Science; it’s complicated

I mentioned, a while ago, that I’d been at a meeting and had an idea for a post. Well, this is my attempt to articulate what I thought at that meeting. A good deal of my own research involves trying to understand discs around very young stars, in particular discs that are quite massive, relative to the mass of the central protostar.

Spiral density waves in a disc around a young protostar.

For example, consider a young Sun with a disc that extends to just beyond where the edge of Solar System is today and that has a mass greater than 10% that of the central young Sun. Such a disc is likely to be susceptible to the growth of a gravitational instability, which can manifest itself as spiral density waves, as illustrated by the figure on right.

One reason why this is interesting is that it might provide a mechanism for transporting mass through the disc onto the central protostar. The waves grow and are then damped, which essentially acts to heat the disc by converting kinetic/rotational energy into thermal energy. In doing so, this must transport angular momentum, which has to go outwards, allowing mass to flow inwards. In fact, this might be the primary way in which mass is transported onto stars during the earliest stages of star formation.

A circumstellar disc undergoing fragmentation.

Now, there is another possible outcome; if the disc becomes very unstable, it might undergo what we call fragmentation. The spiral density waves start to form clumps that ultimately contract to form bound objects (see the Figure on the left). For a while this was regarded as a possible mechanism for the formation of gas giant planets, like Jupiter and Saturn.

Although I wasn’t the first, I was amongst one of the first to try to quantify the conditions under which a disc would fragment. Essentially, it would need to be susceptible to the growth of the gravitational instability and it would need to be able to cool rapidly. If it didn’t cool rapidly, the dense regions would heat up, and they would be unable to contract to form bound clumps.

Much of this work was done using numerical simulations. About 6 years ago, another group pointed out that if you ran some of these simulations at much higher resolution than we could have done, fragmentation occurred even when the discs cooled slowly. You might think that the immediate response would be to change our general view of this process. However, there had been a lot of other work to try to understand this process. People had shown that the wave amplitudes would depend on how fast the disc cools; if it cools slowly, the waves have a small amplitude. If the amplitude of the waves are small, how can they then form dense regions that clump to form bound objects? People had looked at the power spectra and shown that most of the energy was at scales that were well-resolved by the original simulations. If we were originally resolving these scales, why didn’t we see the fragmentation that was later seen in higher resolution simulations?

Also, if this process was easy, we’d expect to find many giant planets on wide orbits around stars, and we don’t. They do exist, but they are relatively rare. I also published a paper suggesting that if fragmentation were common, we’d expect to see some of the planets that form this way contaminating the known exoplanet population, and – again – we don’t see much evidence for this. So, there was immediately some suspicion, and most of the subsequent work has suggested that there is a numerical issue with some of the simulations, rather than it being likely that fragmentation can happen even if the discs cool slowly.

This isn’t definitive, but the general view – despite some uncertainty – is that if you want to form clumps in discs around young stars, you need a disc that is gravitationally unstable and one that is able to cool rapidly. Consequently, this process is unlikely to play a dominant role in the formation of giant planets.

The point I’m getting at is that science (research, really) can be complicated. It may be that some things challenge our understanding and so we have to consider the weight of the evidence and also whether or not there are reasons why there might be issues with this new evidence. Some amount of expert judgement is often required. There may be cases where the challenge is so convincing that you modify your understanding, and others where it appears that there are problems with the challenge, even if it’s not completely clear what it is.

Of course, it is always possible that our current understanding will change and we should be open to that as a possibility. However, we also shouldn’t change our views the instant someone comes along with some kind of challenge. Often, our current understanding builds up over time and is based on many lines of evidence. Challenging this is, of course, a key part of science, but it also requires that these challenges themselves stand the test of time. In fact, even if these challenges don’t overthrow the current paradigm, they still act to strengthen our understanding, because they force us to look more closely at all the lines of evidence.

The bottom line, though, is that if there appears to be some challenge to a scientific consensus that seems to be being ignored by the scientific community, it’s probably because they’ve looked at all of the evidence and concluded that, despite this challenge, the mainstream view is still probably broadly correct. It can be complicated, in other words.

