What is the Eddington luminosity, and why is it important for accretion onto black holes?

• A. The maximum luminosity a black hole can produce.
• B. The speed of light around the black hole.
• C. The threshold above which radiation pressure balances gravity.
• D. The balance point where outward radiation pressure equals inward gravity; it limits stable accretion rates onto black holes.

Answer: (D)

The Eddington luminosity is the luminosity where the outward push of radiation on the infalling gas exactly balances the inward pull of gravity. In the usual picture, photons scatter off electrons in the accreting material, and the radiation force increases with luminosity. When this radiation force matches the gravitational force pulling the gas toward the black hole, you reach a tipping point: you can’t sustain higher, steady accretion without blowing gas away or stifling the inflow. So this luminosity sets a characteristic cap on how bright an accreting black hole can be in a simple, spherical-flow scenario, effectively limiting stable accretion rates for a given mass. The concept is mass-dependent, with the exact value given by L_Edd ∝ M, derived by equating gravity and radiation pressure forces. Keep in mind that more complex accretion geometries can lead to apparent luminosities above this limit, but the fundamental idea remains the balance between radiation pressure and gravity that governs steady fueling.