Physics Help Forum How to calculate the density of a star's core

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 Jan 29th 2017, 03:21 AM #1 Junior Member   Join Date: Jan 2017 Posts: 2 How to calculate the density of a star's core For school I have to calculate certain aspects of a star but now I am stuck at the density. My teacher gave me a formula but I got an unrealistic answer from it. (I know that the formula is very basic) The formula is: P=Md*P(core pressure)/kT Md=average mass of a particle:1,03E-27 Kg Pressure=4,68E8 Pa k=1,38E-23 J/K T=5,87E5 K Inserting these numbers in the formula gives 0,06kg/m3 If somebody has an idea of what I'm doing wrong it would be much appreciated. Last edited by Migroh; Jan 29th 2017 at 03:23 AM.
 Jan 29th 2017, 07:36 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,354 Your math is correct. I think the reason you are getting such a low number for a result is that the values for P (pressure) and T are way off. For our sun you would use values of P = 26.5x10^12 Pa, and T = 1.5 x 10^7 K. Try using those values and see what you get. topsquark likes this.
 Jan 29th 2017, 08:42 AM #3 Junior Member   Join Date: Jan 2017 Posts: 2 Good to know that the calculation is correct, that the values for T and P are off must have something to do with my previous calculations(I'm not using the sun as reference)
 Jan 30th 2017, 03:10 PM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,086 Are you sure the pressure is in Pascals? Might it be in KPa? You are obviously looking at a smaller and cooler star than our Sun, but I would not expect the pressure to be over 5000 times less. KPa gives a core pressure that is a more believable 5 times less, although a density of 60kg/m³ still seems a bit low. On Further Investigation: I plugged the numbers ChipB gives into the equation and got about 130kg/m³ However this Wikipedia article gives the density as 150g/cm³ which equates to 150000kg/m³ and a pressure of 26.5 petapascals or 2.65e15 Pa So I think the pressure from ChipB should be in KPa and your pressure should be in MPa! topsquark likes this. Last edited by Woody; Jan 30th 2017 at 03:13 PM.
Jan 31st 2017, 09:56 AM   #5
Physics Team

Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,354
 Originally Posted by Woody On Further Investigation: I plugged the numbers ChipB gives into the equation and got about 130kg/m³ However this Wikipedia article gives the density as 150g/cm³ which equates to 150000kg/m³ and a pressure of 26.5 petapascals or 2.65e15 Pa So I think the pressure from ChipB should be in KPa and your pressure should be in MPa!
You are correct - good catch! My source for pressure at the center of the sun was Wikipedia, which said "26.5 petapascals." But I'm afraid I converted to Pa by multiplying by E+12, whereas I should have multiplied that by E+15. So my calculation is low by a factor of 1000.

 Originally Posted by Migroh the values for T and P are off must have something to do with my previous calculations(I'm not using the sun as reference)
Just for reference the pressure you are using of 4.68E+8 Pa is significantly less than the pressure at the center of the Earth, which is is about 3.6E+11 Pa. I doubt that there is any star that could sustain fusion at such low pressures.

Last edited by ChipB; Jan 31st 2017 at 10:05 AM.

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