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Thread: Correcting errors in the "Explore" encyclopedia

  1. #151
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    Quote Originally Posted by Ken G View Post
    Nope, it certainly is incorrect as in physically wrong.
    I was talking about education in general not an "it". We have a total of 1 physically wrong explanation in this thread: What has been demonstrated is that there some (five) web pages that attribute increases in luminosity with mass to increases in core pressure and density.

    That is a "lie to children". If we taught high school children and undergraduates the details of stellar models and made them run a few then they would find that core pressure decreases with mass. Or the teacher could just assert it and the students would have to just believe it.

    It is physically correct to tell a student that a higher mass star has a higher core temperature due to compression because more mass = more compression = higher temperatures. That would be incomplete since it does not mention hydrostatic equilibrium, e.g. the higher temperature cause more fusion which expands the star lowering temperatures to balance out the compression and thermal pressure. We have no evidence that this happens in science lessons or even in textbooks.

    Every lesson about classic mechanics is incorrect because we know that classical mechanics is a low velocity approximation. It is appropriate to teach classical mechanics as a first step to learning this.
    Every lesson about Newtonian gravity is incorrect because we know that Newtonian gravity is a low gravity and velocity approximation. It is appropriate to teach Newtonian gravity as a first step to learning this.
    Teachers should mention that they are teaching an approximation.

  2. #152
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    Quote Originally Posted by Reality Check View Post
    What has actually been shown, using real physics equations even, is that there is not compression involved in the explanation of higher luminosities for larger masses, there is less compression involved. It's a simple error of forgetting that gravity also depends on radius, I just don't see how you don't get that. But what is also true, and you are of course welcome to test this or pretend it isn't true, that's up to you, is that it is also in every intro astro textbook you can find that offers any explanation at all. This is a fact, whether you know it or not. It is the reason for the thread. The intro material has totally dropped the ball on the mass-luminosity relation, one of the most fundamentally important relations in all of astronomy. Yup, just that. So we are not talking about children and bedtime stories, we are talking about college students trying to learn what scientific thinking is, and they need to be shown what it actually is. You just don't get it, what the rest of us are talking about here is an error, a mistake, a commonly repeated blunder-- not some intentional oversight. Any author exposed to this error should change it immediately, and they would of course do so, if they are doing an updated edition.
    It is physically correct to tell a student that a higher mass star has a higher core temperature due to compression because more mass = more compression = higher temperatures.
    Totally wrong. You can lie to children, but don't try to lie to us, we are too savvy. I already gave you a perfectly normal physical situation that completely destructs your claim here-- the Algol binary. There, you have a main-sequence star gaining mass slowly, and obeying the mass-luminosity relation all the while. That star over time exhibits more mass and a higher core temperature-- but is always expanding, no compression anywhere. It's just bunk.

    Now, you can hold any personal opinion you like. You can think it's fine to say that little fairies cause the mass-luminosity relation, and call it a "lie to children." But to people who want to understand the mass-luminosity relation, their first step is rejecting the wrong explanation that higher mass leads to stronger gravity, more compression, higher core pressure, higher core temperature, and faster fusion. Rejecting that should be easy once they understand these facts, all of which are quite well known to stellar experts:

    The correct reasons that higher-mass main-sequence stars have much higher luminosity have essentially nothing to do with any of these, as long as we select from the range 0.5 to 50 solar masses:
    1) fusion (this one does produce a small effect because of how it weakly feeds back onto the stellar structure, but this can be safely ignored, especially for CNO-cycle fusion)
    2) stronger gravity (i.e., a higher g, this should be ignored because it's wrong)
    3) compression (also wrong)
    4) higher pressure (just plain wrong)

    On the other hand, it has everything to do with these:
    1) radiative diffusion
    2) weaker gravity
    3) expansion
    4) lower pressure

    Once that baloney is finally cleared up, discarding volumes of intro texts and websites on the way, someone who actually wants to understand the mass-luminosity relation can begin to assemble the key parts:
    1) the rate that radiation leaks out of a star by diffusing out from great depth (here we must recognize that the mass-luminosity relation is poorly fit by stars of less than about 0.5 solar masses, or above about 50, which are convective).
    2) the internal structure that determines what the radiation is diffusing through is set by force balance between gravity and gas pressure, which can be summarized by the virial theorem (here we must recognize that the mass-luminosity relation is poorly fit by stars of greater than about 50 solar masses, for whom radiation pressure dominates gas pressure).
    3) combining these two key elements with some simplified treatment of the opacity in the star gives you the mass-luminosity relation without telling any lies at all, just a few simplifying idealizations. You hardly even need to know the radius, gravity, or temperature of the star-- the luminosity can be determined almost without any of those, depending on how rough of an opacity idealization you are willing to make!

