The Universe

• Universe - The universe is all the heavenly bodies such as stars and planets and the enormous space between them.
• Galaxy - Galaxies are collections of hundreds of stars around a common centre.
• 3 Shapes of Galaxies

1. Irregular Galaxy - overall globular shape
2. Elliptical Galaxy - egg-shaped
3. Spiral Galaxy - spiral shape; most common form of galaxy; examples are our Milky Way Galaxy and the Andromeda Galaxy

• Solar System - A solar system is any star orbited by planets. Our solar system is our Sun orbited by the planets Mercury, Venus, Earth and so on.
• Planet - A planet is a large body that does not produce its own light and revolves around a star.
• Moon - A moon is a natural satellite that revolves around a planet.
• Asteroid - An asteroid is a celestial body made of rock that orbits the sun between Mars and Jupiter.
• Comet - A comet is a celestial body made of frozen rock and gas that revolves in an orbit around the sun. Its orbit is not the same as the planets. It has a nucleus and a hazy tail which always points away from the sun because the tail is composed of evaporated gases heated by the sun.
• Light Year - A light year is a unit of astronomical measurement. It is the distance light travels in one year. A light year is equal to 9 461 000 000 000 kilometres.

Theories on the Origin of the Universe

• Age - The universe is estimated to be between 7 and 20 billion years old. There are 2 main scientific theories about how the universe formed.

1. Big Bang Theory - This is the more widely accepted theory. It explains that the universe was formed from the gigantic explosion of a central large and dense mass. In the first few minutes after the big bang, many elements such as hydrogen and helium were formed. Other heavier elements were formed later in the nuclear fusion processes of the stars. The universe is expanding, as is shown by the Red Shift of many stars (Red Shift is the spectrum showing longer wavelengths that form when stars are receding away from us).
2. Steady-State Theory - This theory states that matter is continually being created at the outer parts of the universe, and is condensing into galaxies. It is not generally accepted now.