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56 Responses to Science; it’s complicated

  1. Here´s a challenge for you:
    Heat up a surface to 14 degrees celsius. Add a cold fluid at -33 degrees celsius. Now make an argument about how hot the solid surface gets from the cold fluid.

    Then, consider the fact that solar irradiation is equal to gravity. Like this: TSI/(4/3)=4/3*g^2. And surface temperature is equal to 4g^2 as thermal resistance. And tropopause temperature is equal to 4/3*g^2 as thermal resistance. With the geometry of a sphere and thermodynamic book-keeping of energy, the greenhouse theory is smashed into [Snip. Shirt ripping. – Willard]

  2. Willard says:

    If someone takes on Life’s challenge, I’ll delete the comments a make a post out of it.

  3. BBD says:

    Can someone explain in relatively simple terms why protostellar discs begin to rotate? Sorry if this is facepalm-dumb territory.

  4. BBD,
    Consider a big cloud of gas and dust; much bigger than the size our Solar System. Starts to collapse under the influence of its own gravity. Even if it has a small amount of initial rotation it will start to spin faster and faster (conservation of angular momentum). There is a limit (determined by it’s own gravity) as to how far it can collapse before it would start to break itself apart. However, the mass can still collapse parallell to the rotation axis without violating angular momentum transport. Hence, the expectation is that star formation will involve some mass (that with little initial angular momentum) forming the protostar, with the rest forming a rotationally supported protostellar disc.

    This is one of the main problem in star formation – how do you remove the angular momentum, allowing mass to fall onto the central protostar? The gravitational instability may well be one of the processes that does exactly that. The other solution is a magnetic instabilty – it may well be that it is a combination of both (gravitational instability when it’s very young, magnetic instability when it is older).

  5. Jon Kirwan says:

    I really enjoyed this contribution, today. Partly because I have some limited experiences with orbital mechanics and simple perturbation theory. (One book’s worth of study.) But mostly because you made me wonder about a different connection.

    I believe I’ve read that perhaps some 85% of stars are binary systems, with some triples and even “higher-multiple” systems. I’m curious about additional thoughts you might have that address this very specific quantitative detail about the relatively large percentage of stellar systems that are binaries (or higher), if my recollection is accurate here.

  6. BBD says:

    (conservation of angular momentum)

    Thank you, ATTP.

    This is one of the main problem in star formation – how do you remove the angular momentum, allowing mass to fall onto the central protostar.

    How indeed? Wonderful stuff. But as you say, complicated 🙂

  7. Jon,
    Thanks. There are many systems that are binary, or higher-multiple, but it’s only for very massive stars (~10 Solar mass, or greater) that it’s something like 85%. For Sun-like stars, it is more like 30%. There are certainly some (including some work I’ve been involved with) that has suggested that if you end up with a very massive, very extended disc then it will fragment to form a stellar companion rather than a planetary-mass companion. There is in fact, a recent ALMA observation that does indeed show what might be a stellar-mass companion forming via fragmentation in a protostellar disc (although a disc around what is already a binary star system).

  8. Phil says:

    The bottom line, though, is that if there appears to be some challenge to a scientific consensus that seems to be being ignored by the scientific community, it’s probably because they’ve looked at all of the evidence and concluded that, despite this challenge, the mainstream view is still probably broadly correct.

    Is ATTP inviting comparisons with the recent history of methods to determine ECS in Climate Science, I wonder ?

  9. Jon Kirwan says:

    Thanks! You’ve provided important refining details I hadn’t appreciated before.

    I gather from your discussion, namely the fact that it has taken the advent of having access to powerful computing and the ability to break up the steps finely in both spatial and time dimensions in order to continue advancements in this field of yours, that it would be nearly pointless for me to sit back down to try and apply the usual reductionist approaches I’m more familiar with.

