Stars and Pressure

By Michael on September 26, 2006 at 12:47 am | In Astrophysics, Blog Posts |

In my last post I talked about the balance of forces that make stars, like the Sun, stable. I said we could learn things about stars by understanding this balance. Here is an example.

We ended up with this equation:

This is not as complex as it looks. The big G is just a number based on the units we are using. The M is the mass of the star. The funny looking p is the greek letter roh and it is the density of the star. r is the radius and the big P is the pressure. We can make this more simple if we assume the pressure is zero at the surface of the star. We’ll also change the density back to mass divided by volume so we can combine some terms.

The pressure P at the center of the star is proportional to the square of the mass and inversely proportional to the 4th power of the radius.

So if two stars are the same size and one is twice as massive as the other (and obviously much more dense as a result), the pressure in the center increases by a factor of 4. If two stars are the same mass but one is half as big as the other, the pressure at the center will be 16 times more.

If you Google the mass and radius of the sun and the value of the constant G you can calculate the pressure of the sun with the equation above. Try it!