    The bottom line is, the physical reason main-sequence stars between about 0.5 and 50 solar masses obey a single simple mass-luminosity relation is because those stars are exactly the ones whose energy transport in their interior (the key to understanding their luminosity) is primarily by radiative diffusion. This means they are essentially "big leaky buckets of light." The rate the light leaks out gives the luminosity, and this turns out to depend only on the mass, and in a simple way, for reasons that are crucial to understand, if one is to understand the first thing about stars. Hint: don't look at intro textbooks, they don't have that understanding. Use more advanced books, or read this thread-- very carefully.
    Last edited by Ken G; 2016-Nov-17 at 09:23 PM.

  3. #153
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    My coldly logical mind says that astrophysicists who have it right are remiss in not challenging textbook writers who may be leading entry level college students astray. Another side of me acknowledges that I have not challenged the multitude of map publishers and GPS programmers who have made it hard for plumbers and electricians to find my house, so perhaps I should not complain about the astrophysicists.

  4. #154
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    There may be a parallel with map-making. It is said that maps sometimes include "paper towns", which are made-up landmarks that don't exist, placed on a map as a kind of trap for anyone who would copy that map without proper copyright privileges. Whoever first inserted the bogus explanation for the mass-luminosity relation, it seems to have served the same purpose-- it's poor physics, but it shows who copied who if you can find where it first appears!

  5. #155
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    It might be an interesting research project to try to trace the paper trail back over the past century and find the smoking gun, that is, a book or paper that either misquotes an astrophysicist who had it right or else made up the wrong explanation out of thin air.

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    Indeed, it would be fascinating for a lot of reasons just to see what was in intro texts 50 years ago!

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    Quote Originally Posted by Hornblower View Post
    It might be an interesting research project to try to trace the paper trail back over the past century and find the smoking gun, that is, a book or paper that either misquotes an astrophysicist who had it right or else made up the wrong explanation out of thin air.
    Funny you should mention that...

    [I'm curious if my logic makes sense, so I will throw this out there in hopes I have a causal view of Ken's claims.]

    "The Quiet Sun", 1973, Edward Gibson written to educate those going to Skylab with focus on the surprising temp gradient of the corona. It states:

    P = NkT for a unit volume, no surprise, but notes that ionization doubles N for hydrogen. This implies pressure doubles, though he goes into mean atomic weights. But increasing pressure will cause expansion and, with ionization, lower opacity, further causing expansion. This, I think, would reduce density and move the tacholine zone (where recombination occurs and opacity increases) farther from the core.

    Helium ionization triples the paticle count. It is unclear how the transmutation to He would affect density given all the variables, but, that aside, it looks like density would decrease due to the expansion since local gravity is reduced with expansion.

    I haven't read enough of this fairly comprehensive book to be too critical, but I suspect it had the opportunity to address what seems would become misleading regaerding density, but ignored it.

    My Jupiter case was to serve as the easy example one could hold for greater density with mass addition. Ionization, I think, makes the flip. [ I sure hope this is so because I'm wearing weak borrowed glasses typing poorly on an iPad.]
    Last edited by George; 2016-Nov-20 at 07:51 PM. Reason: Grammae
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  8. #158
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    Quote Originally Posted by George View Post
    Funny you should mention that...

    [I'm curious if my logic makes sense, so I will throw this out there in hopes I have a causal view of Ken's claims.]

    "The Quiet Sun", 1973, Edward Gibson written to educate those going to Skylab with focus on the surprising temp gradient of the corona. It states:

    P = NkT for a unit volume, no surprise, but notes that ionization doubles N for hydrogen. This implies pressure doubles, though he goes into mean atomic weights. But increasing pressure will cause expansion and, with ionization, lower opacity, further causing expansion. This, I think, would reduce density and move the tacholine zone (where recombination occurs and opacity increases) farther from the core.

    Helium ionization triples the paticle count. It is unclear how the transmutation to He would affect density given all the variables, but, that aside, it looks like density would decrease due to the expansion since local gravity is reduced with expansion.

    I haven't read enough of this fairly comprehensive book to be too critical, but I suspect it had the opportunity to address what seems would become misleading regaerding density, but ignored it.

