Question

In: Mechanical Engineering

Sirius has a mass of 2 solar masses and radius of 1.71 solar radius, determine average...

Sirius has a mass of 2 solar masses and radius of 1.71 solar radius, determine average
density of Sirius.

a) pressure at half radius of Sirius b) temperature at half radius of Sirius c) radiation pressure at half radius.

Hint: You may find the ideal gas law helpful. Mean molecular weight for outer layers of stars is ~ 0.61.

Solutions

Expert Solution

Mass of sun = 1.988 x 1030. kg

2 solar mass = 2 * 1.988x 1030 kg mass of sirius.

Radius of sirius = 1.7 * 696340 km = 1183778 km

Volume of Sirius = 4/3 r3. = 6.95×1018 Km3.

Density = mass / volume = 3.976x1030 / 6.95×1018 = 5.721 x 1011 kg/km3.

Density of sirius = 5.721 x 1011 kg/km3.

Pressure = g2.

g = M/ r2.

P central = g central2.

ghalf = Mass half / radius half2.

Half radius = 1183778/2 =591889 km

= 1.988 x 1030. / 591889 = 5.675 x 1018.

Pressure = ghalf 2 = 3.22x1037kg/km2.

Ideal gas equation

PV = nRT

Volume = 4/3 (r/2)3. = 8.69×1017

(3.22x1037 * 8.69×1017 ) = 0.61 * 287.058 * T

T = (3.22x1037 * 8.69×1017 ) / 0.61 * 287.058 = 1.6 x 1053K.

Considering a perfect reflection raidatio

Radiation pressure Prad = Pressure * 2 = 2 *3.22x1037kg/km2. = 3.22 x 1074.kg/km2.


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