Starship Asterisk*

Ann wrote: ↑Fri May 20, 2022 6:15 pm

johnnydeep wrote: ↑Fri May 20, 2022 4:19 pm

Ann wrote: ↑Fri May 20, 2022 3:51 am Well, stupid me!!!!

I wrote:



The answer is, of course, one. Because a parsec is less than four light-years, and the distance to the nearest star after the Sun is more than four light-years. So there is exactly one star in the cubic parsec centered on the Sun, namely, the Sun itself.

Well, groan. So, if there are 500 times more stars per cubic parsec in globular clusters than in the cubic parsec centered on the Solar system, then there must be 500 x 1 stars = 500 stars per cubic parsec in globular clusters, right?

Except that - no. I'm still thinking of "stars" as if they all had the same mass as the Sun. (Double-dummy, Ann.) Because the Sun isn't an average star at all - it really isn't! - because some 90-95% of the stars in the Solar neighborhood are less massive than the Sun. And globular clusters are going to lack stars more massive than the Sun altogether, apart from a few blue stragglers with, at best, up to a little more than twice the mass of the Sun.

Let's go to Wikipedia and see what it wrote about Omega Centauri!



So, I asked Google. How many parsecs are 150 light-years? Google said about 45.9. I simplified it to 45, because my brain can't take complicated numbers. So how many cubic parsecs do you get from 45 parsecs? I daringly asked Google to multiply 45 x 45 x 45, and Google gave me 91,125. I'm going to simplify that to 90,000 and say that there are 10 million stars in 90,000 cubic parsecs.

So, groan. I asked Google to divide 10,000,000 by 90,000. Google wasn't too interested. So I asked Google to divide 1000 by 9. Google said 111.1111111....

Does that mean that there are about 100 stars or a little more per cubic parsec in Omega Centauri?

It doesn't seem right though, does it? Because Wikipedia said that there are 10 million stars in Omega Centauri, but their total mass is about 4 million solar masses. So the mass of a hundred average Omega Centauri stars per cubic parsec should be only 40 solar masses per cubic parsec. That's a far cry from 500 solar masses per cubic parsec.

Does that mean that I should multiply 40 by 12.5 to get 500? And does it mean I should also multiply 100 by 12.5 to get 1250 stars per cubic parsec in Omega Centauri?

Please help the math idiot!!!

Ann

Just going back to the Wikipedia quote: "Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way."

The simplest way to look at that is to realize it means there are 1e7 stars totaling 4e6 solar masses in the volume of a 150 ly diameter sphere. To calculate the average stellar density throughout the entire volume of that sphere, it's simply 1e7 / 4/3 pi 150*150*150 stars/ly³, or 0.7 stars/ly³. And since there are 3.26 ly/pc, that's 0.7 * 3.26³ = 24 stars/pc³. And again, that's just the average. The density would be much higher toward the center and much lower toward the periphery.

And since those 1e7 stars total 4e6 solar masses, the average star would be 4e6 / 1e7 = 0.4 solar masses. So, there are more small stars than large, which comports well with dwarf stars being the most common. Of course, this also ignores any dust, gas or dark matter which might also be contributing to the total mass of the cluster. But perhaps clusters have a paucity of all three?

Thanks, Johnny, I admire your math.

I really think that almost all Milky Way globulars are practically gas- and dust-free. That is because they are so old, and they were so unbelievably energetic when they were young, that the monstrous O-type and Wolf Rayet stars that these globulars once contained blew all the gas and dust clear out of the globulars.


Let's compare two massive clusters, Westerlund 1 whose estimated age is 4 - 5 million years, and Westerlund 2, whose age is 1 - 2 million years.

Star cluster Westerlund 1 is home to some of the largest and most massive stars known. It is headlined by the star Westerlund 1-26, a red supergiant star so big that if placed in the center of our Solar System, it would extend out past the orbit of Jupiter. Additionally, the young star cluster is home to 3 other red supergiants, 6 yellow hypergiant stars, 24 Wolf-Rayet stars, and several even-more unusual stars that continue to be studied. Image Credit: ESA/Hubble & NASA.

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The star cluster Westerlund 2 in the Milky Way galaxy, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known. Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team.

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Westerlund 1 and Westerlund 2 are two very young and very massive clusters. Westerlund 1 appears to be the most massive of them, or as Wikipedia wrote:

Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster about 2.6 kpc (8480 ly) away from Earth. It is one of the most massive young star clusters in the Milky Way and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster.

When it comes to Westerlund 2, Wikipedia doesn't stress its total cluster mass but rather the high mass and temperatures of some of its member stars:

Westerlund 2 is an obscured compact young star cluster (perhaps even a super star cluster) in the Milky Way, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known.

Do note that there is no apparent nebulosity in or near Westerlund 1. The massive cluster has undoubtedly blown the gas and dust away.

The situation is quite different for the very young cluster Westerlund 2. Much gas remains all around the cluster:

Westerlund 2 surrounded by stellar nursery RCW 49. Image credit: NASA.

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But as you can see, even though there is a lot of gas and dust surrounding Westerlund 2, the cluster has cleared a cavity around itself, where there is a lot less gas and dust than in its surroundings.

