Starship Asterisk*

rstevenson wrote:...And thanks, Ann, for sharing your knowledge of and enthusiasm for the subject of star colour. I always find your explanations clear and thorough.

Rob

I concur!
Margarita

Thanks, Rob and Margarita!

Ann

I agree with Rob and Margarita, too
Ann, your explanation about stars are excellent

Ann wrote, at the beginning of this thread
In the diagram of the stellar content of globular cluster M55,

At the time, I just looked at the horizontal bar of blue stars in the top left, but today, focusing a bit more closely, I notice blue stars really low down at bottom left and also some greenish ones also scattered around in the quadrant bottom left. Are these all blue dwarfs?

As you know, I still have a very l o n g way to go in getting an understanding of how to read a H-R diagram clearly.
(But - I know a lot more than I did two months ago! )

Margarita

Any stars at the bottom of the color-magnitude diagram must be faint. Blue stars are hot, so in order to be faint, they must be very small.
Take a look at this picture (an artist's conception) of Sirius A and B. Sirius A is the star that twinkles so brightly in our skies, 23 times brighter than the Sun and barely nine light-years distant. Sirius B, on the other hand, is a white dwarf.







Jim Kaler wrote about Sirius B:

(Sirius') greatest claim to fame may be its dim eighth magnitude (8.44) companion, Sirius B, which is visually nearly 10,000 times fainter than the bright star, Sirius A. Sirius B, however, is actually the hotter of the two, a blue-white 24,800 Kelvin. Though typically separated from each other by a few seconds of arc, Sirius B is terribly difficult to see in the glare of Sirius A. The only way the companion star can be both hot and dim is to be small, only 0.92 the size of Earth, the total luminosity (including its ultraviolet light) just 2.4 percent that of the Sun.

Sirius B is so small because it has shed "all of itself" except its small hot core. Jim Kaler wrote:

Sirius B may once have been a hot class B3-B5 star that could have contained as much as 5 to 7 solar masses, the star perhaps losing over 80 percent of itself back into interstellar space through earlier winds.

Margarita, you asked about the green stars. There are, technically, no green stars, simply because the human eye is incapable of seeing stars as green. The reason for this is undoubtedly that the Sun is the perfect example as a green star, since it reaches the peak of its light curve in the green part of the spectrum. To us, however, the total light of the Sun is white, and the light of the Sun that passes through the atmosphere gets separated in one blue component (the sky) and one bright yellowish point source (the solar disk as seen through the atmosphere). And because the Sun doesn't look green to us, no other stars whose light curves are similar to the Sun's look green to us either.
Take a look at this color-magnitude diagram again. The "fat diagonal line of stars" that seems to run from lower right and up towards the upper left is the so called main sequence. The stars that are on the main sequence fuse hydrogen to helium in their cores. The more massive these stars are, the bluer they are. The more light-weight they are, the redder they are.

The more massive the stars are, the sooner they use up the hydrogen in their cores. When that happens, they reach the turn-off point. At the turn-off point, the stars start to shine by other means that core hydrogen fusion, for example by core contraction. I believe they also fuse hydrogen to helium in a shell around their cores. At this stage, the stars grow larger, brighter and redder. They are now on the red giant branch.
As the stars move up along the red giant branch, their cores get hotter and hotter due to core contraction. At a certain stage, their cores become so hot that they can support core helium fusion. The stars now fuse helium to oxygen and carbon in their cores. This causes the outer layers of the star to contract and become hotter. Why is that? I don't know.

The blue helium-burning stars are on the blue horizontal branch. However, only very metal-poor stars ever become hot enough to be blue during their helium burning phase. The diagram on the right shows the infrared J-K color-magnitude diagram of globular cluster 47 Tuc. Although 47 Tuc is very metal-poor compared with the Sun, it is metal-rich compared with most globulars. It is too metal-rich to support a blue horizontal branch. You can see a tiny red little "wing" pointing to the left in the red part of the diagram. (Don't ask me why part of the diagram is red and the other part is blue.) Anyway, that red little wing is the red horizontal branch of 47 Tuc. These stars are also called the "red clump stars", and since they are all more or less equally bright, they have been used as distance indicators in attempts to refine distances to nearby galaxies.

The stars on the horizontal branch, whether on the red or the blue part of it, eventually become unstable. I'm not sure how it happens, but I believe that the stars begin "switching" between core helium fusion and hydrogen fusion in a shell around the core. They now leave the horizontal branch. The stars become variable, and the variability becomes worse and worse. The stars grow ever larger and redder. They are now on the AGB branch, the Asymptotic Giant Branch.