    I will read the links you’ve provided and anything interesting developing from those, as well. This is something I can sink my teeth into, at least a little bit anyway. I do enjoy exploring mathematical insights that might develop. (I see that your first link is available in PDF form without a pay-wall. Thanks!! Regarding the 2nd article, I was able to bypass Elsevier, so I have that as well.)

    Thanks again!!

    P.S. I’m currently studying Gilmore’s books on Catastrophe Theory and Lie Groups and Algebras and Coxeter’s Regular Polytropes, as part of slowly getting up to speed on M-theory (with periodic visits to the home of a theorist at UofO who occasionally works with Ed Witten.)

  10. Steven Mosher says:

    Of course none of this is science, it’s just models.
    I will go out in my backyard, get some cardboard and styrofoam, and make a proto star and let you know how those things forms.I read a book. star formation for dummies. We need real measurements and real experiments, like the good old days before all this computer mumbo jumbo. back when I a kid we used slide rules.

    Willard, pass me the duct tape.. I’m going for my Nobel, maybe Sir Rud will help me.

    ah wait forget it, it’s probably got some Chaos in it, so we will never know.

    maybe Unicorns could help your theory ATTP? can you rule them out?

  11. Jon Kirwan says:

    The atomic model is just a model. It’s still science. It predicts and it explains. And does so a lot better than reading tea leaves or playing with styrofoam.

  12. Jon,
    We are actually developing a semi-analytic approximation (of course, based on the more complex numerical model results). Essentially, it’s a combination of assuming that the system settles to a state where it is just gravitationally stable (stability criterion ~ 1), using energy balance (heating balancing cooling), and some understanding of the expected wave amplitudes. See, this paper for where we first introduced the approach, and this paper for where we suggest that you can use this to understand whether or not such discs could be directly observed.

  13. BBD says:

    @Jon

    Steven was, I think, playfully mocking certain ‘sceptic’ memes rather than expressing his own views.

  14. Susan Anderson says:

    Lovely. All beyond my skills, but entirely admirable. Would that people would realize that science is not a plaything but a field of fine hopes and dreams blended in with mundane efforts to understand, some of the best of which humanity is capable. As for modeling, Gavin explains why it makes sense here. I hope gallopingC is not “above” taking a look:

    I own a lovely astronomy tome (coffee table book style) that opens with some spectacular illustrations, from wonderful former Astronomer Royal Martin Rees:
    https://en.wikipedia.org/wiki/Universe_%28book%29

    I can only understand a minute portion of this, but I am certainly a fan. I see Martin Rees has also produced not one, but two Ted Talks: https://www.ted.com/talks/martin_rees_can_we_prevent_the_end_of_the_world and https://www.ted.com/talks/martin_rees_asks_is_this_our_final_century

    Ten years ago, I wrote a book which I entitled “Our Final Century?” Question mark. My publishers cut out the question mark. The American publishers changed our title to “Our Final Hour.” Americans like instant gratification

    Please pardon this descent into rank amateurism. But GC, it might be worth noting that wherever you look people like Martin Rees take the trouble to say things like that.

  15. Susan Anderson says:

    Sorry, should not have put Gavin’s link on a line by itself; the other links are more humble in respect of those who wish to remain at least vaguely connected to astronomy (would welcome a repair).

  16. [Playing the ref. – Willard]

    I made a mistake, TSI/(4/3)=4/3*8g^2 is the right balance equation. For effective radiation=
    (TSI-(TSI/(4/3)^2))/4.
    Surface temperature=TSI/(4/3)^2=4g²

    (TSI/4)-T(effective)=g²

    Units for g is Nm² for thermal resistance.

    Do you think greenhouse equations are more reasonable? And analogies with blankets?

  17. Shouldn’t write on the phone.

    Should be TSI/(4/3)^2=8g²

  18. Jon Kirwan says:

    To @BBD: Granted. My comment then applies to people who might say such things and is just as appropriate to include immediately afterwards. But thanks!

  19. lifeisthermal,

    Here´s a challenge for you:
    Heat up a surface to 14 degrees celsius. Add a cold fluid at -33 degrees celsius. Now make an argument about how hot the solid surface gets from the cold fluid.

