Some of these are "Blue Stragglers"- stars that should have turned off the main sequence long ago but which have remained as a result of likely collision with another star within the cluster (like a binary pair colliding), resulting in a star with increased mass, increased luminosity, and higher temperature (blue). A nice description of blue stragglers can be found here: https://www.mso.anu.edu.au/~jerjen/rese ... ggler.html. I also describe them more briefly on my website from a few years ago, in the context of M13 and M5: http://www.starrywonders.com/m132019small.html.
Well, it's simple. Sort of simple. It has to do with metallicity, or, how pure the hydrogen-helium gas was that the stars originally formed from, or how "contaminated" the birth cloud was by heavier elements formed by supernovas and even red giants shedding their outer layers.
When a star goes supernova, it has already, even before it exploded, built up a lot of heavier elements in its interior. The star has gone through several fusion processes in its core in order to support itself against gravity, and each fusion process has left behind certain "ashes" of heavy elements, like oxygen, carbon, neon, silicon and iron. When the star develops an iron core, that's the curtain fall - or the shrill bugle horn command to explode - for this star.
A star that is massive enough to go supernova will typically shed more than half of its mass even before going out in a blaze of glory. This expelled mass will itself be enriched in heavier elements. And when the star eventually explodes for real, still more heavy elements are created in the explosion itself, and these elements are explosively blasted out into the surrounding cosmos, where these heavy element-rich gases will eventually mix with other gas clouds and become the birth clouds for new stars.
But stars that are as old as the ones that make up the oldest globular clusters were formed from gas that was very pristine and contained very small amounts of heavy elements. Astronomers still haven't found a single star that is completely uncontaminated by heavy elements, so that it consists of hydrogen and helium only (that is, a star that is so pristine that it contains only the elements that, along with lithium, were created in the Big Bang). But old stars will contain very small amounts of heavy elements.
And this affects a star's evolutionary track. Consider how a solar mass, solar metallicity star will evolve:
The Sun will stay on the main sequence for about 9 billion years, fusing hydrogen to helium in its core. But after 9 billion years the Sun will have used up its core hydrogen, and then it will start fusing hydrogen in a shell around its core. As it does so, the Sun's outer atmosphere will expand and cool (and get ever redder), while simultaneously its core will shrink and become ever hotter.
Eventually the Sun's core will become hot enough to start fusing helium. The moment when this happens is called the helium flash (no, don't ask me to explain). The Sun will now fuse helium to oxygen and carbon in its core, and simultaneously it will shrink somewhat, and its outer layers will become hotter. As this happens, the Sun will become "more yellow" and "not so red". As long as the Sun is fusing helium to oxygen and carbon in its core, its evolutionary stage is on the so called horizontal branch. Even though the Sun will shrink from its red gianthood when it enters the horizontal branch, it will still be larger, cooler and redder than the Sun is today.
But a metal-poor solar mass star's evolutionary track is different. Just like the Sun, it will expand and cool and grow red when it has run out of hydrogen in its core, and it will support itself by fusing hydrogen to helium in a shell around its core. But when a metal-poor star's core becomes hot enough to start fusing helium, the star shrinks dramatically, and its outer layers become very much hotter, hotter than the Sun's outer layers are today.
Or to put it differently: When a metal-poor star is on the horizontal branch, it will become blue for real.
After a solar mass star has used up the helium in its core, it will become a red giant for the second time. It will become larger, redder and cooler than it was during the first time when it was a red giant.
As for today's APOD, NGC 6355 is apparently one of the most metal-poor globular clusters in the Milky Way, which explains why it contains many blue stars. The horizontal branch stars are evolved and much brighter than they were when they were on the main sequence, which explains why they stand out in a good color picture.
However, we must always remember: The final appearance of a color image is due to a combination of the filters used for the image acquisition and the processing done to create the final image. If the picture that is today's APOD had been acquired with different filters and processed differently, all the blue stars that we see in today's APOD may not have looked blue at all.
Ann
Last edited by Ann on Mon Jan 30, 2023 6:57 pm, edited 2 times in total.
Some of these are "Blue Stragglers"- stars that should have turned off the main sequence long ago but which have remained as a result of likely collision with another star within the cluster (like a binary pair colliding), resulting in a star with increased mass, increased luminosity, and higher temperature (blue). A nice description of blue stragglers can be found here: https://www.mso.anu.edu.au/~jerjen/rese ... ggler.html. I also describe them more briefly on my website from a few years ago, in the context of M13 and M5: http://www.starrywonders.com/m132019small.html.
With the very high resolution of HUBBLE, you can also see the star chains very well. The more wavelengths are combined, the more the holes in the chains fill up. There are practically no normal (white) stars in the cluster that cannot be assigned to a chain. Interesting that this phenomenon of "magnetized" star chains is not really discussed in the literature.
AVAO wrote: ↑Mon Jan 30, 2023 9:38 pm
With the very high resolution of HUBBLE, you can also see the star chains very well. The more wavelengths are combined, the more the holes in the chains fill up. There are practically no normal (white) stars in the cluster that cannot be assigned to a chain. Interesting that this phenomenon of "magnetized" star chains is not really discussed in the literature.
Because I don't think it has any physical meaning. It's just our eyes finding patterns... patterns that are just coincidental. I did an experiment years ago where I created random dot patterns, and people would find the same kinds of "chains", and sometimes other patterns, too. But our brains are very tuned into linear patterns.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Gacrux has the MK system stellar classification of M3.5 III.[3] It has evolved off of the main sequence to become a red giant star, but is most likely on the red giant branch rather than the asymptotic giant branch.
