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The Question

(Submitted June 12, 1997)

What would happen to Earth's orbit if the Sun became a black hole?

The Answer

Thanks for your question about the orbit of the Earth if the Sun became a black hole. The Sun is not massive enough to ever evolve into a black hole; it will end its life in about 4.5 billion years as a white dwarf star. But, perhaps you are asking what would happen to the Earth's orbit if the Sun's mass was suddenly collapsed into a black hole (not plausible,given our understanding of stellar evolution, but an interesting theoretical question).

If a one solar mass black hole were to suddenly replace the Sun at the center of our solar system, the orbits of the planets would not change. This is because the physical laws that determine the orbital motion of the Earth depend only on the actual mass of the Sun, and not on whether it is distributed within a sphere (like the Sun) or at a point (like a black hole). I hope that answers your question.

Regards,
Padi Boyd,
for the Ask an Astrophysicist

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