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Thread: Correcting errors in the "Explore" encyclopedia

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    Correcting errors in the "Explore" encyclopedia

    I'm wondering what is the method for correcting errors in the "Explore" encylopedia that this forum links to? There are a number of misconceptions about stars that are propagated there (some really blatant, such as it claims that the pressure inside stars increases as the mass of a main-sequence star increases, and that light pressure is what balances gravity). Is there some Wiki-like mechanism for fixing those?

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    The data is feeding in from the http://www.teachastronomy.com/ site. It doesn't look like a wiki style page.
    Solfe

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    I was wondering about that-- it links to that site, but it also seems to have its own content. It felt like cosmoquest was responsible for that content, so it could be replaced by cosmoquest as well. For example, the entry at http://cosmoquest.org/x/explore/2014...main_sequence/ links to "Astropedia", which is what you are talking about, but the link I gave seems to be a kind of blog entry on main sequence stars that contains different information than the astropedia entry on main sequence stars (the differences contain their own different errors, actually!).

    One might naturally assume that all these different sources of information about something as basic as "main sequence stars" would be both correct and mutually consistent, but closer examination reveals that neither of those assumptions are true. I don't see any way to comment on those blog entries, I'm wondering if a thread on here could result in the replacement of the incorrect text? This doesn't seem like a satisfactory state of affairs, much better knowledge exists about stars that can be stated just as simply, but be correct.
    Last edited by Ken G; 2014-Nov-29 at 06:39 PM.

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    Hi there! We'd appreciate comments and corrections here on this thread. I'll subscribe to it so we stay updated. We're happy to take them at educate@cosmoquest.org as well. Thank!

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    Sorry I missed this post, my bad! I would be happy to present my comments here. The article is generally good, but does have some problems that need correcting. Some could be considered minor, so perhaps can be let slip (I'd be happy to go into them, they center on the fact that fusion doesn't provide radiation pressure, it simply replaces the heat being lost, so the regular old gas pressure can stay constant-- radiation pressure is always pretty negligible in the Sun). The real problem comes later on, with:

    "Why does luminosity depend so strongly on mass? A modest increase in mass corresponds to a star with a substantially higher pressure and temperature in the core. Since the rate of nuclear reactions depends sensitively on temperature, more massive stars have much larger rates of nuclear fusion. This in turn leads to a much higher luminosity."

    That argument is quite wrong, on several counts. First of all, the more massive is a main sequence star, the lower is its pressure, not the higher. This is why high-mass stars don't go degenerate and make white dwarfs, they are low-density, low-pressure objects. Secondly, the luminosity is not high because the fusion happens faster, the fusion rate happens faster because the luminosity is higher. Fusion is self-regulated to simply replace whatever heat is being lost, and that's true at any mass for the star. The real reason that higher mass stars are more luminous is that light escapes from them faster, as was known by Eddington (even before he had ever heard of nuclear fusion).

    ETA: The error is compounded later with:
    "More massive stars have greater gravity that creates higher pressure in the stellar interior. The higher pressure results in higher temperature that causes higher energy output by the fusion process, giving both higher luminosity and higher surface temperature."
    The gravity, generally defined as the gravitational acceleration at the surface, of high-mass stars is lower than for low-mass stars. The gravitational energy per particle, on the other hand, is about the same for all masses on the main sequence, because the core temperatures are similar. The reason high-mass stars have higher core temperatures has nothing directly to do with their gravity, it has to do with their aforementioned need to self-regulate their fusion rate to be much faster than low-mass stars, because they "leak light" faster.
    Last edited by Ken G; 2015-Jan-11 at 10:31 PM.

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    Now I am curious about how a star would continue to evolve in the absence of nuclear fusion after reaching the luminosity predicted by Eddington. I would expect it to continue contracting slowly as the heat leaks away and is not replenished. As it gradually becomes denser and more opaque, would the luminosity decrease? Would a supermassive star eventually settle into a black hole without any fireworks?

