Starship Asterisk*

NGC 4372 and the Dark Doodad

Explanation: The delightful Dark Doodad Nebula drifts through southern skies, a tantalizing target for binoculars toward the small constellation Musca, The Fly. The dusty cosmic cloud is seen against rich starfields just south of the Coalsack Nebula and the Southern Cross. Stretching for about 3 degrees across the center of this telephoto field of view, the Dark Doodad is punctuated near its southern tip (upper right) by yellowish globular star cluster NGC 4372. Of course NGC 4372 roams the halo of our Milky Way Galaxy, a background object some 20,000 light-years away and only by chance along our line-of-sight to the Dark Doodad. The Dark Doodad's well defined silhouette belongs to the Musca molecular cloud, but its better known alliterative moniker was first coined by astro-imager and writer Dennis di Cicco in 1986 while observing Comet Halley from the Australian outback. The Dark Doodad is around 700 light-years distant and over 30 light-years long.

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There you are!

Ann

Clicking on the image to get a larger picture, there are some dust clouds and a pink nebula to the bottom left of the image. What are these?

Eclectic Man wrote: ↑Thu Mar 30, 2023 10:50 am Clicking on the image to get a larger picture, there are some dust clouds and a pink nebula to the bottom left of the image. What are these?

I think - think, mind you - that I may have found the position of the nebula. I can see now that I didn't draw the nebulosity that is sort of semi-encircling Beta Musca in the ESO/Beletsky image very well, but I don't have the energy to re-do it. Still, I think the position of the pink nebula that I have indicated may be correct. Two dust lanes meet here, which enhances the chances of star formation or other energetic processes taking place, which may indeed lead to hydrogen being ionized into a pink nebula.

What is the nebula called? I have absolutely no idea! I haven't found the nebula in any other pictures.

Ann

Eclectic Man wrote: ↑Thu Mar 30, 2023 10:50 am Clicking on the image to get a larger picture, there are some dust clouds and a pink nebula to the bottom left of the image. What are these?

Ann wrote: ↑Thu Mar 30, 2023 11:46 am What is the nebula called? I have absolutely no idea! I haven't found the nebula in any other pictures.

Looking at Rick Stevenson's Crux and Musca wide field image from Ann's first post,
I'm thinking we may be looking at the outer edges of the Coalsack Nebula.

The "Discuss" link is missing on my APOD page for today. https://apod.nasa.gov/apod/astropix.html

When you Doodle; you can do the Doodad

So is the "dark doodad" just the visibly darker part of the longer arc? And does the entire "musca molecular cloud" consist of that plus all the other lighter colored dust regions visible here? None of the links made that clear to me.

bls0326 wrote: ↑Thu Mar 30, 2023 1:01 pm The "Discuss" link is missing on my APOD page for today. https://apod.nasa.gov/apod/astropix.html

Same for me. Puzzling: Had to take the scenic route to find the discussion.

johnnydeep wrote: ↑Thu Mar 30, 2023 8:23 pm So is the "dark doodad" just the visibly darker part of the longer arc? And does the entire "musca molecular cloud" consist of that plus all the other lighter colored dust regions visible here? None of the links made that clear to me.

Let's take the second question first. Yes, all the dust bunnies you can see in the Musca region, as well as the Dark Doodad itself, are certainly part of the Musca molecular cloud.

To answer the second question, I'd like to draw your attention to the Sco-Cen association of mostly B-type stars:


In this picture, you can see the very orange Antares at far left (at "9.30 o'clock") and the very pink Carina Nebula at far right (at 5 o'clock). The disk of the Milky Way, slightly brownish in color, runs along the lower part of the image. The brightest star in the picture, very white in color, is Alpha Centauri. To the upper right of Alpha Centauri is Beta Centauri, and to the right of Beta Centauri is Crux, the Southern Cross. Just to the left (east) of Crux is the dark nebula known as the Coalsack. And just below the Coalsack is Alpha and Beta Musca.

