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

105 points · 61 comments · jandrewrogers

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://en.wikipedia.org/wiki/VY_Canis_Majoris" rel="nofollow">https://en.wikipedia.org/wiki/VY_Canis_Majoris
<a href="https://nineplanets.org/vy-canis-majoris/" rel="nofollow">https://nineplanets.org/vy-canis-majoris/
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's traces of exosphere past 500km.
  1. chabska · 1789863215 · focus · HN ↗
    The radius is based on visual observation. We have good theories and models about what's happening inside the star. We are quite sure that it's mostly empty space inside the radius. But that's not a good enough reason to say that the star's radius is anything other than what we see through our telescope.
    zamadatix · 1789928502 · focus · HN ↗
    Importantly, this works for stars relatively close to us (the farthest I can find is 16,000 light years for one hyper giant) but the majority of the stars catalogued in even our own galaxy have only been assigned a radius via taking the temperature (via color) and luminosity.

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