Star

Stars are extremely large, extremely luminous bodies of gas at great distances. They are the most obvious features found in the universe. They are principally composed of hydrogen that is undergoing nuclear fusion to become helium. Our Sun is the nearest star to Earth, at a distance averaging 93 million miles. The Earth orbits the Sun in a period of approximately 365.25 days, and this defines the year. The diameter of the Sun, which is a typical star, is about 870,000 miles and its power output is about 10²⁶ watts. The temperature inside the Sun is estimated to be in excess on ten million degrees, and this is hot enough for nuclear reactions to occur.

The Magnitude scale of brightness

The Greeks devised a rough and ready brightness scale for stars. The brightest stars were designated 'stars of the first magnitude', less bright stars were designated 'stars of the second magnitude', etc., down to stars of the sixth magnitude, which were barely visible to the naked eye. The magnitude scale is still in use, but modern telescopes and photometers have made it very much more exact and rigorous. A star of magnitude 1.0 is now 2.51188643 times as bright as a magnitude 2.0 star, which is 2.51188643 times as bright as a magnitude 3.0 star, and so on. This number has been chosen so that a magnitude 6.0 star is exactly 100 times as bright as a magnitude 1.0 star, i.e. 2.51188643 = 10^0.4.

Variable stars

Some stars vary in brightness and are known as variable stars. The star Algol in the constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The most famous of these is delta Cepheus.

Stars are also often used as symbols on flags, such as the American flag (50 stars) and the flag of the European Union (12 stars).