alt Hacker NewsOne 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.”
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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.
My layman definition of what a star is is "it's somewhere in the sky and it gives off light". The word "density" wouldn't even come to my mind
> therefore an average density of 5.33 to 8.38 mg/m3
That's nuts. How on earth does fusion happen at that density? Is there a denser core that actually fuses, and the outer fluffy bits just glow from the inner heat?
Yes and no. It’s fluffy on the outside, but at the core it’s likely denser than our sun.
Average density for non-uniform objects is pretty useless, since the cubic volume scaling makes a mess of things.
As an example, I pulled the stats for an electric locomotive. 19x3x4.4 meters, mass 90 metric tonnes. That's an average density of about 1/3rd of water.
These objects (black hole stars) seem to be far larger than that,
https://en.wikipedia.org/wiki/Quasi-star#Formation_and_prope... (Caption: "Size comparison of a hypothetical quasi-star to some of the largest known stars")
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.