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What is a light-year? -

Before we go any further, a word about light-years. For most space objects, light-years are used to describe their distance. A light-year is the distance light travels in one Earth year. And get this: one light-year is about nine trillion km (six trillion miles).

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Dark matter -

Incredibly, the numerous planets, stars, galaxies, and everything else we can see make up less than 5% of the total Universe. The odd thing is that 26.8 % is a substance scientists label as "dark matter." It doesn't interact with light or visible matter, but can be detected through its gravitational influence on the movements and appearances of other objects in the Universe, for example stars or galaxies.

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Dark energy -

And what of the other 68% of the Universe? Cosmologists call this "dark energy," the mystery force that rules the Universe. This phenomenon overwhelmed gravity and gained control of the Universe about five billion years ago. It is invisible, fills all of space, and its repulsive gravity is speeding up the expansion of the Cosmos.

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Milky Way -

Our own galaxy, the Milky Way, is a barred spiral-shaped galaxy and includes our own solar system. The oldest stars in the Milky Way are nearly as ancient as the Universe itself, probably formed after the Big Bang.

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Stellar jet -

Stellar jets are beams of ironized matter ejected from some astronomical objects, but usually produced by young stars still in the process of forming.

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The Big Bang -

The Big Bang is the most widely accepted theory for the origin of the Universe. But what is the Big Bang?

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What caused the Big Bang? -

Around 13.7 billion years ago, everything in the entire Universe was condensed in an infinitesimally small singularity, a point of infinite denseness and heat. Suddenly, from this mere dot, a huge explosion expanded space like a balloon. From a single atom, the Universe grew to bigger than a galaxy. It is still expanding today.

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Crab Nebula -

The Crab Nebula is a six-light-year-wide remnant of a supernova explosion. First observed by Chinese and other astronomers in the year 1054, it is 6,500 light-years from Earth.

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Supernova -

Supernovae are violent stellar explosions that litter the Cosmos. One of the brightest, named by scientists as LMC N49 (pictured), is approximately 160,000 light-years from Earth.

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Spiral galaxy -

Spiral galaxies consist of a flat, rotating disk containing stars, gas, and dust. These galaxy forms were originally described by American astronomer Edwin Hubble (1889–1953). Suitably, this image of Spiral Galaxy M81 is viewed from the Hubble Space Telescope.

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Thousands of galaxies -

And there are thousands of galaxies out there. Even the small dots seen in this image are entire galaxies, and are about 13 billion light-years away.

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Mapping the Big Bang -

This image from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite shows the first all-sky microwave image of the Universe soon after the Big Bang. It was published by a team of astronomers from NASA and Princeton University.

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Black hole -

A black hole is a place in space where gravity is so strong that even light can't get out. This is because matter has been squeezed into a hugely compressed area. This happens, for example, when a star is dying. Scientists reckon there are millions of these cosmic bodies in our galaxy, but no one knows what they contain. This image is of a black hole in Messier 87, a supergiant elliptical galaxy in the constellation Virgo.

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Dark clouds -

A dark cloud in space is a cloud of gas and dust that blocks light from the regions of space behind it. Pictured are newborn stars peeking out from beneath their natal blanket of dust in this dynamic image of the Rho Ophiuchi dark cloud from NASA's Spitzer Space Telescope. Located near the constellations Scorpius and Ophiuchus, the nebula is about 407 light-years away from Earth.

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Exoplanets -

An exoplanet is a planet beyond our solar system. Most exoplanets orbit stars, but free-floating exoplanets, called rogue planets, orbit the galactic center in permanent darkness and are untethered to any star. Kepler-186f (pictured as an artist's concept), is the first exoplanet discovered in what scientists call the habitable zone—the region around the host star where the temperature is right for liquid water.

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Kepler-452b -

Kepler-452b (sometimes nicknamed Earth 2.0 or Earth's Cousin) is an exoplanet orbiting the Sun-like star Kepler-452, about 1,400 light-years from Earth in the constellation Cygnus. It is the first potentially rocky super-Earth planet discovered orbiting within the habitable zone of a star very similar to the Sun.

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Are we alone? -

In this artist's impression, the planetary system around the red dwarf, Gliese 581, is pictured showing what astronomers believe is another as yet unnamed Earth-like planet found outside our solar system, a planet that could have water running on its surface. And it's a lot nearer to us, a mere 20.5 light-years away, in the constellation Libra. Suddenly, the potential for extraterrestrial life seems a lot more real.