Please note the "river" of blue stars that seem to float above the disk of the Milky Way. These stars all belong to the Sco-Cen association of stars. An association is larger and more spread out than a cluster, and the stars were not necessarily born at the same time. But they are definitely related.

Here is another of my points. All these stars of the Sco-Cen association must have been born from a large dust cloud. This dust must be at more or less the same distance from us as the stars that were born from it. I'd argue that the Coalsack is a remnant of the dust cloud that gave birth to the two brightest stars of Crux. It wouldn't surprise me at all if the long and upward-curving dust lane to the left of Alpha Centauri is a remnant of the dust cloud that gave birth to other stars of the Sco-Cen association.

Here's my point. All these stars (except Alpha Centauri and orange Gacrux at the top of Crux, but including Antares) were born at more or less the same time, and they are at more of less the same distance from us, some ~400 light years away. And Alpha and Beta Musca are part of the Sco-Cen association.

Now let's consider the Dark Doodad and its possible connection to Alpha and Beta Musca.


Can you see that the Dark Doodad seems to "spread out" and then end at the position of Alpha Musca? The dust lane then resumes some distance away from Alpha Musca. Near Beta Musca, the dust lane curves into an arc.

This is what I think. The Dark Doodad is at the same distance from us as Alpha and Beta Musca. Not only that, but Alpha and Beta Musca were born from the cloud whose main remnant is the Dark Doodad.

I think that the Dark Doodad dust lane ends at Alpha Musca, because not only was Alpha Musca born from it, but Alpha is sufficiently powerful to blow the remaining gas and dust around it away. And the dust lane curves away at Beta Musca, because Beta is not quite as powerful as Alpha, but powerful enough to blow the dust near it into an arc.

But the other end of the Dark Doodad has also been "blown into an arc". Why is that?


Well, how about the influence of Gamma Musca? Gamma is less powerful than Alpha and Beta Musca, but probably powerful enough to blow the "back end" of the Dark Doodad into an arc. Gamma, of course, must also have been born from the dust cloud whose present-day remnant is the Dark Doodad.

I'd say that we are seeing the glory of starbirth as well as the messy bits of the remnants of starbirth in constellation Musca.

Ann

bls0326 wrote: ↑Thu Mar 30, 2023 1:01 pm The "Discuss" link is missing on my APOD page for today. https://apod.nasa.gov/apod/astropix.html

mostly cloudy wrote: ↑Fri Mar 31, 2023 3:08 am Same for me. Puzzling: Had to take the scenic route to find the discussion.

The "Discuss" link is always of the form:
discuss_apod.php?date=YYMMDD

For example, today's link is:
discuss_apod.php?date=230330

Thanks, Ann. So, Alpha, Beta and Gamma Musca are at 315, 340 and 325 ly distant per Wikipedia, with ages of 18, 15 and 68 My respectively. Although it seems likely that they arose from the same dust cloud, do any of those numbers put that in doubt? I suppose not...

johnnydeep wrote: ↑Fri Mar 31, 2023 2:46 pm Thanks, Ann. So, Alpha, Beta and Gamma Musca are at 315, 340 and 325 ly distant per Wikipedia, with ages of 18, 15 and 68 My respectively. Although it seems likely that they arose from the same dust cloud, do any of those numbers put that in doubt? I suppose not...

Personally I doubt that Gamma Musca is 68 million years old. I'd guess it is between 15 and 20 million years old, like Alpha and Beta.

Admittedly it is nevertheless possible that Gamma Musca belongs to an earlier generation of stars than Alpha and Beta.

Ann

Ann wrote: ↑Fri Mar 31, 2023 2:51 pm

johnnydeep wrote: ↑Fri Mar 31, 2023 2:46 pm Thanks, Ann. So, Alpha, Beta and Gamma Musca are at 315, 340 and 325 ly distant per Wikipedia, with ages of 18, 15 and 68 My respectively. Although it seems likely that they arose from the same dust cloud, do any of those numbers put that in doubt? I suppose not...

