APOD: Millions of Stars in Omega Centauri (2021 Jun 03)

[APOD Robot]

Millions of Stars in Omega Centauri

Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter. It's the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. Omega Centauri's red giant stars (with a yellowish hue) are easy to pick out in this sharp, color telescopic view.

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[Ann]

Great globular cluster Omega Centauri. Note the lack of concentration
toward the center. Photo: Ignacio Diaz Bobillo.

Great globular cluster 47 Tuc. Note the dramatically rising stellar density toward the center. Photo: Don Goldman.

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Omega Centauri is certainly a great and fascinating cluster. For myself, I can't get over how "loose" it is, and lacking in a central concentration. Compare Omega Centauri with the second brightest globular cluster in the Milky Way, 47 Tuc, and note how the stellar concentration rises steeply toward the center in that other great globular.

I get the impression that if you were dropped somewhere close to but still some distance away from the center of Omega Centauri, you wouldn't be quite sure which way the center was, because much of Omega Centauri looks the same "all over"! But it sure is packed with stars. According to Wikipedia, the average distance between stars in Omega Centauri is only 0.1 light-years.

Central part of Omega Centauri in highly saturated colors. Credit: NASA, ESA, and the Hubble SM4 ERO Team.

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Take a look at the "wildly colored" Hubble image of stars near the center of Omega Centauri. The colors are highly saturated and not "realistic". Nevertheless, the colors are "sort of true".

1) Note that the brightest stars are red. Indeed, the very brightest star in the image is also the reddest, and the reason why it is so red is because it is so bloated. This very bright red star is on its last legs, and its demise is near. It is a so-called asymtotic giant branch star of spectral class M. This star has already started pulsating, and eventually the pulsations will be so strong that the star will "shrug off" all of its outer atmosphere, after which it will become a white dwarf.

2) There are other red stars in this part of Omega Centauri, and there are orange stars there, too. It is not possible to tell at a glance if these are red giant stars or asymtotic giant branch stars, but in general the red giant branch stars are likely to be a little less red and a little more orange in color than the asymtotic branch stars, and also a bit less bright (although that is not always the case).

3) The red giant stars have more life in them than the asymtotic branch stars. They are not facing death yet, because first they will undergo a so called "helium flash", shrink rather dramatically and become so hot that they become blue. At this stage they are horizontal branch stars, fusing helium to carbon and oxygen in their cores. They are the blue stars in the picture. And when the blue stars run out of steam, they will turn into asymtotic giant branch stars.

4) Note the profusion of small white stars in the picture. They are, I would say, in most cases stars similar to the Sun. They are close to exhausting the hydrogen in their cores, after which they will become red giants.

6) Look carefully, and you can also see a lot of tiny red dots. They are red dwarfs.

7, 5)There are also a few tiny blue dots. They may be white dwarfs. Only the really tiny blue dots are likely to be white dwarfs. The "merely very small" blue stars are likely to be "extreme horizontal branch stars", which are hotter, bluer and much smaller than most other stars on the horizontal branch.

I really have more to say about Omega Centauri, but I'm getting a bit tired of myself here, and I suspect that you are too...

Ann

[JohnD]

Thank you Ann, always interesting!
And stars 0.1LY apart, on average? You don't tell us what the SD is, but that sounds awful close, even if it is 6^11miles (10^11kilometers)!
If there are planets around those stars,and there must be, and there is life on those planets, as there may be, or has been, then isn't Omega Cetauri a prime target for SETI?

But considering my usual apocalyptic scenario, in a cluster of so-close stars - could a nova set off a chain reaction?
John

[orin stepanek]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

[Chris Peterson]

Thank you Ann, always interesting!
And stars 0.1LY apart, on average? You don't tell us what the SD is, but that sounds awful close, even if it is 6^11miles (10^11kilometers)!
If there are planets around those stars,and there must be, and there is life on those planets, as there may be, or has been, then isn't Omega Cetauri a prime target for SETI?

