The young star RY Tau is like a finicky spinning top. Around it whirls a flat disk of gas and dust — the leftovers of the cloud from which the star was born. The star pulls some of the disk's matter toward itself like a vacuum cleaner, and flings some back out in a narrow wind cone like a fountain. For 11 years, from 2013 to 2024, astronomers recorded the star's spectrum and brightness on every clear night. A spectrum is a rainbow stretched into a line, and the dark and bright bands in it show what is moving toward us and what is moving away.
It turned out that the inflow and outflow are not constant. Roughly every 20 days, the star either gulps down more gas or blows out more wind. And there is a clear order: first the inflow strengthens, and two days later the wind weakens. It's like a seesaw: give one side a push, and a moment later the other side flies up. The cause is the tilt of the star's magnetic field. In RY Tau, the magnetic dipole (an imaginary bar with north and south poles) is tilted relative to the rotation axis. So our line of sight alternately crosses the region where gas falls onto the star and the region where gas streams away in the wind.
The oscillations in inflow and outflow are most likely caused by density waves in the disk — compressions that run across the disk like ocean waves across water. When such a compression approaches the star, the inflow strengthens. Two days later, as the compression moves past, the wind weakens. This delay is like an echo: first a loud clap, then silence.
The bottom line is simple: RY Tau is not a calm star but a living mechanism that breathes matter. And by the rhythm of this breathing, we can understand how its magnetic field is arranged and how stars like our Sun are born.
RY Tau lies in the constellation Taurus, in a cloud of dark dust where dozens of stars are being born right now. If we could see in infrared light, this cloud would glow like a Christmas tree.