Personally I doubt that Gamma Musca is 68 million years old. I'd guess it is between 15 and 20 million years old, like Alpha and Beta.

Admittedly it is nevertheless possible that Gamma Musca belongs to an earlier generation of stars than Alpha and Beta.

Ann

You could be right. How are these stellar ages determined anyway? Is it based solely on class, luminosity and spectra?

johnnydeep wrote: ↑Fri Mar 31, 2023 3:02 pm

Ann wrote: ↑Fri Mar 31, 2023 2:51 pm

johnnydeep wrote: ↑Fri Mar 31, 2023 2:46 pm Thanks, Ann. So, Alpha, Beta and Gamma Musca are at 315, 340 and 325 ly distant per Wikipedia, with ages of 18, 15 and 68 My respectively. Although it seems likely that they arose from the same dust cloud, do any of those numbers put that in doubt? I suppose not...

Personally I doubt that Gamma Musca is 68 million years old. I'd guess it is between 15 and 20 million years old, like Alpha and Beta.

Admittedly it is nevertheless possible that Gamma Musca belongs to an earlier generation of stars than Alpha and Beta.

Ann

You could be right. How are these stellar ages determined anyway? Is it based solely on class, luminosity and spectra?

Frankly, I don't know.

What I do know is that massive stars really don't live long, and there are many stars of spectral class B2 in the Sco-Cen association. The way I understand it, main sequence stars of spectral class B2 can't be much older than, perhaps, 25 million years old. Interestingly, both Alpha and Beta Musca belong to spectral class B2. Alpha is supposedly somewhat evolved, which is to say that it has probably shut down hydrogen fusion in its core. The way I understand it, its luminosity class of IV means that no fusion at all is going on inside Alpha Musca right now, and the core is just shrinking and generating heat that way. In any case, Alpha Musca has grown somewhat bigger and brighter. Beta Musca, on the other hand, is still fusing hydrogen to helium in its core.

Gamma Musca is a less massive than Alpha and Beta, and therefore it will live longer. It is not surprising that a star of 5 solar masses, like Gamma Musca, could still be fusing hydrogen in its core at an age of 68 million years.

But the only reason that I can think of for ascribing an age of 68 million years to Gamma Musca, in view of the fact that its neighbors Alpha and Beta are so clearly a lot younger, is that Gamma might be considered to be too bright for its spectral class of B5V if it is only around 20 million years old. Stars do grow brighter all throughout their main sequence life time, and a star of 5 solar masses may possibly spend a 100 million years on the main sequence.

But frankly, I don't know how the age of Gamma Musca was determined.

Ann

Ann wrote: ↑Fri Mar 31, 2023 3:33 pm

johnnydeep wrote: ↑Fri Mar 31, 2023 3:02 pm

Ann wrote: ↑Fri Mar 31, 2023 2:51 pm

Personally I doubt that Gamma Musca is 68 million years old. I'd guess it is between 15 and 20 million years old, like Alpha and Beta.

Admittedly it is nevertheless possible that Gamma Musca belongs to an earlier generation of stars than Alpha and Beta.

Ann

You could be right. How are these stellar ages determined anyway? Is it based solely on class, luminosity and spectra?

Frankly, I don't know.

What I do know is that massive stars really don't live long, and there are many stars of spectral class B2 in the Sco-Cen association. The way I understand it, main sequence stars of spectral class B2 can't be much older than, perhaps, 25 million years old. Interestingly, both Alpha and Beta Musca belong to spectral class B2. Alpha is supposedly somewhat evolved, which is to say that it has probably shut down hydrogen fusion in its core. The way I understand it, its luminosity class of IV means that no fusion at all is going on inside Alpha Musca right now, and the core is just shrinking and generating heat that way. In any case, Alpha Musca has grown somewhat bigger and brighter. Beta Musca, on the other hand, is still fusing hydrogen to helium in its core.

