The Universe, Galaxies, Stars, Constellations

The universe is all of the light, matter, and energy that exist. A planet is an object that orbits a sun but doesn't make its own light. The Solar System is all of the planets, moons, and other matter that orbit a sun. A star is a giant ball of glowing gas. A galaxy is a grouping of billions of stars, solar systems, gas, and dust held together by gravity. The Sun is a star at the center of a planetary system. A moon is an object the orbits a planet.

The universe contains everything that exists. It is believed to have begun 10 to 20 billion years ago in a massive surge of energy called the Big Bang Theory. Some scientists believe the universe is expanding.
A black hole is a large amount of matter packed into a very small space. A black hole has a huge gravitational pull on other objects so objects drawn toward a black hole can never escape, even light. A light-year is the time it takes light to travel in one year. Light can travel around the Earth seven and a half times in a single second. A light-year is equal to 5.9 trillion miles. The nearest star to us, not including the sun), is Proxima Centauri, which is 4.3 light years away, or 25 trillion miles.

The galaxy where our Solar System resides is called the Milky Way. There are trillions of galaxies in the universe. The Milky Way is spiral-shaped. It gets its name from the milky bands of light that its spiral sometimes casts into the sky. The Andromeda Galaxy, also a spiral galaxy, is the closest major galaxy. It is 2.2 million light years away and is sometimes called our twin galaxy. Dark matter is the stuff in the universe that we believe is there but is not visible.

Stars and constellations have been studied for thousands of years. Stars are huge balls of glowing gas. The largest stars known to us are more than 100 times bigger than our sun. Constellations are imaginary pictures in the stars. They are used to help us remember where certain stars are in the night sky. Officially, there are 88 constellations. A nova is a bright flash of light occurring between two stars which occurs when a smaller "dwarf" star steals matter and gas from a larger star. A supernova is an extremely bright explosion of an old, massive star. Alpha Centauri is a nearby star system, consisting of 3 stars, at a distance of 4.4 light years away. The North Star is called Polaris because it is almost directly above the Earth's North Pole. It is 431 light-years away from Earth. It can be located by finding the cup of the Big Dipper, where it is the brightest star above. Polaris has helped travelers find their way across sea and land for centuries. Orion, or the Hunter, looks like a gladiator drawing back his sword. It is best seen in January from North America. Canis Major, The Greater Dog, is said to be one of Orion's hunting dogs. One of the stars that make up this constellation is Sirius, also called the Dog Star, which is the brightest star in the sky, not including the sun. Draco, the Drago, looks like a snake in the sky. Draco appears head-down in winter, and head-up in summer. Camelopardalis, The Giraffe, looks like a giraffe. The brightest star is Camelopardalis, is Beta Cam, which is about 100 times the size of our sun. Ursa Major, The Great Bear, looks like a big bear, whereas Ursa Minor, The Lesser Bear, looks like a small bear.