Picturepedia, Second Edition: An Encyclopedia on Every Page
The Universe is everything that exists—all of space, matter, energy, and time. It is a huge wide-open space with billions of galaxies, each containing billions of stars, and yet it is at least 99.99 percent empty space. It has been expanding constantly since its beginning 13.8 billion years ago, when it exploded into life with the Big Bang.
WHAT MAKES UP THE UNIVERSE?
The Universe contains matter and energy. Matter is generally physical “stuff” that can be seen, like the planets, but galaxies also contain invisible matter called “dark matter.”
This does not give off light or heat and can be detected only by the effects of its gravity on visible objects. Between and beyond both types of matter is “dark energy,” a mysterious thing that scientists know almost nothing about.
THE SCALE OF THE UNIVERSE
The Universe is so vast that it is hard to appreciate its size. This series of pictures “zooms in” on the Universe to show how our Solar System and planet relate to the rest of the Universe. Space is so huge that astronomers use the speed of light to measure distances. One light-year is the distance light travels in a year, which is approximately 6 trillion miles (nearly 10 trillion km).
A black hole is a region of space where matter has collapsed in on itself. This means there is nothing to be seen, but astronomers know, black holes exist because they have such a strong gravitational pull that nothing can escape them—not even light.
Dwarf planets, like major planets, have enough selfgravity to make them form into a round shape, but they are smaller and orbit the Sun with many similar small objects. Pluto was once listed as a major planet, but now it is classed as a dwarf planet
A moon is a rocky or icy body that orbits a planet. Some planets have many moons, but Earth has only one. Moons are also known as natural satellites
Asteroids are small rocky bodies that orbit the Sun. There are millions of them in space, and they are mainly made of materials that were left over from the formation of planets.
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