APOD: The Milky Way's Black Hole (2022 May 13)

[johnnydeep]

Now that we've seen the blackhole of our galaxy; if there is danger of a pending emission will we not have 27,000 years to figure out what to do?

No. All we know is that it isn't currently very active. We don't know enough about the interaction between SMBs and their surrounding stars to have much idea how long a conversion to high activity would take. That would be how much time we have to prepare (not that it's likely this SMB could do anything that would present any risk to us).

Why couldn't there be a "high energy emission" from the BH that has been travelling to us for 26999 years - and is now just a light year away - that we will be hit with in one year's time? I'm not understanding how you can rule that out.

[Chris Peterson]

No. I'm saying that every star in the image is either in the foreground or the background.

(FWIW, the black hole image is 1/100 of one pixel of the larger image!)

I certainly realize that the black hole is way, way to small to be visible in the larger image.

But when you say that all the stars seen in the larger image have to be either in the foreground or in the background (and not, say, within a few light-years of the black hole), I frankly don't get it.

Ann

Me neither. Why couldn't there be stars to either side (in the larger image), yet be the same distance from us as the black hole?

There can't be. There could be some stars at a similar distance. But for the question to be meaningful, you need to define how similar a distance. And what value knowing that even provides. Obviously, we're looking at some columnar volume of space. What does the distance to the stars matter?

[AVAO]

(I know you're aware that the SMB is too small to be seen. I was just pointing out how incredibly small, at 1% of a single pixel.)

It's really incredibly small...

100'000LY

10'000LY

1'000LY
https://live.staticflickr.com/65535/520 ... b878_k.jpg

100LY
https://live.staticflickr.com/65535/520 ... cda6_k.jpg

10LY
https://live.staticflickr.com/65535/520 ... 0250_b.jpg

1LY
https://live.staticflickr.com/65535/520 ... ee9c_b.jpg

18Lightdays
Jac Berne (flickr)

The white pixel in the middle has a size of 260 light minutes. This means that the SMB is again 26 times smaller. that is really extremely small.

[Don't point this thing at me]

What's the most annoying if we're really looking down the barrel: That we could get fried at any instant (I'm being ironic - please don't pay attention) or that the Fermi bubbles, the X-ray chimney and all that stuff aligned with the Galaxy's Y axis that would be so neatly explained by jets out of Sgr A*, might in fact be in the wrong direction for that? In fact, if it's aimed at the disk and not poleward, where in the disk is the damage from past activity?

Unless we really can't tell anything about the orientation from the "image"? The ApJ paper says face-on is more likely, but that's two BH imaged so far, and both of them look face-on. Aren't they supposed to look pretty much the same from all sides anyway?

And, OK. Let's be positive. M87* was neat, but it's in somebody else's galaxy. This is our very own SMBH, and it's very nice. I'll want the stuffed toy when they make it.

[Chris Peterson]

What's the most annoying if we're really looking down the barrel: That we could get fried at any instant (I'm being ironic - please don't pay attention) or that the Fermi bubbles, the X-ray chimney and all that stuff aligned with the Galaxy's Y axis that would be so neatly explained by jets out of Sgr A*, might in fact be in the wrong direction for that? In fact, if it's aimed at the disk and not poleward, where in the disk is the damage from past activity?

Unless we really can't tell anything about the orientation from the "image"? The ApJ paper says face-on is more likely, but that's two BH imaged so far, and both of them look face-on. Aren't they supposed to look pretty much the same from all sides anyway?

And, OK. Let's be positive. M87* was neat, but it's in somebody else's galaxy. This is our very own SMBH, and it's very nice. I'll want the stuffed toy when they make it.

Again, there's no reason to think that even if a jet from this SMB crossed our path, it would cause any harm. And "face-on" is not a number. What is the angle and the uncertainty? Two gunfighters are each looking down the barrels of each other's guns, but a degree of error means they will miss each other.

[Ann]

No. I'm saying that every star in the image is either in the foreground or the background.

(FWIW, the black hole image is 1/100 of one pixel of the larger image!)

I certainly realize that the black hole is way, way to small to be visible in the larger image.

But when you say that all the stars seen in the larger image have to be either in the foreground or in the background (and not, say, within a few light-years of the black hole), I frankly don't get it.

Ann

I fail to see what else they could be. They're either between us and the black hole, or they are behind it. (I didn't say anything about a few light years.)

Perhaps the question was whether any of the stars in the image are orbiting the black hole? The handful of stars that we have observed orbiting Sgr A* range in distance from about 10 AU to about 2000 AU, most being at the lower end of that range. A star 2000 AU from the black hole would be 15 pixels away in the large image. Which would be lost in a region of saturated pixels. So the answer is no, none of the stars in this image are likely to be orbiting Sgr A*, except to the extent that being so close to the center of the galaxy, a few might be nearly orbiting it (while actually orbiting the galaxy's center of mass, which is likely very close to Sgr A*). What is actually orbiting what can be difficult to define clearly in closely interacting multiple body systems.

