Figure 21-10: X-rays Generated by Accretion of Matter Near a Black Hole Stellar-remnant black holes, such as Cygnus X-1, V404 Cygni, and probably A0620–00, are detected in close binary star systems. This drawing (of the Cygnus X-1 system) shows how gas from the 30-M_⊙ companion star, HDE 226868, transfers to the black hole, which has about 14.8 M_⊙. This process creates an accretion disk. As the gas spirals inward, friction and compression heat it so much that the gas emits X-rays, which astronomers can detect.

(NASA, ESA, Martin Kornmesser [ESA/Hubble])