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I Is the boundary of a star objective or subjective?

  1. Aug 3, 2016 #1
    What I mean by this question is the following: If, just for example, we define the surface region of a star as that where the matter undergoes a phase transition from plasma to radiation, then that boundary has an objective physical meaning (let's not bother with the fact that the transition is not definite and there are flares and fluctuations and so on, that's not my point).

    On the other hand, if we define the surface of the star, again as just an example, by the region where its radiation falls below the visible spectrum and becomes infrared, the 'visible diameter of the star for us humans', then that description is subjective, any living being with infrared vision would say that the diameter of the star is larger than what we humans say, and any living being with vision shifted the other way might say that the star diameter is smaller.

  2. jcsd
  3. Aug 3, 2016 #2


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    Matter does not undergo a phase transition at the surface of a star.
    This definition is not operative in astronomy.

    The surface of a star is defined as the surface of last scattering. We are typically interested in optical photons, so it's these photons that define the surface
  4. Aug 3, 2016 #3
    TX DrSteve, so I understand that the definition of a star's diameter is completely objective. That was precisely what I was asking, so tx a lot.

    But just to reconfirm it completely, if we imagine sentient beings with vision going into the infrared (so I assume they might perceive a bigger star visually), would they anyway agree that the surface of last scattering is the same as that defined by us? and therefore agree with a same diameter for the star even if visually they see it larger than we do?
  5. Aug 3, 2016 #4

    Ken G

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    They wouldn't see it larger than we do. The opacity in the infrared and the optical is not much different, at least not between 0.8 and 4 eV. The density falls off so quickly that you need spectacular changes in opacity to get any kind of significant difference in the apparent size of the Sun, especially since you have essentially a roiling surface with all kinds of bumps and projections. The Sun looks more or less exactly the same in the infrared, consider these pictures (taken at different times, you can seek cotemporal ones if you like):
    Sun in the infrared: http://umbra.nascom.nasa.gov/images/latest_nsoHe.gif
    Sun in the optical: http://www.space.com/images/i/000/0...311?interpolation=lanczos-none&downsize=640:*
    Notice all the detail at the sharp surface, and though you cannot necessarily see from these pictures, that surface is not going to look significantly different. In fact, you may see more in the way of scattered light off low density gas in the optical, giving perhaps more of a "glow" to the Sun in the optical, though the glowing regions are not what we consider to be the "surface". So once we adopt an objective definition, it's not much different if we use optical or infrared light to create that objective definition, but the choice to use that objective definition is certainly subjective.
  6. Aug 4, 2016 #5
    Thanks a lot, clear!
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