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B The Sun is cool or hot?

  1. May 20, 2018 #21

    stefan r

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    76.45% of main sequence stars are classified as M-type. So median temperature is much lower than the sun. If you list the brightest visible stars then the sun is closer to median.

    The milky way looks redder than the zodiacal light. That is not a precise number but you can collect the ballpark data yourself on a clear night using no equipment (if you can get away from city lights).


    Most astronomers use the star as a point source. For far away stars it is impossible to separate the corona and photosphere except during eclipses. The temperature is determined by the spectrum. A star's spectrum is compared to radiation from a black body.

    Core temperatures in red giants are much higher than core temperatures in similar mass main sequence stars. Red giants are considered cooler stars. The classification is entirely based on the light that is observed.
     
  2. May 20, 2018 #22
    No.
    On the other side of the median.
    93 brightest stars:
    https://en.wikipedia.org/wiki/List_of_brightest_stars
    include:
    6 M stars (all giants or supergiants)
    22 K stars (ditto)
    2 G stars (both dwarfs)
    Grand total of 30 stars as cool or cooler than Sun, including Sun itself, and 63 hotter.
    Obviously Sun is a cool star.
    As just pointed out, by another criterion Sun is a hot star.
     
  3. May 21, 2018 #23

    stefan r

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    There are 9,500 objects visible to (some) human eyes, magnitude 6.5. You could also use stars within 10 parsecs. This list has 4 As , 6 Fs ,22Gs and 35Ks
     
  4. May 21, 2018 #24
    ... which would give very different results!

    Is Sun bright or dim?
    See:
    http://www.atlasoftheuniverse.com/stars.html
    Of the brightest 50 (49 of the list, plus Sun), all of them apparent magnitude +2,00 or less, 10 have positive absolute magnitude. 11 have absolute magnitudes under -5.
     
  5. Jun 15, 2018 #25
    This star regulus is hotter than sun.We conclude that blue stars are hotter than red and Orange.Why only "blue" stars make it images (15).jpeg
     
  6. Jun 16, 2018 #26

    russ_watters

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    Make what?
     
  7. Jun 16, 2018 #27
    Why only blue stars are hotter than red and Orange? What's the actual relationship between this "blue colour " and "hotness"
     
  8. Jun 16, 2018 #28

    russ_watters

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  9. Jun 16, 2018 #29
    'Hotter' means more energetic photons are being emitted.
    Usually this is a simple consequence of how massive the star is.
    However other factors such as the composition of the star's elements play a part in stars luminosity profile.
     
  10. Jun 16, 2018 #30
    And internal structure. Red giants may be a modest fraction of all stars, but they are a large part (though not a majority) of bright ones.
     
  11. Jun 17, 2018 #31
    So Does hotter star mean a massive star?

    More hotter the star , more massive it is?
    Or

    More massive the star, more hotter it is?
     
  12. Jun 17, 2018 #32
    In general yes, more massive stars emit much more energetic EM radiation (= hotter).
    One consequence of this is that more massive stars have short lives compared to smaller ones.
     
  13. Jun 17, 2018 #33
    Yes. That's right. By the equation e= mc^2 , every second , the Sun slims down by 5 million tonnes. This may sound like a huge amount , but during the life of the Sun it only amounts to 0.01% of its mass. So Is Sun's mass average?

    (As hotness depends on mass)
     
  14. Jun 18, 2018 #34
    Average over what?

    Of the 50 brightest stars, 14 are dwarfs. All of them are more massive than Sun, save Sun itself.
    Of the remaining 36 subgiants, giants, bright giants and supergiants, 12 are G or cooler. The rest are F or hotter.

    All the dwarfs brighter than Sun are also more massive. And hotter. On main sequence, it is fairly consistent tat a more massive star has bigger total luminosity, higher temperature as well as larger area.
    About the giants: as stars exhaust part of their fuel (protium in centre first), they tend to fuse the remaining fuel at increasing rate. At equal mass, an older star is brighter.
    The temperature varies widely. Many old stars cool down - they become bright, but acquire a large area and low temperature. Yet most giants are still hotter than Sun.
    Most of the giants that are bright in sky are still more massive than Sun. Not all. Some, notably Arcturus, are suspected of being only as massive as Sun, but older, brighter, bigger and cooler.
     
  15. Jun 18, 2018 #35
    Here mass is same , but why does the area varies? download (8).jpeg
     
  16. Jun 18, 2018 #36
    Because the luminosity varies.
     
  17. Jun 18, 2018 #37
    High luminosity or low luminosity ?
    How surface area depends on the luminosity?
     
  18. Jun 18, 2018 #38
    As the core of the star produces increasing amounts of heat, the surface layers of the star must emit the light somehow.
    They might emit it by remaining the same size and getting hotter. But gases expand on heating, so surface area necessarily increases at least somewhat.
    For red giants, the expansion is actually so large that the star finally comes to equilibrium where its surface area has increased even more than luminosity and the surface temperature has therefore fallen.
     
  19. Jun 18, 2018 #39

    stefan r

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    The core of Arcturus is not the same as the core of the sun. When astronomers say "temperature of a star" they mean the observable surface of the star. The Sun's corona is much hotter(5 million°K) than the Sun's photosphere(5772°K). The Sun's core is closer to 16 million°K. The hydrogen at the core in Arcturus burned up. Hydrogen fusion occurs in a shell around a much smaller core made of mostly helium. The core in red giant branch stars is less than around 100 to 200 million °K. When the temperature gets higher the helium will fuse and Arcturus will move to the next stage.

    Because Arcturus has a hotter core the fusion rate in the shell is higher than the fusion rate in the Sun's core. The surfaces of both stars are held up by radiation pressure. Increased reaction rates inflate the star and makes the surface puff outward.


    If you take any gas and compress it you change the temperature. Gas law. Arcturus has around 25 times the Sun's radius so around 16,000 times the volume. If you created a sample of Arcturus gas in a laboratory and then you compacted it to the density of the Sun's gas you would expect the temperature/pressure to rise by a factor of 16,000 if it was an "ideal gas". The internal structure is different and the gases are not ideal but the general idea applies. Air conditioners and refrigerators work using temperature change in a gas that expands and compresses. If you took a sample of solar gas at 5772°K in a laboratory container and expanded to 16,000 times the volume the temperature will be much lower than Arcturus (4286°K).
     
  20. Jun 18, 2018 #40

    Ken G

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    You're right that there is a high shell fusion rate in Arcturus, but neither the Sun nor Arcturus experiences any significant radiation pressure anywhere in the star. It's well over 99% gas pressure.
     
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