Pulsars are some of the most accurate clocks in the entire universe. Except, apparently, when they’re not.

A pulsar is the rapidly spinning corpse of a once-mighty star. It’s up to a couple of times the mass of the Sun, but only as wide as a city. It can spin anywhere from about once a second to hundreds of times a second. It emits beams of energy, usually in the form of radio waves, that can sweep across Earth with each spin. That makes it look like a cosmic lighthouse, flashing on and off.

Those flashes pulse on and off with an extremely consistent beat. As a result, pulsars are the most accurate clocks in the universe — better than the best atomic clocks here on Earth.

But astronomers have discovered that some pulsars periodically turn off and on — the clock suddenly stops ticking, then starts again. Some of them spend most of their time switched on, then go quiet for a while. Others are just the opposite.

Only a few of these on-again, off-again pulsars have been found. But a recent study suggested they could be common — perhaps more common than constant pulsars.

The study also suggested that the pulsars switch off because something stops the beams of energy, which are created by charged particles trapped in the pulsar’s magnetic field. So far, though, scientists haven’t determined what’s interfering with the beams — stopping the ticking of some of nature’s most accurate clocks.

More about pulsars tomorrow.

Script by Damond Benningfield