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    Quote Originally Posted by Hornblower View Post
    Now I am curious about how a star would continue to evolve in the absence of nuclear fusion after reaching the luminosity predicted by Eddington.
    It turns out the luminosity of a star, usually both before and during the main sequence, is mostly dependent only on the mass of the star and little else. So if there were no fusion, stars would maintain the same luminosity they have on the main sequence for a long time, basically until they become white dwarfs (for lower mass) or go supernova (for higher mass). This is generally called the "Henyey track."
    I would expect it to continue contracting slowly as the heat leaks away and is not replenished. As it gradually becomes denser and more opaque, would the luminosity decrease?
    Not much, it would stay fairly constant if the mass stays fairly constant.
    Would a supermassive star eventually settle into a black hole without any fireworks?
    Yes I think that's true, the current supernova fireworks have to do with a rapid instability that occurs when stars get so hot and dense that strange processes eat up kinetic energy as their cores shrink. Those processes include the "Urca" process (which involves neutrino emission), the photodisintegration of the nuclei (which is kind of like undoing all the fusion the core has done), and neutronization (which also involves escape of neutrinos). When these processes remove kinetic energy, the star loses support, and collapses, only for the envelope to "bounce back" when the core converts to the strange forces available to a neutron star (like strong forces and relativistic gravity), leading to the "fireworks". But if we were going back into the days of Eddington, and give him the quantum mechanics of a white dwarf, but take away all knowledge of nuclear processes so everything always stays protons and electrons, then you're right-- there wouldn't be much fireworks, the core would just shrink away into a black hole as mass was added to it.

    For some reason, Eddington thought stars were always stable, so I don't think he imagined models where stars shrink away into black holes. He thought the Sun was much younger than it is, of course, because he didn't know about fusion, and when quantum mechanics and observations of white dwarfs came along, he would have realized that the Sun would eventually become one of those. But I don't know why he didn't think more massive stars would shrink down into black holes as they lost heat-- maybe he did, he just thought it would be gradual, like you are saying.
    Last edited by Ken G; 2015-Jan-12 at 04:41 AM.

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    Perhaps it would be good to put this discussion in the Astronomy forum.

    My educated guess is that when Eddington was doing this theoretical work, nobody knew enough about subatomic structure to carry stellar evolution to the bitter end. Suppose fusion could not happen and we have nothing but hydrogen. I can imagine a star slowly contracting and getting hotter at the photosphere, and staying about the same luminosity. Eventually it would reach some compression limit, at which time it would start cooling and fading. White dwarfs were known by then, and it would not have been unreasonable to expect stars of all masses to end up that way. If I am not mistaken, nobody yet knew about electron and neutron degeneracy, let alone their upper limits on how much collapsed mass they could support.

    It is noteworthy that low-mass stars such as Proxima Centauri, at about 0.1 solar mass, are expected to gradually contract and get hotter and somewhat brighter, and never swell into a red giant stage (Sky and Telescope, November 1997, p. 20). That is because they are convective throughout and thus never get stratified. As hydrogen fuses into helium, the two gases become blended throughout the star. As the fusion progresses the result is fewer particles, and that appears to me to explain the contraction on the basis of gas laws. When the fusion finally runs out the contraction and brightening hasten until electron degeneracy is reached, after which time the star is a fading "white dwarf". My educated guess now is that a hypothetical one-solar-mass star with no fusion would do something similar at a faster rate and end up resembling Sirius B. Perhaps high-mass stars would collapse into neutron stars or black holes, with no red giant stage before the final crunch.