My point is that there is no appreciable amount of gas or dust in the age-old globular clusters, and whatever gas and dust might be present there doesn't contribute significantly to the mass of the cluster.

Ann

johnnydeep wrote: ↑Fri May 20, 2022 4:19 pm

Ann wrote: ↑Fri May 20, 2022 3:51 am Well, stupid me!!!!

I wrote:

(Or, alternatively, does anyone know how many stars there is in the cubic parsec centered on the Sun?)

The answer is, of course, one. Because a parsec is less than four light-years, and the distance to the nearest star after the Sun is more than four light-years. So there is exactly one star in the cubic parsec centered on the Sun, namely, the Sun itself.

Well, groan. So, if there are 500 times more stars per cubic parsec in globular clusters than in the cubic parsec centered on the Solar system, then there must be 500 x 1 stars = 500 stars per cubic parsec in globular clusters, right?

Except that - no. I'm still thinking of "stars" as if they all had the same mass as the Sun. (Double-dummy, Ann.) Because the Sun isn't an average star at all - it really isn't! - because some 90-95% of the stars in the Solar neighborhood are less massive than the Sun. And globular clusters are going to lack stars more massive than the Sun altogether, apart from a few blue stragglers with, at best, up to a little more than twice the mass of the Sun.

Let's go to Wikipedia and see what it wrote about Omega Centauri!

Wikipedia wrote:

Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way.

So, I asked Google. How many parsecs are 150 light-years? Google said about 45.9. I simplified it to 45, because my brain can't take complicated numbers. So how many cubic parsecs do you get from 45 parsecs? I daringly asked Google to multiply 45 x 45 x 45, and Google gave me 91,125. I'm going to simplify that to 90,000 and say that there are 10 million stars in 90,000 cubic parsecs.

So, groan. I asked Google to divide 10,000,000 by 90,000. Google wasn't too interested. So I asked Google to divide 1000 by 9. Google said 111.1111111....

Does that mean that there are about 100 stars or a little more per cubic parsec in Omega Centauri?

It doesn't seem right though, does it? Because Wikipedia said that there are 10 million stars in Omega Centauri, but their total mass is about 4 million solar masses. So the mass of a hundred average Omega Centauri stars per cubic parsec should be only 40 solar masses per cubic parsec. That's a far cry from 500 solar masses per cubic parsec.

Does that mean that I should multiply 40 by 12.5 to get 500? And does it mean I should also multiply 100 by 12.5 to get 1250 stars per cubic parsec in Omega Centauri?

Please help the math idiot!!!

Ann

Just going back to the Wikipedia quote: "Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way."

The simplest way to look at that is to realize it means there are 1e7 stars totaling 4e6 solar masses in the volume of a 150 ly diameter sphere. To calculate the average stellar density throughout the entire volume of that sphere, it's simply 1e7 / 4/3 pi 150*150*150 stars/ly³, or 0.7 stars/ly³. And since there are 3.26 ly/pc, that's 0.7 * 3.26³ = 24 stars/pc³. And again, that's just the average. The density would be much higher toward the center and much lower toward the periphery.

And since those 1e7 stars total 4e6 solar masses, the average star would be 4e6 / 1e7 = 0.4 solar masses. So, there are more small stars than large, which comports well with dwarf stars being the most common. Of course, this also ignores any dust, gas or dark matter which might also be contributing to the total mass of the cluster. But perhaps clusters have a paucity of all three?

Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster about 2.6 kpc (8480 ly) away from Earth. It is one of the most massive young star clusters in the Milky Way and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster.

Westerlund 2 is an obscured compact young star cluster (perhaps even a super star cluster) in the Milky Way, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known.

Ann wrote: ↑Fri May 20, 2022 3:51 am Well, stupid me!!!!

I wrote:

(Or, alternatively, does anyone know how many stars there is in the cubic parsec centered on the Sun?)

The answer is, of course, one. Because a parsec is less than four light-years, and the distance to the nearest star after the Sun is more than four light-years. So there is exactly one star in the cubic parsec centered on the Sun, namely, the Sun itself.

Well, groan. So, if there are 500 times more stars per cubic parsec in globular clusters than in the cubic parsec centered on the Solar system, then there must be 500 x 1 stars = 500 stars per cubic parsec in globular clusters, right?

Except that - no. I'm still thinking of "stars" as if they all had the same mass as the Sun. (Double-dummy, Ann.) Because the Sun isn't an average star at all - it really isn't! - because some 90-95% of the stars in the Solar neighborhood are less massive than the Sun. And globular clusters are going to lack stars more massive than the Sun altogether, apart from a few blue stragglers with, at best, up to a little more than twice the mass of the Sun.

Let's go to Wikipedia and see what it wrote about Omega Centauri!

Wikipedia wrote:

Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way.

So, I asked Google. How many parsecs are 150 light-years? Google said about 45.9. I simplified it to 45, because my brain can't take complicated numbers. So how many cubic parsecs do you get from 45 parsecs? I daringly asked Google to multiply 45 x 45 x 45, and Google gave me 91,125. I'm going to simplify that to 90,000 and say that there are 10 million stars in 90,000 cubic parsecs.

So, groan. I asked Google to divide 10,000,000 by 90,000. Google wasn't too interested. So I asked Google to divide 1000 by 9. Google said 111.1111111....

