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Black Holes or Black Hole Stars? Astronomers Spar over 'Little Red Dots'

105 points · 61 comments · jandrewrogers

  1. jahnu · · 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. Sharlin · · focus · HN ↗
      The surface radius of a star is defined based on optical thickness. If you were there, the star would in fact look like a fairly well-defined opaque spheroid with the reported radius. The density of the photosphere plasma is near zero, but there’s a lot of it.
      1. brabel · · focus · HN ↗
        Why doesn’t the stuff just falls in towards the center? It just floats there??
        1. zygentoma · · focus · HN ↗
          Radiation pressure! There is so much heat (= photons) radiating outwards, that it counteracts the gravitational pull.
          1. brabel · · focus · HN ↗
            Wow I knew that stars have that radiation pressure but had no idea it caused mass to get so crazy far away from the ignited area of the star!
            1. Sharlin · · focus · HN ↗
              These giant stars burn so very very bright. And correspondingly only live a few tens of millions years at most.
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