    My Jupiter case was to serve as the easy example one could hold for greater density with mass addition. Ionization, I think, makes the flip. [ I sure hope this is so because I'm wearing weak borrowed glasses typing poorly on an iPad.]
    My bold. Can you elaborate on this? As I see it, a Jupiter-sized ball of hydrogen and helium with several times Jupiter's mass is already many times more dense than the upper limit for having it in the form of normal neutral atoms in their ground state. It must be dissociated into a state with the electrons and nuclei buzzing around much more closely spaced. I would need someone better informed than myself to say whether or not this is considered ionized.

  9. #159
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    Good point. Still shooting from the hip.., perhaps the ionization along a much greater radius (radiative zone and not so much the core region) from the very small core volume yields the doubling of particle count => increasing pressure => causing expansion and lowering opacity (from ionization) => decreasing local gravity => decreasing core density. I would guess the heat transfer from lower opacity would cause greater adjacent ionization as well. I am stumbling along, but it is an interesting adventure.
    Last edited by George; 2016-Nov-21 at 02:07 AM.
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  10. #160
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    Quote Originally Posted by George View Post
    Good point. Still shooting from the hip.., perhaps the ionization along a much greater radius (radiative zone and not so much the core region) from the very small core volume yields the doubling of particle count => increasing pressure => causing expansion and lowering opacity (from ionization) => decreasing local gravity => decreasing core density. I would guess the heat transfer from lower opacity would cause greater adjacent ionization as well. I am stumbling along, but it is an interesting adventure.
    If I am not mistaken, a star is fully ionized throughout, except at and very near the photosphere. That makes high opacity and very slow radiative transfer of energy. In the more rarified envelope of a more massive star the transfer is faster but still a snail's pace compared with what it would be if we had neutral gas instead of plasma throughout the envelope at the same radius and density. If I am off here, I welcome help from Ken G or anyone else who can help.

  11. #161
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    Quote Originally Posted by Ken G View Post
    What has actually been shown,...
    What has been actually shown in this thread is that for larger masses, there is less density and pressure as calculated in solar models.
    21 October 2016 from StupendousMan: Note that as the mass of a star increases from 0.5 to 7 solar masses, the central temperature goes UP by a factor of about 3, but the central density goes DOWN by a factor of about 8. The result is that the central pressure decreases with mass.

    What has been demonstrated is that there some (five) web pages that attribute increases in luminosity with mass to increases in core pressure and density.

    It is physically correct to tell a student that a higher mass star (a star that has mass added to it) has a higher core temperature due to compression because more mass = more compression = higher temperatures because that is basic physics (compressed gases can heat up ). It is not complete because it does not mention that higher temperatures = more fusion = more pressure = the star expands = temperatures decrease. It is especially not complete in undergraduate courses where the teacher may not go into the details of stellar modeling that shows that the resulting balance has the increase in temperature from compression + more fusion mostly canceled by the decrease in temperature caused by the star expanding.

    The topic remains education where "lies to children" are used.

    Unsupported assertions and an accusation of "baloney" in the mass-luminosity relation has nothing to do with the teaching of undergraduate astronomy courses. The physical facts are that if you add mass to a star the star will react with
    1. Stronger gravity (more mass !)
    2. Compression.
    3. Higher core pressure.
    4. Higher core temperatures.
    5. Greater rate of fusion.

    What happens then is that hydrostatic equilibrium is reestablished resulting in the core pressure actually decreasing (see the first link above) possibly because the core stabilizes at a higher core temperature (Tc).
    For that matter - do you realize the core temperatures go up in stellar models with mass (the Tc 21 October 2016 from StupendousMan. And that fusion rates go up wit increasing temperature? And that means that there are more photons released and thus a higher luminosity?

    OTOH, maybe we are getting into ATM territory - an ATM idea that an increasing rate of fusion does not make stars more luminous?

  12. #162
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    Quote Originally Posted by Hornblower View Post
    If I am not mistaken, a star is fully ionized throughout, except at and very near the photosphere. That makes high opacity and very slow radiative transfer of energy. In the more rarified envelope of a more massive star the transfer is faster but still a snail's pace compared with what it would be if we had neutral gas instead of plasma throughout the envelope at the same radius and density. If I am off here, I welcome help from Ken G or anyone else who can help.
    There's no question that ionization plays an important role, including doubling the number of particles and reducing the opacity. It also completely changes the ground state, because electrons are such a low-mass particle that they have a low momentum per kinetic energya, whereas neutral hydrogen is not even a fermion! If the question is being asked is, what would the Sun, or Jupiter, be like if we waved a magic wand and made them both neutral without changing their temperature, then they would indeed be a lot different. The Sun's radius would roughly double to maintain force balance at core fusion temperature, but Jupiter's ground state would be changed so completely it probably requires a very small radius to achieve it. So it's a question of when you are comparing the effects of de-ionization.