But considering my usual apocalyptic scenario, in a cluster of so-close stars - could a nova set off a chain reaction?
John

Where stars are so close, there will certainly be no complex life. The only planets that would have any chance of surviving the tidal interactions would be massive ones in very close orbits around their stars. Probably not capable of forming or sustaining even the simplest of life, let alone complex life. Globular clusters, except possibly in their very outermost regions, are likely sterile.

There's no such thing as a supernova chain reaction. Binary stars regularly see one of the pair supernova, without destroying the other.

[Chris Peterson]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

[neufer]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Omega Centauri is roughly the same apparent size as the Sun
but it is ~30 apparent magnitudes dimmer than the Sun.

Thus the chance of our visual line intersecting an actual
Omega Centauri star is on the order of 100^-{30/5} or 10^-12.

[Chris Peterson]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Omega Centauri is roughly the same apparent size as the Sun
but it is ~30 apparent magnitudes dimmer than the Sun.

Thus the chance of our visual line intersecting an actual
Omega Centauri star is on the order of 100^-{30/5} or 10^-12.

The simulation I conducted utilized a "line" that was the diameter of a star. That makes a collision somewhat more likely.

I'll have to think a bit more about your approach here. Maybe we need to consider the strong density gradient?

[shaileshs]

I wonder if those BIG bluish/white stars (some at inner circular boundary and towards outer circular boundary of photo) are part of this GC ? Are they seen much bigger and brighter because they are closer OR they are really big supergiants ?

[orin stepanek]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Thanks; seems complex; but I trust your Opinion!

[orin stepanek]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Omega Centauri is roughly the same apparent size as the Sun
but it is ~30 apparent magnitudes dimmer than the Sun.

Thus the chance of our visual line intersecting an actual
Omega Centauri star is on the order of 100^-{30/5} or 10^-12.

Thanks Art!

[Chris Peterson]

I wonder if those BIG bluish/white stars (some at inner circular boundary and towards outer circular boundary of photo) are part of this GC ? Are they seen much bigger and brighter because they are closer OR they are really big supergiants ?

Keep in mind that every star in this image is vastly bloated. If we could represent the stars accurately, they would be several orders of magnitude smaller than a single pixel. The apparent size of a star is determined by its brightness, not its actual size.

[johnnydeep]

I was going to comment that I just love these big majestic perfectly symmetrical GCs, but then I read this at the https://esahubble.org/news/heic0809/ link:

Globular clusters consist of up to one million old stars tightly bound by gravity and are found in the outskirts of many galaxies including our own. Omega Centauri has several characteristics that distinguish it from other globular clusters: it rotates faster than a run-of-the-mill globular cluster, its shape is highly flattened and it consists of several generations of stars -- more typical globulars usually consist of just one generation of old stars.

Doesn't look very flattened here, but it all depends on which axis the flattening is perpendicular to! And in "rotating faster" than a typical GC, I would expect it to be rotating predominantly around that same axis. Or is that a poor assumption?

[johnnydeep]

Thank you Ann, always interesting!
And stars 0.1LY apart, on average? You don't tell us what the SD is, but that sounds awful close, even if it is 6^11miles (10^11kilometers)!
If there are planets around those stars,and there must be, and there is life on those planets, as there may be, or has been, then isn't Omega Cetauri a prime target for SETI?

But considering my usual apocalyptic scenario, in a cluster of so-close stars - could a nova set off a chain reaction?
John

What's "SD"? Stellar Distance? Standard Distance? School District? ...

[neufer]

Kapteyn's Star comparison with Sun, Jupiter & Earth

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<<Kapteyn's Star is a class M1 red subdwarf about 12.83 light years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With a magnitude of nearly 9 it is visible through binoculars or a telescope.

Kapteyn's Star came within 7.0 ly of the Sun about 10,900 years ago and has been moving away since that time. Kapteyn's Star orbits the Milky Way retrograde. It is a member of a moving group of stars that share a common trajectory through space, named the Kapteyn moving group. Based upon their element abundances, these stars may once have been members of Omega Centauri, a globular cluster that is thought to be the remnant of a dwarf galaxy that merged with the Milky Way. During this process, the stars in the group, including Kapteyn's Star, may have been stripped away as tidal debris.

