Locating Stars

Part I

Each Star Belongs to a Constellation

When people of long ago in different parts of the world studied the skies, they often imagined the stars formed shapes of people, animals, or objects. They associated the figures they saw with different myths or legends. A group of stars named for a particular figure or shape is a constellation.

The Ancient Greeks named 48 constellations. Today we still use the Greek constellations to describe portions of the sky. Orion is one of the few constellations that actually resemble a figure. By drawing lines to connect the stars in Orion, the figure of a hunter is formed. You might recognize the three stars that form his belt. One of these stars is really a nebula.

One of the best known constellations is Ursa Major or the Big Bear. Notice that Ursa Major includes a star group you might recognize--- the Big Dipper. Such a distinct star group that is part of a constellation is called an asterism. The two stars at the end of the bowl of the Big Dipper point at the North Star (Polaris), which is always in the north. The Little Dipper, in the constellation Ursa Minor, or the Little Bear, is another asterism. The North Star is in the end of the handle of the Little Dipper.

Astronomers now divide the sky into 88 regions, or constellations, including the ones named by the Ancient Greeks. Every star belongs to a constellation. But the stars of a constellation are not necessarily related to each other in any other way. For example, they may appear to be near each other in the sky but not really be near each other in space. A view from space shows that the stars in the Big Dipper are different distances from earth.

Distances to Stars

In order to see an object, light from that object must reach your eyes. Light travels 300,000 km in a second--- a million faster than a jet plane. In fact, light can travel seven times the distance around the earth in a second. Light from the moon reaches us in a little more than a second. The sun is much farther away. Its light takes about eight minutes to reach the earth.

The other stars are much farther away than the sun. Light from these stars takes years

to reach the earth. The distance light travels in one year is a light year, about

9.5 trillion kilometers. Astronomers measure the great distances of space in light years. Proxima Centauri, the star

nearest to our sun, is 4.4 light years away. The light reaching us from Proxima

Centauri left that star more than four years ago. If the star exploded tonight, we would not see the flash for over four years.

Many stars are millions of light years.

Proxima Centauri

Astronomers determine distances to nearby stars, such as Proxima Centauri, in the following way. First they observe a star’s location in the sky in relation to more distant stars. Then they wait a few months while the earth moves in its orbit partway around the sun. When the earth has moved, a nearby star appears to shift position in relation to more distant stars. This shift is called parallax. Nearer stars shift farther than more than distant stars. Very distant stars have parallaxes too small to measure.

To determine the distances of stars farther than 100 light years away, astronomers usually compare the brightness of stars. Most methods use the fact that if two stars have the same brightness, the fainter one is farther away. How bright a star or other object in the sky looks is its apparent magnitude. The absolute magnitude of a star describes how bright the star would be if all stars were placed the same distance from the earth.

Some stars have absolute and apparent magnitudes that vary every few hours or days and are called variable stars. A type of variable star, called a Cepheid variable, appears to change brightness in a regular period. By knowing the period of a Cepheid, astronomers can determine its absolute magnitude. Then, they can calculate its distance by comparing the apparent and absolute magnitudes. Distances to other stars can be determined by comparing the stars to Cepheids.

Cepheid variable

Prepared By:

Mr. Albin B. Caibog

BEED IV-A