    Ken G, as I read your explanations in this thread, and also in that one you did at length several years ago, I am flabbergasted at the mistakes by those who should have known better about the pressure and temperature inside a massive star. We know from observation that main sequence stars of 10 times the Sun's mass are also roughly 10 times its diameter. It follows easily that each increment of mass has only about 1/100 of the gravitational weight it would have if compacted into one solar diameter, so the pressure must be only about 1/10 that inside the Sun. From the familiar gas law equation PV = nRT (R is the gas constant, not a radius), it follows that the temperature inside will not be much different. This is all Physics 101. Keep up the good work in sniffing out these errors and calling them to our attention.

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    Quote Originally Posted by Hornblower View Post
    Perhaps it would be good to put this discussion in the Astronomy forum.
    Yes it is appropriate there, though if the Explore encyclopedia is monitoring it, this discussion could be relevant to their deliberations.
    Suppose fusion could not happen and we have nothing but hydrogen. I can imagine a star slowly contracting and getting hotter at the photosphere, and staying about the same luminosity.
    Yes, that's what would happen. The physics of it is what goes into the "Henyey track," which is not as widely known as it probably should be.
    Eventually it would reach some compression limit, at which time it would start cooling and fading. White dwarfs were known by then, and it would not have been unreasonable to expect stars of all masses to end up that way.
    Since they had special relativity, they knew that if stars can lose enough heat, they have to become black holes eventually (because they eventually have to go relativistic, even the protons, and that will cause extreme shrinking as the star continues to lose heat). Since they also had quantum mechanics, they knew that a star could reach a ground state where it could lose no further heat. So the issue was always, which happens first? Chandrasekhar showed that the mass of the star is the key issue in deciding that, but Eddington just didn't believe that any star could keep shrinking away to a black hole. He felt that special relativity must be wrong since it predicted that this could happen above the Chandrasekhar limit, and indeed he even tried to modify special relativity so that did not happen. Had he been right, he would have been heralded as someone with the insight of Einstein-- winners write history.
    If I am not mistaken, nobody yet knew about electron and neutron degeneracy, let alone their upper limits on how much collapsed mass they could support.
    It depends on what time in history you take. An interesting time to look at is at the time of the Eddington vs. Chandrasekhar controversy, at which time white dwarfs were understood via quantum mechanics, but compact stars like neutrons stars and black holes were not known to exist. Once Eddington had gotten over his incredulity around the observations of white dwarfs, he did not take the lesson and apply skepticism to his incredulity about the existence of black holes! It was as though he felt he could believe in the quantum mechanics of white dwarfs because we observe them and because they will not contract any further, but he could not stomach a type of object that just kept contracting vastly more than a white dwarf already has, until it becomes an object on the scale of everyday human experience! When physics said such could happen, he said, then we need new physics. His stature and credibility in astrophysics at the time were no less than Einstein's, so people generally did not reject his opinion on that, and Chandrasekhar was given a hard time. It was not astronomy's finest hour in its culture clashes with physics (astronomy fared much better in the solar neutrino problem!).
    It is noteworthy that low-mass stars such as Proxima Centauri, at about 0.1 solar mass, are expected to gradually contract and get hotter and somewhat brighter, and never swell into a red giant stage (Sky and Telescope, November 1997, p. 20). That is because they are convective throughout and thus never get stratified. As hydrogen fuses into helium, the two gases become blended throughout the star. As the fusion progresses the result is fewer particles, and that appears to me to explain the contraction on the basis of gas laws.
    Yes, when the lost heat is being replaced, there would not be contraction if not for the lower number of particles. Contraction is usually something that happens mostly when there is not fusion going on, but here's a case where it proceeds even with fusion.
    When the fusion finally runs out the contraction and brightening hasten until electron degeneracy is reached, after which time the star is a fading "white dwarf".
    Yes, now the heat is not replaced so contraction can happen more quickly.
    My educated guess now is that a hypothetical one-solar-mass star with no fusion would do something similar at a faster rate and end up resembling Sirius B.
    Exactly.
    Perhaps high-mass stars would collapse into neutron stars or black holes, with no red giant stage before the final crunch.
    Yes, and no supernova either, because the contraction would be gradual until the very latest stages when there's not much gravitational energy left that can still be extracted. When the core contracts at the very end, it might create a temporary red giant, in place of a supernova, so it would be more like an LBV.
    Ken G, as I read your explanations in this thread, and also in that one you did at length several years ago, I am flabbergasted at the mistakes by those who should have known better about the pressure and temperature inside a massive star. We know from observation that main sequence stars of 10 times the Sun's mass are also roughly 10 times its diameter. It follows easily that each increment of mass has only about 1/100 of the gravitational weight it would have if compacted into one solar diameter, so the pressure must be only about 1/10 that inside the Sun. From the familiar gas law equation PV = nRT (R is the gas constant, not a radius), it follows that the temperature inside will not be much different. This is all Physics 101. Keep up the good work in sniffing out these errors and calling them to our attention.
    Thanks, I was pretty amazed as well. So far, I have not seen any significant corrections appearing-- the same errors are still out there, which is why I am trying to raise this issue to the attention of the Explore encyclopedia. They only seem to check the thread infrequently, but the main thing is that the corrections eventually get made.
    Last edited by Ken G; 2015-Jan-18 at 04:26 PM.

