Very small, single-celled plants that exist in large numbers almost everywhere on Earth.



A thousand million, written as 1,000,000,000.


binary star

Two stars that are close together and circle around one another; according to some estimates, roughly half of all stars are members of binary systems.


black hole

A region of space in which the pull of gravity is so strong that nothing, not even light, can escape. A black hole may form from the squashed remains of a very heavy star after the outer parts of the star have been blown away in a supernova explosion.



The smallest living unit making up an animal or plant. Cells are usually too small to be seen without the help of a microscope. They come in many forms and serve many different purposes. Examples include red blood cells, bone cells, and brain cells.



In a star, the extremely hot region in the middle where the star produces its light and heat.


deja vu

A French expression meaning "already seen;" deja vu is the name for the strange feeling people have that what is happening has happened once before.



A very large collection of between a few billion and a few hundred billion stars. The galaxy in which we live, the Milky Way Galaxy, is a spiral in shape, contains more than 200 billion stars, and measures more than 100,000 light-years across.



A force exerted by any object that has mass. The Earth's gravity is the force that prevents the atmosphere, and people, from floating away into space.



A unit used by astronomers to measure distances in the Universe. It is the distance that light, traveling at 186,282 miles per second, covers in a year. One light-year equals 5.85 trillion miles.



Mass can be thought of in two ways. It is a measure of the amount of matter in an object. Mass is also a measure of how hard it is to make an object move or make it move in a different way.



The supposed ability of some people to be able to know what is going to happen or to sense clearly certain events in the future. It can take various forms, from "fortune telling" to dreaming about disasters before they occur.



Similar to precognition, but more of a vague feeling about the future than certain knowledge. There is still not enough evidence to show whether premonition and precognition are real or not. Most scientists assume they are not.



There are two types of supernova. The one discussed in this book involves the explosion of an old, very massive type of star known as a supergiant. One of the possible results of a supernova is a black hole.



A tiny, water-dwelling creature that can survive when most of the water in its body is lost. This allows the tardigrade to live through periods of drought and then become active again when water returns to its surroundings.



A million million; written as 1,000,000,000,000.



"Unidentified Flying Object." A UFO is any object seen in the sky for which there is no immediate explanation. Although many UFOs do turn out simply to be airplanes, bright stars or planets, or unusual clouds, a small number remained unexplained.


white dwarf

The type of object that an average-sized star like the Sun becomes after it stops producing light and heat in its core. Most white dwarfs are from one to three times the size of the Earth.


white hole

In theory, possibly the other end of a black hole. Instead of being like a whirlpool that sucks everything into it that comes too close, a white hole would look like a geyser in space from which matter and light would gush out.



In theory, a sort of tunnel that connects a black hole to some other region of space and time. Wormholes may or may not exist. If they do, they might be like subways connecting different parts of our own universe or even other universes.



Rays that are similar to ordinary light but which carry much more energy. X-rays may be produced in a variety of ways in space. One of these is by gas heated to millions of degrees.