Mysteries of Astronomy
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What Is Dark Energy?
Adrian Cho Fourteen years ago, the discovery of dark energy rocked astronomy. Two teams of astronomers and astrophysicists studied distant stellar explosions called type Ia supernovae to measure how the universe has expanded over its 13.7-billion-year lifetime. They expected that the expansion would be slowing as galaxies pull toward one another with their gravity. To their shock, they found that the expansion is accelerating as if some bizarre "dark energy" is stretching space. The nature of dark energy is now perhaps the most profound mystery in cosmology and astrophysics. And it may remain forever so.
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How Hot Is Dark Matter?
Adrian Cho For decades, astronomers have thought that some unseen "dark matter" provides the gravity that holds the galaxies together. Scientists still don't know what dark matter is, but that could soon change. Within years, physicists might be able to detect particles of the stuff. Even if they can't, astronomers and astrophysicists may be able to narrow in on dark matter's most basic properties by astronomical means alone.
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Where Are the Missing Baryons?
Yudhijit Bhattacharjee To describe the universe, you need to know what's in it and where the components reside. So far, astronomers are a long way from completing that inventory. It's not just that they can't pin down dark energy and dark matter, the two invisible components that make up 95% of the cosmos (see pp. 1090 and 1091). More than half of the remaining 5%—"baryonic matter," the ordinary atoms and ions that make up stars, planets, dust, and gas—remains unaccounted for as well.
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How Do Stars Explode?
Yudhijit Bhattacharjee When a star's fuel has been spent, it explodes into a giant fireball known as a supernova, producing the brilliance of multiple suns and sometimes outshining entire galaxies. How these explosions occur has been a subject of observational and theoretical study for decades. In recent years, advances in supercomputing have enabled astronomers to simulate the internal conditions of stars with increasing sophistication, helping them to better understand the mechanics of stellar explosions. Yet, many details of what goes on inside a star leading up to an explosion, as well as how that explosion unfolds, remain a mystery.
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What Reionized the Universe?
Edwin Cartlidge Some 400,000 years after the big bang, protons and electrons had cooled off enough for their mutual attraction to pull them together into atoms of neutral hydrogen. A few hundred million years later, something stripped the electrons off the atoms again. This time, however, the expansion of the universe had dispersed the protons and electrons enough so that the new energy source kept them from recombining. The "particle soup" was also dilute enough so that most photons could pass through it unimpeded. As a result, most of the universe's matter turned into the light-transmitting ionized plasma that it remains today. What caused this cosmic reionization? No one is sure.
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What's the Source of the Most Energetic Cosmic Rays?
Daniel Clery Astrophysicists know that 89% of cosmic rays are simple protons, or hydrogen nuclei. Most of the rest are helium nuclei with a smattering of heavier nuclei, electrons, and antimatter. Astrophysicists think most of the lower-energy particles—up to 1010 eV—come from the sun and that those from 1010 eV up to 1015 or even 1018 eV come from elsewhere in our galaxy. Because cosmic rays with even higher energies seem to come equally from all parts of the sky rather than bunching in the plane of our galaxy, they probably originate outside the Milky Way. Data taken from detectors in the past few years have provided some clues to their origin but, as yet, no smoking gun.
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Why Is the Solar System So Bizarre?
Richard A. Kerr All manner of planets circling other stars have been popping up of late: big ones, little ones; gassy ones, rocky ones; hot ones, cold ones. But the freakish diversity of worlds starts much closer to home. From the 1960s to the 1980s, space probes returned the first close-up looks at eight of the then-nine planets. To researchers expecting a simple story that would explain what shaped our solar system, the observations sent a sobering message: in your dreams. Today, enigmas such as Mercury's makeup (mostly iron core, with a thin veneer of rock) and Uranus's skewed magnetic field continue to bedevil planetary scientists, and no tidy resolution is in sight.
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Why Is the Sun's Corona So Hot?
Richard A. Kerr Yes, the sun is hot—really hot. It's 16 million kelvin at its fusion-fueled core, cooling, as the second law of thermodynamics requires, to a still-blistering 5780 K at its visible surface. But for the better part of a century, solar physicists have been mystified by the sun's ability to reheat its corona, the encircling wispy crown of light that emerges from the glare during a total solar eclipse. There, temperatures again soar to 1 million K and more. How would heat dissipating from the core out beyond the surface abruptly punch temperatures up by a factor of 200 and more?
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