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April 26

Mass-radius relations of stars and mean molecular weight

Are there analytical expressions that describe how the radii of white dwarfs change with mean molecular weight? JoJo Eumerus mobile (main talk) 19:06, 26 April 2024 (UTC)[reply]

See White dwarf#Mass–radius relationship. The radius goes down fairly quickly as the mass increases. NadVolum (talk) 20:13, 26 April 2024 (UTC)[reply]

The radius depends on the mean molecular weight through the number of electrons per unit mass N. Ruslik_Zero 20:27, 26 April 2024 (UTC)[reply]

Thx. Is there a similar approximation for nondegenerate matter like brown dwarfs? JoJo Eumerus mobile (main talk) 13:56, 27 April 2024 (UTC)[reply]

Brown dwarfs don't really change in radius as their mass changes. Radius stays nearly constant as mass increases from the onset of significant self-compression (about Saturn's mass) to the onset of hydrogen burning (turning into a red dwarf at about 75 Jupiter masses). So they'd all more or less be one Jupiter in radius – you already see the low end of this in the Solar System, with Jupiter not being much larger in radius than Saturn. Double sharp (talk) 18:10, 27 April 2024 (UTC)[reply]

@Double sharp: Does the radius change with mean molecular mass, though? JoJo Eumerus mobile (main talk) 13:07, 28 April 2024 (UTC)[reply]

In terms of their major components, wouldn't brown dwarfs all resemble Jupiter in composition? Particularly so since many would have depleted their deuterium through fusion by the time we observed them. So I kind of doubt mean molecular mass would vary that much between them to produce noticeable changes. But admittedly I'm guessing here. Double sharp (talk) 05:46, 1 May 2024 (UTC)[reply]

Nope - brown dwarfs around Black Widow Pulsar-like pulsars may be stripped down remains of stars and thus consist mainly of helium (or carbon, oxygen etc.). Brown dwarfs formed from white dwarf debris (around black holes) or kilonova ejecta may also contain other elements. Jo-Jo Eumerus (talk) 09:08, 3 May 2024 (UTC)[reply]

Interesting: you learn something new every day, I suppose! OTOH, I guess most brown dwarfs would still be directly produced and thus indeed resemble Jupiter in composition. Anyway, it doesn't seem like the companion to the Black Widow Pulsar has a known radius, so the question may be unanswerable for now. Double sharp (talk) 10:14, 3 May 2024 (UTC)[reply]

Equations of state have been derived for numerous elements, and even scaled for non-hydrogen compositions. So there should be a theoretical estimate somewhere. Jo-Jo Eumerus (talk) 08:57, 4 May 2024 (UTC)[reply]