    Not quite sure why to take up this challenge, because I’m not really seeing the relevance. I’m assuming that your suggesting that the surface of the Earth is warmed by something that is colder and hence that this violates basic thermodynamics. However, this isn’t what happens, so you challenge doesn’t have any relevance.

  20. Willard says:

    > I’m not really seeing the relevance.

    The only one I can is that if and when one commenter responds, I move the comments to a DragonSlayers’ Lair thread.

  21. The only one I can is that if and when one commenter responds, I move his comments to a DragonSlayers’ Lair thread.

    Okay, I’ll wait and see if someone else wants to have a go.

  22. Ragnaar says:

    BBD:

    https://en.wikibooks.org/wiki/General_Astronomy/Protostars_and_Stellar_Nurseries

    Any movement of stuff towards the disk’s center of mass will be an arc, but this assumes that either the center of mass or the piece of stuff is non-stationary.

    If a piece of stuff asks the question, how did I get here? Mostly likely through movement related to a center of mass. Most of the time this results in an arc.

    Assume only a stationary Earth and Sun. The two would approach each other and collide. If the Earth had enough momentum perpendicular to the Sun, it would assume an elliptical orbit.

    Movement plus gravity is good plan to create disks.

  23. Jon Kirwan says:

    Quick question about your reply to me, where you mentioned that perhaps about 30% of the stars like our sun are binary systems. (And that the higher percentage I’d ignorantly suggested before was more towards the more massive star types.)

    I was just glancing through some paper abstracts and found “Angular momentum and the formation of stars and black holes” by Larson, 2009 (some copy of it I found here: http://www.astro.yale.edu/larson/papers/AngMom09.pdf). In the abstract, he says, “… (1) low-mass stars, most of which probably form in binary or multiple systems; …” That caught my attention.

    Is his comment in the abstract there consistent with your statement to me? Or does it differ? If consistent, I’ll go read the paper more thoroughly and see where that takes me. If it differs, perhaps you could explain more about why?

    Thanks again for putting out this topic as I’m more interested than I’d like to admit and will probably spend way too much time reading papers, now.

  24. russellseitz says:

    Tanks to ATTP for the density wave illustrations, which vividly recall what one unexpectely saw two decades agn on turning from naked eye viewing of comet Hale-Bopp to examining its bright nucleus with a substantial telescope- it was wreathed in a Catharine Wheel of visibly pulsing light , that appeared to rotate far faster than the leisurely rate of the comet’s tumbling core.

  25. angech says:

    Ragnaar says:
    “Assume only a stationary Earth and Sun.”
    Yes.
    “The two would approach each other and collide.”
    No.
    They are stationary, remember.

    “Movement plus gravity is good plan to create disks”.
    If and only when combined the right way

  26. angech says:

    The answer to the first part of the problem seems to be that a natural cloud of space dust cannot and must not, by the laws of physics aggregate into a disc.
    Caveats.
    As always.
    Natural would be a cloud of dust existing as it always has just drifting along in space stationary as Ragnar says.
    Of course this is itself physically difficult [impossible to conceive].

    The answer lies in two aspects.
    One billions of years ago when the very large original and their second and third, tenth offspring exploded scattering the dust outwards not to return. Escape velocity.
    Combined with an expanding universe? theoretically each particle would be most unlikely to congeal with other particles.
    Gravity would however tend to draw some adjacent particles into files and rows of outward extending streaks so one can imagine over time streaks/streams of related matter traveling semi adjacent to each other.
    Time is the factor here.
    The universe was a lot smaller and these particles were impeded by the other stars and exploded star materials ending up in swirls around the other stars til they too exploded and clumping together with the other debris and interacting with the debris from the newer exploding stars.
    These of course included heavy metal particles with more gravitational attraction.
    It is not the gravitational attraction that causes the formation of discs and protostars. It is the left over differential movements of the particles that have come from different stars in different directions being forced into a mass that is assumed to all have the same angular momentum but doesn’t.
    The bits that travel in opposite directions collide lose velocity and become subject to the effect of their gravity and start pulling everything in that was previously happily moving in unison with its own debris pattern [and not coming together]

    The problem is somewhat similar to the discussions on Carbonate build up in the crust of the earth. On massive time scales we have a crust impregnated with billions of tons of inorganic matter that was once organic, now unrecognizable.
    The universe is at least 4 1/2 times older than the earth with all that extra time to fashion the stars we see today.