Does that mean that there are about 100 stars or a little more per cubic parsec in Omega Centauri?

It doesn't seem right though, does it? Because Wikipedia said that there are 10 million stars in Omega Centauri, but their total mass is about 4 million solar masses. So the mass of a hundred average Omega Centauri stars per cubic parsec should be only 40 solar masses per cubic parsec. That's a far cry from 500 solar masses per cubic parsec.

Does that mean that I should multiply 40 by 12.5 to get 500? And does it mean I should also multiply 100 by 12.5 to get 1250 stars per cubic parsec in Omega Centauri?

Please help the math idiot!!!

Ann

Ann wrote: ↑Fri May 20, 2022 3:51 am Well, stupid me!!!!

I wrote:

(Or, alternatively, does anyone know how many stars there is in the cubic parsec centered on the Sun?)

The answer is, of course, one. Because a parsec is less than four light-years, and the distance to the nearest star after the Sun is more than four light-years. So there is exactly one star in the cubic parsec centered on the Sun, namely, the Sun itself.

Well, groan. So, if there are 500 times more stars per cubic parsec in globular clusters than in the cubic parsec centered on the Solar system, then there must be 500 x 1 stars = 500 stars per cubic parsec in globular clusters, right?

Ann

(Or, alternatively, does anyone know how many stars there is in the cubic parsec centered on the Sun?)

Wikipedia wrote:

Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, and a total mass equivalent to 4 million solar masses, making it the most massive-known globular cluster in the Milky Way.

Ann wrote: ↑Thu May 19, 2022 5:59 am

johnnydeep wrote: ↑Wed May 18, 2022 8:04 pm

Ann wrote: ↑Wed May 18, 2022 5:51 pm

Thanks a bunch!

Galactic center stellar cluster from Near and mid infrared observations of the galactic center.png

´
The galactic center is a crowded place! Indeed.

As David Bowman said in the movie 2001: A Space Odyssey:

My God, it's full of stars!

Ann

So now let's compare the density of stars in this pic with that of a globular cluster. In this pic, I'd guesstimate there are at least 10*10*10 stars in the 5x5x5 arcsec volume around the BH at the center plus mark. That would be a density of 1000 / 0.2³ stars/pc³, or 1 star per 0.000008 pc³

Per Wikipedia,

https://en.wikipedia.org/wiki/Stellar_density wrote:The locations within the Milky Way that have the highest stellar density are the central core and the interior of globular clusters. A typical mass density for a globular cluster is 70 MSun pc⁻³, which is 500 times the mass density near the Sun.[2] In the solar neighborhood, the stellar density of a star cluster must be greater than 0.08 MSun pc⁻³ in order to avoid tidal disruption[3]

That would mean the stellar density in the vicinity of the BH (excluding the BH itself!) is much higher than that at the center of GCs, namely, a density of 125000 stars per pc³ around the BH! No? Unless I got my math screwed up...(which I admit to not being entirely confident in).

Johnny, if your math can sometimes screw up, mine is non-existent.

I get, more or less, the expression 1 star per 0.000008 pc³. If nothing else, I certainly know how to ask Google to explain exactly what it means. So, for example, how many stars would that be per cubic light-year? Google should be able to tell me.

But I don't get this: A typical mass density for a globular cluster is 70 MSun pc⁻³, which is 500 times the mass density near the Sun. Yeah? This just talks about stellar mass, not about the number of stars.

Is there a way to figure out how many stars per cubic parsec, on average, it would take to create a 70 MSun pc⁻³ situation in a globular cluster?

(Or, alternatively, does anyone know how many stars there is in the cubic parsec centered on the Sun?)

We should keep a few things in mind here. First of all, stars in globular clusters are typically low in mass. With the exception of a handful of blue stragglers, my guess is that the most massive members of globular clusters are about as massive as the Sun. On the other hand, there may be a bit of a shortage of very low-mass red dwarfs, because the lowest-mass stars are the ones that are most likely to be kicked out of the globular cluster due to tidal interactions. So what is the average mass of a stellar member of a globular cluster? How about, say, 0.4 MSun?

So... if every star in a globular cluster contains a mass of 0.4 MSun, then there would be 175 stars in every cubic parsec of a globular cluster. But the stars inside a globular vary in mass, of course. And even if there is a (relative) scarcity of the most light-weight stars inside globulars, the light-weight red dwarfs are still very likely to be the most common denizens.

Closeup of globular cluster M4. Image: ESA/NASA and Hubble.

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Color-magnitude diagram of globular cluster M55. The smallest, reddest, most light-weight stars (lower right) are the most common. Illustration: B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope

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Go to this page to see a 13 MB Hubble closeup of globular cluster M4. Try to make an assessment of how many tiny little red dots you can spot among the brighter stars. There aren't that many, in my opinion, but there certainly are a number of them. Could there even be quite a few little red dwarfs that are too faint to show up even in this Hubble closeup?

There are also a few tiny blue dots in the globular cluster, which are white dwarfs.

