) Ann's posts, I like the pic of M53 from https://en.wikipedia.org/wiki/Messier_53 better, since it seems to more clearly show individual "blue stragglers":

M53

But are they actually "blue stragglers"? And frankly, I seem to see the same number of blue stars as orange stars, both of which seem to be fewer in number than the majority of whiter stars. The filters used no doubt play a role...

[ta152h0]

When the James Webb telescope comes online, would this image be drastically different and how? pass the ice cold one

[Ann]

To add to (or perhaps misunderstand!) Ann's posts, I like the pic of M53 from https://en.wikipedia.org/wiki/Messier_53 better, since it seems to more clearly show individual "blue stragglers":

---

But are they actually "blue stragglers"? And frankly, I seem to see the same number of blue stars as orange stars, both of which seem to be fewer in number than the majority of whiter stars. The filters used no doubt play a role...

No, the blue stars in that picture are blue horizontal stars, not blue stragglers.

We can tell that they are blue horizontal stars because

a) They are "medium-bright" (they stand out, but they are not as bright as the brightest red stars)

d) They are all more or less the same brightness

c) There are a lot of them

d) They are spread evenly all over the globular


As for the blue stragglers...

a) They are practically always fainter than the blue horizontal stars, which are a kind of "giants"

b) They are not all the same brightness, because they are not all the same mass

c) They are often less blue than the blue horizontal branch stars

d) They are not evenly spread throughout the cluster, because they are comparatively massive, so they tend to sink towards the cluster center

Blue straggler stars (center) in NGC 6397, and blue horizontal branch stars scattered over the globular. Photo: ESA/Hubble and NASA.

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Closeup of the blue straggler stars in NGC 6397.

---

Ann

P.S. I'm sure you are right that there are a lot more white stars than either red or blue. The white stars are in all probability main sequence Sun-like stars, or else they are Sun-like in temperature but more lightweight than the Sun in mass. That is a possibility in metal-poor globulars.

P.P.S. I missed my nice number 11111 posts. A pity.

[johnnydeep]

Thanks as usual, Ann!

[shaileshs]

To add to (or perhaps misunderstand!) Ann's posts, I like the pic of M53 from https://en.wikipedia.org/wiki/Messier_53 better, since it seems to more clearly show individual "blue stragglers":


M53.JPG


But are they actually "blue stragglers"? And frankly, I seem to see the same number of blue stars as orange stars, both of which seem to be fewer in number than the majority of whiter stars. The filters used no doubt play a role...

No, the blue stars in that picture are blue horizontal stars, not blue stragglers.

We can tell that they are blue horizontal stars because

a) They are "medium-bright" (they stand out, but they are not as bright as the brightest red stars)

d) They are all more of less the same brightness

c) There are a lot of them

d) They are spread evenly all over the globular


As for the blue stragglers...

a) They are practically always fainter than the blue horizontal stars, which are a kind of "giants"

b) They are not all the same brightness, because they are not all the same mass

c) They are often less blue than the blue horizontal branch stars

d) They are not evenly spread throughout the cluster, because they are comparatively massive, so they tend to sink towards the cluster center

Blue straggler stars (center) in NGC 6397, and blue horizontal branch stars scattered over the globular. Photo: ESA/Hubble and NASA.

---

Closeup of the blue straggler stars in NGC 6397.

---

Ann

P.S. I'm sure you are right that there are a lot more white stars than either red or blue. The white stars are in all probability main sequence Sun-like stars, or else they are Sun-like in temperature but more lightweight than the Sun in mass. That is a possibility in metal-poor globulars.

P.P.S. I missed my nice number 11111 posts. A pity.

Ann, You are tooooooooooooooo kind to take efforts to explain again what you explained in deep detailed thoughts above already. Seems JohneyDepp didn't read carefully your detailed thoughts/analysis

[Ann]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

The article was too detailed for me. Does it actually say that GC rotation is what prevents them from collapsing sooner? And does it say how much sooner collapse would happen without rotation? And how fast do GCs rotate on average?

Yes, the orbital rotation of all those stars in globular clusters is precisely what prevents them from just plunging to the center and collapsing into one monstrous black hole.

Note that it is the individual stars in the globular that must rotate to prevent a huge collapse of the globular.

Frankly, everything in space rotates, orbits, or is on the move in other ways. What prevents the Earth from just plunging into the Sun and getting heated and fried into blisteringly hot silicon gas?

You know the answer. It is the Earth's orbit, its rotation around the Sun. We don't plunge into the Sun because we orbit around it.

We orbit, therefore we are.

Ann

[Ann]

I couldn't find an interesting-looking animation of the rotation of stars inside a globular cluster, but I want to show you three other things: The plane of the Solar system versus the plane of the Milky Way, the Sun's motion along the plane of the Milky Way, and the orbital motion of two million stars in the Milky Way.

The plane of the Solar system is inclined to the plane of the Milky Way by about 60°. Illustration possibly by David Handy.

---

The Sun bobs up and down as it orbits the center of the Galaxy along the galactic plane. Illustration: I don't know.

---

As you can see, the plane of the Solar system is not aligned with the plane of the Milky Way at all. The Sun orbits more or less along the plane of the Milky Way, but it bobs up and down as it does so.

Click to play embedded YouTube video.

Finally, take a look at the motion of two million stars in the Milky Way, measured by the European space telescope Gaia.

