Physicists think that neutron stars can range from roughly one to two and a half times the mass of the sun and that they probably consist of at least three layers. Initially some scientists were skeptical that such extreme objects could exist, and it was not until Jocelyn Bell Burnell and her colleagues observed pulsars in 1967-and researchers over the next year determined they must be spinning neutron stars-that the idea was widely accepted. It had only been two years since British physicist James Chadwick discovered the neutron. “A neutron star is held together by gravity.”Īstronomers Walter Baade and Fritz Zwicky proposed neutron stars in 1934 as an answer to the question of what might be left over after a supernova-a term they coined at the same time for the extra-bright explosions being spotted across the sky. “A nucleus is held together by nuclear interactions,” Lattimer says. It is much like a single giant atomic nucleus, says James Lattimer, an astronomer at Stony Brook University-with an important difference. (A femtometer is a millionth of a nanometer, which is itself a billionth of a meter.) When the star has finished collapsing, it contains about 20 neutrons for every proton. “The atom goes from being a tenth of a nanometer across to just a blob of neutrons a few femtometers wide.” That is like shrinking Earth down to the size of a single city block. “The iron is compressed by a factor of 100,000 in each direction,” says Mark Alford, a physicist at Washington University in St. The gravity is so strong it quite literally crushes the atoms, pushing the electrons inside the nucleus until they fuse with protons to create neutrons. Suddenly gravity has no opposition, and it slams down on the star like a piston, blowing the outer layers away and smashing the core, which at this point in a star’s life is mostly iron. Neutron stars are forged in the cataclysms known as supernovae, which occur when stars run out of fuel and cease generating energy in their cores. If scientists can do that, they will have a handle not just on one class of cosmic oddity but on the fundamental limits of matter and gravity as well. Through these experiments and others, the prospect of understanding what is inside a neutron star finally looks possible. As these beams pass over Earth, we see pulsars blink on and off at more than 700 times a second. NICER watches pulsars, which are highly magnetic, furiously rotating neutron stars that emit sweeping beams of light. These waves carried information about the masses and sizes of the stars right before the crash, which scientists have used to place new limits on the properties and possible compositions of all neutron stars.Ĭlues are also coming from the Neutron Star Interior Composition Explorer (NICER), an experiment that started at the International Space Station in June 2017. A big break came in August 2017, when terrestrial experiments detected gravitational waves-undulations in spacetime produced by the acceleration of massive objects-from what looked like a head-on collision of two neutron stars. Short of cutting open a neutron star and looking inside, there is no easy way to know which of these theories is right. And it is possible that the interiors of these stars are made of even more exotic stuff, such as hyperons-weird particles composed not of regular “up” and “down” quarks (the kind found in atoms) but their heavier “strange quark” cousins. Perhaps the neutrons inside neutron stars dissolve further into their constituent particles, called quarks and gluons, which swim untethered in a free-flowing sea. Others propose much stranger possibilities. Some ideas suggest that neutron stars really are just full of regular neutrons and maybe a few protons here and there. There are several competing theories about what goes on at that threshold. “What goes on at that threshold,” Arzoumanian says, “is what we’re trying to explore.” They are also the most strongly gravitating form of matter known-add just a bit more mass, and they would be black holes, which are not matter at all but rather purely curved space. “They are matter at the highest stable density that nature allows, in a configuration that we don’t understand,” says Arzoumanian, who works at NASA’s Goddard Space Flight Center. Astronomers have never seen a neutron star up close, and no laboratory on Earth can create anything even approaching the same density, so the inner structure of these objects is one of the greatest mysteries in space. Below the star’s surface, under the crush of gravity, protons and electrons melt into one another to form a bulk of mostly neutrons-hence the name. A chunk of neutron star the size of a Ping-Pong ball would weigh more than a billion metric tons. When a star the size of 20 suns dies, it becomes, in the words of astrophysicist Zaven Arzoumanian, “the most outrageous object that most people have never heard of”-a city-size body of improbable density known as a neutron star.
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