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    Thanks for filling in my information gaps. I was thinking in terms of about 1905, though I knew about Eddington's treatment of Chandra at later dates, which I thought was pretty shabby.

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    It certainly serves as a cautionary tale against getting so much self-confidence that it amounts to hubris that nature will behave the way we expect her to, as though nature must defer to our great insight. I think nature gets quite a few laughs at our expense on that issue, though we have had some impressive successes (most recently, the Higgs, and probably soon, gravitational waves). The lesson seems to be that most central one in science: follow the evidence, not the preconceptions of the authorities, but the authorities aren't always wrong either!
    Last edited by Ken G; 2015-Jan-18 at 06:08 PM.

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    The explore encyclopedia at http://cosmoquest.org/x/explore/2014...main_sequence/ still says "More massive stars have greater gravity that creates higher pressure in the stellar interior." That's totally wrong, and needs replacing.

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    That link still says "A modest increase in mass corresponds to a star with a substantially higher pressure and temperature in the core. Since the rate of nuclear reactions depends sensitively on temperature, more massive stars have much larger rates of nuclear fusion. This in turn leads to a much higher luminosity." The error is doubled-down on in the next paragraph: "More massive stars have greater gravity that creates higher pressure in the stellar interior. The higher pressure results in higher temperature that causes higher energy output by the fusion process, giving both higher luminosity and higher surface temperature."

    Both the quoted remarks are just as completely wrong as when I pointed it out three months ago. Perhaps the author and administrator's of the encyclopedia don't care about accuracy, but that surprises me. The problems with the statement are:

    1) The second statement claims that higher pressure causes higher temperature, but in fact temperature and pressure are independent quantities in the ideal gas law, and examples abound where higher temperatures can be associated with either lower or higher pressure. Consider the stratosphere of the Earth, for example. This would be a minor complaint, if not for the two much worse problems:
    2) It makes easily falsifiable claims. Higher-mass main-sequence stars have lower pressures every where in the star, which is a basic consequence of something called "the virial theorem," as explained in more detail above.
    3) The argument is incorrect, even if it had gotten the pressure difference right, and even if it had treated temperature and pressure as independent variables. The reason higher-mass main-sequence stars have higher luminosities has little to do with the physics of fusion, though fusion physics could be used in an iterative way to make small corrections to the simplest argument (if such detailed corrections were warranted, which they generally are not). It is clear that fusion physics is not particularly relevent to the issue of the luminosity, for two reasons:

    i) astronomers like Russell and Eddington were able to account for the higher luminosity of higher-mass main-sequence stars without even knowing that fusion exists, and
    ii) solar-like and more massive main-sequence stars reach their approximate main-sequence luminosity before fusion even initiates in the star, as can be seen from any H-R diagram showing stellar evolution models. This makes it clear that the basic processes that are mostly responsible for setting the luminosity have nothing at all to do with nuclear fusion, except for the lowest-mass stars (and for them one needs to include electron degeneracy to have anything close to a correct description of the processes that set the luminosity).