The mass of the faintest red dwarfs in the Hubble image might be, I guess, around 0.3 or 0.2 solar masses. The mass of a typical white dwarf might be, say, 0.6-0.7 solar masses or so. The mass of the multitude of white-looking stars would be, I guess, a bit less than the mass of the Sun, because these stars are metal-poor, and therefore they shine whiter and brighter than a metal-rich star of the same mass. Let's say that the mass of the white-looking stars are some 0.9 solar masses, or in some cases a little more. Most of the evolved, bright-looking stars are probably also less massive than the Sun, because they have lost mass during their evolution. Let's say that their mass is perhaps 0.6-1.1 solar masses.

So, what do you think? How many stars will there be in a cubic parsec of a globular cluster, if the stellar mass inside it is 70 MSun? Okay, let's define "stars" as objects that sustain themselves through some kind of fusion, but let's also count white dwarfs as stars. But let's forget about the brown dwarfs, even if they can, at least in theory, contribute a non-negligent amount of mass to the globular.

Ann

johnnydeep wrote: ↑Wed May 18, 2022 8:04 pm

Ann wrote: ↑Wed May 18, 2022 5:51 pm

Fred the Cat wrote: ↑Wed May 18, 2022 4:12 pm
Give the credit to Ann. She asks interesting questions that I get a gut reaction to look for answers.

Thanks a bunch!

Galactic center stellar cluster from Near and mid infrared observations of the galactic center.png

NADEEN BASSAM IZZAT SABHA wrote:
Figure 1.5: L-band (3.8 µm) mosaic of the Galactic Center stellar cluster obtained with VLT NaCo in
2012. Most sources are identified based on Viehmann et al. (2005). One arcsec translates to ∼0.04 pc
for an 8 kpc distance to the GC. The position of the supermassive black hole Sagittarius A* (Sgr A*) is
marked by a cross. North is up and east is to the left.

´
The galactic center is a crowded place! Indeed.

As David Bowman said in the movie 2001: A Space Odyssey:

My God, it's full of stars!

Ann

So now let's compare the density of stars in this pic with that of a globular cluster. In this pic, I'd guesstimate there are at least 10*10*10 stars in the 5x5x5 arcsec volume around the BH at the center plus mark. That would be a density of 1000 / 0.2³ stars/pc³, or 1 star per 0.000008 pc³

Per Wikipedia,

https://en.wikipedia.org/wiki/Stellar_density wrote:The locations within the Milky Way that have the highest stellar density are the central core and the interior of globular clusters. A typical mass density for a globular cluster is 70 MSun pc⁻³, which is 500 times the mass density near the Sun.[2] In the solar neighborhood, the stellar density of a star cluster must be greater than 0.08 MSun pc⁻³ in order to avoid tidal disruption[3]

That would mean the stellar density in the vicinity of the BH (excluding the BH itself!) is much higher than that at the center of GCs, namely, a density of 125000 stars per pc³ around the BH! No? Unless I got my math screwed up...(which I admit to not being entirely confident in).

Ann wrote: ↑Wed May 18, 2022 5:51 pm

Fred the Cat wrote: ↑Wed May 18, 2022 4:12 pm

johnnydeep wrote: ↑Wed May 18, 2022 3:22 pm

How DO you manage to find these interesting papers? This one begins with a very nice well-written summary and description of the Milky Way. Thanks for the link! So, the upshot of this picture (figure 1.5 in the thesis) is that there are many stars - perhaps over 100? - within 5 arcsecs of the central BH, putting them within 0.2 pc (5 * 0.04), or 0.65 ly. Pretty crowded down there!

Give the credit to Ann. She asks interesting questions that I get a gut reaction to look for answers.

Thanks a bunch!

Galactic center stellar cluster from Near and mid infrared observations of the galactic center.png

NADEEN BASSAM IZZAT SABHA wrote:
Figure 1.5: L-band (3.8 µm) mosaic of the Galactic Center stellar cluster obtained with VLT NaCo in
2012. Most sources are identified based on Viehmann et al. (2005). One arcsec translates to ∼0.04 pc
for an 8 kpc distance to the GC. The position of the supermassive black hole Sagittarius A* (Sgr A*) is
marked by a cross. North is up and east is to the left.

´
The galactic center is a crowded place! Indeed.

As David Bowman said in the movie 2001: A Space Odyssey:

My God, it's full of stars!

Ann

https://en.wikipedia.org/wiki/Stellar_density wrote:The locations within the Milky Way that have the highest stellar density are the central core and the interior of globular clusters. A typical mass density for a globular cluster is 70 MSun pc⁻³, which is 500 times the mass density near the Sun.[2] In the solar neighborhood, the stellar density of a star cluster must be greater than 0.08 MSun pc⁻³ in order to avoid tidal disruption.[3]

Fred the Cat wrote: ↑Wed May 18, 2022 4:12 pm

johnnydeep wrote: ↑Wed May 18, 2022 3:22 pm

Fred the Cat wrote: ↑Wed May 18, 2022 2:56 pm
Just how many seems not to be easily digested but one could try to assimilate this thesis.

In it, one image does illuminate his studies.
Galactic Center (2).png

How DO you manage to find these interesting papers? This one begins with a very nice well-written summary and description of the Milky Way. Thanks for the link! So, the upshot of this picture (figure 1.5 in the thesis) is that there are many stars - perhaps over 100? - within 5 arcsecs of the central BH, putting them within 0.2 pc (5 * 0.04), or 0.65 ly. Pretty crowded down there!

