Basic Concepts Needed for the Study of Astronomy
Universe
The universe is everything that is--- including time, space, matter, and energy.
Astronomy
Astronomy is the study of the universe.
Light year
A light year is not a unit of time, but rather a unit of distance.
A light year is the distance that light travels in one year. Since light travels at 300,000 kilometers per second, it should be no surprise that a light year is quite a distance---about 10 trillion kilometers, or 6 trillion miles.
Planet
A planet is a celestial body that is in orbit around the Sun or another star, and that meets minimum mass/size requirements. Examples of planets are the Earth, Saturn, Jupiter, and Uranus.
Galaxy
A galaxy is a huge collection of stars (typically about 100 billion) that are bound by gravity. A galaxy also contains material such as interstellar dust. The Sun lies on the outer edge of a galaxy called the Milky Way.
Star
A star is a compact sphere of gas that has a mass sufficient for maintaining nuclear fusion. Our Sun is an example of a star.
Constellation
A constellation is a group of stars that form a pattern in the night sky.
The stars that make up a constellation have no logical or scientific connection. In fact, many of the stars in a single constellation are actually quite far apart-- but they appear to be in close proximity when viewed from the human vantage point on earth.
The celestial sphere is the canopy of about 6000 stars that are visible to the naked eye. The stars in the celestial sphere are grouped into 88 distinct constellations.
