A lighthouse blinks every few seconds, but then begins to slowly brighten and dim over the course of a year. That is exactly what about a third of red giants do — huge stars in the late stages of life. Their normal pulsations take days or weeks, but on top of that, long secondary periods are visible: months and years. This phenomenon was noticed nearly a hundred years ago, but has never been explained.

For a long time, the main suspect was an invisible companion: a small star or brown dwarf orbiting nearby, whose gravity makes the giant wobble slightly. Then the brightness would change due to tidal forces or dust raised by the companion. The Gaia space telescope, which precisely measures the positions and velocities of stars, helped test this. Scientists took a sample of nearby red giants with long periods and looked at how they move. If a companion were present, the star would trace a tiny circle on the sky, and its velocity would vary with the orbital period.

But the biggest surprise was yet to come. If such companions existed, Gaia would have detected a slight jitter in the star's position — astronomers call this an astrometric signal. They predicted it based on observed velocity variations and then compared it with real data. The real stars jittered noticeably less than expected. Moreover, most of them looked single. So the companion hypothesis fails for the majority of cases.

That means the cause of long secondary periods lies hidden within the red giants themselves. Perhaps some slow processes are happening in their interiors that we have not yet been able to see. The mystery, nearly a century old, remains open, but now we know where not to look for the answer.