Okay, one more.
Why is it that very metal-poor moderate-mass stars (stars that are around solar mass or a bit less) become
blue during a part of their evolution,
after they have left the main sequence? Particularly if they were not blue when they were on the main sequence, like the Sun?
Just how can an evolved star become bluer when it is some kind of giant than it was when it was on the main sequence? This flies in the face of everything we expect from "normal metal-rich stars" like the Sun.
Clearly it has to do with the star's metallicity, since we only see this effect in very metal-poor stars.
I'm going to speculate now. I think that a very metal-poor star is more "transparent" than a metal-rich star. And by "transparent", I mean how easy it is for a photon that has been generated inside a star's core to make its way to the star's surface, where the photon can escape so that its light becomes visible.
Because it's really hard for a photon to make its way through the mass of a star's interior. The photon keeps colliding with all kinds of particles inside the star, as it is making its way towards the surface. This process is called a "random walk".
There are a few points to remember here.
1) The photons that are generated in a star's core are gamma rays, extremely hot and energetic. If the Sun bathed us in gamma rays, we would be dead in an instant.
2) The hotter the core of a star is, the more energetic and hot the gamma rays generated there will be.
3) During their long random walk through its sun, the photons lose energy. In fact, every collision with a particle robs the photons of some energy. After bouncing around inside the Sun for thousands or even millions of years and undergoing billions of collisions, the photons that emerge from the Sun's photosphere have a "typical" wavelength of perhaps 520 nm, which corresponds to perfectly harmless green light.
I recommend this video, where you can see a photon's random walk inside a star being generated:
Click to play embedded YouTube video.
So let's say that a metal-poor star is more "transparent" than a metal-rich star. And let's say that this relative "transparency" means that the photons generated inside a metal-poor star's core will undergo fewer collisions on their way to the star's surface than the photons generated inside a metal-rich star's core.
In other words, the "random walk" of a photon will be shorter inside a metal-poor than inside a metal-rich star, and the photon will lose less energy on its way to the surface of a metal-poor star. (Or so I think anyway.) Therefore, the photons will emerge more energetic and "bluer" from a metal-poor star than from a metal-rich one.
But how can metal-poor stars become bluer when they are on the horizontal branch than they were on the main sequence?
Remember that a star that is fusing helium to oxygen and carbon in its core (like stars do when they are on the horizontal branch) have a hotter core than non-blue stars that are on the main sequence. Our own Sun, which is on the main sequence, has a core whose temperature is about 15 million K. That's fine for fusing hydrogen into helium, but it's not enough to fuse helium into oxygen and carbon. That sort of fusion can only occur when the core temperature becomes significantly higher.
So stars on the horizontal branch have very hot cores, and the metal-poor stars are sufficiently transparent that the photons that emerge from them are typically bluer than the photons that emerged from these stars when they were on the main sequence (assuming the stars were non-blue when they were on the main sequence).
Take a look at this color-magnitude diagram of stars in globular cluster M55 again:
Look at the horizontal branch with the blue stars at upper left. Note that the horizontal branch slopes downward. Note that the bluest stars (the leftmost stars) are also fainter than the other stars on the horizontal branch (which is why they are lower down).
The fact that the bluest of the stars on the horizontal branch are the faintest can only meant that the bluest stars on the horizontal branch are also the physically smallest of the stars on the HB branch.
For one reason or another, these stars have shrunk to a very small size. Therefore, the random walk that the photons inside them have to walk is shorter than the walk of the photons in any other evolved star in a globular cluster. (Because we typically see that the larger a star is, the redder is its light, all other things being equal. And
the smaller a hot star is, the bluer it is.) But the stars on the horizontal branch have cores that are hotter than the core of any non-blue star that is still on the main sequence.
And you star, how "clean" is your stellar interior? Because all that determines what color your photosphere and the light that emerges from you is going to be.
Ann
Okay, one more. [b][i]Why[/i][/b] is it that very metal-poor moderate-mass stars (stars that are around solar mass or a bit less) become [b][i][color=#0040FF]blue[/color][/i][/b] during a part of their evolution, [b][i]after[/i][/b] they have left the main sequence? Particularly if they were not blue when they were on the main sequence, like the Sun?
Just how can an evolved star become bluer when it is some kind of giant than it was when it was on the main sequence? This flies in the face of everything we expect from "normal metal-rich stars" like the Sun.
Clearly it has to do with the star's metallicity, since we only see this effect in very metal-poor stars.
I'm going to speculate now. I think that a very metal-poor star is more "transparent" than a metal-rich star. And by "transparent", I mean how easy it is for a photon that has been generated inside a star's core to make its way to the star's surface, where the photon can escape so that its light becomes visible.