(I know you're aware that the SMB is too small to be seen. I was just pointing out how incredibly small, at 1% of a single pixel.)

Couldn't some of the stars in the larger image Be more than 2000 AU from the black hole, but less than a few light-years?

Andromeda, which has a much more massive black hole than the Milky Way, has stars orbiting within a light-year of the BH:

This artist's concept shows a view across a mysterious disk of young, blue stars encircling a supermassive black hole at the core of the neighboring Andromeda Galaxy (M31). The region around the black hole is barely visible at the center of the disk. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies. Spectroscopic observations by the Hubble Space Telescope reveal that the blue light consists of more than 400 stars that formed in a burst of activity about 200 million years ago. The stars are tightly packed in a disk that is only a light-year across. Under the black hole's gravitational grip, the stars are traveling very fast: 2.2 million miles an hour (3.6 million kilometers an hour, or 1,000 kilometers a second). Image: NASA, ESA and A. Schaller.

---

Couldn't there be stars orbiting within a light-year of the central black hole of the Milky Way? What prevents it?

Are we to understand that the Milky Way black hole has cleared a stellar void within a few light-years of itself, apart from the stars that have been found orbiting the black hole within 2000 AU of it?

Ann

[Chris Peterson]

I certainly realize that the black hole is way, way to small to be visible in the larger image.

But when you say that all the stars seen in the larger image have to be either in the foreground or in the background (and not, say, within a few light-years of the black hole), I frankly don't get it.

I fail to see what else they could be. They're either between us and the black hole, or they are behind it. (I didn't say anything about a few light years.)

Perhaps the question was whether any of the stars in the image are orbiting the black hole? The handful of stars that we have observed orbiting Sgr A* range in distance from about 10 AU to about 2000 AU, most being at the lower end of that range. A star 2000 AU from the black hole would be 15 pixels away in the large image. Which would be lost in a region of saturated pixels. So the answer is no, none of the stars in this image are likely to be orbiting Sgr A*, except to the extent that being so close to the center of the galaxy, a few might be nearly orbiting it (while actually orbiting the galaxy's center of mass, which is likely very close to Sgr A*). What is actually orbiting what can be difficult to define clearly in closely interacting multiple body systems.

(I know you're aware that the SMB is too small to be seen. I was just pointing out how incredibly small, at 1% of a single pixel.)

Couldn't some of the stars in the larger image Be more than 2000 AU from the black hole, but less than a few light-years?

Sure. But like I asked, so what? And what does that have to do with the original question? Any of those would still either be in the foreground or the background, like every other star, regardless of distance.

Couldn't there be stars orbiting within a light-year of the central black hole of the Milky Way? What prevents it?

Maybe... but I'm not sure a star that far away would be gravitationally bound to the black hole, as opposed to the galactic center of mass.

Are we to understand that the Milky Way black hole has cleared a stellar void within a few light-years of itself, apart from the stars that have been found orbiting the black hole within 2000 AU of it?

Cleared? No. But all of the stars we observe to be orbiting the black hole are much closer than a few light years.

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

[Ann]

I fail to see what else they could be. They're either between us and the black hole, or they are behind it. (I didn't say anything about a few light years.)

Perhaps the question was whether any of the stars in the image are orbiting the black hole? The handful of stars that we have observed orbiting Sgr A* range in distance from about 10 AU to about 2000 AU, most being at the lower end of that range. A star 2000 AU from the black hole would be 15 pixels away in the large image. Which would be lost in a region of saturated pixels. So the answer is no, none of the stars in this image are likely to be orbiting Sgr A*, except to the extent that being so close to the center of the galaxy, a few might be nearly orbiting it (while actually orbiting the galaxy's center of mass, which is likely very close to Sgr A*). What is actually orbiting what can be difficult to define clearly in closely interacting multiple body systems.

(I know you're aware that the SMB is too small to be seen. I was just pointing out how incredibly small, at 1% of a single pixel.)

Couldn't some of the stars in the larger image Be more than 2000 AU from the black hole, but less than a few light-years?

Sure. But like I asked, so what? And what does that have to do with the original question? Any of those would still either be in the foreground or the background, like every other star, regardless of distance.

Couldn't there be stars orbiting within a light-year of the central black hole of the Milky Way? What prevents it?

Maybe... but I'm not sure a star that far away would be gravitationally bound to the black hole, as opposed to the galactic center of mass.

Are we to understand that the Milky Way black hole has cleared a stellar void within a few light-years of itself, apart from the stars that have been found orbiting the black hole within 2000 AU of it?

Cleared? No. But all of the stars we observe to be orbiting the black hole are much closer than a few light years.

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Chris, you talk about stars as if they were electrons that obey the Pauli exclusion principle which disallows two identical half-integer spin particles from simultaneously occupying the same quantum state. So if the black hole is at a distance of 27,000 light-years, then there can't be any stars "occupying the same distance state" to the east or west of it at the same distance!

But okay, this is your best argument so far:

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Okay. I'll settle for that for now.

Ann

[Ann]

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:

Omega Centauri. The image shows only the central part of the cluster. Photo: ESO.