Ann

[Ann]

>> If our Sun were part of this star cluster, the night sky would glow like a jewel box of bright stars.
I'm wondering Is there a simulation video or even a photo of artist's imagination of this ? What a sight it'd be. What's the closest 2 stars are near the core of this cluster ? I'm guessing closer than our Sun and Alpha Centauri ? Maybe within light hours ? Wow!

A view from a hypothetical planet inside globular cluster 47 Tuc. Illustration by William Harris and Jeremy Webb.

---

The center of globular clusters 47 Tuc. Photo: Macedon Ranges Obervatory.

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The Small Magellanic Cloud and far more nearby Milky Way clusters 47 Tuc (right) and NGC 362 (top). Photo: Alan Dyer.

---

I love the illustration of what giant globular cluster 47 Tuc might look like from a planet inside it. We can be sure that we are inside 47 Tuc, because we can see the bright central condensation of this cluster just above the mountain range at right in the picture at top left. Also, in the same illustration we can see another, fainter globular at upper left, which is undoubtedly the apparent "neighbour" of 47 Tuc, globular cluster NGC 362. Moreover, we can vaguely see the shape of the Milky Way at left. Not that I think anyone would be able to see much of the Milky Way from inside 47 Tuc!

This is my favorite illustration of what a globular cluster might look like from inside.

Ann

[VictorBorun]

I

The Sun bobs up and down as it orbits the center of the Galaxy along the galactic plane. Illustration: I don't know.

---

As you can see, the plane of the Solar system is not aligned with the plane of the Milky Way at all. The Sun orbits more or less along the plane of the Milky Way, but it bobs up and down as it does so.

Click to play embedded YouTube video.

Finally, take a look at the motion of two million stars in the Milky Way, measured by the European space telescope Gaia.
Ann

I am not quite satisfied with this picture.
1. The bobbing of the Sun above and under the disk of the Milky Way is approximately 2.7 times per Sun's orbit around the galactic center, not so many as in the first picture.
2. The complex moving of nearby stars in the video is but for 5 million years. The Sun bobs with a period of 83 million years and is more or less fixed for the duration of the video. We can see the random stars change their velocities and directions, but I think it's what a constant velocity looks like in a panoramic view when a star passes close to the observer.

[JohnD]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

To be clear, GCs may or may not show a degree of common rotation, but their stability has nothing to do with rotation. They are (mostly) stable because each of their member stars exists in its own orbit. That was understood from as early as GCs were observed and understood to be collections of stars.

Ah! Thank you, Chris! So GCs are fundametally different from an 'ordinary' galaxy, where all the stars, dust and gas rotate around the Centre? How does this difference arise? A galaxy, like a solar system, 'congealing' from a cloud of dust and gas will retain any motion in that cloud and amplify it into the spinning stars or planets. If, as I read, GCs are also formed from primordial gas clouds, the contraction down to form stars will have been enormous, and any small motion of the cloud would be massively amplifyed, however small originally.

[Ann]

I

The Sun bobs up and down as it orbits the center of the Galaxy along the galactic plane. Illustration: I don't know.

---

As you can see, the plane of the Solar system is not aligned with the plane of the Milky Way at all. The Sun orbits more or less along the plane of the Milky Way, but it bobs up and down as it does so.

Click to play embedded YouTube video.

Finally, take a look at the motion of two million stars in the Milky Way, measured by the European space telescope Gaia.
Ann

I am not quite satisfied with this picture.
1. The bobbing of the Sun above and under the disk of the Milky Way is approximately 2.7 times per Sun's orbit around the galactic center, not so many as in the first picture.
2. The complex moving of nearby stars in the video is but for 5 million years. The Sun bobs with a period of 83 million years and is more or less fixed for the duration of the video. We can see the random stars change their velocities and directions, but I think it's what a constant velocity looks like in a panoramic view when a star passes close to the observer.

Thanks for your clarification, Victor.

Ann

[johnnydeep]

I asked a similar question to AZSteve about globular clusters awhile ago, and it was Chris who taught me that they rotate, about a common center of mass, just like a full sized galaxy.
But this picture made me wonder - a planet of one of these stars would have wondrous night sky, but would it have constellations? How fast is the rotation? Our galaxy spins slowly enough so that we haven't seen a significant change in them for three or more thousand years. Would the observed positions from a cluster star change more quickly? That led me to this paper https://www.aanda.org/articles/aa/pdf/2 ... 709-14.pdf, whose introduction outlines the research that established this rotation - and it's all in the last twenty years!

What did astronomers think prevented clusters from collapsing under gravity, before they knew they rotated? Or was it not of interest, and ignored?
John

The article was too detailed for me. Does it actually say that GC rotation is what prevents them from collapsing sooner? And does it say how much sooner collapse would happen without rotation? And how fast do GCs rotate on average?

Yes, the orbital rotation of all those stars in globular clusters is precisely what prevents them from just plunging to the center and collapsing into one monstrous black hole.

Note that it is the individual stars in the globular that must rotate to prevent a huge collapse of the globular.

Frankly, everything in space rotates, orbits, or is on the move in other ways. What prevents the Earth from just plunging into the Sun and getting heated and fried into blisteringly hot silicon gas?

You know the answer. It is the Earth's orbit, its rotation around the Sun. We don't plunge into the Sun because we orbit around it.

We orbit, therefore we are.

Ann

I thought we were distinguishing the random orbital motions of individual stars in a GC (which is indisputable) from any overall common motion of all the stars in a GC about a shared axis (which the paper was about).