Wednesday, April 16, 2008

Stars: Points of Light in the Cosmos

Cosmic Bolts of light
Often during a clear night what we call meteors or shooting stars streak briefly across the sky. These bright trails of light that move across the sky at lightning speed are caused by tiny metallic and rock particles that enter Earth's atmosphere from outer space. The particles become extremely hot, excite the air to incandescence with their great speed, and are incinerated completely in the process. A grain of dust weighing less than one gram can give rise to a meteor that momentarily appears brighter than the brightest stars.

The Stars: Points of light in the Cosmos
If one looks up at the sky during a clear night and far from the air pollution of cities, one has the impression of seeing innumer­able stars. In fact, "only" about 2,500 stars are visible to the naked eye. When one looks through binoculars or a telescope, the number of visible stars increases to hundreds of thousands or several million. All the stars rise and set in the sky just like
the sun, moon, and planets, because Earth rotates once a day. Apart from this nightly rising and setting, the stars exhibit practically no motion, their position in relation to each other remains essentially constant. Because of this apparent immobility the astronomers of antiquity called them the "fixed stars."
The reason why the stars seem to be glued to their places in the sky is that they are separated from Earth by incredibly vast distances. The stars are so very far removed from Earth that astronomers had to invent a new unit for measuring distance, the light-year.
One light-year is the distance that light travels (at a speed per second of 186,282 miles or 300,000 km, a distance equal to 7 1/2 times around Earth's equator) in exactly one year. It is equivalent to 5.88 trillion miles (9.46 trillion km). (A trillion is a "1" followed by 12 zeroes.)

How is it possible for us to see stars at all at such incredible, "astronomical" distances? Because they emit huge amounts of heat and light, just like our sun. All fixed stars are suns, some bigger, some smaller than ours.
Fixed stars and planets are two entirely different things. Stars are spheres of incandescent gas that produce their own light, the planets are solid, cold bodies that orbit around the sun and are illuminated by it. Th
e stars are light ­years away from us, trillions of miles - while the plan­ets move much closer to Earth, being separated from us by "mere" millions or billions of miles. The other big difference between stars and planets is that the latter do not stay in one place in the sky. They revolve around the sun and keep changing their positions as seen from Earth, appearing now in one constellation, now in another.