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Surface of the Sun

  1. Jun 30, 2007 #1
    Why does the sun appear to have such low surface gravity? Most would say the the nuclear reactions inside the sun continually pushes matter outward against the force of gravity, but this does not account for the slow fall of solar flares and speed of solar wind.

    According to the current theory of gravity, the sun should have immense surface gravity. It simply does not appear to be so.
  2. jcsd
  3. Jun 30, 2007 #2
    wow, is that so?? i didnt knew it was!!
    but how do you know the fall of solar flares and solar wind on sun's surface are low?? i mean, are there any stats supporting this??
  4. Jun 30, 2007 #3
    according to wikipedia, the cause of solar wind (at least the high speed and low deceleration) is still unknown.
    as for solar flares, just looking at pictures and reading about them shows you they dont behave like they should
  5. Jun 30, 2007 #4


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    You can tell that by looking at pictures? Besides the obvious problem that still pictures aren't moving, do you have any concept of how big they actually are?

    The sun's surface gravity is 28g. Whether that "appears" low, I don't know...
  6. Jun 30, 2007 #5
  7. Jun 30, 2007 #6
    i dont know, the video does not seem to make any statement on the gravity of the sun.
    i mean, you cant tell it by seeing the video
  8. Jun 30, 2007 #7
    I know...

    forgive me for making such an unverifiable and subjective claim in this topic
  9. Jun 30, 2007 #8
    Here's a fun project for you: By studying video of solar flares (in conjunction with the solar radius, from wikipedia), figure out how high one goes. Then, by knowing how long it takes to fall, you should be able (2s=at^2) to estimate the sun's surface gravity yourself. o:) Then tell us how you went.
    Last edited: Jun 30, 2007
  10. Jul 2, 2007 #9
    Stupid question:
    Does the sun have a surface? I thought that it would be just a big ball of gas that gets thinner and thinner and finally just peters out. Wouldn't the surface we see be just an apparent surface due to photon scattering (where the atmosphere finally gets thin enough that most photons are no longer scattered)?
  11. Jul 2, 2007 #10


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    That is right, the sun does not have a sharp surface. The surface of the sun is just a convention, that the surface is where the optical depth is 25 (or something, I have forgotten the exact numbers)
  12. Aug 19, 2007 #11


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    http://video.google.com/videoplay?docid=708234991989726743 - very large prominence. There is the matter of frame rate - time lapse - which would artificially speed up the event.

    http://video.google.com/videoplay?docid=140644097329538951 - fast moving solar flare - I believe real time.

    Please quantify 'immense'.

    How about magnetohydrodynamics and the interaction of a hot plasma with intense magnetic fields.

    try g=GM/R2 to see what the g should be in the chromosphere.

    I come up with 274 m/s2 or 28g as Russ mentioned.

    The escape velocity is ~620 km/s, and the solar flares are moving at 1000's of km/sec. Also be careful not to confuse particle (solar wind) velocity with the plasma wave velocities.

    Temperature of the photosphere is about 5500 K, while that of the chromosphere is about 30,000 by some measurements and nearly 1 MK by others. The corona has a temperature of about 5 MK.


    See also - http://zebu.uoregon.edu/~js/ast122/lectures/lec12.html
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