    Or if you just de-ionize Jupiter right now, and don't wait, it will immediately have its temperature go way up because the heavy neutrals will start acting like an ideal gas, barring van der Waals forces and the like, but it still won't expand unless you include all that ionization energy. Did the ionization energy magically disappear when we magically de-ionized Jupiter, or is it still going to be in there? The reason it won't expand without the ionization energy being included is that monatomic nonrelativistic gas pressure is 2/3 the internal kinetic energy divided by the volume, so simply doubling the particles doesn't change either the internal energy or the volume, you just have half the kinetic energy per particle. So the overall question is complicated, one must say what one is comparing when one waves the magic wand. In the solar atmosphere, you can assume that conduction maintains something like a smooth temperature, so going from neutral to ionized does double the pressure if there is not expansion. What happens is the "scale height" roughly doubles as the gas becomes ionized, but there the comparison is at fixed temperature.
    Last edited by Ken G; 2016-Nov-21 at 03:33 PM.

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    Quote Originally Posted by Reality Check View Post
    What has been actually shown in this thread is that for larger masses, there is less density and pressure as calculated in solar models.
    Nope, those are stellar models for zero-age main-sequence stars. I wouldn't highlight the wrong parts of your statements if I were you! And yes, I know the core pressure and density both drop with mass, that's what I've been telling you throughout.

    It is physically correct to tell a student that a higher mass star (a star that has mass added to it) has a higher core temperature due to compression because more mass = more compression = higher temperatures because that is basic physics (compressed gases can heat up ).
    Wrong. I already told you how wrong that claim would be for the Algol binary, right now. I have no idea what part of that you still aren't getting, but perhaps this challenge will help you think: let's say you just told a student that nonsense you quoted above, perhaps someone on this very forum. The student, being a scientific thinker, asks you "but then how come the main-sequence star in Algol is always obeying the mass-luminosity relation as its mass increases, yet the star is always only expanding, not contracting anywhere?" What say you to that student?

    You see, a good question will always destroy a poor explanation, and the poor instructor simply hopes the students are not smart enough to ask that good question. The good instructor says, "my goodness, you're completely right, the explanation I've been handing out all these years is sheer nonsense." Yup, that's just what the good instructor must say, it's part of the job.
    Last edited by Ken G; 2016-Nov-21 at 03:39 PM.

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    Quote Originally Posted by Hornblower View Post
    If I am not mistaken, a star is fully ionized throughout, except at and very near the photosphere.
    I don't have that impression because isn't the main reason for convection at the tacholine recombination (even if partial), which increases opacity and creates hot cells that become buoyant? [I read that somewhere.] There is a critical luminosity that, if exceeded, causes convection, which may explain the massive star core convections, but may not, seemingly, apply to the sun-like stars.

    Your much earlier post that shows how much greater in size are the massive stars (in ratio with their mass) would seem to be enough to verify Ken's arguments, but I am still curious about the cause, since ionization was a simpleton shot that fizzled....

    Ken's reference to the Henyey track is interesting. But, the PMS (not main sequence admittedly) for this track reveals a decrease in size since temperatures increases and luminosity remains about the same, so radius must shrink and, therefore, density must increase. But this is with a star with a very low density to begin with, so I don't think this in any way counters your arguments, Ken. What stands out to me is that massive stars have convective cores, thus greater heat transfer to the radiative zone, which, IMO (as an amateur), would simulate something like the red giant phase for a one solar mass star where fusion takes place farther from the original core radius. This should yield low density for massive stars even during their main sequence period.

