The Weirdest Object in the Universe

Matthew R. Francis

The Universe is full of strange things. That’s what gets many astronomers out of bed: knowing that somewhere something really weird is waiting to be discovered.

Even though I’m a black hole aficionado, I would choose neutron stars as the weirdest things in the cosmos. These are the dense remnants of stars at least eight times more massive than the Sun. When these stars run out of nuclear fuel, they explode in supernovas, but their cores collapse under intense gravity.

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As a result, neutron stars have the mass of a star, but diameters of roughly 20 kilometers (12 miles)—about the length of Manhattan. Neutron stars have a solid surface, though I wouldn’t recommend trying to stand on it: they are about 100 times hotter than the Sun, and the gravity is so strong that scaling a one-centimeter bump would take as much energy as climbing Mount Everest on Earth.

The relatively large mass and tiny size mean that the density of a neutron star is enormous: roughly 400 trillion times the density of water (4 × 1014 grams per cubic centimeter). That’s denser than any material we can make. In fact, it’s about as dense as the nucleus of an atom, but far larger than any atom can be.

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The gravity is enough to squeeze atoms until electrons combine with protons to make neutrons. That’s where the name “neutron star” comes from, though they aren’t entirely made of neutrons and they aren’t anything like normal stars. (Technically a “star” is an object that shines by nuclear fusion, which neutron stars don’t. We’re stuck with the name, though.)

That combination of intense gravity, large density, strange composition, strong magnetic fields, and high temperature make neutron stars weird. In fact, researchers don’t have a complete physical model for neutron star interiors: we need our most sophisticated particle physics, gravitational physics, and knowledge of materials at high density.

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To make matters worse, every neutron star is too far away to observe in detail, and so far we can’t make one in the lab, even in miniature. The combination of pressure, temperature, and density means it’s difficult to create even a small drop of the stuff neutron stars are made of. That’s not to say we never will—scientists are clever, after all—but it slows us down if we can’t either perform experiments or look at a neutron star up close.

Even so, astronomers know a lot about neutron stars. They were first predicted in the 1930s, but British astronomer Jocelyn Bell first observed them in 1967 as regular pulses of radio waves during what was supposed to be a routine calibration of a telescope. The pulses are from a beam of light produced by the intense magnetic field, which sweeps across Earth as the neutron star rotates. For that reason, such objects are called pulsars, and they have provided our best data about neutron stars.

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Since 1967, researchers have identified many pulsars. Some of these are among the most reliable clocks in the cosmos, thanks to their incredibly regular rotations. The fastest can spin around hundreds or thousands of times each second. Even for an object that small, that’s an incredible rate of rotation.

But we’re not done with neutron star weirdness yet.

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While every neutron star has an intense magnetic field, the ones known as magnetars are exceptional. They are rare: astronomers have found only 21 magnetars so far (with 5 more potential candidates). And they are more magnetic than any known object, with fields more than 100 trillion times stronger than Earth’s magnetic field.

One magnetar is only about a light-year from the black hole at the center of the Milky Way. Light emitted by the hot gas swirling around the black hole passed near this magnetar before reaching us. During that passage, the intense magnetic field twisted the light. (This is known as “Faraday rotation” for 19th century physicist Michael Faraday, which was discussed in the May 11 episode of Cosmos.) By measuring this twisting, astronomers mapped the magnetic environment near the galactic center—something they wouldn’t be able to do easily without the magnetar’s presence.

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The power of a “normal” neutron star’s magnetic field is fairly comprehensible: the remaining protons and electrons that didn’t get squished into neutrons create powerful electric currents inside. But why are magnetars so much stronger?

That question has puzzled astronomers for decades, but a new study published just last week may have the answer. Researchers using the creatively named Very Large Telescope (VLT) in Chile discovered a magnetar in the vicinity of another, rapidly moving star. A little astronomical forensics suggests that these objects used to be a binary: two stars in mutual orbit. The interaction between them caused one of the pair to spin faster and faster … until it exploded in a supernova, leaving a neutron star behind and kicking the other star out. That extra spin in the progenitor star might have been enough to give the neutron star more magnetic power, making it a magnetar.

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We don’t know yet if this hypothesis is correct, or if all magnetars were made this way. Time—and more observations—will tell. However, it’s another indication of how weird and wonderful neutron stars are: they continue to challenge us and give us another reason to keep looking at the sky.

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