Thread

1. jahnu · 1789828933 · focus · HN ↗

   One thing worth noting is our popular notion of what a star seems to be is quite different from what astronomers have.

   One example, Canis Majoris has a radius of about 1420 solar radii. But its mass is roughly 17±8 times the mass of the Sun and therefore an average density of 5.33 to 8.38 mg/m3. “It is over 100.000 times less dense than Earth’s atmosphere at sea level.”

   <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;VY_Canis_Majoris" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;VY_Canis_Majoris

   <a href="https:&#x2F;&#x2F;nineplanets.org&#x2F;vy-canis-majoris&#x2F;" rel="nofollow">https:&#x2F;&#x2F;nineplanets.org&#x2F;vy-canis-majoris&#x2F;

   1. FranOntanaya · 1789831189 · focus · HN ↗

      I always wonder when do they stop considering the outer layers part of the star and not relatively dense solar wind soup. Nobody says Earth has 7000km radius, even tho there&#x27;s traces of exosphere past 500km.

      1. antognini · 1789834819 · focus · HN ↗

         The outer boundary is the point where a photon has a ~50% chance of escaping without encountering another particle.

         1. encrypted_bird · 1789843009 · focus · HN ↗

            Do you have a source? I&#x27;d love to read more. :)

            1. zamadatix · 1789852613 · focus · HN ↗

               Amateur stargazer&#x27;s understanding:

               Depending what kind of reading material you&#x27;re looking for (e.g. high level details or mathematically&#x2F;jargon dense papers) look for things discussing &quot;Rosseland optical depth&quot; and &quot;grey atmosphere approximation&quot;.

               At a high level, the common convention is to define the radius by finding where the optical depth is 2&#x2F;3 when using Rosselands clever way of calculating a weighted mean of the opacity from that layer in the star to space across different wavelengths of light. 2&#x2F;3 being a clever derivation from Eddington where, in an idealized model of a star, that&#x27;s when the actual temperature of the star should equal its blackbody equivalent temperature.

               Pedantically, this distance to the point of equality is an ever so slightly different value than the &quot;distance from the center where there is a 50% chance a photon traveling directly outward will escape the star without another interaction&quot; rule of thumb (in the same idealized grey model). Practically, that the difference is so small is why it&#x27;s a fantastic rule of thumb explanation.

            2. antognini · 1789951697 · focus · HN ↗

               I&#x27;d recommend Carroll &amp; Ostlie&#x27;s Introduction to Modern Astrophysics, though it requires some basic calculus and physics.

               1. encrypted_bird · 1790151105 · focus · HN ↗

                  Thanks!

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