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I have a mass of 1.3 solar masses (For a white dwarf), and my aim is to find the radius of this mass.

I have used the relation R $\propto$ $M^{\frac{-1}{3}}$.

From here I multiplied using the solar mass and radius.

*$R_D =$ Radius of white dwarf, $R_S$ = Radius of the sun.

$\frac{R_D}{R_S}$ = ${(\frac{1.3M_S}{M_S}})^{\frac{-1}{3}}$

After multiplying across by $R_S$ I get a value of $R_D$ = 0.916 $R_S$ which is incorrect.

I would appreciate if someone could point out where I'm going wrong.

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You need to know the mass density of a white dwarf. Your error was using the density of the sun. Wikipedia gives a reasonable calculation: "A white dwarf, then, packs mass comparable to the Sun's into a volume that is typically a million times smaller than the Sun's; the average density of matter in a white dwarf must therefore be, very roughly, 1,000,000 times greater than the average density of the Sun, or approximately 10^6 g/cm^3, or 1 tonne per cubic centimetre." https://en.wikipedia.org/wiki/White_dwarf

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