    Thanks for the interesting post. I hope my contribution is not perceived as token.

  27. angech,

    “The two would approach each other and collide.”
    No.
    They are stationary, remember.

    Pretty sure he means held stationary and then let go. Gravity, remember.

  28. Andrew Dodds says:

    russellseitz –

    If you want to observe density waves spontaneously forming, simply look at a pint of Guinness settling after being poured and as it separates into head and liquid.

    (You may need to run the experiment several times)

  29. angech says:

    “and then let go.”
    I know that’s what he meant.

    We are talking about sun formation from space dust and stationary objects in space tend to be stationary with regard to each other because they are trying to move apart as fast as they are being pulled together.
    Which is why you have the problem of sun formation in the first place.
    Nobody put them near each other just for gravity to work on them.
    Or they would not have been apart in the first p[lace.

  30. angech,

    We are talking about sun formation from space dust and stationary objects in space tend to be stationary with regard to each other because they are trying to move apart as fast as they are being pulled together.

    This is very confused. The universe is expanding, so on large scales everything is moving apart (which can be thought of as an overall expansion of spacetime). However, the universe is also structured on all scales. On small scales, local gravity dominates and the overall expansion of the universe isn’t relevant (I’m not a cosmologist, so maybe there is a better way to explain this). The point is that within galaxies, individual stars, clouds of gas and dust, planets, etc all interact via their gravity and are not influenced by the expansion of the universe. Hence the local dynamics can be very complex. Objects can be moving towards each other, away, there can be rotation, etc.

    A cloud of gas and dust will start to collapse if its gravity dominates over its pressure. Given that it is initially optically thin, it will lose any thermal energy (from the conversion of gravitational potential energy into heat) very quickly and once it starts to collapse it will continue until it becomes dense enough to prevent this thermal energy from continuing to radiate away. This is called the opacity limit and tends to happen when a central clump has a density of about 10^{-13} g cm-3 and a mass of a few Jupiter masses. The material around it will continue to collapse but there is a centrifugal barrier (that depends on the initial mass and rotation of the cloud) which means that it can’t all fall onto the dense object in the centre. Instead it tends to fall parallel to the rotation axis to form a rotationally supported disc.

  31. dikranmarsupial says:

    ATTP, is there an approachable (first or second year undergradate) textbook on this sort of thing (or at least the sort of thing that leads into this sort of thing) that you would recommend?

    “I know that’s [held stationary and then let go. Gravity, remember.] what he meant.”

    in that case, one wonders why you replied

    “No. They are stationary, remember.”

    in response to

    ““The two would approach each other and collide.”?

    If it is some attempt at cryptic humour, it is probably a good idea to use a ;o)

  32. dikran,

    is there an approachable (first or second year undergradate) textbook on this sort of thing (or at least the sort of thing that leads into this sort of thing) that you would recommend?

    The Cosmic Perspective is pretty good (I use it in one of my classes). It’s expensive new, but the older editions are still fine.

  33. dikranmarsupial says:

    cheers, will add to my list!

  34. BBD says:

    Good tip. I will keep an eye out for a s/h copy too. Thanks ATTP.

  35. BBD says:

    @angech

    I know when I need to read rather than post (unless it is a question). It’s a useful thing to learn.

  36. angech says:

    Only trying to help.
    “Assume only a stationary Earth and Sun.”
    With all the caveats.OK?
    Like it is really an elliptical orbit, the earth is rotating and there is an earth moon dynamic etc.
    The point is the earth is stationary to the sun, not falling into it, not moving away.
    “If the Earth had enough momentum perpendicular to the Sun, it would assume an elliptical orbit”
    A circle is one form of an ellipse where the orbiting body would be truly stationary.
    Precisely because it has angular momentum.
    Which is why it was misguided to say that gravity would make them collide.
    Yes they both have gravity.
    Gravity is what is keeping them stationary by balancing the angular momentum.
    If the gravity had been stronger they would have collided long ago. If the gravity was weaker then they would be moving away, like voyager.

