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Are any supergiants translucent? Some have volumes thousands of times more than the Sun's while having maybe twenty times the mass of the Sun which makes them sound rather diffuse. If there was a very bright distant star behind the supergiant would we be able to see it?

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    $\begingroup$ You might find this question helpful: astronomy.stackexchange.com/questions/26622/… $\endgroup$ – fasterthanlight Apr 30 at 2:22
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    $\begingroup$ Great question! Roughly speaking we can say that the part outside of it's photosphere is and the part below isn't, roughly speaking. Certainly when simulating eclipsing binaries one might want to include light from one passing through the supra-photosphere part of the other, but I don't know how often or carefully that's done. Slightly related: What is the spectral reflectance of starlight in a close binary? $\endgroup$ – uhoh Apr 30 at 3:30
  • $\begingroup$ A red giant may have a huge non-luminous atmosphere, but that atmosphere can contain dust that's been dredged up from the interior. And they spew out a lot of dust too. Some red giants are almost completely obscured by their dust in optical wavelengths, but they're quite bright in infrared photos. $\endgroup$ – PM 2Ring Apr 30 at 9:26
  • $\begingroup$ I think there is possibly a misunderstanding on how opacity sets up. It seems to me like asking if one can see the fog, in the sense that it should depend on the sight direction and what portion of the disk one look at. But I am not sure, myself. It is just to say that being within the periphery of the suorrgiant one should be able to see the outside and turning to the back, a huge star. $\endgroup$ – Alchimista Apr 30 at 10:12
  • $\begingroup$ Another related question: Photosphere is relatively transparent. Is that right? $\endgroup$ – B--rian Jun 18 at 19:27
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Although I will only tackle one part of the question, I find the following part of a picture from NRAO/AUI/NSF, S. Dagnello, cited from space.com worth sharing:

NRAO/AUI/NSF, S. Dagnello

You see the radial structure of Antares, a red supergiant of spectral type M1.5Iab-Ib, and more specifically

  1. The average temperatures of photosphere, chromosphere, and above are given.
  2. One can see how far the different zones would stretch out if Antares would be the center of our solar system.
  3. We learn which parts can be studied with the Atacama Large Millimeter/submillimeter Array (ALMA) or the Very Large Array (VLA).

How is that related with you question? Well, when we speak about transparency or translucency, we should also specify the wavelength(s) we are considering, here a quote from the above mentioned space.com article:

While Antares' diameter is about 700 times larger than the sun in visible light, this map revealed that, as seen in radio light, the star's atmosphere stretches even farther and is even more enormous.

So, for the case of Antares and radio-frequencies, I would suspsect, that it could be possible to actually see other radio-objects within the radio-glow (e.g. the "Wind Acceleration Zone") of Antares, but I have not found any publication on that yet, maybe somebody could suggest search terms in the comments.

References

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    $\begingroup$ Just a small addition from wikipedia: "Like most cool supergiants, Antares's size has much uncertainty due to the tenuous and translucent nature of the extended outer regions of the star." which suggests this is true for most other 'cool' supergiants. +1 btw $\endgroup$ – Aryan Bansal Jun 13 at 23:04
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    $\begingroup$ +1, and potentially related: Do stars have “radio photospheres”? Are they different from their optical photospheres? $\endgroup$ – uhoh Jun 14 at 0:07
  • $\begingroup$ So if the radius of this or any similar star were established via radio astrometric techniques for some reason (which is not typically how the size of a star is defined), the star has the potential to be optically translucent or even transparent in its outer layers? Is that a tl;dr of your answer? $\endgroup$ – uhoh Jun 18 at 2:16
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    $\begingroup$ Your TL;DR more or less what I meant, I feel it is a matter of wavelengths. I do not see any conceptual arguments why some chromosphere (definition) would not allow translucency in of some kind. $\endgroup$ – B--rian Jun 18 at 19:21
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No mass blob of stellar mass is transparent at any wavelength of interest. Opacities $\kappa_{\nu}$(inverse transparency) as function of wavelength becomes really high and broad band at pressures above > 0.1 bars, for all wavelengths.
This leads to the optical depths $\tau_{\nu}$ being enormous and as transmission is $T=1-\exp(-\tau)$, you won't be able to see through any star, ever.

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    $\begingroup$ I feel like this question could be elaborated. What would the pressure be at the photosphere of a supergiant; what kind of transition would you expect there? $\endgroup$ – antlersoft Apr 30 at 14:25
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    $\begingroup$ -1 While this answer sounds authoritative there's no way to see if it is right or wrong. There's nothing here but "No, because I said so." The relation $T=1-\exp(-\tau)$ alone doesn't tell us anything without a $\tau$ and you haven't cited one. $\endgroup$ – uhoh Apr 30 at 15:59
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    $\begingroup$ This is Stack Exchange; answers should and do support their assertions. $\endgroup$ – uhoh Apr 30 at 17:41
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    $\begingroup$ I modestly agree with the answer. It just seems to me that my comment under the question is of relevance. One cannot see through a star passing through its center, but being inside the star at a sufficient peripheral point could allow translucency. I am not sure about the density of the outer parts of a supergiant but it should be low. $\endgroup$ – Alchimista May 1 at 12:17
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    $\begingroup$ Nothing with a photosphere is transparent. $\endgroup$ – ProfRob May 10 at 19:05

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