    Am I close?
    Last edited by George; 2016-Nov-23 at 06:53 PM.
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    Quote Originally Posted by George View Post
    I don't have that impression because isn't the main reason for convection at the tacholine recombination (even if partial), which increases opacity and creates hot cells that become buoyant? [I read that somewhere.]
    You are probably thinking about helium, which is not fully ionized throughout. But hydrogen is-- except near the surface.
    There is a critical luminosity that, if exceeded, causes convection, which may explain the massive star core convections, but may not, seemingly, apply to the sun-like stars.
    Yes, very massive stars are convective over most of their interior.
    Your much earlier post that shows how much greater in size are the massive stars (in ratio with their mass) would seem to be enough to verify Ken's arguments, but I am still curious about the cause, since ionization was a simpleton shot that fizzled....
    The reason is simple-- stars contract until their cores are hot enough to fuse hydrogen. A more massive star doesn't have to contract as much to get there, so they are bigger and have lower density. It's a simple consequence of the core temperature being proportional to the gravitational potential energy per particle, which is proportional to M/R.

    What stands out to me is that massive stars have convective cores, thus greater heat transfer to the radiative zone, which, IMO (as an amateur), would simulate something like the red giant phase for a one solar mass star where fusion takes place farther from the original core radius.
    The fusion is still in the core, but you''re right that if the mass gets too big, there is too much convection, and the radiative diffuion argument breaks down. These tend to also be stars where radiation pressure is important, and they have their own mass-luminosity relation more like luminosity proportional to mass (this is also called "the Eddington luminosity").

    This should yield low density for massive stars even during their main sequence period.
    Just use T ~ M/R, that's the easy way to see that R is nearly proportional to M.

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    Quote Originally Posted by Ken G View Post
    You are probably thinking about helium, which is not fully ionized throughout. But hydrogen is-- except near the surface.
    Me thinks I too quickly jump onto causal explanations. Opacity increases exponentially with radius, and I imagined recombination as a big part of it.

    The reason is simple-- stars contract until their cores are hot enough to fuse hydrogen. A more massive star doesn't have to contract as much to get there, so they are bigger and have lower density.
    But this happens when it is very close, or equal to, the same mass as any star that begins hydrogen fusion. So a more massive stars simply continues accretion, which changes things, creating the incorrect view you address.

    It's a simple consequence of the core temperature being proportional to the gravitational potential energy per particle, which is proportional to M/R.
    But what does this look like in a physical explanation? If you don't mind another hip shot....if I understand the virial theorem, half the PE becomes heat, increasing temperature, increasing pressure. But pressure is free to do its thing -- push. So density would decrease as a result of the expansion. But by how much seems to be the crux (of the matter)? The ideal gas law doesn't tell us what happens to P & V as T increases. If P increases more than V increases, density increases, but the opposite would lower density. You are using other tools that reveal these degrees of change, which are still unclear to me, though I haven't read all your posts here, largely due to time constraints, even today.

    Just use T ~ M/R, that's the easy way to see that R is nearly proportional to M.
    But even here one could say that R decreases when T increases, which increases density. That's not what happens, but why?

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    Quote Originally Posted by George View Post
    Me thinks I too quickly jump onto causal explanations. Opacity increases exponentially with radius, and I imagined recombination as a big part of it.
    When opacity comes into play, you need to consider "metals" too, so they do recombine.
    But this happens when it is very close, or equal to, the same mass as any star that begins hydrogen fusion. So a more massive stars simply continues accretion, which changes things, creating the incorrect view you address.
    Not sure what you mean-- you only need 0.1 solar masses for fusion, so we are talking about stars with plenty of mass. The issue is more how much they need to contract before they can reach fusion temperature.
    But what does this look like in a physical explanation? If you don't mind another hip shot....if I understand the virial theorem, half the PE becomes heat, increasing temperature, increasing pressure. But pressure is free to do its thing -- push. So density would decrease as a result of the expansion.
    Yes, no matter how you think of it, the virial theorem gives you that T ~ M/R, and that has density consequences.
    But by how much seems to be the crux (of the matter)? The ideal gas law doesn't tell us what happens to P & V as T increases.
    You don't need the ideal gas law, you can just use P ~ GM2/R4 if you know M and R and want P. Then you get T ~ M/R from the virial theorem, but that does require the ideal gas law because of the reference to T.
    You are using other tools that reveal these degrees of change, which are still unclear to me, though I haven't read all your posts here, largely due to time constraints, even today.
    The key tool is the virial theorem with the ideal gas law, giving T ~ M/R. The consequences for density follow.
    But even here one could say that R decreases when T increases, which increases density. That's not what happens, but why?
    That is what happens if you fix M and consider higher T, you will be talking about more dense stars. As stars contract toward fusion temperature, their density increases.