    It has been that way for 4 billion years [+/_].
    Ditto the rings around Saturn.
    Things don’t move spontaneously in space just because gravity exists.
    Gravity helps hold them “stationary”.
    So a cloud of space dust is essentially that, drifting along, rotating along, all in place.
    It did not come from a super weak hiccough of a proto sun thinking I’ll just push all this space dust out so far and then let gravity bring it back in to reconstitute itself.
    Hence the conundrum that ATTP is trying to explain. Since gravity is doing all it can just to keep the space dust where it is [Saturn’s rings again], where does it find the energy to suddenly start forming into a collapsing space dust cloud.
    I feel that my explanation is cogent and to the point.
    “A cloud of gas and dust will start to collapse if its gravity dominates over its pressure.”
    Is true for a cloud that originated from a small explosion in space that did not cause particles to reach escape velocity.
    It has a ease of explanation that is actually not possible in the context of how the space dust collected into clouds in the first place and what rules of physics then bring it together.

  37. angech,
    I don’t even know how to respond to the above. It seems remarkably confused.

  38. dikranmarsupial says:

    “The point is the earth is stationary to the sun, not falling into it, not moving away.”

    You have misunderstood Ragnaar’s thought experiment. He didn’t say “stationary to the sun”, he said “stationary”, i.e. not moving at all.

    “Only trying to help.”

    Trying to help is a good thing, but to do so you need to try and understand the person you are trying to help and bear in mind that if what they said doesn’t make sense to you, then it may be (and is indeed likely to be) that the error lies with you rather than them.

    ” Which is why it was misguided to say that gravity would make them collide.”

    is a classic example. Ragnaar said nothing about the two being in orbit, so it is in no way misguided to say that gravity would make them collide, and indeed AFAICS is trying to explain why some initial angular momentum is required for the formation of a protoplanetary disk (and indeed a stable orbit).

    “I feel that my explanation is cogent and to the point.”

    a bit of self-skepticism is often a good idea as well.

    ATTP wrote “A cloud of gas and dust will start to collapse if its gravity dominates over its pressure.”

    angech wrote “Is true for a cloud that originated from a small explosion in space that did not cause particles to reach escape velocity. It has a ease of explanation that is actually not possible in the context of how the space dust collected into clouds in the first place and what rules of physics then bring it together.”

    you do realise that you are trying to correct a professor of astronomy, right? No angech, what ATTP wrote is (AFAICS) true for the collapse of gas/dust clouds in general, for instance in the formation of stars. See “An Introduction to the Sun and Stars” (second edition) by Green and Jones, Cambridge University Press, 2015 (ISBN 9781107492639, chapter 5 in general, but the first paragraph of section5.3.1 in particular.

  39. angech says:

    I don’t even know how to respond to the above. It seems remarkably confused.

    Sorry, I do get carried away at times.
    Thanks for the reference, Dikran.

  40. dikranmarsupial says:

    angech, an explicit admission that Ragnaar’s statement was correct would be better. If you can’t accept your misunderstandings, you won’t make any progress.

  41. The Very Reverend Jebediah Hypotenuse says:

    angech, the basic physics of the collapse of interstellar gas clouds and subsequent star formation was worked out over a century ago.

    You may wish to have a look at this.

    ATTP is looking at how smaller-scale, but similar, astrophysical processes, can lead to planet formation.

  42. The Very,
    Indeed, the Jeans Instability is essentially what I was describing at the end of this comment. It’s fairly easy to estimate the mass (or size) a cloud of gas and dust of temperature T and density \rho would need in order to start collapsing. This is the Jeans Mass (or Jeans length, if describing size, rather than mass).

