# Are red dwarfs really 30-100 times our Sun's density?

This article by K. E. Edgeworth claims red dwarfs tend to be around 30-100 times our Sun's density. However, this seems a bit high.

With smaller stars, mass tends to be proportional to radius but have a notably smaller volume, leading to higher densities. This Wikipedia article labels red dwarfs as massing between 0.08 and 0.45 solar masses.

However, following mass being proportional to radius, a 0.25 solar radii star with 0.25 solar masses has a density of 22.5003 g/cm³, or about 16 times our Sun's density.

A star 1/4 the size and mass of ours is smack in the middle of the red dwarf range, but is only about halfway to the 30 times our Sun's density minimum the above article outlines, while based on the above article I would expect a red dwarf 1/4 the size and mass as our Sun to have a density more like 60 times our Sun's.

What is the cause of this discrepancy?

• This is a 1946 paper. Commented Feb 23, 2021 at 6:46

Red dwarfs, depending on your definition, can range from 2.5 to 150 times more dense than the Sun.

What is the cause of this discrepancy?

They give no calculations, so I can only guess.

• The article is from 1946 and we've gotten a lot better at science.
• It's 1946 and information exchange is limited. No internet, no TV, and long distance calls are expensive.
• Red dwarfs are tricky to observe and we've gotten a lot better at that.
• Their hypothesis is that red dwarfs do not fuse hydrogen which is incorrect.

Edgeworth's other contemporary works might provide insight.

# "Red dwarf"

There is no strict definition of a red dwarf. Some use class M stars below a certain temperature and mass. Others include some class K stars. Fortunately they give a definition.

low luminosity (say, not more than one tenth that of the sun), small mass (say, not more than three quarters that of the sun) and high density (perhaps 30–100 times the density of the sun).

Luminosity and mass are basically correct for the modern definition of a red dwarf, M-class, but red dwarf density runs a considerably larger range from 2.5 to 150 times more dense. Still, they're inside the range.

They give no rationale for their density calculations. Being 1946 its probable their mass and radius calculations were considerably off. Red dwarfs, being so faint, are very difficult to observe.

# Red dwarfs fuse hydrogen

In the case of stars belonging to the main sequence, it seems to be necessary to assume that the general conditions in the interior of the star are such that convection currents occur which are of sufficient importance to convey hydrogen in adequate amounts from the outer layers to the central core...

Main sequence stars are convective, but it's complicated.

...although this does not imply that the convection currents are of sufficient magnitude to be the dominating factor in determining the distribution of temperature.

Incorrect. The lower the mass the more convection is the "dominating factor" over radiation.

...red dwarfs probably contain a normal proportion of hydrogen...

Correct.

...but that convection currents in these stars have never been of sufficient magnitude to convey the hydrogen in adequate amounts from the outer layers to the central core...

Incorrect. The lower the mass the more convection. Low-mass red dwarfs are fully convective with no helium accumulating in the core.

When the mass of the star is less than [0.75 M☉], the hydrogen which the star contains never reaches the core in sufficient quantity to initiate and sustain the process of transforming hydrogen into helium, and dynamical equilibrium is secured only by contraction involving comparatively high densities.

Incorrect. Red dwarfs fuse hydrogen. What they're describing is more like a brown dwarf.

On this theory the red dwarfs are not stars of great age which have consumed their substance in riotous living; they are simply feeble things which have never been able to make use of the supplies of fuel which they actually possess.

Red dwarfs are of great age, but they aren't bigger stars which have run through their fuel. They also are not "feeble things" which can't fuse hydrogen. They are low-mass stars which use their fuel very efficiently and so live to a great age.

With an incorrect model they will get incorrect answers.

• Now I feel dumb. I didn't even see the date, I just figured it was up-to-date info since it was literally the first thing to pop up when I searched "red dwarf densities" in Google and the website looked well formatted. My bad. However, this begs the question, if the "regular" density of a 0.25 stellar mass/radii star's density is about 16 times our Sun's density, what would cause that density to vary? Temperature? Could a star's composition, or metallicity, impact that?
– Xi-K
Commented Feb 23, 2021 at 6:34
• @Xi-K Totally understandable, it looks like a modern paper. Stars are a <strike>mass of incandescent gas</strike> miasma of ionized plasma and temperature will effect their density. Cooler stars will be more dense, and red dwarfs are quite cool. Red giants are extremely hot and "fluffy". That's the simple explanation. Ask that as a new question. Commented Feb 23, 2021 at 7:08

This is a brief letter to Nature from 1946, containing no quantitative justification of the density estimate

In 1946, whilst the radius of some of the nearest red dwarfs could be estimated from their luminosities and blackbody temperatures, there would be little information about masses.

There is little else to say. Modern models and measurements of masses and radii in eclipsing binaries give more accurate densities.

Hydrostatic equilibrium combined with the virial theorem means that the central temperatures of stars are proportional to $$M/R$$. Given the steep temperature dependence of nuclear reactions, the mass-dependence of the central temperature is weak, so $$M$$ is approximately proportional to $$R$$ and so density is proprtional to $$M/R^3$$, which is proportional to $$M^{-2}$$. Hence a $$0.1$$ solar-mass star would likely be about 100 times as dense as the Sun.