Scott M. Ransom

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.

In a dense ball of hundreds of thousands of stars, what astronomers call a globular cluster, radio telescopes have picked up a faint but very steady rhythm. Every 2.45 milliseconds, that's 408 ticks per second, a radio pulse arrives.

Picture a lighthouse on the shore at night. Its lamp is off almost all the time, but once every few days, for a split second, it blazes so brightly that the light is visible beyond the horizon.

At night in fog, a searchlight works worse than in daylight: tiny droplets scatter the beam, and distant objects drown in the murky haze. Much the same way, radio waves from pulsars get lost in the interstellar medium when there are many free electrons between us and the star.