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. Astronomers used this principle to peer into a binary system where a neutron star (the ultra-dense remnant of a massive star, sometimes manifesting as a pulsar (a source of periodic radiation pulses)) pulls matter from a neighboring, ordinary star. The infalling gas spirals into an accretion disk, heated by friction to temperatures at which it emits X-ray radiation.

Using spectroscopy — the method of splitting light into its constituent colors (spectrum)—scientists measured the Doppler shift of the iron emission line. From the magnitude of the shift, the line-of-sight velocity of the gas at each point in the system can be calculated. This is how a velocity map was constructed. It was expected that the X-ray radiation would come mainly from the disk itself or from a hot corona above it, but the main source turned out to be a stream of matter overflowing the disk's edge, like water spilling from an overfilled cup. Interestingly, the same "waterfall" is also observed in the optical range, confirming its reality.

This new method, called X-ray Doppler tomography, allows us to directly see the motion of gas in such systems — a true ultrasound of cosmic flows. Just as medical ultrasound reconstructs internal structure from reflected waves, here the picture of flows, inaccessible to direct observation, is reconstructed from Doppler shifts.