Density of a small red dwarf star is 57,000 kg/m3?!?!
Constantine Thomas
Radius of sun = 696265 km = 696265000 metres
Volume = (4/3)*pi*(r^3) = 1.41e27 m3
Mass of sun = 1.9891e30 kg
Density of sun = 1406 kg/m3 (close enough to the official value)
That sounds about right.
Then I tried to figure out the density of Proxima Centauri (our most
famous friendly neighbourhood red dwarf), which according to
http://www.solstation.com/stars/alp-cent3.htm has a radius of 14.5%
that of Sol and 12.3% of its mass.
So... for Proxima we'd have
Radius = 696265000*0.145 = 100958425 metres
Volume = 4.31e24 m3
Mass of Proxima = 1.9891e30 * 0.123 = 2.45e29 kg
so Proxima density = 56760 kg/m3.
56,760 kg/m3?!! I can't see anything I'm doing wrong here. Are red
dwarfs really that dense?! It sounds like they'd actually have a solid
surface that one could land on!
Am I doing something wrong here, that's so obvious I can't see it? Or
am I missing something obvious? Or, dare I say it, am I right?
Constantine Thomas constanti...@shaw.ca
Mad (Planetary) Scientist, Space Artist, Horseman of the Apocalypse
http://members.shaw.ca/evildrganymede/home.htm
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R Hendricks
>Then I tried to figure out the density of Proxima Centauri (our most
>famous friendly neighbourhood red dwarf), which according to
>http://www.solstation.com/stars/alp-cent3.htm has a radius of 14.5%
>that of Sol and 12.3% of its mass.
>
>So... for Proxima we'd have
>
>Radius = 696265000*0.145 = 100958425 metres
>Volume = 4.31e24 m3
>Mass of Proxima = 1.9891e30 * 0.123 = 2.45e29 kg
>so Proxima density = 56760 kg/m3.
>
>56,760 kg/m3?!! I can't see anything I'm doing wrong here. Are red
>dwarfs really that dense?! It sounds like they'd actually have a solid
>surface that one could land on!
>
>Am I doing something wrong here, that's so obvious I can't see it? Or
>am I missing something obvious? Or, dare I say it, am I right?
>
>
Looks like everything is right to me. From what I've read, from about
Jupiter's mass to the
mass of a red dwarf, the radius stays "fairly" constant, so the density
has to increase. ProxCen has ~100x the mass of Jupiter, so should be
~100x denser. Jupiter's density is 1.33 G/cm^3 (www.solarviews.com), so
Prox Cen is about 133 G/cm^3, or 133,000 kg/m^3. But Jupiter is really
71,500 km in radius, and prox cen is 101,000 km in radius. Ratio of the
cubes is about 1/3, and 1/3 of 133,000 is about 50,000. So 56,760 as
an exact number sounds right.
I think it's because more of the sun's gas is in a very hot state,
puffing it out more than a red dwarf that can barely sustain fusion.
Another interesting tidbit is that the force of gravity on the "surface"
of the red dwarf is much greater than on the sun, by a factor of 6.
I suspect the surface wouldn't be any more "solid" than Jupiter.
Carsten Nielsen
> I just calculated the density of our own sun:
>
> Radius of sun = 696265 km = 696265000 metres
> Volume = (4/3)*pi*(r^3) = 1.41e27 m3
> Mass of sun = 1.9891e30 kg
> Density of sun = 1406 kg/m3 (close enough to the official value)
>
> That sounds about right.
>
>
> Then I tried to figure out the density of Proxima Centauri (our most
> famous friendly neighbourhood red dwarf), which according to
> http://www.solstation.com/stars/alp-cent3.htm has a radius of 14.5%
> that of Sol and 12.3% of its mass.
>
> So... for Proxima we'd have
>
> Radius = 696265000*0.145 = 100958425 metres
> Volume = 4.31e24 m3
> Mass of Proxima = 1.9891e30 * 0.123 = 2.45e29 kg
> so Proxima density = 56760 kg/m3.
>
> 56,760 kg/m3?!! I can't see anything I'm doing wrong here. Are red
> dwarfs really that dense?! It sounds like they'd actually have a solid
> surface that one could land on!
>
> Am I doing something wrong here, that's so obvious I can't see it? Or
> am I missing something obvious? Or, dare I say it, am I right?
>
> Constantine Thomas
I think you have the math right.
Density of Proxima = Mass / Volume = (0.123 * Ms) /(4/3)*pi*((0.145*rs)^3)
Density of sun = Mass / Volume = Ms / (4/3)*pi*(r^3)
Density(Proxima) / Density(Sun) = 0.123 / (0.145)^3 = 40.34
> Density of sun = 1406 kg/m3 (close enough to the official value)
Density of Proxima = 1406 kg/m3 * 40.36 =56726 kg/m3
Perhaps with the lower temperatures, the interior is more compressed ?
Or solstation has a typo somewhere.
I'll ask on a local NG.
Regards
Carsten Nielsen
Denmark
Carsten Nielsen
> Volume = 4.31e24 m3
> Mass of Proxima = 1.9891e30 * 0.123 = 2.45e29 kg
> so Proxima density = 56760 kg/m3.
>
> am I missing something obvious? Or, dare I say it, am I right?
>
> Constantine Thomas
Now I got some answers back.
Apparently it makes sence that Proxima is denser, because a red dwarf
can't be very active, so the radiaton pressure, which keeps the Sun
expanded, must be lower.
Apparently Proxima has 150 times the mass of Jupiter, It has about 45
times the density of Jupiter, which is because of the greater
overlaying masses of hydrogen and helium. It's diameter or radius is
1.5 times Jupiter.
Regards
Carsten Nielsen
Constantine Thomas
<carste...@hotmail.com> wrote:
>Apparently Proxima has 150 times the mass of Jupiter, It has about 45
>times the density of Jupiter, which is because of the greater
>overlaying masses of hydrogen and helium. It's diameter or radius is
>1.5 times Jupiter.
OK, thanks everyone. I had to check, it just caught me totally off
guard :).
Would these lowmass stars be structured like normal stars? I
understand they've got convective cores and an atmosphere, but I'm
just kinda boggling at the thought of 50,000 kg/m3 materrial
convecting... is that a solid? Or some wacky form of hydrogen
(metallic, perhaps?). Or a plasma (can't be surely, M Dwarfs aren't
*that* hot are they?). I'm just trying to get a grip on what the
stellar material is like, if anyone knows.
Cheers,
Consty
Carsten Nielsen
> Would these lowmass stars be structured like normal stars?
I should think so.
>I understand they've got convective cores and an atmosphere, but I'm
> just kinda boggling at the thought of 50,000 kg/m3 materrial
> convecting... is that a solid? Or some wacky form of hydrogen
> (metallic, perhaps?). Or a plasma (can't be surely, M Dwarfs aren't
> *that* hot are they?).
The inside is, or there would be no fusion. The Earth is also hotter inside.
>I'm just trying to get a grip on what the
> stellar material is like, if anyone knows.
>
> Cheers,
> Consty
Regards
Carsten Nielsen
Denmark