Sunday Science: Neutron Stars and Pulsars

A rupture in the crust of a highly magnetised neutron star, shown here in an artist's rendering, can trigger high-energy eruptions. (Credit: NASA's Goddard Space Flight Center/S. Wiessinger)

When a suitably massive star comes to the end of its life, it collapses in on itself in a massive supernova explosion. If that original star was between 10 and 29 solar masses it collapses to the point where the protons and electrons in its core are squashed together leaving behind a neutron star.

Neutron stars are incredibly small and incredibly dense. They cram roughly 1.3 to 2.5 solar masses into a sphere of roughly a few miles in diameter.

If we could extract just one sugar-cube sized piece of a neutron star, it would weigh more than one billion tons. That's equivalent to the weight of Mount Everest.

If the neutron star weighs more than three solar masses, it will continue to collapse in on itself to form a black hole.

Neutron stars also have very large magnetic fields (approximately one trillion times stronger than the Earth's magnetic field) and are the strongest magnets in the universe. They have a surface temperature of one million Kelvin (the Sun's surface temperature is 5,778 Kelvin).

Introducing pulsars...

Most detected neutrons stars belong to a subclass called "pulsars".

Pulsars spin extremely quickly (hundreds of times per second), which is about the same rate as your kitchen blender.*

Pulsars also beam out radio waves. On Earth, these radio beams sweep across us like lighthouse beacons, with an incredibly precise period.

​In fact, the precision of these pulses seemed a little suspicious when they were first noticed in 1968 and they were given the acronym "LGM" - which stands for "Little Green Men". Unfortunately, they turned out not to be first contact, but you can read more about the discovery in the extra reading at the end of this post. 

...and magnetars

Magnetars are another type of neutron star. The magnetic field of a neutron star is one trillion times that on Earth, but the magnetic field of a magnetar is another 1,000 times stronger.

In all neutron stars, the star's crust is linked to its magnetic field so that any change in one affects the other.

So, a small movement in the crust will cause ripples in the magnetic field. For a magnetar, such movement cause a huge burst of electromagnetic radiation. In December 2004, a massive magnetar blast blinded satellites and partially ionised the Earth's atmosphere. The cosmic blast only lasted one-tenth of a second but it released more energy than the Sun has emitted in 100,000 years.

* No Ironmen were harmed in the making of this Sunday Science.

Extra reading

Jocelyn Bell ca. 1970

When two neutron stars smashed into each other in August, we weren't quite sure what was left over. But it's looking increasingly likely that it could be the biggest neutron star we've ever detected. The unprecedented celestial collision produced a gamma-ray burst and astronomers at LIGO detected the resulting gravitational waves.

If you'd like to find out more about why pulsars spin so fast and so regularly, you can read more here.

The story of the discovery of pulsars is also one of my favourite astronomy anecdotes as it features a great female scientist, Jocelyn Burnell (nee Bell). I've copied and pasted this fantastic article from the American Society of Physics covering the discovery:

"In 1967, when Jocelyn Bell, then a graduate student in astronomy, noticed a strange “bit of scruff” in the data coming from her radio telescope, she and her advisor Anthony Hewish initially thought they might have detected a signal from an extraterrestrial civilization. It turned out not be aliens, but it was still quite exciting: they had discovered the first pulsar. They announced their discovery in February 1968.

Bell, who was born in Ireland in 1943, was inspired by her high school physics teacher to study science, and went to Cambridge to pursue her Ph.D. in astronomy. Bell’s project, with advisor Anthony Hewish, involved using a new technique, interplanetary scintillation, to observe quasars. Because quasars scintillate more than other objects, Hewish thought the technique would be a good way to study them, and he designed a radio telescope to do so.

Working at the Mullard Radio Astronomy Observatory, near Cambridge, starting in 1965 Bell spent about two years building the new telescope, with the help of several other students. Together they hammered over 1000 posts, strung over 2000 dipole antennas between them, and connected it all up with 120 miles of wire and cable. The finished telescope covered an area of about four and a half acres.

They started operating the telescope in July 1967, while construction was still going on. Bell had responsibility for operating the telescope and analyzing the data — nearly 100 feet of paper every day–by hand. She soon learned to recognize scintillating sources and interference.

Within a few weeks Bell noticed something odd in the data, what she called a bit of “scruff.” The signal didn’t look quite like a scintillating source or like manmade interference. She soon realized it was a regular signal, consistently coming from the same patch of sky.

No known natural sources would produce such a signal. Bell and Hewish began to rule out various sources of human interference, including other radio astronomers, radar reflected off the moon, television signals, orbiting satellites, and even possible effects from a large corrugated metal building near the telescope. None of those could explain the strange signal.

The signal, a series of sharp pulses that came every 1.3 seconds, seemed too fast to be coming from anything like a star. Bell and Hewish jokingly called the new source LGM-1, for “Little Green Men.” (It was later renamed.)

But soon they managed to rule out extraterrestrial life as the source of the signal, when Bell noticed another similar signal, this time a series of pulses arriving 1.2 seconds apart, coming from an entirely different area of the sky. It seemed quite unlikely that two separate groups of aliens were trying to communicate with them at the same time, from completely different locations. Over Christmas 1967, Bell noticed two more such bits of scruff, bringing the total to four.

By the end of January, Bell and Hewish submitted a paper to Nature describing the first pulsar. In February, a few days before the paper was published, Hewish gave a seminar in Cambridge to announce the discovery, though they still had not determined the nature of the source.

The announcement caused quite a stir. The press jumped on the story–the possible finding of extraterrestrial life was too hard to resist. They became even more excited when they learned that a woman was involved in the discovery. Bell later recalled the media attention in a speech about the discovery: “I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records. Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret, and how many boyfriends did I have at a time?”

Other astronomers were also energized by the finding, and joined in a race to discover more pulsars and to figure out what these strange sources were. By the end of 1968, dozens of pulsars had been detected. Soon Thomas Gold showed that pulsars are actually rapidly rotating neutron stars. Neutron stars were predicted in 1933, but not detected until the discovery of pulsars. These extremely dense stars, which form from the collapsed remnants of massive stars after a supernova, have strong magnetic fields that are not aligned with the star’s rotation axis. The strong field and rapid rotation produces a beam of radiation that sweeps around as the star spins. On Earth, we see this as a series of pulses as the neutron star rotates, like a beam of light from a lighthouse.

After discovering the first pulsars, Jocelyn Bell finished her analysis of radio sources, completed her PhD, got married and changed her name to Burnell. She left radio astronomy for gamma ray astronomy and then x-ray astronomy, though her career was hindered by her husband’s frequent moves and her decision to work part time while raising her son. Anthony Hewish won the Nobel Prize in 1974 for the discovery of the first pulsars. Over 1000 pulsars are now known.**

As for little green men, they haven’t been found yet, but projects such as the Search for Extra Terrestrial Intelligence (SETI) are still looking for them."
​
** Now, more than 2,000 pulsars have been discovered, but this figure was accurate in 2006 when the article was originally published.

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What is Sunday Science?

Hello. I’m the freelance writer who gets tech. I have two degrees in Physics and, during my studies, I became increasingly frustrated with the complicated language used to describe some outstanding scientific principles. Language should aid our understanding — in science, it often feels like a barrier.
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So, I want to simplify these science sayings and this blog series “Sunday Science” gives a quick, no-nonsense definition of the complex-sounding scientific terms you often hear, but may not completely understand. 

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