Eventually, the stars can't grow any bigger, and they reach the top of the red giant branch (which is the same as the top of the Asymptotic Giant Branch, I think). An example of a star at the very top of the AGB branch is Mira. Jim Kaler wrote about Mira:

The star is approaching the last stages of its life. Long ago, the hydrogen fusion that powered its core ran out, and then the by-product of that fusion, helium, fused to carbon and oxygen, and now the helium has also run out. The result of these internal changes is a hugely distended, very luminous star. The light variations are caused by pulsation, changes in size that also affect the star's temperature and thus the amount of light that leaks out at visual wavelengths (the infrared variation nowhere near so large).

When the star has reached the top of the AGB branch, it starts to die. Jim Kaler wrote:

Mira's great size and instability promote a dusty wind that blows at a rate of about a tenth of a millionth of a solar mass per year (10 million times that of the solar wind) that will soon evaporate away its outer envelope to produce an ephemeral planetary nebula (such as the Ring or Saturn nebulae), the inner nuclear burning portions of the star eventually condensing into a burnt-out white dwarf, a tiny star the size of Earth, the rest of the star lost to interstellar space.

So the bright red AGB star is just preparing to become a planetary nebula and a white dwarf. I think it is important to understand that this transformation doesn't happen instantly. I think that some of the blue-green stars that appear to be anomalously placed near the bottom of the color-magnitude diagram are at stages where they have not fully exposed their hot cores and haven't become as blue as they will be when they are "full-fledged white dwarfs". Then again, I don't pretend to understand the location of every star in this color-magnitude diagram!

Looking at the color-magnitude diagram of globular cluster M55 again, we may note what seems like a scarcely populated extension of the main sequence extending to the upper left beyond the turn-off point. These stars seem to be too hot and massive for their age. They should have used up their core hydrogen at this stage of their evolution and entered the red giant branch. Why haven't they?

These stars, which are called blue stragglers, are believed to have received an extra helping of hydrogen into their cores and thus have gained an extended lease of main sequence life. Astronomers believe that they may have been interacting with a companion star, "stealing" hydrogen from their companion.

Ann

Thanks again, Ann.

When I wrote "green stars" that was a somewhat lazy shorthand for "dots that seem to my eyesight to be a somewhat greenish colour - possibly teal? - that represent stars"! I recall you telling me that there are NO green stars, which is why I wondered what these dots really signified. (Just a note about my colour vision: my husband has noticed that I often describe some colour which is in the turquoise/teal range as green which he sees as being more in the blue range of hues. So I wasnt certain exactly what colour was intended on this diagram.)

Margarita

Take a look at this picture of the Summer Triangle. At center right, at about three o'clock, you can see one star that could probably be described as "teal-colored". It is Altair, one of the bright stars of the Summer Triangle. This star is of spectral class A7IV-V. Its temperature is 7550 degrees Kelvin, and its B-V index is +0.221. Altair is 11 times more luminous than the Sun.

At about eleven o'clock in the picture you can see a bright blue star. This is Vega, a main sequence star of spectral class A0. Vega is 48 times brighter than the Sun, with a temperature of 9500 degrees Kelvin and a B-V index of -0.001. At about eight o'clock in the picture, just above the red North America Nebula, you can see Deneb. This is a supergiant star of spectral class A2, with a whooping luminosity of some 50,000 times that of the Sun and a B-V index of +0.092.

Altair is smaller, fainter and cooler than Vega, and it is cooler and very much smaller than Deneb. It is, however, larger, brighter and hotter than the Sun. I's say that stars like Altair, stars of spectral class F and late class A, may qualify as "teal-colored" stars.

Ann

P.S. If Jerry Lodriguss' delightful picture should disappear - because Jerry Lodriguss doesn't like his pictures being hotlinked - then google Jerry Lodriguss Catching the Light Summer Triangle. Then you should find the picture again.

Speaking of HD 140283, showed that the star I could silently consumed, without explosion.
Ann, corrected me, saying that HD 140283 is the usual sequence and must
become in a red giant,etc.
But I found that if there may be a case where at the end of life of a star, there
isn´t explosion
When at the end of his life, a super star of more than 50 solar masses,
created inside a super massive black hole, this practically off the light
from the star, like turning off a light bulb!!

