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B The Sun is cool or hot?

  1. May 7, 2018 #1
    Red dwarf stars are main sequence stars but they have such low mass that they’re much ' cooler' than stars like our Sun.

    Then Simple question here : Sun is cool or hot??
     
  2. jcsd
  3. May 7, 2018 #2

    davenn

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    what do you think ?
    have you googled something like ...
    1) the surface temperature of the Sun or
    2) the core temperature of the Sun ?

    there's 2 things for you to do an report back with your findings :smile:


    Dave
     
  4. May 7, 2018 #3

    Ken G

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    Although it is a strange term, the Sun is indeed regarded as a "cool star." The basic distinction is the different atmospheric physics you encounter when a star has a convection zone around its exterior, rather than a quiet zone of radiative diffusion there. "Hot stars" typically have convection in their cores, but not so much their exteriors. The discovery of magnetic fields in some "hot stars" has blurred the distinction somewhat, but there is a general tendency for higher mass stars to behave quite differently from lower mass stars, so this basic bimodal distinction between "hot stars" and "cool stars" is likely to persist. (For example, there is a yearly meeting called "
    Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun", which more or less answers your question right there!)
     
  5. May 7, 2018 #4

    stefan r

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    The terms cool and warm can be ambiguous. Artists, fashion designers and painters would say that "cool" colors have more blue in them. This is the opposite of a star's surface temperature.
     
  6. May 8, 2018 #5
    Quite different
     
  7. May 12, 2018 #6
    A simple question with no simple answer.
    What is "cool" and what is "hot"?
    Of the 93 brightest stars, 28 are cooler than Sun, 2 are about equal (Sun itself and Alpha Centauri A), 63 are hotter.
    Of the 64 nearest stars, 6 are hotter than Sun, 2 are about equal (the aforesaid), 56 are cooler.
     
  8. May 12, 2018 #7

    Ken G

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    Nevertheless, "cool" and "hot" are not really arbitrary distinctions for stars, they are an effort to notice that stars can be divided into two very different flavors. Of course any such division is artificial, and more divisions can certainly be made, but in a coarse sense you have stars that are more massive, less convective near the surface, and ultimately have core-collapse supernovae, and stars that are less massive, more convective near the surface, and become white dwarfs. So that's "hot" versus "cool" in a nutshell, though even the massive stars can puff out so much as they evolve that they temporarily become "cool" and develop surface convection. In that way of slicing things, the Sun is a "cool star." It's about helping researchers group together objects that may have similar physics that they are interested in, moreso than some arbitrary line in the sand of surface temperature.
     
  9. May 12, 2018 #8

    stefan r

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    Wikipedia on Wolf-Rayet stars:
    For example V429 Carinae has surface temperature 44,700K and is "
     
  10. May 13, 2018 #9

    Ken G

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    The general trend is that if you start at the coolest surface temperatures and start increasing it, at first the core gets more and more convectively stable, while the outer layers remain convectively unstable. This continues until the outer convection zone gets smaller and smaller and eventually nearly disappears, but then the core starts to become unstable. At the highest surface temperatures (like the Wolf-Rayet phase), the core convection zone has expanded to include most of the star. But the pattern generally holds-- high surface temperature implies core convection, low surface temperature implies envelope convection. At the highest end, the core convection merely encompasses most of the star, and at the lowest end, the envelope convection encompasses most of the star. But the main point is, the people who are most interested in how convection affects the atmosphere are generally people who study cool stars, and it is far from established that all Wolf-Rayet stars have convection zones that reach the surface. Indeed, the surfaces of last scattering in Wolf-Rayet stars tend to be found within a supersonic wind, which is certainly not convective.
     
  11. May 13, 2018 #10
    They are grossly arbitrary, because more distinctions can be made.
    Counting nearby stars, I found that roughly half of them are hotter than M3.
    It seems that it is roughly around M3...M4 that stars hotter than this possess radiative stagnant core and accumulate some helium isotopes in core. Thus, all stars hotter than M3, including Sun, are hot stars in one important sense - but just one of many possible senses, therefore arbitrary.
     
  12. May 13, 2018 #11

    Ken G

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    But all distinctions are arbitrary, it doesn't make them useless. Most of the stellar astrophysics community is divided into "hot star" and "cool star" studies. Better if everyone went to the same conferences, but then they'd just be too big and varied.
     
  13. May 13, 2018 #12

    stefan r

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    The sun has coronal holes which are not convective. The corona further complicates the hot vs cool question.

    The temperature of a star is a real number. There is no obvious gap in the hertzsprung-Russell diagram. The transition from not having a core to having a tiny core is very difficult to see. Somewhere in the M3 to M4 range. CNO burning of hydrogen is taking place in the Sun but at a rate too slow to cause core convection. You could find an overlap where a star has some core convection and some surface convection.

    Are they divided by star types or by telescope/detector type? It would also be reasonable for Astronomers to group by the age of the stars they are studying. It is hard to see M-dwarfs that are far away.

    Would a study of the Orion Nebula go to the "hot star" conference or the fit in with the "cool" astronomers?
     
  14. May 13, 2018 #13

    Ken G

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    Subfields are generally divided by star type, but astronomers often characterize their expertise by wavelength. It's odd, but you often find "radio astronomers" attending a "cool star meeting", or "X-ray astronomers" attending a "hot star meeting," or vice versa. The lines have historical purposes, but aren't always logical.
    Possibly "hot stars," possibly "interstellar medium." If you are interested in the stars that illuminate the nebula, or the history of the supernovae that made it, you might hear about these at a "hot star" meeting. I doubt cool star astronomers care much about the Orion nebula!
     
  15. May 15, 2018 #14
    They found a lot of young cool stars besides the young hot stars. Including (or excluding?) an eclipsing binary of two young brown dwarfs.
     
  16. May 15, 2018 #15

    Ken G

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    Good point, I guess the real message is the unity of astronomy, and how the distinctions we make for convenience should never let us forget that it's all one cosmos.
     
  17. May 15, 2018 #16
    I guess it is possible for two Jupiter sized objects to merge when a planetary disk collapses.
    At that stage fusion starts, and it can be called a distinct star and not just a nebula..
    The Sun is a bigger star than average, yes it is hotter than small stars.
    It is not exceptionally bigger and hotter though, there are blue supergiants. although they are rare
     
    Last edited: May 15, 2018
  18. May 15, 2018 #17

    Ken G

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  19. May 17, 2018 at 8:42 PM #18

    stefan r

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    A paper link. Also a video from NASA.

    They also found the temperatures reversed. The more massive brown dwarf is cooler. This could be caused by spots.

    If you are interested in cold spots on hot stars do you go to the cool conference or the hot conference?
     
    Last edited: May 17, 2018 at 10:39 PM
  20. May 17, 2018 at 9:28 PM #19

    Ken G

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    Mind blown.
     
  21. May 20, 2018 at 6:04 PM #20
    I assume you are referring to the hottest part of the sun, which lies above the surface of the sun. As we get to the surface, temperatures drop significantly. Observations suggest that below the surface temperatures drop even more. Indeed, looking into holes indicates lower temperatures.

    According to emmited heat, the consensus seems to be that our sun may be average or slightly below. It is about at the median. This is also complicated by the definition of a star.

    There appears to be no consistent temperature throughout the sun and it is very difficult to get emissions deep in the sun. So, the answer is difficult. For most other stars I think we only measure emission temperatures.

    At least that is my understanding.
     
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