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    Update: It looks like the author of the teachastronomy textbook this forum links to, Chris Impey, has made some important revisions, perhaps due to the comments in this thread (and/or an email discussion I had with him). At https://www.teachastronomy.com/textb...Main-Sequence/ it now says: "The luminosity of the star depends on the way radiation diffuses out from the hot central regions, subject to hydrostatic equilibrium at every point. The result of these calculations is a luminosity that depends strongly on temperature, and so on mass. Intriguingly, the energy generation mechanism does not feature prominently in this calculation." This is a vast improvement, correcting the problem I alluded to, and refuting claims by certain others that my position was "against the mainstream." No more. (It could be noted that the luminosity doesn't depend sensitively on internal temperature either, because as the star contracts, its temperature rises in such a way as to largely compensate for the rising density and retain a consistent total radiative diffusion rate, but this is kind of a detail that is not an essential correction. The main point is, the luminosity does not hinge on the fusion physics, but it does hinge on the radiative diffusion physics.)
    Last edited by Ken G; 2018-Apr-15 at 05:57 AM.

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    Quote Originally Posted by Ken G View Post
    Update: It looks like the author of the teachastronomy textbook this forum links to, Chris Impey, has made some important revisions, perhaps due to the comments in this thread (and/or an email discussion I had with him).
    Perhaps. But, success, you say
    At https://www.teachastronomy.com/textb...Main-Sequence/ it now says: "The luminosity of the star depends on the way radiation diffuses out from the hot central regions, subject to hydrostatic equilibrium at every point. The result of these calculations is a luminosity that depends strongly on temperature, and so on mass. Intriguingly, the energy generation mechanism does not feature prominently in this calculation." This is a vast improvement, correcting the problem I alluded to, and refuting claims by certain others that my position was "against the mainstream." No more. (It could be noted that the luminosity doesn't depend sensitively on internal temperature either, because as the star contracts, its temperature rises in such a way as to largely compensate for the rising density and retain a consistent total radiative diffusion rate, but this is kind of a detail that is not an essential correction. The main point is, the luminosity does not hinge on the fusion physics, but it does hinge on the radiative diffusion physics.)

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    It helps restore my faith in the self-correcting nature of scientific thought.

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    Quote Originally Posted by Ken G View Post
    Update: It looks like the author of the teachastronomy textbook this forum links to, Chris Impey, has made some important revisions, perhaps due to the comments in this thread (and/or an email discussion I had with him). At https://www.teachastronomy.com/textb...Main-Sequence/ it now says: "The luminosity of the star depends on the way radiation diffuses out from the hot central regions, subject to hydrostatic equilibrium at every point. The result of these calculations is a luminosity that depends strongly on temperature, and so on mass. Intriguingly, the energy generation mechanism does not feature prominently in this calculation." This is a vast improvement, correcting the problem I alluded to, ...
    A couple of years ago we had More massive stars have greater gravity that creates higher pressure in the stellar interior" and your unsupported assertion that was wrong. The scientific evidence in this thread that central pressure does go down with mass came 9 months later from StupendousMan
    One good source for information on stellar interiors is a website run by Lionel Siess, a stellar astronomer at the Institute of Astronomy and Astrophysics at the Université Libre de Bruxelles. Here's his main home page's address:

    http://www.astro.ulb.ac.be/~siess

    Since Dr. Siess has published some 30+ papers in the technical literature as first author, in addition to many as co-author, I think we can accept his work as authoritative. Again, his research area is stellar interiors and evolution.

    Now, he provides many very handy tools on his website that visitors can use to generate models of stars of various ages, masses, and chemical compositions. I used this one

    http://www.astro.ulb.ac.be/~siess/pm...ols/Isochrones

    ...
    Note that as the mass of a star increases from 0.5 to 7 solar masses, the central temperature goes UP by a factor of about 3, but the central density goes DOWN by a factor of about 8. The result is that the central pressure decreases with mass.

    Many thanks to Lionel Siess for providing these tools for all of us, in addition to his many papers!
    However the new text you quote does not actually say central pressure reduces with mass in stellar models.

    Understanding the Main Sequence
    Why does luminosity depend so strongly on mass? An increase in mass corresponds to a star with a substantially higher temperature in the core. The luminosity of the star depends on the way radiation diffuses out from the hot central regions, subject to hydrostatic equilibrium at every point. The result of these calculations is a luminosity that depends strongly on temperature, and so on mass. Intriguingly, the energy generation mechanism does not feature prominently in this calculation. That's why astrophysicists like Arthur Eddington were able to understand the main sequence even before the theory of the fusion process was developed!
    The bit that may be included from your comments is that knowledge of fusion is not needed (if I remember correctly all Eddington needed was gravity + gas laws + entropy?).
    Last edited by Reality Check; 2018-Apr-16 at 12:42 AM.