This thread is very informative. Thank you for beginning it Ann. I just found it today. It seemed a familiar subject, and sure enough, Ann’s first comment and several others link to an S&T newsblog about HD 140283 and its great age:

Ann wrote:Read about HD 140283 here, where you can also see a picture of the star.
Ann

I had made some of the comments on that article’s thread, and I was wondering about the veracity of something I had written therein:

“…Before the very first stars formed the gas that they were to form from had to cool off. This early gas only contained Hydrogen, Helium and some Lithium, as no heavier elements were generated in the big bang. Stars of today actually can begin fusion reactions at lower core temperatures than the first stars due to the presence of the elements Carbon, Oxygen and Nitrogen. These elements serve as thermonuclear catalysts, allowing H to be fused into He at lower pressures than what is required if the C, O & N are not present. As these elements were missing from the first generation of stars the first stars had to be much more massive than the stars of today to generate the higher pressures and temps required to fuse H directly. The masses of the first stars are thought to have been up to 200 suns, but once they ignited they would have burned through the elements up to iron very quickly and then they would have all went supernova relatively quickly. None of these earliest generation of stars still exist today. Galaxies would have started out just as denser gas pockets in the early universe. Any old stars in the Milky Way such as this HD 140283 would have either always been a member of this galaxy or it may have come from one of the numerous smaller galaxies that the Milky Way has cannibalized over the millennia.”

I respect the advanced astronomical knowledge of many of those who post here, and I want to improve the accuracy of my statements as much as possible. Is what I wrote correct? I wonder most as to the part about C,O and N serving as thermonuclear catalysts. Is this correct for solar and lesser massed stars? Since the time of my writing the above I have learned that the main reaction in the solar sized stars is the proton-proton chain, and that C-O-N reactions predominate in stars more massive than ours. Do I need to make a correction to the quoted comment?

Bruce

Bruce, you might find this link helpful Jim Kaler's Osher Lifelong Learning Institute course lectures
I've certainly found them helpful. I've worked through them once and am now re- reading them.
Margarita

MargaritaMc wrote:Bruce, you might find this link helpful Jim Kaler's Osher Lifelong Learning Institute course lectures
I've certainly found them helpful. I've worked through them once and am now re- reading them.
Margarita

Thank you for that Margarita. First I want to point out how very impressed I have been with how much and how quickly you have learned about astronomy. You’re climbing the steep learning curve at an amazing rate!

Jim Kaler’s link you provided is an excellent reference; however it is targeted toward beginners. I looked through his site and found that my question went beyond the scope of what he covered. The answers I’m seeking get into the realm of theoretical nuclear astrophysics. I’ve been reading about this subject off and on for over four decades now, since the question of how all the elements came to be has fascinated me ever since I was a child. But, at the rate you’re learning Margarita, you may very be able to answer my questions very soon yourself!

It may help if I rephrase what I’m wondering about. The very first stars to form (called Population III) contained only H, He and Li and the Li would have quickly fused with H, and then split, forming He. It is theorized, (since no Pop III stars have been found yet) that these completely metal-less stars could have been extremely massive, which would account for the fact that none are still around in this epoch. But would these stars have HAD TO HAVE BEEN more massive than typical main sequence metal enriched stars are today? Does the complete absence of metals require stars to have higher core temps/pressures and therefore greater masses for fusion to continue?

It may seem that my inquiry is off the topic of this thread, but I don’t think that it is. HD 140283 is one of the most metal poor stars ever found, and yet it is LESS massive than the Sun. Apparently, the meager metal content of HD 140283 still allows this star to shine and evolve more or less normally (assuming that metal content is a requirement for low mass stars to fuse at lower core temps). Would that be the case if it had no metal content at all? If so, then where are all the low mass Pop III stars?

Bruce

I didn't reply before because I don't feel qualified to do so, Bruce, but you may want to listen to my musings anyway.

I think we should be careful when we make assumptions about the very first stars and before we conclude that they were necessarily all very massive.

The way I understand it, dust - that is, elements that have been created inside previous generations of stars - is considered necessary for star formation to occur in the present-day universe. Today, star formation takes place inside cold, dense clouds of gas and dust. Without the dust content, gas clouds are not believed to be able to cool and condense to the point where the force of gravity makes them collapse so that they can make new stars.

This creates a conundrum, however. The very first stars had to form out of gas clouds that contained no heavier elements at all. How could these stars form in the first place? It would seem that they couldn't exist at all. It's like asking what came first, the chicken or the egg.

But I think we should be careful about making very strong assumptions about things that we don't know how to check in the first place. How do we really know that all the "pioneer stars" that formed in the universe were very massive? Why is that necessary?

