What’s a pulsar? Why does it pulse?

What is a pulsar?

A pulsar is a rapidly spinning neutron star. So, what’s a neutron star? A neutron star is the small, incredibly dense remnant of a much more massive star. Neutron stars are so dense that if you could scoop up a teaspoon from off the surface of the star, it would weigh as much as Mount Everest. A neutron star of about 15 miles (24 km) across would contain more matter than our sun. So why are some neutron stars – the ones we see as pulsars – pulsing?

How a pulsar is born

To learn how a pulsar is born, first you have to learn how a neutron star is born. When a large star – its core is about 1-3 times the mass of our sun – goes supernova, the result will be a neutron star. Much of the star billows outward, but the core collapses inward. Protons and electrons fuse into neutrons in the star. When the star shrinks but retains its mass, it begins to spin quickly, like a skater that pulls in their arms. The star follows the law of conservation of angular momentum.

Rapidly spinning neutron stars have strong magnetic fields. Such a neutron star emits high-energy beams from its north and south magnetic poles. When these beams are pointed toward Earth and flash across us as the neutron star rotates, we see pulses. So astronomers named these beasts pulsars. Most neutron stars are observed as pulsars. So, all pulsars are neutron stars, but not all neutron stars are necessarily pulsars. But most neutron stars appear as pulsars from the proper vantage point – with their beams aimed at Earth as they rotate – as long as they are emitting enough radiation to be detectable.

The first discovery and little green men

Jocelyn Bell discovered the first pulsar on August 6, 1967. Initially, some thought the pulsar could be a signal from aliens. But on December 21, Bell discovered a second pulsar. Still, Bell and her supervisor, Antony Hewish, playfully nicknamed the first signal LGM-1 for little green men.

The first pulsar is now known as CP 1919 or PSR B1919+21. This pulsar is in the constellation Vulpecula and has a period of 1.3373 seconds.

The most famous pulsar

The most famous pulsar is probably the one that sits inside the Crab Nebula. The Crab Nebula is the remnant of a supernova explosion. In 1054 CE, Chinese, Japanese, Korean and Arab astronomers all reported sighting a new star in the sky.

It took almost another 700 years before someone from Earth saw the dusty, nebulous remains of this exploded star. The pulsar near the center of the Crab Nebula spins about 30 times per second.

A small black and white image showing 3 stars, with 1 flashing on and off.
The flashing of the Crab Nebula pulsar in infrared wavelengths. This view is considerably slower than its 30 times per second period. Image via Cambridge University Lucky Imaging Group/ Wikimedia Commons.
Pulsar: Red and blue nebulous features with bluish-white rings around a very bright star in the middle.
The Hubble Space Telescope imaged the center of the Crab Nebula in 2016. There’s a rapidly spinning neutron star at the center of the nebula, known as a pulsar. It’s the rightmost of the 2 stars near the center of the image. The bluish light is radiation emitted by electrons speeding at close to the speed of light along the neutron star’s powerful magnetic field. Scientists think the wispy circular features move out of the pulsar due to a shockwave that piles up highly energetic particles coming from high-speed winds emanated from the neutron star. Image via NASA/ ESA/ J. Hester/ M. Weisskopf.

Bottom line: A pulsar is a neutron star with its poles aimed toward Earth so that we can see pulses of light resulting from the star’s strong emission when it’s rapidly spinning.

July 15, 2022

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