All about the universe.

Essay by vivJunior High, 9th gradeA+, July 2003

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i.Explain how stars can be classified according to size, colour and temperature.

Astronomers use The Hertzsprung-Russell Diagram, which is a graphical tool to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. In this diagram, the brightest stars lie near the top of the diagram and the hottest stars lie to the left. On the H-R diagram, most of the stars, including the Sun, fall along a diagonal line that goes from the upper left to the lower right of the diagram. This line is called the main sequence. The great majority of stars neighboring the Sun fall on the lower part of the H-R diagram's main sequence, and relatively few lie on the portion of the main sequence above the Sun. This means that most of the Sun's neighboring stars are both cooler and fainter (in absolute magnitude) than the Sun.

A smaller population of brighter but cooler stars known as supergiants occupies the uppermost region of the diagram. Some stars, which are difficult to discover because they are so faint, lie near the bottom of the H-R diagram. These faint stars are called white dwarfs. Below is the Diagram of Main Sequence.

ii.Explain what a galaxy is and describe, with examples, the four main types.

A galaxy is a collection of billions of stars and matter held loosely together by gravity. They are often spread over thousands of light years. Astronomers estimate that there are about 125 billion galaxies in the universe. All the stars visible to the unaided eye from Earth belong to Earth's galaxy, the Milky Way. The Sun, with its associated planets, is just one star in this galaxy.

There are four main types of galaxies:

Spiral Galaxies: a galaxy consisting...