Every star we know is made of matter: atoms, or in the case of neutron stars, tightly packed neutrons. These are fermions, particles that refuse to share the same state, and that refusal is part of what holds stars up. A boson star would be different. It would be made of bosons, particles that can pile into the same state, forming a single smooth field held together by its own gravity.
An idea from 1968
In the 1950s John Wheeler imagined “geons”, lumps of electromagnetic energy held together by gravity, but they turned out to be unstable. In 1968 David Kaup showed that a field of bosons of a certain kind could form a stable, self-gravitating object instead of collapsing, and Remo Ruffini and Silvano Bonazzola followed in 1969. Such an object has no surface: its density simply fades away towards the edge.
How large and heavy a boson star could be depends on the mass of the boson. The lighter the boson, the heavier the star it can form.
Black hole impostors
Boson stars interest astronomers for two reasons. First, they could be made of dark matter: if dark matter consists of light bosons such as axions, they might clump into such objects. Second, a heavy, compact boson star could look surprisingly like a black hole. It would bend light around it and could appear to cast a dark shadow. Unlike a black hole, though, it has no event horizon, and light could in principle pass through its centre.
GW190521
In May 2019 the LIGO and Virgo detectors recorded GW190521, the merger of two unusually heavy black holes. In 2021 Juan Calderón Bustillo and colleagues showed that the signal fits slightly better if it came instead from a head-on collision of two Proca stars, a type of boson star made of vector bosons. They presented it as a demonstration that the two are hard to tell apart, not as a detection. No boson star has been confirmed.