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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.
          2. stouset · · focus · HN ↗
            What’s particularly interesting to me is that for stars the size of our sun, regular old gas pressure dominates. The sun’s atmosphere is held up essentially just from the temperature (and therefore high kinetic energy) of the plasma.

            Only once you get to 10+ solar masses does radiation (light) pressure begin to become significant, and at 50+ solar masses is when it dominates and the atmosphere is held up by the momentum of light.

          3. sega_sai · · focus · HN ↗
            In most star photospheres the role of radiation pressure is negligible. They are supported by the pressure gradients. The exception is very hot stars.
        2. kulahan · · focus · HN ↗
          To add on to the other response, think of it like the star “leaking” into space. It’ll be defined as a pretty radically different size at some point in the future, even as it’s still undergoing fusion.
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