Attention was first drawn to what is now known as Kapteyn's Star by the Dutch astronomer Jacobus Kapteyn in 1898. While he was reviewing star charts and photographic plates, Kapteyn noted that a star, previously catalogued in 1873 by B.A. Gould, seemed to be missing. However, R.T.A. Innes found an uncatalogued star about 15 arc seconds away from the absent star's position. It became clear that the star had a very high proper motion of more than 8 arc seconds per year and had moved significantly. At the time of its discovery it had the highest proper motion of any star known, dethroning Groombridge 1830. In 1916, Barnard's Star was found to have an even larger proper motion. In 2014, two super-Earth planet candidates in orbit around the star were announced.

Kapteyn's Star is a variable star of the BY Draconis type with the identifier VZ Pictoris. This means that the luminosity of the star changes because of magnetic activity in the chromosphere coupled with rotation moving the resulting star spots into and out of the line of sight with respect to the Earth. In 2014, Kapteyn's Star was announced to host two planets, Kapteyn b and Kapteyn c. Kapteyn b is the oldest-known potentially habitable planet, estimated to be 11 billion years old. However, Robertson et al. (2015) noted that the orbital period of Kapteyn b is an integer fraction (1/3) of their estimated stellar rotation period and thus the planetary signal is most likely an artifact of stellar activity. The authors do not rule out the existence of Kapteyn c, calling for further observation. Guinan et al. (2016) (as well as earlier authors) found a lower value for the stellar rotation, which lends support to the original planetary finding. (In 2021, the existence of planets was questioned after the rotational period of the star was refined, with a rotational period very similar to that of candidate c.)

Guinan et al. (2016) suggest that the present day star could potentially support life on Kapteyn b, but that the planet's atmosphere may have been stripped away when the star was young (~0.5 Gyr) and highly active. The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.>>

[JohnD]

Thank you, Chris, for the words about "sterile" globular clusters - "learn something every day" - Aristotle?

For Orin, I looked for online animations of the movement of a cluster, but many seem to show an axial rotation, or else what they show is a simulated 'fly-by' of a relatively still cluster.

But this one seems to show what Chris describes, and more, as it includes the sequestering of large stars at the centre, while small are ejected:
https://www.sciencephoto.com/media/5274 ... -animation

A true depiction?
JOhn

[XgeoX]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Thanks Chris for all your input on this.
I’ve often thought about the same experiment for the galaxy.
It’s great that you actually ran a simulation for this.
Again your posts continue to educated me, than you so much!

Eric

[Chris Peterson]

Thank you, Chris, for the words about "sterile" globular clusters - "learn something every day" - Aristotle?

For Orin, I looked for online animations of the movement of a cluster, but many seem to show an axial rotation, or else what they show is a simulated 'fly-by' of a relatively still cluster.

But this one seems to show what Chris describes, and more, as it includes the sequestering of large stars at the centre, while small are ejected:
https://www.sciencephoto.com/media/5274 ... -animation

A true depiction?
JOhn

Orbital dynamics is a subject that is extremely well understood. Indeed, as much as we can say it about everything, it is completely understood. Modern n-body simulations are essentially perfect models of reality. So yes, it is all but certain that this correctly shows the evolution of a GC.

[Chris Peterson]

I've often thought that the stars must be in orbit in order to keep from bumping into each other; yet how do you get the stars on top and bottom of the globe to orbit without going against the grain?

The stars all orbit around the center point. Not some center axis (which seems to be what your drawing suggests). They have random semimajor axes (but with more stars having small values than large) and random inclinations. Picture the obsolete drawings of atoms with their electrons orbiting at different angles.

They don't often collide because the stars are tiny compared with the distances between them. I worked out a problem many years ago involving this GC. If you started projecting lines through it randomly (like shooting bullets or arrows), you'd have to do so thousands of times before your line intersected a single star. A GC is, by a large factor, mostly empty space.

Thanks Chris for all your input on this.
I’ve often thought about the same experiment for the galaxy.
It’s great that you actually ran a simulation for this.
Again your posts continue to educated me, than you so much!

