# Mass of a star given orbital radius and period

## Homework Statement

In recent years, a number of nearby stars have been found to possess planets. Suppose, the orbital radius of such a planet is found to be 4.3 times 1011 m, with a period of 1080 days. Find the mass of the star.

?????

## The Attempt at a Solution

I don't even know where to start with this question. Without the mass of the planet I am clueless. ANy help would be much appreciated, Thanks a lot.

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## Homework Statement

In recent years, a number of nearby stars have been found to possess planets. Suppose, the orbital radius of such a planet is found to be 4.3 times 1011 m, with a period of 1080 days. Find the mass of the star.

?????

## The Attempt at a Solution

I don't even know where to start with this question. Without the mass of the planet I am clueless. ANy help would be much appreciated, Thanks a lot.
Let's start by looking at what interactions are relevant. So erm what are the relevant interactions? Or more to the point, what forces are acting on our planet?
After that we'll need to see what the motion of the planet means in terms of forces. So again can you think of a relation between the period, mass and radius for an object in circular motion to the force exerted on it?
after that we should be at a point to get an answer after a bit of algebra

(mv^2)/r
am i on the right track?

(mv^2)/r
am i on the right track?
So that's the equation for the centripetal force, you will need to relate v to the period and radius. Also you need to recognize what force is causing the circular motion and what the equation for that force is

Look up "Kepler's third law" in the index of your book. You are given numbers to substitute into the formula.

I'm surprised they've not covered Kepler's laws first? Did you skip a chapter?

Seems a little advanced to expect you to know how to find mass without it?

You don't really need to know the mass of the planet since it will be much smaller than the star generally so you can approximate it ignoring the planets mass to all intents and purposes.

Even Jupiter's mass is only ~1/1000 of the Suns.

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