Top: Gas from the companion star is drawn by gravity onto the black hole in a swirling pattern. As the gas nears the event horizon, a strong gravitational redshift makes it appear redder and dimmer. When the gas finally crosses the event horizon, it disappears from view. Because of this, the region within the event horizon appears black.
Bottom: As above, gas from the companion star flows down onto the collapsed star--in this case a neutron star instead of a black hole. As the gas approaches the neutron star, a similar gravitational redshift makes the gas appear redder and dimmer. However, when the gas strikes the solid surface of the neutron star, it glows brightly.