Give the credit to Ann. She asks interesting questions that I get a gut reaction to look for answers.

NADEEN BASSAM IZZAT SABHA wrote:
Figure 1.5: L-band (3.8 µm) mosaic of the Galactic Center stellar cluster obtained with VLT NaCo in
2012. Most sources are identified based on Viehmann et al. (2005). One arcsec translates to ∼0.04 pc
for an 8 kpc distance to the GC. The position of the supermassive black hole Sagittarius A* (Sgr A*) is
marked by a cross. North is up and east is to the left.

johnnydeep wrote: ↑Wed May 18, 2022 3:22 pm

Fred the Cat wrote: ↑Wed May 18, 2022 2:56 pm

Chris Peterson wrote: ↑Wed May 18, 2022 1:54 pm


I'm sure there are many stars within a few light years of the black hole! I haven't suggested otherwise.

Just how many seems not to be easily digested but one could try to assimilate this thesis.

In it, one image does illuminate his studies.
Galactic Center (2).png

How DO you manage to find these interesting papers? This one begins with a very nice well-written summary and description of the Milky Way. Thanks for the link! So, the upshot of this picture (figure 1.5 in the thesis) is that there are many stars - perhaps over 100? - within 5 arcsecs of the central BH, putting them within 0.2 pc (5 * 0.04), or 0.65 ly. Pretty crowded down there!

Fred the Cat wrote: ↑Wed May 18, 2022 2:56 pm

Chris Peterson wrote: ↑Wed May 18, 2022 1:54 pm

Ann wrote: ↑Wed May 18, 2022 1:51 pm

So why shouldn't we assume that the stellar density is also very high in the core of the Milky Way, so that there may be many stars just a few - say, 1-7 light-years - from the black hole?

Ann

I'm sure there are many stars within a few light years of the black hole! I haven't suggested otherwise.

Just how many seems not to be easily digested but one could try to assimilate this thesis.

In it, one image does illuminate his studies.
Galactic Center (2).png

Chris Peterson wrote: ↑Wed May 18, 2022 1:54 pm

Ann wrote: ↑Wed May 18, 2022 1:51 pm

Chris Peterson wrote: ↑Wed May 18, 2022 1:19 pm

Because the stellar density in the cores is very high. Much higher than in the bulge in many cases.

So why shouldn't we assume that the stellar density is also very high in the core of the Milky Way, so that there may be many stars just a few - say, 1-7 light-years - from the black hole?

Ann

I'm sure there are many stars within a few light years of the black hole! I haven't suggested otherwise.

Ann wrote: ↑Wed May 18, 2022 1:51 pm

Chris Peterson wrote: ↑Wed May 18, 2022 1:19 pm

Ann wrote: ↑Wed May 18, 2022 5:32 am I know that this discussion should be over now, but there is a question that you haven't answered.

Why is it that so many galaxies are so bright in their cores?

Because the stellar density in the cores is very high. Much higher than in the bulge in many cases.

So why shouldn't we assume that the stellar density is also very high in the core of the Milky Way, so that there may be many stars just a few - say, 1-7 light-years - from the black hole?

Ann

Chris Peterson wrote: ↑Wed May 18, 2022 1:19 pm

Ann wrote: ↑Wed May 18, 2022 5:32 am I know that this discussion should be over now, but there is a question that you haven't answered.

Why is it that so many galaxies are so bright in their cores?

Because the stellar density in the cores is very high. Much higher than in the bulge in many cases.

Ann wrote: ↑Wed May 18, 2022 5:32 am I know that this discussion should be over now, but there is a question that you haven't answered.

Why is it that so many galaxies are so bright in their cores?

Ann wrote: ↑Wed May 18, 2022 5:32 am

Chris Peterson wrote: ↑Sat May 14, 2022 5:52 pm

Ann wrote: ↑Sat May 14, 2022 5:37 pm

But Omega Centauri is believed to be the core of a disrupted dwarf galaxy. Why would it be denser than the inner core of the Milky Way, the region near Sgr A*?

And why wouldn't the bulge of our galaxy have a stellar gradient, so that it is denser near the central black hole?

Consider NGC 1512:

NGC 1512 is a barred spiral galaxy whose core is surrounded by a 2,400 light-year-wide circle of infant star clusters, called a circumnuclear starburst ring. Authors: D. Maoz (Tel-Aviv University/Columbia University), A. J. Barth (Harvard CfA), L. C. Ho (Carnegie Obs.), A. Sternberg (Tel-Aviv University and A. V. Filippenko (UC Berkeley).

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As you can see, the nuclear region is much brighter than the extended yellow region that might be compared with a galactic bulge. Why is the nucleus so bright? It could be because the central black hole is active and is surrounded by a brilliantly luminous accretion disk, but I have seen nothing to suggest that NGC 1512 has an active nucleus. Therefore, if the black hole isn't active and isn't surrounded by a luminous accretion disk, the most likely explanation is that the stellar density increases sharply near the black hole.

Also check out pictures of M51, M61, M94, NGC 488 and in the brightest galaxy of the interacting pair known as NGC 6050. The increased brightness at very core is not so obvious in their cases as it is in NGC 1512, but you can still see it. I don't think that any of these galaxies has an active core. Therefore, the increased brightness in the nuclear region right next to the central black hole is most likely caused by an increased stellar density.

