The dipole array telescope—a mass of wires and poles stretched across an area the size of 57 tennis courts—took Cambridge University students more than two years to build. But after the telescope was finished in July 1967, it took only a few weeks for graduate student Jocelyn Bell Burnell to detect something that would upend the field of astronomy.

The giant net-like telescope produced enough data to fill 700 feet of paper each week. By analyzing this, Bell Burnell noticed a faint, repetitive signal that she called “scruff”—a regular string of pulses, spaced apart by 1.33 seconds. With help from her supervisor Antony Hewish, Bell Burnell was able to capture the signal again later that fall and winter.

The signal looked like nothing any astronomer had ever seen before. Yet before long, Bell Burnell discovered more little beacons out there, just like the first but pulsing at different speeds in different parts of the sky.

After eliminating obvious explanations like radio interference from Earth, the scientists gave the signal the fanciful nickname LGM-1, for “little green men” (it later became CP 1919 for “Cambridge pulsar”). Although they didn’t seriously think it might be extraterrestrials, the question remained: what else in the universe could emit such a steady, regular blip?

Fortunately, the field of astronomy was collectively ready to dive into the mystery. When the discovery appeared in the prestigious Nature journal on February 24, 1968, other astronomers soon came up with an answer: Bell Burnell had discovered pulsars, a previously unimagined form of neutron star that spun rapidly and emitted beams of x-ray or gamma radiation.

“Pulsars were completely unanticipated, so it was remarkable for a discovery of something that we hadn’t ever thought about in theory-driven terms,” says Josh Grindlay, a Harvard University astrophysicist who was a doctoral student at Harvard while excitement swirled around the discovery. “The discovery of pulsars stands out as telling us that the world of compact objects was very real.” In the past 50 years, researchers have estimated there are tens of millions of pulsars in our galaxy alone.

By compact objects, Grindlay means those exotic celestial objects that include black holes and neutron stars. Neutron stars were proposed in 1934 by physicists Walter Baade and Fritz Zwicky, but were thought to be too dark and minute for scientists to identify in reality. These incredibly small, dense stars were thought to be the outcome of the supernova process—when an enormous star explodes and the remaining matter collapses in on itself.

Baade and Zwicky were right. As astrophysicists discovered, pulsars were a small subset of neutron stars—and, since they were visible, proved the existence of other neutron stars. Made of tightly packed neutrons, pulsars can have a diameter of only about 13 miles, yet contain twice the mass of the sun. To put that in perspective, a portion of neutron star the size of a sugar cube would weigh the same amount as Mount Everest. The only object in the universe with a higher density than neutron stars and pulsars is a black hole.

What makes pulsars different from other neutron stars is the fact that they spin, like tops, some so rapidly they approach the speed of light. This spinning motion, combined with the magnetic fields they create, results in a beam shooting out of them on either side—not so much like the constant glow of our Sun, but more like the rotating spotlight of a lighthouse. It was this flicker that allowed astrophysicists to observe and detect pulsars in the first place, and infer the existence of neutron stars, which remain invisible.

“At the time this was happening, we didn’t know that there was stuff between the stars, let alone that it was turbulent,” Bell Burnell told the New Yorker in 2017, reflecting back on her historic observation. “That is one of the things that has come out of the discovery of pulsars—more knowledge about the space between stars.”

In addition to proving the existence of neutron stars, pulsars also honed our understanding of particle physics and provided more evidence for Einstein’s theory of relativity. “Because they’re so dense, they impact space time,” says San Diego State University physicist Fridolin Weber. “If you have good data on pulsars, then Einstein’s theory can be tested against competing theories.”

As for practical applications, pulsars are nearly as precise as atomic clocks, which measure time more accurately than anything else through the regular movements of energized atoms. If we were ever to send astronauts deep into space, pulsars could function as navigational points, Weber says. In fact, when NASA launched the Voyager probes in the 1970s, the spacecraft included a map of our Sun’s location in the galaxy based on 14 pulsars (though some scientists have critiqued the map because we’ve learned there are many more pulsars in the galaxy than previously believed).

More recently, scientists have become optimistic about using pulsars to detect gravitational waves, by monitoring them for minute abnormalities. These ripples in space-time, which vindicated Einstein and helped scientists understand how super massive and dense objects impact space, earned their discoverers the 2017 Nobel Prize for Physics—just as Antony Hewish had won the Physics Prize in 1974. (Bell Burnell was not awarded the prize, perhaps because of her status as a grad student, as she claims, or for being a woman, as others have suggested.) Now, scientists plan to use pulsars to find gravitational waves that even LIGO can’t detect.

Yet plenty of questions remain when it comes to the behavior of pulsars and their place in the galaxy. “We still don’t completely understand the exact electrodynamics of what produces the radio pulses,” Grindlay says. If scientists could observe a pulsar in a binary system with a black hole—the two objects interacting with each other—that would provide even more insight into the nature of physics and the universe. Thanks to new telescopes like the Square Kilometer Array in South Africa and the Five-hundred-meter Aperture Spherical Telescope (FAST) in China, physicists are likely to have much more data to work with soon.

“We have lots of models about super dense matter and objects [like pulsars], but to know what really goes on and how to describe them in detail, we need high quality data,” Weber says. “This is the first time we’re about to have these data. The future is really exciting.”