Gamma Musca is a less massive than Alpha and Beta, and therefore it will live longer. It is not surprising that a star of 5 solar masses, like Gamma Musca, could still be fusing hydrogen in its core at an age of 68 million years.

But the only reason that I can think of for ascribing an age of 68 million years to Gamma Musca, in view of the fact that its neighbors Alpha and Beta are so clearly a lot younger, is that Gamma might be considered to be too bright for its spectral class of B5V if it is only around 20 million years old. Stars do grow brighter all throughout their main sequence life time, and a star of 5 solar masses may possibly spend a 100 million years on the main sequence.

But frankly, I don't know how the age of Gamma Musca was determined.

Ann

You both made me curious so I looked. Gamma Musca's age determination was not as easily readily apparent.

Fred the Cat wrote: ↑Fri Mar 31, 2023 4:08 pm

Ann wrote: ↑Fri Mar 31, 2023 3:33 pm

johnnydeep wrote: ↑Fri Mar 31, 2023 3:02 pm

You could be right. How are these stellar ages determined anyway? Is it based solely on class, luminosity and spectra?

Frankly, I don't know.

What I do know is that massive stars really don't live long, and there are many stars of spectral class B2 in the Sco-Cen association. The way I understand it, main sequence stars of spectral class B2 can't be much older than, perhaps, 25 million years old. Interestingly, both Alpha and Beta Musca belong to spectral class B2. Alpha is supposedly somewhat evolved, which is to say that it has probably shut down hydrogen fusion in its core. The way I understand it, its luminosity class of IV means that no fusion at all is going on inside Alpha Musca right now, and the core is just shrinking and generating heat that way. In any case, Alpha Musca has grown somewhat bigger and brighter. Beta Musca, on the other hand, is still fusing hydrogen to helium in its core.

Gamma Musca is a less massive than Alpha and Beta, and therefore it will live longer. It is not surprising that a star of 5 solar masses, like Gamma Musca, could still be fusing hydrogen in its core at an age of 68 million years.

But the only reason that I can think of for ascribing an age of 68 million years to Gamma Musca, in view of the fact that its neighbors Alpha and Beta are so clearly a lot younger, is that Gamma might be considered to be too bright for its spectral class of B5V if it is only around 20 million years old. Stars do grow brighter all throughout their main sequence life time, and a star of 5 solar masses may possibly spend a 100 million years on the main sequence.

But frankly, I don't know how the age of Gamma Musca was determined.

Ann

You both made me curious so I looked. Gamma Musca's age determination was not as easily readily apparent.

And then there are isochrones - though I can't quite understand why I never see these isochrones plotted ON TOP of the standard H-R diagram. Is it because the metallicity needs to be the same? :

https://en.wikipedia.org/wiki/Stellar_isochrone wrote:In stellar evolution, an isochrone is a curve on the Hertzsprung-Russell diagram, representing a population of stars of the same age but with different mass.[1]

The Hertzsprung-Russell diagram plots a star's luminosity against its temperature, or equivalently, its color. Stars change their positions on the HR diagram throughout their life. Newborn stars of low or intermediate mass are born cold but extremely luminous. They contract and dim along the Hayashi track, decreasing in luminosity but staying at roughly the same temperature, until reaching the main sequence directly or by passing through the Henyey track. Stars evolve relatively slowly along the main sequence as they fuse hydrogen, and after the vast majority of their lifespan, all but the least massive stars become giants. They then evolve quickly towards their stellar endpoints: white dwarfs, neutron stars, or black holes.

