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Q & A: changing mass of Sun

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Most recent answer: 03/22/2013
Q:
Since its the suns gravitational pull that keeps us in our orbit. And an objects mass is what determines how much of a gravitational pull it creates. As the sun slowly burns away its gases and decreases in mass will our orbit change with it? Will we float further and further away?
- Dan Langdon (age 24)
Manchester, England.
A:
The effect isn't large, but yes as the Sun loses mass it will change the Earth's orbit. I'll neglect complications, like the interaction between the Earth and the material gradually blowing away, because I don't know enough to calculate them. The simplest model would just have the Earth's angular momentum stay fixed while its energy went down, both the negative gravitational energy and the positive kinetic energy. Their ratio doesn't change, thanks to the virial theorem. So if v is the speed and R is the distance, we'd have vR staying constant while v2 went down. That would mean R would have to go up, as you guessed.

The Sun wouldn't gradually throw off all its gases. It's supposed to collapse to some sort of cooler star after spending some time in an expanded redder state. I think most of the mass will remain. The Earth will get fried during the expanded state.


Mike W.

(published on 03/22/2013)

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