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Step Right Up to See the Incredible, Unbelievable Neutron Star

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If a black hole is the senior starting quarterback on your high school football team—dark, handsome, well-known, and admired by everyone—then a neutron star is the starting quarterback’s kid brother, a gangly freshman who marches to the beat of his own drummer and can’t help being overshadowed by the sheer magnitude of his older sibling.

Like a black hole, a neutron star forms when the core of a massive star collapses in on itself, but because a neutron star is less weighty than a black hole, it has its own unique set of properties. A neutron star was originally an enormous glowing ball of plasma, a star 8 to 25 times more massive than our sun that used hydrogen in constant fusion reactions to produce energy. Eventually it burned through all its hydrogen and so transitioned to using helium in these reactions, then carbon, then oxygen, then silicon—each element in the sequence producing less energy than the one before. Once it gets to iron, the stellar core can no longer produce enough energy to resist the pull of its own gravity. It collapses inward, rebounding a bit to create a shock wave that flings the rest of the star into space—the spectacular light show we call a supernova.

The explosion leaves behind the dense core—the neutron star. The creation of a neutron star is actually quite a feat; it forms when protons and electrons, subatomic particles that normally refuse to get too close to each other, are forced to merge by gravity. They combine to form neutrons (neutral particles) that are compacted together into a small nugget. All the extra space between them is squeezed out, much like compressing the air out of a sponge in your fist.

A nugget of neutrons may not sound particularly impressive, especially considering that it is only about a dozen miles across; small enough to fit inside the city of Denver. However, this nugget, due in large part to the massive amount of pressure that creates it, is armed with properties so extreme that they are completely unlike any on earth. Perhaps one of the most remarkable things about a neutron star is its density—a mass twice the size of the sun crammed into its 12-mile diameter. In earth terms, you would have to compress 50,000 blue whales into a single teaspoon to achieve a similar density. In fact, the only thing denser than a neutron star is its big brother, a black hole.

A neutron star also has an immensely strong gravitational field—200 billion times stronger than that of earth. This means that if you dropped your cell phone three feet above the surface of the star (assuming that you could somehow resist the gravitational pull yourself), it would hit the ground one microsecond later, leaving you no chance of even seeing the phone fall, let alone grabbing it in midair. Nor will your phone survive the fall no matter what kind of protective case you have. It will be traveling 4.5 million miles per hour by the time it lands.

The heat of a neutron star is impossible to fathom in terms of any temperature we experience. It starts out 1.5 million times hotter than the earth’s core, and slowly cools to 1,600 times hotter, or some 18 million degrees Fahrenheit. All the while this super dense, incredibly hot, high-gravity nugget of neutrons is spinning many times per second, a dizzying rate of rotation only possible because it is so compact (think of a figure skater pulling her arms in towards her body to spin faster.)

Astronomers Walter Baade and Fritz Zwicky in 1934 first speculated that neutron stars were created when stars exploded. However, as is often the case, it was many more decades before science could confirm their hypothesis. The confirmation came from the neutron star’s ability to keep time—a metronome in the sky.

A neutron star is also known as a pulsar, short for pulsating radio star, because it gives off beams of electromagnetic radiation from its poles as it spins, which we can detect from earth. Because pulsars spin at such a constant rate, and the electromagnetic radiation can only be detected when the beam is pointing towards earth, the interval between the electromagnetic pulses is incredibly regular—in fact, we can actually use it to keep time.

When graduate student Jocelyn Bell Burnell and her advisor Anthony Hewish in 1967 first detected a regular electromagnetic pulsation emanating from the celestial sky, they didn’t know what to make of it. They used the process of elimination to rule out the usual suspects—regular stars, radio frequency interference—and finally were driven to consider one more possibility, that extraterrestrials were producing the signal. They went so far as to nickname the signal LGM-1 for “little green men,” a name that stuck around until a second candidate was discovered. It wasn’t until later that theorists figured out that they had stumbled across something just as weird. That little green man was a neutron star marching to the beat of its own drummer in the middle of the galaxy.

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