In a Schwarzschild (non-rotating) black hole, where is the photon sphere located?

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Study for the NOVA Black Hole Apocalypse Astronomy Test. Explore astronomy with our rich content. Use flashcards and multiple-choice questions to enhance your knowledge. Prepare effectively and uncover the mysteries of black holes. Get ready for your exam!

Multiple Choice

In a Schwarzschild (non-rotating) black hole, where is the photon sphere located?

Explanation:
The key idea is the photon sphere—the special radius around a Schwarzschild black hole where gravity is just strong enough for light to orbit in a circular path. For a non-rotating black hole, this radius is at r = 3GM/c^2, which is 1.5 times the Schwarzschild radius r_s = 2GM/c^2. That places the photon sphere outside the event horizon (which sits at r = r_s) but still very close to it. The circular photon orbits here are unstable: a tiny inward or outward nudge sends the photon either into the black hole or away to infinity. So the location of the photon sphere is 1.5 Schwarzschild radii.

The key idea is the photon sphere—the special radius around a Schwarzschild black hole where gravity is just strong enough for light to orbit in a circular path. For a non-rotating black hole, this radius is at r = 3GM/c^2, which is 1.5 times the Schwarzschild radius r_s = 2GM/c^2. That places the photon sphere outside the event horizon (which sits at r = r_s) but still very close to it. The circular photon orbits here are unstable: a tiny inward or outward nudge sends the photon either into the black hole or away to infinity. So the location of the photon sphere is 1.5 Schwarzschild radii.

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