Isochrones can be used to date open clusters because their members all have roughly the same age.[2] One of the first uses of an isochrone method to date an open cluster was by Demarque and Larson in 1963.[3] If the initial mass function of the open cluster is known, isochrones can be calculated at any age by taking every star in the initial population, using numerical simulations to evolve it forwards to the desired age, and plotting the star's luminosity and magnitude on the HR diagram.[4] The resulting curve is an isochrone, which can be compared against the observational color-magnitude diagram to determine how well they match. If they match well, the assumed age of the isochrone is close to the actual age of the cluster.

johnnydeep wrote: ↑Fri Mar 31, 2023 7:02 pm
And then there are isochrones - though I can't quite understand why I never see these isochrones plotted ON TOP of the standard H-R diagram. Is it because the metallicity needs to be the same? :

https://en.wikipedia.org/wiki/Stellar_isochrone wrote:In stellar evolution, an isochrone is a curve on the Hertzsprung-Russell diagram, representing a population of stars of the same age but with different mass.[1]

The Hertzsprung-Russell diagram plots a star's luminosity against its temperature, or equivalently, its color. Stars change their positions on the HR diagram throughout their life. Newborn stars of low or intermediate mass are born cold but extremely luminous. They contract and dim along the Hayashi track, decreasing in luminosity but staying at roughly the same temperature, until reaching the main sequence directly or by passing through the Henyey track. Stars evolve relatively slowly along the main sequence as they fuse hydrogen, and after the vast majority of their lifespan, all but the least massive stars become giants. They then evolve quickly towards their stellar endpoints: white dwarfs, neutron stars, or black holes.

Isochrones can be used to date open clusters because their members all have roughly the same age.[2] One of the first uses of an isochrone method to date an open cluster was by Demarque and Larson in 1963.[3] If the initial mass function of the open cluster is known, isochrones can be calculated at any age by taking every star in the initial population, using numerical simulations to evolve it forwards to the desired age, and plotting the star's luminosity and magnitude on the HR diagram.[4] The resulting curve is an isochrone, which can be compared against the observational color-magnitude diagram to determine how well they match. If they match well, the assumed age of the isochrone is close to the actual age of the cluster.

Theoretical isochrones for near-solar metallicity and a range of ages

---

That top (blue) isochrone irritates me a little. It clearly claims that after 5 million years, the most massive star of a massive cluster has not only turned into a red supergiant, but also exploded as a core-collapse supernova and turned into a - white dwarf??? Come on, a massive star that has gone core-collapse supernova won't turn into a white dwarf. They turn into neutron stars, or, in rare cases, into black holes. But the isochrone post-supernova line going straight to the left without dipping down suggests that the stellar post-supernova remnant is very bright!

But neutron stars, let alone black holes, aren't bright. By contrast, I believe that newborn white dwarfs are bright, because their freshly exposed cores are millions of degrees hot. Also the white dwarfs, although small, are at least as large as the Earth, whereas neutron stars are the size of a city. There is no way that a neutron star, even a newborn one, can be even nearly as bright as a red (or a blue) supergiant!

Anyway. Let's talk a little about isochrones for specific clusters! Let's look at their isochrones (or at least color magnitude diagrams) and see pictures of the clusters. Let's start with the easiest one, the Pleaides:



Take a look at this picture showing the HR (Hertzsprung-Russell, or color magnitude) diagrams for open clusters of different ages:


Let's look at the two youngest and brightest clusters, NGC 2362, which has no red giants, and h and chi Persei, which does:


Finally, I highly recommend this Astrobite on how hard it is to determine stellar ages, how important the nearby Hyades cluster has been for calibrating the age of other clusters, and how we may have gotten the Hyades age quite wrong!

Here’s the thing though: stellar ages are really really hard to measure for main sequence field stars, but much easier for cluster stars. That’s because they are stellar populations – they’re all the same age and roughly the same metallicity. You can fit an isochrone to an ensemble of stars —that gives you a much tighter constraint on its age (assuming the stellar models are correct, of course). For this reason, clusters are used to calibrate other stellar dating methods.

Ooops. Well, if the Hyades is the key for calibrating the ages of other clusters, then we really don't want to get the Hyades age wrong!

