The pulsar at the heart of the Crab Nebula is a compact object. (Credit: NASA.)

When astronomers refer to “compact objects,” they are generally referring to objects significantly more dense than a typical star or a planet. For example, white dwarfs and neutron stars are extremely dense objects that result when their progenitors stars—Sun-like stars or smaller in the case of white dwarfs, and giants in the case of neutron stars—have run out of fuel for fusion. The stars…

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dark matter

Roughly 13.7 billion years ago, the universe started expanding from a dense, hot volume. In the early universe, some 400,000 years after the Big Bang, conditions cooled enough to allow the formation of hydrogen atoms from free protons and electrons. After this early phase, known as "recombination," the universe began to take shape as objects—galaxies, stars, planets—coalesced from the…

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dark energy

One of the most important and surprising scientific discoveries of the twentieth century is that the expansion of space is not slowing down, but speeding up—contrary to what we expect the gravitational pull of all the matter in the Universe to do. The driver of this accelerating expansion has been labeled "dark energy," but there is much about the phenomenon that researchers don’t understand.…

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dark matter image

Only about five percent of the total matter and energy of the Universe is made of the same familiar matter that makes up everything from stars to planets to human beings. The identity of the remaining 95 percent, roughly one-third known as "dark matter" and roughly two-thirds as "dark energy," is unknown. Though scientists have not yet detected it directly in laboratories on Earth, dark matter…

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Computer simulation of one of the earliest stars in the Universe. (Credit: Abel, et al.)

Much in the same way archeologists reconstruct past civilizations by looking at remains in the present, cosmologists reconstruct the Universe’s past by looking at the constituents of the current Universe. By doing so, they can infer how the Universe began, how it evolved into its present state, and how it will continue to change over time. The study of the early Universe is one of the most…

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particle acceleration

The Universe is awash in highly energetic particles with velocities approaching the speed of light. Though these particles exist in many places throughout the Universe—and, as cosmic rays, can even be found slamming into our own atmosphere—scientists don’t yet fully understand their origins.

Nature’s High-Performance Accelerators

Certain cosmic phenomena can eject particles…

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size of saturn orbit around the sun

More than 4000 planets have been discovered orbiting other stars. These extrasolar planets, or exoplanets, span a vast range of properties, and most form systems very different than our own, ranging from "hot Jupiters," gas giants that are closer to their stars than Mercury is to our Sun, to tightly-packed systems of multiple "super-earths" orbiting faint red stars.

Planets are…

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