Do we know that the stellar density doesn't increase (a lot) near the central black hole of our own galaxy?

Ann

The density gradient is not very strong in galactic cores or in globular clusters. This is related to there being mass both inside and outside most stars orbits. I have no problem with the fossil core of a tidally disrupted galaxy having a higher density than the original core. A lot went on to rip that away.

I know that this discussion should be over now, but there is a question that you haven't answered.

Galaxy M91. Image credit: ESA/Hubble & NASA, J. Lee and the PHANGS-HST Team.

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Why is it that so many galaxies are so bright in their cores?

Yes, I realize that some galaxies have active nuclei, and some have an accretion disks that can become extremely luminous. But in the latter cases we are talking about quasars.

The caption on the image of M91 doesn't say anything about this galaxy having an active core. And if it doesn't have an active core, surely it doesn't have a brilliantly bright central accretion disk either?

So why is its core so bright?

And why do so many other galaxies also have very bright cores?

Isn't it because there are so many stars crowded together in the core region? So that the star density is many, many times higher in the small core than in the larger bulge or bar region?

I'd like to hear a comment from you.

Ann

Chris Peterson wrote: ↑Sat May 14, 2022 5:52 pm

Ann wrote: ↑Sat May 14, 2022 5:37 pm

Chris Peterson wrote: ↑Sat May 14, 2022 1:50 pm
Not a good comparison.

We are looking along a pyramid of space that extends to somewhere beyond 27,000 ly, and which has an intermediate base that is 49 sq ly at that distance. We are not looking at a cluster, but through a density gradient that is about 10x our local density where we cross an arm, and about 100x our local density through the galaxy's central bulge, which includes thousands of light years on either side of the central black hole.


The core of our galaxy is estimated to be about 100 times the stellar density of our local region. Globular clusters are much denser than galactic bulges.

But Omega Centauri is believed to be the core of a disrupted dwarf galaxy. Why would it be denser than the inner core of the Milky Way, the region near Sgr A*?

And why wouldn't the bulge of our galaxy have a stellar gradient, so that it is denser near the central black hole?

Consider NGC 1512:

NGC 1512 is a barred spiral galaxy whose core is surrounded by a 2,400 light-year-wide circle of infant star clusters, called a circumnuclear starburst ring. Authors: D. Maoz (Tel-Aviv University/Columbia University), A. J. Barth (Harvard CfA), L. C. Ho (Carnegie Obs.), A. Sternberg (Tel-Aviv University and A. V. Filippenko (UC Berkeley).

---

As you can see, the nuclear region is much brighter than the extended yellow region that might be compared with a galactic bulge. Why is the nucleus so bright? It could be because the central black hole is active and is surrounded by a brilliantly luminous accretion disk, but I have seen nothing to suggest that NGC 1512 has an active nucleus. Therefore, if the black hole isn't active and isn't surrounded by a luminous accretion disk, the most likely explanation is that the stellar density increases sharply near the black hole.

Also check out pictures of M51, M61, M94, NGC 488 and in the brightest galaxy of the interacting pair known as NGC 6050. The increased brightness at very core is not so obvious in their cases as it is in NGC 1512, but you can still see it. I don't think that any of these galaxies has an active core. Therefore, the increased brightness in the nuclear region right next to the central black hole is most likely caused by an increased stellar density.

Do we know that the stellar density doesn't increase (a lot) near the central black hole of our own galaxy?

Ann

The density gradient is not very strong in galactic cores or in globular clusters. This is related to there being mass both inside and outside most stars orbits. I have no problem with the fossil core of a tidally disrupted galaxy having a higher density than the original core. A lot went on to rip that away.

Ann wrote: ↑Sat May 14, 2022 5:37 pm

Chris Peterson wrote: ↑Sat May 14, 2022 1:50 pm

Ann wrote: ↑Sat May 14, 2022 5:48 am Wait! I just changed my mind!





How can a panel that is 7 light-years across correspond to a volume on the order of a million cubic light-years? All right, you are saying that we are looking at the black hole through an at least 27,000 light-year long "tube" (or rather through a telescope's field of view), so that we are seeing stars all along the length of that "tube of a field of view", stars that are are perhaps thousands of light-years in front of the black hole, and - perhaps? - also stars that are thousands of light-years behind the black hole, as seen from our perspective.

But consider Omega Centauri:

Not a good comparison.

We are looking along a pyramid of space that extends to somewhere beyond 27,000 ly, and which has an intermediate base that is 49 sq ly at that distance. We are not looking at a cluster, but through a density gradient that is about 10x our local density where we cross an arm, and about 100x our local density through the galaxy's central bulge, which includes thousands of light years on either side of the central black hole.

Omega Centauri is believed to be the core of a disrupted dwarf galaxy. Why should the core of a a disrupted dwarf galaxy be more densely populated than the core of a large spiral galaxy like the Milky Way?

The core of our galaxy is estimated to be about 100 times the stellar density of our local region. Globular clusters are much denser than galactic bulges.

But Omega Centauri is believed to be the core of a disrupted dwarf galaxy. Why would it be denser than the inner core of the Milky Way, the region near Sgr A*?

