Picturepedia, Second Edition: An Encyclopedia on Every Page

Book Preface

The Universe

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).

BLACK HOLES
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
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

MOONS
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
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.