Jocelyn Bell, Neutron Stars, and the Little Green Men

By Emily Levesque, University of Washington

In 1967, astronomer Jocelyn Bell discovered a strange signal in data from a radio telescope in Cambridge, England. Pulses of radio emission had appeared in her data, arriving just over once per second. The pulses appeared with astonishing regularity, like a perfectly ticking clock. Their timing was so precise that Bell and her colleagues jokingly nicknamed the mysterious source of these pulses LGM, for little green men.

Neutron Stars

In the next two months, Bell found three more of these ‘little green men’, all perfectly pulsing radio sources, and all coming from different distant places in our own galaxy.

We know today that the signals discovered by Bell were not coming from extraterrestrial timekeepers. Instead, these pulses were the very first observations of something called a neutron star, the tiny and dense remnant left behind by the supernova death of an enormous star many times the mass of our own Sun.

Neutron stars have been intensely studied in the 50 years since Bell’s observations, and her discovery sparked a new wave of interest in searching for their long-predicted and mysterious cousins: black holes.

Synchrotron Radiation

When a neutron star is born during a supernova, not only are the neutrons and matter compressed, the magnetic field of the original star is also compressed. Most stars have pretty weak magnetic fields, but as we squeeze them into a neutron star, they can become incredibly powerful. These strong magnetic fields can funnel jets of particles out along the two magnetic poles of the neutron star.

The extreme acceleration of jets of particles along the two magnetic poles of the neutron star, speeding away from the star at close to the speed of light and whirling along the curved lines of the star’s magnetic field, actually emits photons as they move. This phenomenon is known as synchrotron radiation, and in a neutron star it generates powerful beams of radio frequency light shooting out of the magnetic poles.

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True North and Magnetic North

We know that the magnetic poles of a star or planet aren’t always aligned with its rotational axis. We’ve seen this here on Earth! Our magnetic poles aren’t at the northern and southern poles but tipped slightly—this is why we sometimes talk about the difference between true north and magnetic north.

Now, if we imagine this in a neutron star, spinning around its rotational poles and shining bright beams of radio frequency light along its tilted magnetic poles, we get this. It acts like the searchlight from a lighthouse or the flashing light on top of an emergency vehicle, producing a flash every time one of those bright radio beams sweeps past.

We detect these flashes as radio pulses coming from the neutron star, with the rate of pulsation telling us how fast the neutron star is spinning. For the slowest-spinning stars, one will detect maybe a couple pulses a minute, but the fastest will emit hundreds of pulses per second. That’s faster than a hummingbird beats its wings!

Pulsars

These rapidly flashing neutron stars are known as pulsars. This is what Jocelyn Bell and her colleagues had found in their data. Of course, they didn’t know this at first; at the time neutron stars had been predicted by theoretical physics but nobody had actually seen any sign of them. And nobody had predicted that they might produce these perfectly timed pulsations. The signal at first was so perfect that Bell’s first step was to ensure that she hadn’t found something too good to be true.

This sort of caution makes sense. It’s always important for astronomers to be careful when they find something new and unexpected! We want to be absolutely sure that the data we’re collecting is truly a signal from deep space and not something a bit closer to home.

Challenges Faced by Radio Astronomy

This is particularly challenging in radio astronomy when one remembers that so many objects used in our everyday life on Earth can produce radio interference—fluorescent lights, cordless phones, and even the spark plugs used in gas-powered car engines, can all be picked up by radio telescopes.

The Parkes radio telescope in Australia spent years detecting strange short bursts of radio emission. It wasn’t until 2015 that a group of researchers realized these bursts tended to happen around mealtimes and tracked them back to microwaves at the facility.

Some radio telescopes, like the Green Bank Observatory in West Virginia, avoid this by operating in radio quiet zones, banning cell phones and microwaves and even requiring diesel-only vehicles near the facilities, since they don’t have spark plugs.

Jocelyn Bell

Jocelyn Bell, despite skepticism from her colleagues, quickly realized that her pulses were the real deal. She tracked the first signal that she discovered for months and connected it to a source that rose in the evenings along with the rest of the night sky. She and her colleagues realized that these pulsations were likely connected to objects like neutron stars and published their results.

The discovery was met with enormous excitement and curiosity, and ultimately received a Nobel Prize in 1974. Unfortunately, Bell herself was not awarded this Nobel—her thesis adviser and another colleague received the prize instead.

Yet, Jocelyn Bell had a long and distinguished career and was awarded the 2018 Special Breakthrough Prize in Fundamental Physics in recognition of her extraordinary scientific achievements.

Common Questions about Jocelyn Bell, Neutron Stars, and the Little Green Men

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