And why wouldn't the bulge of our galaxy have a stellar gradient, so that it is denser near the central black hole?

Consider NGC 1512:

NGC 1512 is a barred spiral galaxy whose core is surrounded by a 2,400 light-year-wide circle of infant star clusters, called a circumnuclear starburst ring. Authors: D. Maoz (Tel-Aviv University/Columbia University), A. J. Barth (Harvard CfA), L. C. Ho (Carnegie Obs.), A. Sternberg (Tel-Aviv University and A. V. Filippenko (UC Berkeley).

---

As you can see, the nuclear region is much brighter than the extended yellow region that might be compared with a galactic bulge. Why is the nucleus so bright? It could be because the central black hole is active and is surrounded by a brilliantly luminous accretion disk, but I have seen nothing to suggest that NGC 1512 has an active nucleus. Therefore, if the black hole isn't active and isn't surrounded by a luminous accretion disk, the most likely explanation is that the stellar density increases sharply near the black hole.

Also check out pictures of M51, M61, M94, NGC 488 and in the brightest galaxy of the interacting pair known as NGC 6050. The increased brightness at very core is not so obvious in their cases as it is in NGC 1512, but you can still see it. I don't think that any of these galaxies has an active core. Therefore, the increased brightness in the nuclear region right next to the central black hole is most likely caused by an increased stellar density.

Do we know that the stellar density doesn't increase (a lot) near the central black hole of our own galaxy?

Ann

Chris Peterson wrote: ↑Sat May 14, 2022 1:50 pm

Ann wrote: ↑Sat May 14, 2022 5:48 am Wait! I just changed my mind!

APOD Robot wrote:
the main panel is about 7-light years across

Chris wrote:
Keep in mind that we're seeing a volume of space here on the order of a million cubic light years

How can a panel that is 7 light-years across correspond to a volume on the order of a million cubic light-years? All right, you are saying that we are looking at the black hole through an at least 27,000 light-year long "tube" (or rather through a telescope's field of view), so that we are seeing stars all along the length of that "tube of a field of view", stars that are are perhaps thousands of light-years in front of the black hole, and - perhaps? - also stars that are thousands of light-years behind the black hole, as seen from our perspective.

But consider Omega Centauri:

Not a good comparison.

We are looking along a pyramid of space that extends to somewhere beyond 27,000 ly, and which has an intermediate base that is 49 sq ly at that distance. We are not looking at a cluster, but through a density gradient that is about 10x our local density where we cross an arm, and about 100x our local density through the galaxy's central bulge, which includes thousands of light years on either side of the central black hole.

Omega Centauri is believed to be the core of a disrupted dwarf galaxy. Why should the core of a a disrupted dwarf galaxy be more densely populated than the core of a large spiral galaxy like the Milky Way?

The core of our galaxy is estimated to be about 100 times the stellar density of our local region. Globular clusters are much denser than galactic bulges.

Chris Peterson wrote: ↑Sat May 14, 2022 3:56 pm

johnnydeep wrote: ↑Sat May 14, 2022 3:40 pm

Chris Peterson wrote: ↑Sat May 14, 2022 3:13 pm

Actually, I was just laughing at the absurdity of the question! And it is, indeed, funny. Obviously every star is either in the foreground or the background!

You mean purely photographically speaking? But there's also "middleground"!

https://www.photographyaxis.com/photography-articles/foreground-middleground-and-background-photography/ wrote: What is Foreground?
The Foreground is the section of the image which lies closer to the viewer’s or photographer’s eye. It is generally located in the bottom of the frame (not always).

What is Background?
The Background is generally the top section of an image in the case of Landscape photography (but not always). When it comes to portrait photography, the Background is usually the out of focus area behind the subject/person in the frame.

What is Middleground?
Middleground comes into the picture in the case of landscape photography. For portrait images, only the foreground and background are the essential sections.

Middleground is the area lying between the foreground and the background. As the name suggests, it lies in the center of the frame.

But now I'm being silly.

Of course, the middleground is in the foreground of the background, and in the background of the foreground!

johnnydeep wrote: ↑Sat May 14, 2022 3:40 pm

Chris Peterson wrote: ↑Sat May 14, 2022 3:13 pm

johnnydeep wrote: ↑Sat May 14, 2022 3:09 pm

True, as long as it is unlikely (or impossible) for any of those stars that look like they could be less than about 1/4 ly from the BH to be orbiting it. And I would consider any stars actually orbiting it to be at the same distance, and not in the foreground or background as those terms are typically used!

[ I think this whole discussion only became prolonged because you said categorically far above that all stars in the image were either in the foreground or background (implying there were none at the "same" distance), and we didn't understand that you were apparently defining "same" to be within some hard to measure (and also fairly meaningless in the context of stellar sizes) granularity. ]

Actually, I was just laughing at the absurdity of the question! And it is, indeed, funny. Obviously every star is either in the foreground or the background!

You mean purely photographically speaking? But there's also "middleground"!

https://www.photographyaxis.com/photography-articles/foreground-middleground-and-background-photography/ wrote: What is Foreground?
The Foreground is the section of the image which lies closer to the viewer’s or photographer’s eye. It is generally located in the bottom of the frame (not always).

