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IT HAS TO BE SOME NEW KIND OF STAR

In the late 1950s, astronomers across the world started to find mysterious, compact sources of radio signals in the sky without any corresponding visible objects. Eventually a source of these radio waves was identified—a faint point of light, which became known as a quasar. In 1963, Dutch astronomer Maarten Schmidt discovered a quasar that was hugely distant (2.5 billion light-years away). The fact that it was so easily detected meant it must be pouring out energy.

Searching for quasars

By the mid 1960s, many radio astronomers were searching for new quasars. One such figure was Antony Hewish, part of a radio astronomy research group at Cambridge University. Hewish had been working on a new technique in radio astronomy based on a phenomenon called interplanetary scintillation (IPS), which is a “twinkling,” or fluctuation, in the intensity of radio emissions from compact radio sources. The twinkling of sources of visible light, such as stars, is caused by disturbances in Earth’s atmosphere that the light has to pass through. The twinkling of radio sources, however, is caused by streams of charged particles emanating from the sun. As radio waves pass through this “solar wind,” they are diffracted, meaning that the waves spread out, making the radio source appear to twinkle.

Hewish hoped that IPS could be used to find quasars. Radio waves coming from a compact source, such as a quasar, twinkle more than radiation from a less compact source, such as a galaxy, and so quasars should twinkle more than other radio sources. Hewish and his team built a large radio telescope designed specifically to detect IPS. It covered an area of nearly 4.5 acres (2 hectares), took two years to construct, and required more than 120 miles (190 km) of cable to carry all the signals.

Members of the Cambridge radio astronomy group built the new telescope themselves. Among them was a Ph.D. student named Jocelyn Bell. When the telescope started operating in July 1967, Bell was made responsible for operating it and analyzing the data, under the supervision of Hewish. Part of her job was to monitor output data from the telescope, made by pen recorders on chart-recorder printouts. Examining about 100 ft (30 meters) of chart paper every day, Bell quickly learned to recognize scintillating sources.

Little Green Man 1

About two months into the project, Bell noticed an unusual pattern of signals, which she described as “scruff.” It looked far too regular and had too high a frequency to be coming from a quasar. Checking back through her records, she found it had appeared in the data before and always came from the same patch of sky. Intrigued, Bell started making more regular chart recordings of the same area of sky. At the end of November 1967, she found the signal again. It was a series of pulses, equally spaced and always 1.33 seconds apart.

Bell showed the signal, dubbed “Little Green Man 1” (LGM-1), to Hewish. His initial reaction was that a pulse occurring every 1.33 seconds was far too fast for something as large as a star, and the signal must be due to human activity. Together, Bell and Hewish ruled out various human-related sources, including radar reflected from the moon, Earth-based radio transmissions, and artificial satellites in peculiar orbits. A second telescope was also found to pick up the pulses, which proved that they could not be due to an equipment fault, and calculations showed that they were coming from well outside the solar system.

Hewish had to revise his opinion that the signals had a human origin. The possibility that they were being sent by extra-terrestrials could not be ruled out. The team measured the duration of each pulse and found it was only 16 milliseconds. This short duration suggested that the source could be no larger than a small planet. But a planet—or an alien civilization living on a planet— was unlikely, since the signal would show slight changes in frequency, called Doppler shifts, as a planet orbited its star.

This image of a pulsar in the Crab nebula, a well-known supernova remnant, was taken in space by the Chandra X-ray Observatory. The white dot at the center is the neutron star.

Publishing dilemma

Hewish, Bell, and their colleagues were unsure how to publish their findings. While it seemed unlikely that the signals were being sent by an alien civilization, no one had any other explanation. Bell returned to her chart analysis, and soon found another “scruff” in a different part of the sky. She discovered it was due to another pulsating signal, this time slightly faster, with pulses every 1.2 seconds. Now she was reassured that the pulses must have some natural explanation—two sets of aliens in different places would surely not be sending signals to Earth at the same time and at nearly the same frequency.

By January 1968, Hewish and Bell had found four pulsing sources in total, which they decided to call “pulsars.” They wrote a paper describing the first source, suggesting that it might be due to pulsed emissions from a theoretical type of superdense collapsed star called a neutron star. Objects of this type had been predicted as long ago as 1934, but up to that time had never been detected.

“My eureka moment was in the dead of night, the early hours of the morning. But when the result poured out of the charts … you realize instantly how significant this is—what it is you’ve really landed on—and it’s great!” Jocelyn Bell Burnell

Explaining the pulses

Three months later, Thomas Gold, an Austrian−American astronomer at Cornell University in the US, published a fuller explanation for the pulsed signals. He agreed that each set of radio signals was coming from a neutron star, but proposed that the star was rapidly spinning. A star like this would not need to be emitting pulsed radiation to account for the pattern of signals observed. Instead, it could be emitting a steady radio signal in a beam that it swept around in circles, just like a beam of light from a lighthouse. When the pulsar’s beam (or perhaps one of its two beams) was pointing at Earth, a signal would be detected, which would show up as the sort of short pulse that Bell had noticed on printouts. When the beam had passed by Earth, the signal would stop until the beam came around again. Challenged about the pulsation rates, which implied extremely rapid spinning, Gold explained that neutron stars could be expected to behave in this way because of the way in which they form—from the collapse of stellar cores in supernova explosions.

A pulsar is a spinning neutron star with an intensely strong magnetic field. It emits beams of radiation from its north and south poles.

Confirming the hypothesis

Initially, Gold’s explanations were not well received by the astronomy community. However, they became widely accepted after the discovery of a pulsar in the Crab nebula, a well-known supernova remnant. Over subsequent years, many more pulsars were found. They are now known to be rapidly rotating neutron stars with intense electromagnetic fields, which emit beams of electromagnetic radiation from their north and south poles. These beams are often, but not always, radio waves and sometimes other forms of radiation, including in some cases visible light. One reason for the excitement regarding the discovery of pulsars was that it increased the likelihood that another theoretical phenomenon—black holes—might also be detected and proven. Like neutron stars, black holes are objects that could result from the gravitational collapse of a stellar core following a supernova explosion.

In 1974, Hewish and Martin Ryle shared a Nobel Prize: “Ryle for his observations and inventions … and Hewish for his decisive role in the discovery of pulsars.” However, Jocelyn Bell Burnell was told that she would not share the award with them because she had still been a student at the time of her work. She graciously accepted that decision.

A spinning neutron star that is emitting radiation beams can be detected as a pulsar on Earth if, as it spins, one or possibly both of its radiation beams recurrently point in Earth’s direction as they are swept around through space. The pulsar will then be detected as a very regular series of signal “blips.”

JOCELYN BELL BURNELL

Jocelyn Bell was born in 1943 in Belfast, Northern Ireland. After earning a physics degree from Glasgow University in 1965, she moved to Cambridge University, where she studied for a Ph.D. There, she joined the team that built a radio telescope to detect quasars. In 1968, Bell became a research fellow at the University of Southampton and changed her last name to Bell Burnell when she married. She has held astronomy and physics-related positions in London, Edinburgh, and at the Open University, where, from 1991 to 2001, she was professor of physics. From 2008 to 2010, she was President of the Institute of Physics. Bell Burnell has received numerous awards for her professional contributions, including the Herschel Medal of the Royal Astronomical Society in 1989. In 2016, she was visiting professor of astrophysics at Oxford University.

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