So Gacrux is probably on its first ascent to red gianthood after using up its core hydrogen, not on its second ascent after using up its core helium.
Wikipedia wrote:
Although only 50% more massive than the Sun, at this stage the star has expanded to 120 times the Sun's radius. It is radiating roughly 760 times the luminosity of the Sun from its expanded outer envelope. With an effective temperature of 3,689 K, the colour of Gacrux is a prominent reddish-orange
It's fascinating, isn't it? Gacrux is not a hugely massive star at all, but it has grown to a size 120 times the Sun's radius, emitting 760 times the Sun's luminosity from its enormous photosphere. That's what red gianthood does to a star!
But Gacrux will become paler, fainter and smaller, although still larger, redder and (much) brighter than the Sun, when it enters the horizontal branch of its life cycle.
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".
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
Last edited by Ann on Tue Jan 31, 2023 7:51 am, edited 3 times in total.
Thanks Ann, Steve, and Chris for explaining why old stars can still be blue. And now for a bit of thread drift, would anyone care to respond to this question?
During its 4.8 billion years of existence, our Sun has likely accreted or "ingested" a significant amount of rocky or metallic elements via planetesimal, asteroid, comet, or even planetary impacts. These dense non-fusible elements descend to the Sun's core, displacing or interfering with the fusion that would otherwise occur there. Does ingestion of non-fusible elements after star formation affect the color, evolution, or life expectancy of the star?
javachip3 wrote: ↑Tue Jan 31, 2023 7:40 am
Thanks Ann, Steve, and Chris for explaining why old stars can still be blue. And now for a bit of thread drift, would anyone care to respond to this question?
During its 4.8 billion years of existence, our Sun has likely accreted or "ingested" a significant amount of rocky or metallic elements via planetesimal, asteroid, comet, or even planetary impacts. These dense non-fusible elements descend to the Sun's core, displacing or interfering with the fusion that would otherwise occur there. Does ingestion of non-fusible elements after star formation affect the color, evolution, or life expectancy of the star?
I have found a single article that appears to answer the question of how the Sun's metallicity will affect its life expectancy. Unfortunately, the article is behind a paywall, and I'm not going to start a free trial in order to read it.
I'll offer you my opinion on your other questions. I don't think it will matter much at all if the Sun ingests a few more asteroids or comets. That is because the Sun vastly outweighs everything else in the Solar system put together, and it already contains much, much more metals than the combined mass of the asteroid belt and the Kuiper belt put together.
Taking into account the nine particularly large TNOs that are known about, and a series of three rings within the Kuiper Belt, Di Ruscio estimates that the total mass of the Kuiper Belt is around 3.6 x 1023 kg – or about 6% that of Earth.
Our Sun has more than a thousand Earth-masses of iron, for example.
So I don't think that the Sun is going to be significantly affected at all by swallowing a large number of comets or wayward asteroids that happen to come its way.
Obviously the Sun is redder in color than it would have been if it had been metal-poor. Its evolution is also affected by it being metal-rich, since it will not turn blue (but rather yellow, with a just a hint of orange) after it has ended its first red giant stage and entered the horizontal branch.
As for the life expectancy of metal-rich stars, I did actually find this snippet of info (after much searching, and after I had started writing this post):
No, quite the contrary. At a fixed mass one can show that the luminosity on the main sequence relates to metal mass fraction Z, as L∝Z−1/6. i.e. High metallicity stars have a lower luminosity than more metal-poor stars of the same mass. As the fuel supply is dominated by hydrogen, and as a mass fraction this is very similar in both metal-rich and metal-poor stars, then the lifetimes of metal-rich stars are longer. e.g. see Figs 1 and 2 of this paper by Bazan & Mathews (1990).
So it appears that the Sun will be granted a slightly longer life thanks to the fact that it is metal-rich.
Why would metal-rich stars live longer than metal-poor ones?
The reason would be that a higher amount of metals in the star's core would interfere somewhat with the fusion processes and slow them down. Given than most stars will never replenish their supply of prime fuel, which is hydrogen, it makes sense that a star can live longer if its uses its fuel more sparingly.
In 1971, a 17-year-old girl crashed with her passenger plane in the Peruvian jungle in the Amazonas, and she had to make her way to som e sort of civilization in order to survive. I read somewhere that she had noting to eat apart from a small bag of sweets, and she survived for 11 days by eating one piece of candy a day and drinking water from the rivers until she found a human settlement.
Tiny stars like Proxima Centauri probably eat the equivalent of one piece of candy a day, while the Sun eats a couple of hamburgers. The most massive stars, by contrast, not only eat everything that is on the menu, but they eat the entire restaurant as well - and they have to ingest a new restaurant every day.
The problem is that the massive stars find their restaurants inside their own bellies. And the Sun finds its hamburgers inside itself, and Proxima Centauri finds its little pieces of candy.
Because stars shine by cannibalizing themselves.They fuse hydrogen (and later helium and sometimes even heavier elements) that they find inside themselves and fuse these elements into more massive elements. But in the process, they "lose" a tiny bit of themselves which turns into photons that later escape the stars as light and heat.
Because the Sun's metals will make the Sun's hydrogen fusion proceed a little bit slower, the Sun will stay on the main sequence a little bit longer than it would have done if it had been a metal-poor star.
Thanks very much Ann. The universe will be a strange place in a hundred billion years, with all of today's stars either gone supernova or turned into cold black dwarfs, each with thousands of Earth masses of heavy elements and crushing gravity.
Globular Star Cluster NGC 6355 from Hubble