    The actual physics that sets the luminosity of main-sequence stars is radiative diffusion and force balance, not nuclear fusion-- google the "Henyey track" or look at the discussion above. This has all been known for 50 years or more, but for some reason does not translate into the popular science textbooks or online resources, and it's high time that it did. Perhaps I am being impatient-- this stuff has been wrong in the textbooks for decades, another three months is probably not a big deal. Still, new young minds are encountering it every day, and getting misled, and I know the highly conscientious and capable people that make this information accessible to the public would not be satisfied with that state of affairs, they need only be made aware of it.
    Last edited by Ken G; 2015-Apr-04 at 12:29 PM.

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    Another month goes by, and yet more young minds misinformed by the claim: "Why does luminosity depend so strongly on mass? A modest increase in mass corresponds to a star with a substantially higher pressure and temperature in the core." No, what the young mind needs to understand is why the higher mass main-sequence star must have a lower core pressure, which is a perfectly well known fact about main-sequence stars. Without understanding why that's true, as the Astropedia entry apparently does not understand, we cannot claim any understanding at all of the guts of main-sequence stars. So I'll keep pinging this, just in case those in a position to fix it, who I know care about correct astronomy education, will have the chance to do it.

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    Ken,

    Did you try e-mailing the address Dr. Gugliucci provided in December? If no one is having the time (or inclination) to visit the Forum (which is sad for a host of reasons), maybe they are less apt to ignore an e-mail?

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    Yes, I had a brief exchange with Pamela Gay back then, and I believe she was in contact with Chris Impey, the author. But I realize these things take time to get changed. I'm just being the squeaky wheel, because this has to start somewhere or it might be a decade before the textbooks get a clue on this matter. I figured if people come through this forum on the way to the Astropedia link, they will have a "heads up" because of this thread. But it sounds like this forum is not the appropriate place because the word is not getting out, so it will make more sense to use email directly. I also don't want to sound too critical of the textbooks-- they do a great job and I know they will want to get this right.

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    I may have asked this in another thread but I will repeat it here. Are college freshman astronomy majors-to-be getting misinformation they will have to unlearn in advanced astrophysics courses?

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    Sure. Having taught such courses, I'm sure I've given them misinformation they needed to unlearn too! But this issue with the luminosity of main-sequence stars is particularly insidious, because it is important, widespread, and easy to fix. In fact, I have one intro textbook that called the reason for the higher luminosity of higher-mass main-sequence stars as one of the most important effects in stellar astronomy, and then promptly went on to completely botch it by saying that the cause of that higher luminosity was the higher fusion rate! (Of course, it also claimed the fusion rate was higher because of the "stronger gravity", so it was a total flub.) This is indeed something that will be hard to unlearn later, it gets hardwired in there pretty deeply, because it seems intuitively correct, but this just means there is a new intuition that needs training. I think the textbooks just get this information from each other, a minor sin that I'm sure we're all party to, but it's time to clean this one up.

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    My hunch is that someone early in the 20th century confused the innards of a main sequence star and an evolved giant of the same mass with the innards of two main sequence stars of different mass. In the former, if I am not mistaken, the core is more dense and hotter, a state enabled by the stratification in stars other than low mass M dwarfs, and the subsequent exhaustion of hydrogen in the core.

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    It is true that when the Sun becomes a red giant, it will have a much higher temperature core. It won't have a higher pressure in its fusion zone though, in fact the main reason that red giants are "giant" is their need to deal with their high fusion temperature-- they have to find a way to drastically lower the pressure in the fusion region or the fusion goes through the roof. So instead, the fusion zone itself goes through its own roof, drastically lowering the density and pressure there as a net result.
    Last edited by Ken G; 2015-May-14 at 08:36 PM.

