A lighthouse blinks steadily once per second. If it moves in a circle, its flashes arrive sometimes a bit early, sometimes a bit late, and they give away an invisible companion. That's exactly how astronomers exposed the hidden partner of the star IY Lyrae.
IY Lyrae is a pulsating variable star in the thick disk of the Galaxy. Its brightness changes with a period of about half a day: the star rhythmically swells and contracts. But this rhythm showed glitches. Comparing the moments of maximum brightness over many years, scientists noticed that the pulsations lag behind or run ahead of predictions, as if the star were approaching us and then receding. This is a classic sign of orbital motion: something massive is circling nearby.
So who is this invisible object? Its mass — about 1.4 solar masses — is too large for a normal main-sequence star in such an old system. A white dwarf at the edge of stability (the so-called Chandrasekhar limit) is possible but unlikely. Much more natural is a neutron star — the core left after a supernova explosion. This is a typical mass for such objects. Additional clues (the chemical composition of IY Lyrae (low metal content, high alpha-element abundance) and its membership in the thick disk) point to a venerable age for the system, ruling out a young massive star.
So an old pulsating beacon in the thick disk of the Galaxy has pointed to a compact remnant of a stellar catastrophe. Such pairs are rare, and each one helps us refine how binary systems live and die in different corners of the Milky Way.
The pulsations of IY Lyrae are so stable that their deviations allow measuring the orbit with an accuracy of a few days over a 4-year period, as if you could determine the length of a table by the ticking of a pocket watch moving in a circle.
In Robert Forward's novel Dragon's Egg, life on the surface of a neutron star is described — an object similar to the companion of IY Lyrae, where gravity is billions of times stronger than on Earth.