But ah, rotation, rotation!!

Ann

Ann wrote: ↑Sat Apr 01, 2023 6:08 am

johnnydeep wrote: ↑Fri Mar 31, 2023 7:02 pm
And then there are isochrones - though I can't quite understand why I never see these isochrones plotted ON TOP of the standard H-R diagram. Is it because the metallicity needs to be the same? :

https://en.wikipedia.org/wiki/Stellar_isochrone wrote:In stellar evolution, an isochrone is a curve on the Hertzsprung-Russell diagram, representing a population of stars of the same age but with different mass.[1]

The Hertzsprung-Russell diagram plots a star's luminosity against its temperature, or equivalently, its color. Stars change their positions on the HR diagram throughout their life. Newborn stars of low or intermediate mass are born cold but extremely luminous. They contract and dim along the Hayashi track, decreasing in luminosity but staying at roughly the same temperature, until reaching the main sequence directly or by passing through the Henyey track. Stars evolve relatively slowly along the main sequence as they fuse hydrogen, and after the vast majority of their lifespan, all but the least massive stars become giants. They then evolve quickly towards their stellar endpoints: white dwarfs, neutron stars, or black holes.

Isochrones can be used to date open clusters because their members all have roughly the same age.[2] One of the first uses of an isochrone method to date an open cluster was by Demarque and Larson in 1963.[3] If the initial mass function of the open cluster is known, isochrones can be calculated at any age by taking every star in the initial population, using numerical simulations to evolve it forwards to the desired age, and plotting the star's luminosity and magnitude on the HR diagram.[4] The resulting curve is an isochrone, which can be compared against the observational color-magnitude diagram to determine how well they match. If they match well, the assumed age of the isochrone is close to the actual age of the cluster.

Theoretical isochrones for near-solar metallicity and a range of ages

---

That top (blue) isochrone irritates me a little. It clearly claims that after 5 million years, the most massive star of a massive cluster has not only turned into a red supergiant, but also exploded as a core-collapse supernova and turned into a - white dwarf??? Come on, a massive star that has gone core-collapse supernova won't turn into a white dwarf. They turn into neutron stars, or, in rare cases, into black holes. But the isochrone post-supernova line going straight to the left without dipping down suggests that the stellar post-supernova remnant is very bright!

...

Ann

I might be misinterpreting what you're saying there, but that's not how I understand what the isochrone graph shows. The lines go through stars of the same age that happen to have different temperatures and luminosities. Thus, taking the blue line, it shows that there are some very hot and luminous stars (presumably massive ones) that are 5 million years old and also some very cool and dim stars (presumably low mass ones) that are 5 million years old. The lines don't track individual stars. But perhaps I am dead wrong because admittedly, I still find the H-R diagram confusing, which might translate to being confused about isochrones as well.

johnnydeep wrote: ↑Sat Apr 01, 2023 2:24 pm

Ann wrote: ↑Sat Apr 01, 2023 6:08 am

johnnydeep wrote: ↑Fri Mar 31, 2023 7:02 pm
And then there are isochrones - though I can't quite understand why I never see these isochrones plotted ON TOP of the standard H-R diagram. Is it because the metallicity needs to be the same? :

Theoretical isochrones for near-solar metallicity and a range of ages

---

That top (blue) isochrone irritates me a little. It clearly claims that after 5 million years, the most massive star of a massive cluster has not only turned into a red supergiant, but also exploded as a core-collapse supernova and turned into a - white dwarf??? Come on, a massive star that has gone core-collapse supernova won't turn into a white dwarf. They turn into neutron stars, or, in rare cases, into black holes. But the isochrone post-supernova line going straight to the left without dipping down suggests that the stellar post-supernova remnant is very bright!

...