  43. The Very Reverend Jebediah Hypotenuse says:

    ATTP:
    As you are no doubt aware, this sort of analysis was also crucial to the early work by Eddington on stellar structure using Emdem polytropic models, where he was able to show that radiation pressure as well as gas pressure is required to prevent collapse.

    This passage from “The Internal Constitution of the Stars”… (p16) must qualify for one of the most understated in the history of science…..


    We can imagine a physicist on a gas-bound planet who has never heard tell of the stars calculating the ratio of radiation pressure to gas pressure for a series of globes of various sizes, starting, say, with a globe of mass 10 gm., then 100 gm., 100 gm., and so on so that his nth globe contains 10^n gm.

    Regarded as a tussle between matter and aether (gas pressure and radiation pressure) the contest is overly one-sided except between nos 33-35 where we may expect something interesting to happen.

    What “happens” is the stars.

  44. The Very,

    As you are no doubt aware, this sort of analysis was also crucial to the early work by Eddington on stellar structure using Emdem polytropic models, where he was able to show that radiation pressure as well as gas pressure is required to prevent collapse.

    There is indeed both gas pressure and radiation pressure, but I think radiation pressure only becomes important in high-mass stars (well above the mass of the Sun).

    On a similar note, I did post this comment about an exchange between Eddington and Jeans, in which Jeans accuses Eddington of ignoring his work.

  45. The Very Reverend Jebediah Hypotenuse says:

    Oops – “EmdeN” not “EmdeM”.

  46. The Very Reverend Jebediah Hypotenuse says:


    I think radiation pressure only becomes important in high-mass stars (well above the mass of the Sun).

    Quite.
    Given that Eddington was ignorant of the processes involved in opacity and nuclear fusion, it is a remarkable passage nevertheless.

    Thanks for the link-back. I’d missed that.

  47. dikranmarsupial says:

    TVRJH wrote “angech, the basic physics of the collapse of interstellar gas clouds and subsequent star formation was worked out over a century ago.”

    In a way it is nice to see that it isn’t only climatology where lay-persons attempt to correct professors on basic concepts that have been well established for over a century. ;o)

  48. angech says:

    “lay-persons attempt to correct professors on basic concepts that have been well established for over a century. )”

    Kelvins classic phrase, There are two clouds obscuring the theory of the structure and mechanism of stars, the persistent discrepancy in absolute amount between the astronomical opacity and the results of calculations base on theoretical and astronomical physics.
    The other is the failure of our efforts to reduce the behaviour of subatomic energy to anything approaching a consistent scheme.

    It seems a bit rough to knock lay people when the professor says the basic principles are not well established even after a century.
    Or as ATTP headed Science , it’s complicated.
    I am allowed to put forward ideas – which I have thought about forward.

  49. angech,

    It seems a bit rough to knock lay people when the professor says the basic principles are not well established even after a century.

    Our understanding of stellar structure and evolution is actually quite remarkable, so I’m not quite sure who you’re suggesting has claimed that they’re not well established.

    I am allowed to put forward ideas – which I have thought about forward.

    Of course, but there is a point where having a reasonable knowledge of our current understanding becomes important – in my view, at least.

  50. dikranmarsupial says:

    “It seems a bit rough to knock lay people when the professor says the basic principles are not well established even after a century.”

    We were talking of whether “A cloud of gas and dust will start to collapse if its gravity dominates over its pressure.”, that has been well established for over a century (Jeans Instability). I don’t see anything ATTP has written that suggests that is not well established.

    angech wrote “I am allowed to put forward ideas – which I have thought about forward”

    Sure, but if you repeatedly try to correct others, and then are unable to admit that you were wrong, and try to evade admitting it (for instance with irrelevant quotes by Kelvin*), then it gives the impression that you are not “putting forward ideas” in good faith, but bullshitting.

