Terzan 5 is a globular star cluster, a tight swarm of hundreds of thousands of stars squeezed into a sphere a few dozen light-years across. In such a crowd, stars exchange matter and spin each other up, and some turn into pulsars — rapidly rotating neutron stars that emit radio pulses like lighthouses. New images taken with the Karl G. Jansky Very Large Array at frequencies from 2 to 4 gigahertz have peered into the heart of this stellar beehive.
The images reveal 38 of the 49 confirmed pulsars, including the bright pulsar Ter5A, whose eclipses were tracked at multiple frequencies. But even after subtracting all visible signals, a noticeable diffuse background remained — light from pulsars too faint to see individually. The sensitivity threshold was about 11 microjanskys, which is like hearing a whisper at the other end of a stadium. By modeling how many such faint sources are needed to produce this background, the astronomers estimate that Terzan 5 contains at least 250 pulsars, and possibly significantly more.
This number is five times higher than previous estimates, making Terzan 5 one of the richest pulsar environments in the Galaxy. Interestingly, the cluster's gamma-ray emission, though not unambiguously interpretable, also points to a large pulsar population. Such an abundance of "beacons" in one place is a real gift for future radio telescopes: they can test how pulsars are born in dense clusters and how they interact with their neighbors.
A pulsar spins so regularly that you could set your clock by it: the period deviation of some pulsars is smaller than that of the best atomic clocks on Earth.