The primordial universe was a very, very, very different place compared with the universe of today. In the extremely early universe, there were incredible amounts of "free gas", for example, and the universe itself was small and cramped compared with what it is like today. How do we know that all that gas inside that small universe wasn't "sloshing around", and how do we know that the resulting "collisions" didn't give rise to stars of many different sizes and masses?

So I feel unconvinced that the first stars were all very massive. But suppose, nevertheless, that they were. These first massive stars must have exploded as supernovae very quickly, rapidly seeding the universe with metals.

HD 140283 is not metal-free. It is metal-poor. Therefore HD 140283 is not a Population III star.

You asked:

But would these stars have HAD TO HAVE BEEN more massive than typical main sequence metal enriched stars are today? Does the complete absence of metals require stars to have higher core temps/pressures and therefore greater masses for fusion to continue?

I'm very sorry that I wouldn't know that at all, Bruce. But think of it like this. HD 140283 is one of the most metal-poor stars known, that's true, but even so I think it can be compared with stars inside many globular clusters.

Here you can see a Hubble picture of globular cluster Omega Centauri. Many of the stars inside this mighty and very old cluster are smaller and a lot less massive than the Sun. Yet their hydrogen fusion is working fine.

We often hear astronomers being surprised by their discoveries. We are told that astronomers are surprised to discover a massive galaxy existing at such an early epoch that no galaxies were supposed to exist at that time at all.

You asked:

Apparently, the meager metal content of HD 140283 still allows this star to shine and evolve more or less normally (assuming that metal content is a requirement for low mass stars to fuse at lower core temps). Would that be the case if it had no metal content at all? If so, then where are all the low mass Pop III stars?

I don't presume to know the answer to this question at all. It could be, indeed, that all the Population III stars that formed were so massive that they have all died by now. It could be that the processes that formed Population III stars mostly formed massive stars, and that only relatively few metal-free low-mass stars were formed. If you bear in mind that star formation in the Milky Way has been going on for twelve billion years or so, it is easy to see that the few Population III stars that remain today (if any), are going to be hard to find in the Milky Way haystack. Remember that the only stars that can have formed with no metals at all are those that formed before the first nearby supernova had polluted their "birth clouds".

HD 140283 is not a Population III star, and its natal cloud was "polluted". The star has no problem fusing its hydrogen into helium. That's all I can really say about the matter.

Ann

Thank you for that very interesting comment Ann. (I apologize for the delay in posting this response, due to computer issues.) Your musings are insightful and did help answer my questions to a large degree. I liked your point about dust being “considered necessary for star formation to occur in the present-day universe”, which somehow I had failed to learn before now. That does make sense, as does your point about the much greater gas density of the early universe contributing to the massive size of the first stars.

I agree that we shouldn’t just assume that ALL Pop III stars were massive, as your point about very old globular clusters well illustrates. While GC stars are not Pop III they are close, as is HD 140283, indicating at least that very little metal content would be required for the formation of Dwarf stars smaller than the sun. I know that the search for the oldest stars is an active field of study, so indeed even older, completely metal-less Pop III stars might pop up, but the fact that they haven’t so far is telling I think. Since the more massive a star is the shorter its lifetime is, extremely massive Pop III stars would have been required to explain the observations of very old Pop II stars. If many smaller than the sun Pop III stars had formed then it seems like at least a few examples should have been found by now since small stars burn through their fuel so slowly.

Neufer’s last post in this discussion was to a fine Astro Bob article on this star entitled “Wise up with a visit to the Methuselah Star”:

Since we know a star can’t be older than the cosmos, the 800 million year “give or take” factor places the star within the margin of error, making it at least 13.2 billion years old. Because HD 140283 contains a small percentage of heavier elements, so we know it can’t be a survivor from the very first generation to form in the aftermath of the Big Bang. Stars then were virtually 100% hydrogen and helium. Still, it’s far older than the sun and the oldest known star for which a reliable age has been determined.
It would appear then that very little time passed between the first generation of stars, which formed about 13.4 billion years ago, and the next generation, which would have included our feature star. Either way, HD 140283 is a fossil in our midst. You can read Bond’s full paper on the topic HERE.

The last link in the above quotation is to the paper that generated this discussion in the first place. An important finding about HD 140283 was that, while it has extremely low iron content, it does have significant oxygen content. Indeed it is this enhanced O content that allowed the age estimate to be shortened enough to resolve the apparent conflict with the age of the universe. Therefore the presence of O and nearby elements, at least in stars from Orange Dwarfs upward, does help them burn H into He at faster rates.

Bruce