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Red dwarf -

A red dwarf is the most common star type in the Universe. It's also the smallest and coolest kind. Not one red dwarf is visible from Earth, even though they account for 50 of the 60 nearest stars to our planet, and make up three-quarters of the stars in the Milky Way.

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Titan -

Titan is the largest moon of Saturn and the second-largest natural satellite in the solar system. Titan is the only moon wrapped in a thick atmosphere and has a surface of rock-hard water ice. Intriguingly, it also likely has a liquid water ocean beneath its surface. Image: NASA/JPL-Caltech/Space Science Institute

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Sunspots -

Sunspots are caused by disturbances in the Sun's magnetic field welling up to the photosphere, the Sun's visible 'surface.' The number of sunspots on our Sun typically ebbs and flows, with each sunspot lasting a few days to a few months, and the total number peaking every 11 years.

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Cosmic rays -

Cosmic rays are high-energy protons and atomic nuclei that zap through space at nearly the speed of light. They most likely originate from the Sun, from outside of the solar system in our own galaxy, and from distant galaxies. This ionizing radiation is harmless to us because it bounces off the Earth's atmosphere. However, cosmic rays have been blamed for electronic circuit problems and data crashes in satellites and other machinery. Image: NASA

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Solar flare -

Similarly, a solar flare is an intense eruption of electromagnetic radiation in the Sun's atmosphere. The phenomenon is associated with sunspots. Flares are our solar system's largest explosive events, and can last from minutes to hours.

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Callisto -

Callisto is possibly the most highly cratered body in the solar system. It's the second-largest moon of Jupiter, after Ganymede, and thought to be a long dead world, with hardly any geologic activity on its surface. It's one of the oldest landscapes in the Cosmos, dated back to at least four billion years. Image: NASA

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Space roar -

A mysterious screen of extra-loud radio noise permeates the Cosmos. Scientists discovered this when they launched an instrument called the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE), fixed to a huge balloon that was sent to space. ARCADE was designed to pick up radio waves from distant stars, but the signal received was much louder than expected, something cosmologists term "space roar." Image: NASA

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Fermi Bubble -

Discovered by cosmologists as recently as 2010, Fermi Bubbles are massive, mysterious structures that emanate from the Milky Way center and extend roughly 20,000 light-years above and below the galactic plane. They emit high-energy gamma rays and X-rays, invisible to the naked eye.

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Magnetic field of the Moon -

When Apollo astronauts brought back Moon rock, scientists were surprised to learn that some of them were magnetic. Researchers believe the magnetism may be a relic of a 193-km-wide (120 mi) asteroid that collided with the Moon's southern pole about 4.5 billion years ago, scattering magnetic material across some parts of the lunar surface.

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Pulsar -

A pulsar is a highly-magnetized rotating neutron star so named because it emits beams of electromagnetic radiation out of its magnetic poles at regular intervals. But no one is quite sure why. Even more puzzling is that occasionally these stars stop pulsing.

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Miranda -

The 11th of Uranus' moons and the fifth largest, Miranda has a surface unlike anything in the solar system. Its broken, haphazard terrain features the Verona Rupes range—10,058 m (33,000 ft) tall and thought to be the highest cliffs in the solar system. They are visible at the bottom of this photograph. Image: NASA/JPL-Caltech

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Atlas -

Saturn's flying saucer moon, Atlas is another solar system oddball. An inner satellite discovered in 1980, Atlas appears like a deep space UFO. Image: NASA/Jet Propulsion Lab-Caltech/Space Science Institute

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Phobos -

Phobos, the inner and larger of Mars' two moons (the other being Deimos), is thought by scientists to be in a "death spiral" in that it's slowly orbiting toward the surface of Mars. The natural satellite is named after the Greek god Phobos, the personification of fear and panic, and the origin of the world phobia. Image: NASA/JPL-Caltech/University of Arizona

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Is space infinite? -

The current width of the observable Universe is about 90 billion light-years. But what lies beyond? The short answer is cosmologists aren't sure. Presumably, there's a bunch of other random stars and galaxies. If the Universe is perfectly geometrically flat, then it can be infinite. If it's curved, like Earth's surface, then it has finite volume. But no one knows.

Sources: (BBC Science Focus) (EarthSky) (NASA) (Jet Propulsion Laboratory) (Space)

See also: Space tourism: would you try it?

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