Because it's really hard for a photon to make its way through the mass of a star's interior. The photon keeps colliding with all kinds of particles inside the star, as it is making its way towards the surface. This process is called a "random walk".
[float=left][img3="A photon's random walk from the core of a star to the star's surface, the photosphere."]https://qph.cf2.quoracdn.net/main-qimg-cb7f7f2cc5ae365158e7f5011b992f10[/img3][/float][float=right][img3="A drunkard's random walk from the pub to his home."]https://miro.medium.com/v2/resize:fit:720/0*WJVuFrA0dhjH_ngR[/img3][/float]
[clear][/clear]
There are a few points to remember here.
1) The photons that are generated in a star's core are gamma rays, extremely hot and energetic. If the Sun bathed us in gamma rays, we would be dead in an instant.
2) The hotter the core of a star is, the more energetic and hot the gamma rays generated there will be.
3) During their long random walk through its sun, the photons lose energy. In fact, every collision with a particle robs the photons of some energy. After bouncing around inside the Sun for thousands or even millions of years and undergoing billions of collisions, the photons that emerge from the Sun's photosphere have a "typical" wavelength of perhaps 520 nm, which corresponds to perfectly harmless green light.
I recommend this video, where you can see a photon's random walk inside a star being generated:
[youtube]https://www.youtube.com/watch?v=re2EkIOrDiI[/youtube]
So let's say that a metal-poor star is more "transparent" than a metal-rich star. And let's say that this relative "transparency" means that the photons generated inside a metal-poor star's core will undergo fewer collisions on their way to the star's surface than the photons generated inside a metal-rich star's core.
In other words, the "random walk" of a photon will be shorter inside a metal-poor than inside a metal-rich star, and the photon will lose less energy on its way to the surface of a metal-poor star. (Or so I think anyway.) Therefore, the photons will emerge more energetic and "bluer" from a metal-poor star than from a metal-rich one.
But how can metal-poor stars become bluer when they are on the horizontal branch than they were on the main sequence?
Remember that a star that is fusing helium to oxygen and carbon in its core (like stars do when they are on the horizontal branch) have a hotter core than non-blue stars that are on the main sequence. Our own Sun, which is on the main sequence, has a core whose temperature is about 15 million K. That's fine for fusing hydrogen into helium, but it's not enough to fuse helium into oxygen and carbon. That sort of fusion can only occur when the core temperature becomes significantly higher.
So stars on the horizontal branch have very hot cores, and the metal-poor stars are sufficiently transparent that the photons that emerge from them are typically bluer than the photons that emerged from these stars when they were on the main sequence (assuming the stars were non-blue when they were on the main sequence).
Take a look at this color-magnitude diagram of stars in globular cluster M55 again:
[img3="M55: Color Magnitude Diagram
B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope"]https://apod.nasa.gov/apod/image/0102/m55cmd_mochejska.jpg[/img3]
Look at the horizontal branch with the blue stars at upper left. Note that the horizontal branch slopes downward. Note that the bluest stars (the leftmost stars) are also fainter than the other stars on the horizontal branch (which is why they are lower down).
The fact that the bluest of the stars on the horizontal branch are the faintest can only meant that the bluest stars on the horizontal branch are also the physically smallest of the stars on the HB branch.
For one reason or another, these stars have shrunk to a very small size. Therefore, the random walk that the photons inside them have to walk is shorter than the walk of the photons in any other evolved star in a globular cluster. (Because we typically see that the larger a star is, the redder is its light, all other things being equal. And [url=https://i0.wp.com/scientificinquirer.com/wp-content/uploads/2019/05/407e291e-9418-480e-9881-6f7672d10714-3957-000002106ad3fbc9.jpg?fit=1280%2C697&ssl=1]the smaller a hot star is, the bluer it is[/url].) But the stars on the horizontal branch have cores that are hotter than the core of any non-blue star that is still on the main sequence.
[float=left][img3="Oh star, how large is your girth? Image credit: UK Schmidt Telescope at Anglo-Australian Observatory." ]https://upload.wikimedia.org/wikiversity/en/thumb/f/f7/Alpha_Capricorni_copy.jpg/250px-Alpha_Capricorni_copy.jpg[/img3][/float][float=right][img3="Oh star, how hot is your core?"]https://www.webexhibits.org/causesofcolor/images/astronomy/Astronomy-sun-layers-NASAZ.jpg[/img3][/float]
[clear][/clear]
And you star, how "clean" is your stellar interior? Because all that determines what color your photosphere and the light that emerges from you is going to be.
Ann