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    Quote Originally Posted by Reality Check View Post
    A couple of years ago we had More massive stars have greater gravity that creates higher pressure in the stellar interior" and your unsupported assertion that was wrong.
    What are you even talking about? My assertions were all correct. Why are you bringing in this non sequitur to the issue that was corrected that I just pointed out? The error you just quoted was a different one that I also pointed out. But do you finally realize you were mistaken all along? Read the thread, you were.
    The scientific evidence in this thread that central pressure does go down with mass came 9 months later from StupendousMan
    That may have been when you understood the situation. I did from the start, as that thread shows. Just read it again.
    However the new text you quote does not actually say central pressure reduces with mass in stellar models.
    Yes we call all read, I don't know if that different error appears somewhere else in the text or not, but it's not one I'm talking about here. Try to keep up, the corrected error was the one about the mass-luminosity relation hinging on fusion physics. That's what has been corrected, in contradiction to the many false claims you made above and seem to have conveniently forgotten about. The part I pointed to did not mention core pressure, and I don't know what it currently claims about that, but hopefully it is correct that the core pressure drops in more massive stars, as I knew all along. I also knew all along that fusion physics was not crucial to the mass-luminosity relation, which is what I'm actually talking about now because it has been corrected.
    The bit that may be included from your comments is that knowledge of fusion is not needed (if I remember correctly all Eddington needed was gravity + gas laws + entropy?).
    The key is radiative diffusion. Read the thread, it's all spelled out quite clearly, a long time ago. Are you on board yet?
    Last edited by Ken G; 2018-Apr-16 at 10:18 AM.

  23. #173
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    Quote Originally Posted by Ken G View Post
    What are you even talking about?...
    I am talking about the documented facts in this thread as in my post:
    Quote Originally Posted by Reality Check View Post
    A couple of years ago we had More massive stars have greater gravity that creates higher pressure in the stellar interior" and your unsupported assertion that was wrong. The scientific evidence in this thread that central pressure does go down with mass came 9 months later from StupendousMan
    You made an unsupported assertion. StupendousMan supported your assertion. Thus your assertion were correct.
    Last edited by Reality Check; 2018-Apr-17 at 12:25 AM.

  24. #174
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    Quote Originally Posted by Reality Check View Post
    You made an unsupported assertion. StupendousMan supported your assertion. Thus your assertion were correct.
    Well I'm very glad we could clear up that I made correct assertions. I try. But what you count as "support" is different from what I count as support-- I include arguments from basic laws of physics, you need authority to do it for you. The problem is, the authorities can disagree with each other, because graduate level textbooks can say one thing, and elementary intro texts another, as in this thread. But yes, I realize it is always a sticky situation when some of the "authorities" are wrong, but you must remember, elementary textbooks and course websites really aren't the kind of authorities that don't make easily exposed mistakes, they just don't make them often.

    A basic physics argument always trumps a weak authority like that, in fact it's how science self-corrects. Citing advanced textbooks often takes the argument out of the sphere where non-experts can understand it, hence the need for simple basic physics arguments like those I provided above. The question of this thread is, what to do when the people providing the simple arguments have stated something that is the opposite of correct, and the people providing detailed arguments are not interested in making simple easily understood statements? Something of a gap is created there, and this thread is a beautiful case study in just exactly that. In my view, when such a gap exists, it's useful to have people who can provide simple physics arguments from basic principles, and that is how I have attempted to fill that gap here. You're welcome.
    Last edited by Ken G; 2018-Apr-17 at 12:36 AM.

  25. #175
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    I did a sanity check on my own and concluded that if a main sequence star of 10 solar masses also has about 10 times the diameter of the Sun, which I think is a pretty good estimate, the pressure throughout, including the center, will be a lot lower. It is easy to fall into the trap of thinking the greater mass means greater gravitational weight, and thus the need for higher pressure to support it. Someone in that trap has overlooked the fact that each increment of the star is 10 times as far out, greatly reducing its gravitational weight. I realize I did not do a rigorous calculation of the density profile, because I do not have all of the math knowhow that Eddington et. al. did, but I am confident I am on the right track.