Ann

I might be misinterpreting what you're saying there, but that's not how I understand what the isochrone graph shows. The lines go through stars of the same age that happen to have different temperatures and luminosities. Thus, taking the blue line, it shows that there are some very hot and luminous stars (presumably massive ones) that are 5 million years old and also some very cool and dim stars (presumably low mass ones) that are 5 million years old. The lines don't track individual stars. But perhaps I am dead wrong because admittedly, I still find the H-R diagram confusing, which might translate to being confused about isochrones as well.

Both of you may be on the right track regarding rotation and metallicity.

Fred the Cat wrote: ↑Sat Apr 01, 2023 3:35 pm

johnnydeep wrote: ↑Sat Apr 01, 2023 2:24 pm

Ann wrote: ↑Sat Apr 01, 2023 6:08 am

That top (blue) isochrone irritates me a little. It clearly claims that after 5 million years, the most massive star of a massive cluster has not only turned into a red supergiant, but also exploded as a core-collapse supernova and turned into a - white dwarf??? Come on, a massive star that has gone core-collapse supernova won't turn into a white dwarf. They turn into neutron stars, or, in rare cases, into black holes. But the isochrone post-supernova line going straight to the left without dipping down suggests that the stellar post-supernova remnant is very bright!

...

Ann

I might be misinterpreting what you're saying there, but that's not how I understand what the isochrone graph shows. The lines go through stars of the same age that happen to have different temperatures and luminosities. Thus, taking the blue line, it shows that there are some very hot and luminous stars (presumably massive ones) that are 5 million years old and also some very cool and dim stars (presumably low mass ones) that are 5 million years old. The lines don't track individual stars. But perhaps I am dead wrong because admittedly, I still find the H-R diagram confusing, which might translate to being confused about isochrones as well.

Both of you may be on the right track regarding rotation and metallicity.

Now I may not understand isochrones at all. An H-R diagram shows the segregation of stars by color (X axis) and luminosity (Y axis) as they exist (in a cluster for example) currently, right? It says nothing (at least directly) about any of the ages of those stars. But an isochrone plot takes that same set of stars, and runs a numerical simulation over time for each one, plotting a point at every 5 My (for example). Over the total period of time considered, a star will appear multiple times on the isochrone plot, each point representing a different age. After all stars are so simulated and plotted, isochrones can then be drawn through all the points of the same age. Is that right?

johnnydeep wrote: ↑Sat Apr 01, 2023 2:24 pm

Ann wrote: ↑Sat Apr 01, 2023 6:08 am

johnnydeep wrote: ↑Fri Mar 31, 2023 7:02 pm
And then there are isochrones - though I can't quite understand why I never see these isochrones plotted ON TOP of the standard H-R diagram. Is it because the metallicity needs to be the same? :

Theoretical isochrones for near-solar metallicity and a range of ages

---

That top (blue) isochrone irritates me a little. It clearly claims that after 5 million years, the most massive star of a massive cluster has not only turned into a red supergiant, but also exploded as a core-collapse supernova and turned into a - white dwarf??? Come on, a massive star that has gone core-collapse supernova won't turn into a white dwarf. They turn into neutron stars, or, in rare cases, into black holes. But the isochrone post-supernova line going straight to the left without dipping down suggests that the stellar post-supernova remnant is very bright!

...

Ann

I might be misinterpreting what you're saying there, but that's not how I understand what the isochrone graph shows. The lines go through stars of the same age that happen to have different temperatures and luminosities. Thus, taking the blue line, it shows that there are some very hot and luminous stars (presumably massive ones) that are 5 million years old and also some very cool and dim stars (presumably low mass ones) that are 5 million years old. The lines don't track individual stars. But perhaps I am dead wrong because admittedly, I still find the H-R diagram confusing, which might translate to being confused about isochrones as well.

That's absolutely correct.

I was just objecting to the top of the blue line, which seemed to imply that (some) massive stars go supernova after 5 million years, after which the remnants turn hot and blue and stay bright.

Ann

I wonder if the yellow light from NGC 4372 does make glitter the segment of the Doodad closest to our line of sight