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Click here for the Universe Quiz.  If you need help, review the notes below.

The universe is all of matter and energy that exists. The study of the what makes up the universe and how it has grown and changed is called cosmology.

How old is the universe? There are several theories or ideas on this:

The matter in the universe is subject to various forces. The strongest force is the force of gravity. This force pulls matter together to form stars. Gravitation also acts to group billions of stars into galaxies and to group galaxies into clusters and superclusters.

The main source of energy in the universe is the changing of the matter of the stars into energy by thermonuclear reactions. These reactions continue throughout the different stages of a star's evolution until the star has used all its available nuclear fuel.

Ways to measure how big the universe is has improved as scientists have better instruments and ways to measure its size. . One method is to use a spectroscope to analyze the light reaching the earth from an object. The distance of a very light or faint object are measured by comparing its brightness to those of objects whose distances are known. Another method depends on the fact that the universe as a whole appears to be expanding in the spectral lines of distant galaxies. Hobble's law makes it possible to estimate their distances from the speed with which they are rushing away from the earth.

The universe is thought to be ten billion light-years in diameter. It is difficult to measure how big the universe is because space may be curved. This was put forth by Einstein in his general theory of relativity. This curve in would affect measurements of distance based on the passage of light through space from objects as far away as five billion light-years or more


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