---

Wikipedia wrote:

Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. 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.

Omega Centauri is very different from most other galactic globular clusters to the extent that it is thought to have an origin as the core remnant of a disrupted dwarf galaxy.

If the diameter of Omega Centauri is 150 light-years, then the volume contained inside it should be on the order of ~3.4 million cubic light-years. But if Omega Centauri contains 10 million stars, then the average stellar density inside it is around 3 stars per cubic light-year. And if Omega Centauri is denser at the center than at the outskirts, which seems highly likely, then the stellar density near the center of the cluster is likely to be higher than 3 stars per cubic light-year.

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?

Ann

[AVAO]

Perhaps some of them could be at the same distance to us as Sgr A*, to the east or left of Sgr A*?
Ann

Couldn't some of the stars in the larger image Be more than 2000 AU from the black hole, but less than a few light-years?

Sure. But like I asked, so what? And what does that have to do with the original question? Any of those would still either be in the foreground or the background, like every other star, regardless of distance.

Couldn't there be stars orbiting within a light-year of the central black hole of the Milky Way? What prevents it?

Maybe... but I'm not sure a star that far away would be gravitationally bound to the black hole, as opposed to the galactic center of mass.

Are we to understand that the Milky Way black hole has cleared a stellar void within a few light-years of itself, apart from the stars that have been found orbiting the black hole within 2000 AU of it?

Cleared? No. But all of the stars we observe to be orbiting the black hole are much closer than a few light years.

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Chris, you talk about stars as if they were electrons that obey the Pauli exclusion principle which disallows two identical half-integer spin particles from simultaneously occupying the same quantum state. So if the black hole is at a distance of 27,000 light-years, then there can't be any stars "occupying the same distance state" to the east or west of it at the same distance!

But okay, this is your best argument so far:

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Okay. I'll settle for that for now.

Ann

In my opinion it's all a matter of definition!


As I understand the question, all possible stars are at the same distance on a spherical surface. If the question is, before or after, refers to a level in the distance of the SMB, all possible variants of positions are before.


The neckline is 1 LY wide. In this area, the white tracking lines show the highly dynamic movement of nearby stars around the SMB over a 20-year period. That one of them crosses the surface of the sphere right now is extremely small.

Jac

[Chris Peterson]

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]

Couldn't some of the stars in the larger image Be more than 2000 AU from the black hole, but less than a few light-years?

Sure. But like I asked, so what? And what does that have to do with the original question? Any of those would still either be in the foreground or the background, like every other star, regardless of distance.

Couldn't there be stars orbiting within a light-year of the central black hole of the Milky Way? What prevents it?

Maybe... but I'm not sure a star that far away would be gravitationally bound to the black hole, as opposed to the galactic center of mass.

Are we to understand that the Milky Way black hole has cleared a stellar void within a few light-years of itself, apart from the stars that have been found orbiting the black hole within 2000 AU of it?

Cleared? No. But all of the stars we observe to be orbiting the black hole are much closer than a few light years.

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Chris, you talk about stars as if they were electrons that obey the Pauli exclusion principle which disallows two identical half-integer spin particles from simultaneously occupying the same quantum state. So if the black hole is at a distance of 27,000 light-years, then there can't be any stars "occupying the same distance state" to the east or west of it at the same distance!

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!

[johnnydeep]

Sure. But like I asked, so what? And what does that have to do with the original question? Any of those would still either be in the foreground or the background, like every other star, regardless of distance.


Maybe... but I'm not sure a star that far away would be gravitationally bound to the black hole, as opposed to the galactic center of mass.


Cleared? No. But all of the stars we observe to be orbiting the black hole are much closer than a few light years.

Keep in mind that we're seeing a volume of space here on the order of a million cubic light years. Given a few thousand stars, there sure can't be many, if any, lying within a few light years of a point 27,000 ly away.

Chris, you talk about stars as if they were electrons that obey the Pauli exclusion principle which disallows two identical half-integer spin particles from simultaneously occupying the same quantum state. So if the black hole is at a distance of 27,000 light-years, then there can't be any stars "occupying the same distance state" to the east or west of it at the same distance!

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).

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

Consider this star

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.

[Chris Peterson]

Chris, you talk about stars as if they were electrons that obey the Pauli exclusion principle which disallows two identical half-integer spin particles from simultaneously occupying the same quantum state. So if the black hole is at a distance of 27,000 light-years, then there can't be any stars "occupying the same distance state" to the east or west of it at the same distance!

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.

[johnnydeep]

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.

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]

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. ]

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!

[johnnydeep]

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!

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

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]

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"!

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]

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"!

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!

Well played, sir, well played.

[Ann]

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.

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 the brightest galaxy of the interacting pair known as NGC 6050. The increased brightness at the 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]

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

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]

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.

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.

---

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

[johnnydeep]

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.

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.

---

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

I'm not Chris, but I think it's a combination of both increased stellar density, and the fact that our line of sight through the core usually passes through the thickest part of the galaxy, so that more stars there contribute to the photons we see coming from a given unit area.

[Chris Peterson]

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]

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