Astronomers use the metric system to study the stars which means the units they
use are grams, centimeters and seconds.  The distances that astronomers are measuring
are so incredibly huge that they have had to come up with some clever ways of measuring
these distances and in doing so they use some other units to more accurately describe
these distances and how they measure these distances.
                                 Lets start little and close to home.

    The first astronomical distance ancient astronomers measured was the distance to the sun even though the Greeks early measurements have been improved upon the unit has survived and is called the astronomical unit or AU. It is approximately 1.50*1011m.
     The next unit you've probably heard of is the light year. It is the distance that
light travels in one year. So you can do the calculation yourself:

 distance = speed * time

 c = the speed of light = 3.00 * 108m/s

 one year = 3.16*107s

 light year = ly = 3.00* 108 m/s * 3.16 * 107s = 9.5*1015m

So a light year is 9.5*1015m which is a great distance, but even the closest star is 3 light
years away so astronomers need even larger measures of distance.

    A parsec is another common unit astronomers use, it has a value of 3.09 *1016m
or 3.26 light years.  The unit comes from the way astronomers measure the distance of
closer stars. As the earth moves around the sun, closer stars shift with respect to the
background stars. This shift is measured in terms of an angle. This angle is called
parallax, and one arc second (1/3600 of a degree) of parallax corresponds to one parsec. It
happens to workout nicely that the nearest star is approximately one parsec away.
 The next logical question would be how do astronomers measure these great
distances. A giant ruler doesn't really seem to be a great way to go about it. In order to
measure these great distances they use the properties of the light emitted by these objects.
Light becomes less intense as its spread out through space.  The original power or
Luminosity and the apparent brightness we actually receive here on earth are related to
how far away the star is:

 Apparent Brightness  =  Luminosity/ 4Pr2

This is called the Inverse Square Law, and will come in very handy in your studies as an
astronomer.