M. Tsujimoto

In the binary system 4U 1624-49, a neutron star pulls gas from its companion star. This gas spirals into a glowing disk, like water in a funnel. And from the edge of this funnel, a hot stream tears away — hydrogen and other elements fly off at speeds up to 320 kilometers per second.

A stone thrown into water creates expanding ripples; if the stone moves, the ripples bunch up in front of it. This effect, discovered by Christian Doppler, applies not only to sound or water waves but also to light: when the source moves, its color shifts — toward the blue end of the spectrum when approaching, toward the red when receding.