Posted by **Me** on Wednesday, November 30, 2011 at 10:49pm.

You are the science officer on a visit to a distant solar system. Prior to landing on a planet you measure its diameter to be 2.070E+7 m and its rotation to be 19.8 hours. You have previously determined that the planet orbits 1.590E+11 m from its star with a period of 368.0 Earth days. Once on the surface you find that the free-fall acceleration is 13.00 m/s2.

What is the mass of the star (in kg)?

I have posted this multiple times and can not figure it out.

- Physics please help! -
**drwls**, Wednesday, November 30, 2011 at 11:05pm
I have answered it once and will not do it again.

http://www.jiskha.com/display.cgi?id=1322287372

- Physics please help! -
**Me**, Wednesday, November 30, 2011 at 11:17pm
Yes, thank you but as I said, I still can not figure out from what you gave me so maybe someone else will explain better (figuring out the mass of the star that is).

- Physics please help! -
**drwls**, Thursday, December 1, 2011 at 12:04am
The reference I gave you for Kepler's Third Law did not solve the problem for stars other than the sun. A better reference would be

http://www.astro.cornell.edu/academics/courses/astro101/java/Finding%20Exosolar%20Planets.htm

There, you will find the formula.

P^2 = 4 pi^2*r^3/[G*(m1 + m2)]

The author also derives it there.

In your case, ignore the planet's mass m2 because it will be negligible compared to the star's mass, m1.

P is the period, 368 days = 3.18*10^7 s

r is the orbital radius, 1.59*10^11 m.

Look up the value of G and solve for the star's mass m1, in kilograms.

It should be not much different from the sun's mass, because the r and P values you are using are close to those of the Earth orbiting the sun.

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