    My question: Why aren't the graduate level experts making a blistering denunciation of the underlings who are continually regurgitating the wrong stuff? Are they unaware that some astronomy majors may be having to unlearn mistakes as they head for graduate school?

  26. #176
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    Quote Originally Posted by Hornblower View Post
    I did a sanity check on my own and concluded that if a main sequence star of 10 solar masses also has about 10 times the diameter of the Sun, which I think is a pretty good estimate, the pressure throughout, including the center, will be a lot lower.
    Yup.
    It is easy to fall into the trap of thinking the greater mass means greater gravitational weight, and thus the need for higher pressure to support it. Someone in that trap has overlooked the fact that each increment of the star is 10 times as far out, greatly reducing its gravitational weight.
    Precisely, weight involves more than mass, it also involves distance from the center. This is also why red giants puff out so much-- they need to reduce the weight of their envelope, or it presses down too hard and induces ghastly fusion rates. But note also that red giants cannot regulate their fusion the way regular stars do, so for red giants, fusion physics is actually quite important, unlike for main-sequence stars. The main point is, science is the one place where "truthy" is not supposed to be enough, we have to take the extra step to make sure it actually makes sense.
    My question: Why aren't the graduate level experts making a blistering denunciation of the underlings who are continually regurgitating the wrong stuff?
    It's a good question. All I can tell you is that it is unbelievably difficult to try to correct basic textbooks, you face equal doses of skepticism from those who can't follow the argument, and apathy from those who can, but don't care what the textbooks say.
    Are they unaware that some astronomy majors may be having to unlearn mistakes as they head for graduate school?
    And some never do, the myths die so hard.
    Last edited by Ken G; 2018-Apr-17 at 03:01 AM.

  27. #177
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    Quote Originally Posted by Ken G View Post
    Quote Originally Posted by Hornblower
    I did a sanity check on my own and concluded that if a main sequence star of 10 solar masses also has about 10 times the diameter of the Sun, which I think is a pretty good estimate, the pressure throughout, including the center, will be a lot lower.
    Yup.
    Thanks, this is georgeeze stuff! To further this, here's a table that articulates y'all's points (I think)...

    Stellar mass vol temp.jpg

    This simply takes the ideal gas law as a general rule of thumb and using it for a list of main sequence stars from O6 to M8 (per Wiki's main sequence page).
    We know time flies, we just can't see its wings.

  28. 2018-Apr-19, 05:53 PM
    Reason
    duplicate

  29. #178
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    Quote Originally Posted by Ken G View Post
    Well I'm very glad we could clear up that I made correct assertions. I try. But what you count as "support" is different from what I count as support-- I include arguments from basic laws of physics, you need authority to do it for you.
    What we have cleared up is that you made unsupported assertions in this thread that someone else later researched and found to be correct. The point I am making is not whether you were correct or incorrect. The point is that you made another person do research to support your assertions when you should already have supporting sources to include in your posts. Or links to posts in another thread (see below). You had 9 months to do that - people produce babies in that time .

    Arguments from basic laws of physics would be stating those laws and deriving that your assertions were valid from them. If you had done that then the assertions would have been supported. I cannot find laws of physics stated in the posts here (maybe in the other thread that is mentioned early on?).

  30. #179
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    Quote Originally Posted by Reality Check View Post
    What we have cleared up is that you made unsupported assertions in this thread that someone else later researched and found to be correct. The point I am making is not whether you were correct or incorrect. The point is that you made another person do research to support your assertions when you should already have supporting sources to include in your posts. Or links to posts in another thread (see below). You had 9 months to do that - people produce babies in that time .

    Arguments from basic laws of physics would be stating those laws and deriving that your assertions were valid from them. If you had done that then the assertions would have been supported. I cannot find laws of physics stated in the posts here (maybe in the other thread that is mentioned early on?).
    He did the work about 12 years ago and posted it here back then.

  31. #180
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    Quote Originally Posted by Hornblower View Post
    He did the work about 12 years ago and posted it here back then.
    Which rather emphases my point - a link to that work in this thread would have short-circuited 9 months (with only a few posts though) of discussion here.

    ETA: A thought - Is that work still on the forum? If not, then memories that it existed is not support for his assertions.
    However the latest started thread link on profiles actually goes back 12 years so here it is: Why are high-mass stars are so luminous?
    Last edited by Reality Check; 2018-Apr-20 at 12:26 AM.

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