What is Background?
The Background is generally the top section of an image in the case of Landscape photography (but not always). When it comes to portrait photography, the Background is usually the out of focus area behind the subject/person in the frame.

What is Middleground?
Middleground comes into the picture in the case of landscape photography. For portrait images, only the foreground and background are the essential sections.

Middleground is the area lying between the foreground and the background. As the name suggests, it lies in the center of the frame.

But now I'm being silly.

Chris Peterson wrote: ↑Sat May 14, 2022 3:13 pm

johnnydeep wrote: ↑Sat May 14, 2022 3:09 pm

Chris Peterson wrote: ↑Sat May 14, 2022 2:37 pm
Which is exactly why I asked (and received no answer) what was meant by "the same distance", and what the relevance of that number might be.


Nothing at all. But so what? We already know there is a distribution of stars covering thousands of light years, which means some are not far from the BH. More interesting, IMO, would be the case where that has some dynamic meaning, as with stars that are orbiting the BH. And I don't think we see any in this image.

True, as long as it is unlikely (or impossible) for any of those stars that look like they could be less than about 1/4 ly from the BH to be orbiting it. And I would consider any stars actually orbiting it to be at the same distance, and not in the foreground or background as those terms are typically used!

[ I think this whole discussion only became prolonged because you said categorically far above that all stars in the image were either in the foreground or background (implying there were none at the "same" distance), and we didn't understand that you were apparently defining "same" to be within some hard to measure (and also fairly meaningless in the context of stellar sizes) granularity. ]

Actually, I was just laughing at the absurdity of the question! And it is, indeed, funny. Obviously every star is either in the foreground or the background!

https://www.photographyaxis.com/photography-articles/foreground-middleground-and-background-photography/ wrote: What is Foreground?
The Foreground is the section of the image which lies closer to the viewer’s or photographer’s eye. It is generally located in the bottom of the frame (not always).

What is Background?
The Background is generally the top section of an image in the case of Landscape photography (but not always). When it comes to portrait photography, the Background is usually the out of focus area behind the subject/person in the frame.

What is Middleground?
Middleground comes into the picture in the case of landscape photography. For portrait images, only the foreground and background are the essential sections.

Middleground is the area lying between the foreground and the background. As the name suggests, it lies in the center of the frame.

johnnydeep wrote: ↑Sat May 14, 2022 3:09 pm

Chris Peterson wrote: ↑Sat May 14, 2022 2:37 pm

johnnydeep wrote: ↑Sat May 14, 2022 2:22 pm

Alright, if "same distance" means identical down to a nanometer (or smaller), then purely as a matter of probability, almost certainly, no two stars are at the same distance from us. Is that what you are driving at here? But of course, even if so, using a distance granularity smaller than, say, an AU is not really meaningful when distances from us are measured in light years (1 LY = 63241 AU).

Which is exactly why I asked (and received no answer) what was meant by "the same distance", and what the relevance of that number might be.

But let me ask the question with respect to a particular star in this 7 ly wide image:

center of MW with black hole shadow pic.JPG

What argument says that the star being pointed at can't possibly be at the same distance from us as the BH, where "same distance" is defined to be within 1 LY.

Nothing at all. But so what? We already know there is a distribution of stars covering thousands of light years, which means some are not far from the BH. More interesting, IMO, would be the case where that has some dynamic meaning, as with stars that are orbiting the BH. And I don't think we see any in this image.

True, as long as it is unlikely (or impossible) for any of those stars that look like they could be less than about 1/4 ly from the BH to be orbiting it. And I would consider any stars actually orbiting it to be at the same distance, and not in the foreground or background as those terms are typically used!

[ I think this whole discussion only became prolonged because you said categorically far above that all stars in the image were either in the foreground or background (implying there were none at the "same" distance), and we didn't understand that you were apparently defining "same" to be within some hard to measure (and also fairly meaningless in the context of stellar sizes) granularity. ]

Chris Peterson wrote: ↑Sat May 14, 2022 2:37 pm

johnnydeep wrote: ↑Sat May 14, 2022 2:22 pm

Chris Peterson wrote: ↑Sat May 14, 2022 1:52 pm
I still don't understand where you are getting the idea that I'm suggesting any sort of star-free zone around the central BH. I'm only pointing out the impossibility of any stars being at the same distance. Every star we see has to be either closer or farther. No two stars in the entire universe are the same distance away from us!

Alright, if "same distance" means identical down to a nanometer (or smaller), then purely as a matter of probability, almost certainly, no two stars are at the same distance from us. Is that what you are driving at here? But of course, even if so, using a distance granularity smaller than, say, an AU is not really meaningful when distances from us are measured in light years (1 LY = 63241 AU).

Which is exactly why I asked (and received no answer) what was meant by "the same distance", and what the relevance of that number might be.

But let me ask the question with respect to a particular star in this 7 ly wide image:

center of MW with black hole shadow pic.JPG

What argument says that the star being pointed at can't possibly be at the same distance from us as the BH, where "same distance" is defined to be within 1 LY.

Nothing at all. But so what? We already know there is a distribution of stars covering thousands of light years, which means some are not far from the BH. More interesting, IMO, would be the case where that has some dynamic meaning, as with stars that are orbiting the BH. And I don't think we see any in this image.