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    Coming up on 9 months, the explore encyclopedia still says:
    Quote Originally Posted by cosmoquest.org/x/explore/2014...main_sequence/
    Why does luminosity depend so strongly on mass? A modest increase in mass corresponds to a star with a substantially higher pressure and temperature in the core. Since the rate of nuclear reactions depends sensitively on temperature, more massive stars have much larger rates of nuclear fusion. This in turn leads to a much higher luminosity.
    That's still just as completely wrong as it was when I pointed it out 9 months ago. Does anybody care what information is out there masquerading as good scientific explanations? It is cause to doubt the reliability of any source that can be left so wrong for so long. I'm sure the whole encyclopedia is not that bad, but how can anyone tell which parts are any good without learning it for themself, and then they would't need an encyclopedia?
    Last edited by Ken G; 2015-Jul-22 at 07:21 AM.

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    (1 month bump)

    Ken G, you might be interested in, although not pleased by, the fact that the Bad Astronomer reiterates the more-mass-more-pressure meme in his Crash Course Astronomy video series (Chapter 29: Low Mass Stars, almost right after the introduction and exposition of hydrogen fusion about 1:00 min).

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    You are correct on all counts. Thanks for pointing that out, it really shows how out of control this meme has gotten-- when even the Bad Astronomer falls for its bad astronomy!

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    I can't stop grinning as I imagine him coming here to discuss it and being told to "take it to ATM."
    "There are powers in this universe beyond anything you know. There is much you have to learn. Go to your homes. Go and give thought to the mysteries of the universe. I will leave you now, in peace." --Galaxy Being

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    Yes, and ironically, if the mods consult intro textbooks, we'd be the ones over there! But if you look at advanced texts, or Eddington's paper, the story is very different. All of which raises the interesting question that thankfully is not so often an issue for science: just what is the "mainstream" anyway?

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    Quote Originally Posted by Ken G View Post
    [...] just what is the "mainstream" anyway?
    But let's not consider that an invitation to discuss this question here! I don't need an intro nor advanced textbook to see what that would do to this thread.
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    Good point. I guess it's clear enough that the mainstream view here is that of the advanced textbooks and research papers. Usually the intro textbooks do a very nice job of summarizing that material at an appropriate level. But it seems that sometimes, an error can creep in, and kind of pick up a life of its own! Then we're faced with the task of getting the error corrected, which can be an uphill battle as we're seeing here. If I wrote a textbook or online encylopedia, I really wouldn't want to have to change it either, but the right answer is really more interesting than the oft-repeated meme-- which is so often true in science!
    Last edited by Ken G; 2015-Aug-25 at 01:57 AM.

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    Is there an "astronomy 101" level text anywhere on the net that gets the story right?

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    If not, will you write one?

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    Actually, the only intro-textbook-like source I've ever found on the web that gets it basically right is the Wiki entry on Main Sequence stars, and even that one requires following some links-- it's never really spelled out to the casual reader. It's not even clear that counts as "Astronomy 101", because the Wiki entries tend to get rather technical!

    Frankly I'm not sure how to write a kind of encyclopedia entry on the topic, where it would be found and used by more people than might come to this forum. The explanation given above is the best I have to offer, so in a sense one could say this thread is such an astronomy 101 explanation, though it's just for that particular question. A full intro textbook is a huge undertaking, and quite frankly most of the ones out there are generally very good and don't need much correction-- so it would seem to make more sense to just point out the occasional boo-boos on places like here, until new editions come out with the problems fixed. I did submit a paper recently to the Journal of College Science Teaching to point out this common error, so maybe that will help, if they send it to good referees. But that paper just points out the problem-- the right explanation is given in better detail on this very thread.

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