Starship Asterisk*

Ann wrote: ↑Mon Jun 07, 2021 3:57 am So the inner part of Omega Centauri is so "fluffy" and "loose" that it is not obvious where the actual cluster center is located.

alter-ego wrote: ↑Mon Jun 07, 2021 1:42 am
I don't know how/if each of you estimated any stellar density gradient, or if you assumed a uniform density within a 150ly diameter sphere. For either uniform or increasing centralized gradients, the maximum, integrated line-of-sight density is within a column of stars passing directly through the center of the spherical cluster.
→ For 10-million stars uniformly distributed within a 150-ly sphere, the max central line-of-sight density ≈ 5 stars/arcsec²
→ For 9 million stars with a varying density that yields ω-Cen surface brightness, the peak density could be ≈ 40 to 50 stars/arcsec².
Clearly this projected density is extremely sparse, and assuming constant-sized stars, their angular diameters ≈ 1.1 microarcsec (uas). The total blocked area from 45 stars is only 43 uas²
→ The probability of the Sun colliding with a star while passing through the center ≈ 3x10^-12
So, though the projected density increases by ~10X over the uniform density case, the resultant collision probability is still infinitesimal.

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In case you're curious, I've questioned this Omega Centauri APOD over the years, and looked at this GC in detail to verify the "10 million" star count claim. I couldn't find a paper that explicitly concluded that so I conducted my own analysis mostly using New Limits on IMBH Mas in Omega Centauri_Paper II and Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in omega Centauri
I decided to use the largest published mass estimate for Omega Cen: ~5.1 million M☉ (Meylan, 1995), and considering the first APOD posting the 10-million stars was in 1996, this legacy quote is likely based on Meylan's mass estimate. Following the published analysis technique, and assuming a constant Mass/Luminosity for all the stars such that the GC surface brightness profile replicated:
→ The result is ~9 million stars, and
→ A peak projected line-of-sight density = 44 stars/arcsec² yielding a central stellar volume density ~1200 stars per cubic light year.

Lastly, the high central volume density requires an upward slope in the projected column density starting at ~100 arcseconds radius. In the paper(s), this "subtle" upward slope is driven by an intermediate black hole postulated to exist in Omega Cen. With the upward slope removed, the maximum projected line-of-sight, centrally-flat, column density ~40 stars/arcsec², which drops the maximum central volume density down to ~235 stars/ly³
 

Click to view full size image

A 2008 study presented evidence for an intermediate-mass black hole at the center of Omega Centauri, based on observations made by the Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile.[24][25] Hubble's Advanced Camera for Surveys showed that stars are bunching up near the center of Omega Centauri, as evidenced by the gradual increase in starlight near the center. Using instruments at the Gemini Observatory to measure the speed of stars swirling in the cluster's core, E. Noyola and colleagues found that stars closer to the core are moving faster than stars farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. By comparing these results with standard models, the astronomers concluded that the most likely cause was the gravitational pull of a dense, massive object such as a black hole. They calculated the object's mass at 40,000 solar masses.[24]

However, more recent work has challenged these conclusions, in particular disputing the proposed location of the cluster center.[26] [27] 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.

Chris Peterson wrote: ↑Thu Jun 03, 2021 3:11 pm

neufer wrote: ↑Thu Jun 03, 2021 2:45 pm

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm

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?

johnnydeep wrote: ↑Sat Jun 05, 2021 11:51 am

JohnD wrote: ↑Sat Jun 05, 2021 9:56 am

Chris Peterson wrote: ↑Fri Jun 04, 2021 1:33 pm 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.

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm 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.

JohnD wrote: ↑Sat Jun 05, 2021 9:56 am

Chris Peterson wrote: ↑Fri Jun 04, 2021 1:33 pm 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!

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.

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.

Chris Peterson wrote: ↑Fri Jun 04, 2021 1:33 pm 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.)

JohnD wrote: ↑Fri Jun 04, 2021 10:32 am 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

JohnD wrote: ↑Fri Jun 04, 2021 1:23 pm "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!

XgeoX wrote: ↑Fri Jun 04, 2021 12:16 pm

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm 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

JohnD wrote: ↑Fri Jun 04, 2021 10:32 am 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

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm 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.

https://en.wikipedia.org/wiki/Kapteyn%27s_Star wrote:

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 wrote: ↑Thu Jun 03, 2021 9:24 am 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

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.

shaileshs wrote: ↑Thu Jun 03, 2021 3:32 pm 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 ?

neufer wrote: ↑Thu Jun 03, 2021 2:45 pm

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm
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 wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm 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 wrote: ↑Thu Jun 03, 2021 2:45 pm

Chris Peterson wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm
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 wrote: ↑Thu Jun 03, 2021 1:36 pm

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm
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.

orin stepanek wrote: ↑Thu Jun 03, 2021 12:07 pm 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?