Eric

Yes, galaxies, too, are mainly empty space. You could fly blindly through the center of a galaxy millions or billions of times and never come very close to a star. We are fooled by the nature of optics, which prevents us from seeing stars at their actual size in relation to their separations from each other. We end up with images, either on our screens or in our eyeballs (through an eyepiece) that seem to show densely packed regions of stars. But it's an illusion created by what are, essentially, massively bloated stars.

[JohnD]

"Flying blindly through the Galaxy" Like Oumuamua, the first extrasolar object detected. But we may already have another one, 2I/Borisov (https://www.nature.com/articles/d41586-019-03530-3 ) and there could be as many as 50 interstellar objects within the Earths orbit ( https://link.springer.com/article/10.11 ... 4621020027 ) Which is a very near miss indeed in Galactic terms!

[Chris Peterson]

"Flying blindly through the Galaxy" Like Oumuamua, the first extrasolar object detected. But we may already have another one, 2I/Borisov (https://www.nature.com/articles/d41586-019-03530-3 ) and there could be as many as 50 interstellar objects within the Earths orbit ( https://link.springer.com/article/10.11 ... 4621020027 ) Which is a very near miss indeed in Galactic terms!

Of course, stellar systems are mainly empty space, too! An alien spacecraft visiting the Sun would have to conduct a very careful survey to determine that we had any planets, and finding the terrestrial ones wouldn't be easy.

(The paper estimates 50 interstellar objects inside a 50 AU radius sphere, not inside Earth's orbit.)

[orin stepanek]

Thank you, Chris, for the words about "sterile" globular clusters - "learn something every day" - Aristotle?

For Orin, I looked for online animations of the movement of a cluster, but many seem to show an axial rotation, or else what they show is a simulated 'fly-by' of a relatively still cluster.

But this one seems to show what Chris describes, and more, as it includes the sequestering of large stars at the centre, while small are ejected:
https://www.sciencephoto.com/media/5274 ... -animation

A true depiction?
JOhn

Thanks John; for the URL & thanks guys for discussing this!

[JohnD]

Of course, stellar systems are mainly empty space, too! An alien spacecraft visiting the Sun would have to conduct a very careful survey to determine that we had any planets, and finding the terrestrial ones wouldn't be easy.
(The paper estimates 50 interstellar objects inside a 50 AU radius sphere, not inside Earth's orbit.)

Ooooooooooops! A tiny error there! 50 AU takes us out to the Oort Cloud, and encloses a much greater volume!

[johnnydeep]

Of course, stellar systems are mainly empty space, too! An alien spacecraft visiting the Sun would have to conduct a very careful survey to determine that we had any planets, and finding the terrestrial ones wouldn't be easy.
(The paper estimates 50 interstellar objects inside a 50 AU radius sphere, not inside Earth's orbit.)

Ooooooooooops! A tiny error there! 50 AU takes us out to the Oort Cloud, and encloses a much greater volume!

Fifty AU is not even double the 30 AU orbit of Neptune, and only a little beyond the 49 AU aphelion of Pluto. The Oort cloud starts much farther out. From https://en.wikipedia.org/wiki/Oort_cloud :

The Oort cloud (/ɔːrt, ʊərt/),[1] sometimes called the Öpik–Oort cloud,[2] first described in 1950 by Dutch astronomer Jan Oort,[3] is a theoretical[4] concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 2,000 to 200,000 au (0.03 to 3.2 light-years).[note 1][5] It is divided into two regions: a disc-shaped inner Oort cloud (or Hills cloud) and a spherical outer Oort cloud. Both regions lie beyond the heliosphere and in interstellar space.[5][6] The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud.

But 50 AU does seem to be at the farther edge of the Kuiper belt. From https://en.wikipedia.org/wiki/Kuiper_belt :

The Kuiper belt (/ˈkaɪpər, ˈkʊɪ-/)[1] is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun.

[VictorBorun]

an intermediate-mass black hole of approximately 40,000 solar masses resides at the center of Omega Centauri

The finding appeared in the April 1 issue of The Astrophysical Journal.

No, seriously?