    Here is a hint, if you are putting forward ideas, then there are some ways of putting them forward than others. For example, if a prof. of astronomy says “A cloud of gas and dust will start to collapse if its gravity dominates over its pressure.”, then rather than making [incorrect] assertions about the situations where this might apply, ask why does it apply to the situation under consideration. In other words, assume that it is you that doesn’t understand something and ask for clarification. That way you are likely to learn from the people who know more than you do, and you don’t look silly when your misunderstanding is exposed (as you actively helped in exposing it, which is a pretty wise thing to do).

    So do you accept that Ragnaar’s statement was correct, yes or no?

    * Jeans seems to have published his result on collapse of dust/gas clouds in about 1902 according to Wikipedia, and Kelvin died in 1907, so it clearly isn’t evidence of any substantive uncertainty relating to Jeans instability today.

  51. BBD says:

    angech

    I wouldn’t normally pile on, but since Ragnaar was originally addressing my question, I will. I can clearly see that what Ragnaar said explains why collapsing gas clouds begin to rotate. Since his explanation was not corrected by ATTP, then I think it’s safe to assume that it was accurate.

    So, I will add to DM’s request that you acknowledge that Ragnaar’s statement was correct.

  52. The Very Reverend Jebediah Hypotenuse says:

    angech:

    It seems a bit rough to knock lay people when the professor says the basic principles are not well established even after a century.

    The other is the failure of our efforts to reduce the behaviour of subatomic energy to anything approaching a consistent scheme.

    I’ll take begging-the-question, twice, for 500, Alex.

    http://www.astro.yale.edu/larson/papers/Physics03.pdf

    I would add that if you think that it’s “knocking lay people” and “rough” to point you to references, then you ought to seriously consider walking away from your keyboard.

    dikranmarsupial:

    In a way it is nice to see that it isn’t only climatology where lay-persons attempt to correct professors on basic concepts that have been well established for over a century. ;o)

    I recall a class from my undergraduate days (i.e. back when they were building Stonehenge) wherein another student kept objecting to the content of the professor’s lecture on the topic of the Protestant Reformation. It quickly became clear that student did not know that Martin Luther and Martin Luther King Jr. were two distinct people.
    The student also did not quite understand the provenance of the religion with which he identified, declaring with 100% conviction: “I’m not Protestant, I’m United!”.
    I had to leave the room.

  53. dikranmarsupial says:

    TVRJH ;o)

    A friend of mine wrote a very interesting book (well I found it interesting) on the history of methodism in Norfolk, which left me wondering if “United” is a jargon term and didn’t have its usual meaning in that context (except perhaps rather briefly).

  54. The Very Reverend Jebediah Hypotenuse says:


    …wondering if “United” is a jargon term and didn’t have its usual meaning in that context…

    The student was a member of the United Church of Canada, and, given the limited evidence available to me, I strongly suspect that he had never considered the history methodism in Norfolk, or anywhere else.

    What was memorable to me was the absolute conviction with which the student confronted the professor.
    When it comes to fervor, climate science and astrophysics have got nothing on religion.

  55. angech says:

    a prof. of astronomy says “A cloud of gas and dust will start to collapse if its gravity dominates over its pressure.”,
    Ragnaar says “Gravity works.”
    Both very clear and true points.

    TVRJH Thank you for your reference which I scanned http://www.astro.yale.edu/larson/papers/Physics03.pdf

    “Thus, star formation processes, like most natural phenomena, probably involve a
    combination of regularity and randomness.
    Possibly all stars form with close companions in binary or multiple systems or clusters, and gravitational interactions between the stars and gas in these systems may cause the redistribution of angular momentum that is necessary for stars to form. Interactions in dense environments, possibly including direct stellar collisions and mergers, may play a particularly important role in the
    formation of massive stars”

    Seems Jeans mass showing a cloud, or part of one, would become unstable and begin to collapse when it lacked sufficient gaseous pressure support to balance the force of gravity is a similar statement to Ragnaar’s.
    A bit like saying a ball on top of a mountain will roll downhill if pushed.
    An semi accurate after the event explanation, if one ignores Jean’s swindle.

  56. Susan Anderson says:

    dikranm: Thanks for the tremendous Harry Frankfort on bullshitting. I can’t help but be reminded of this as well:

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