Even the closest stars are at huge distances from the solar system.  To try to describe their distances with ordinary units such as miles or kilometers would force us to use very large numbers.  Even distances in Astronomical Units would be pretty big.  We choose to use a much larger unit to measure the distance to the stars, a unit called a light year.  It is based on the speed of light.
        We know that light of all wavelengths travels at a constant speed through space.  The speed of light can be expressed in many different units.  Some of them (approximate values) are shown below.

                    Speed of light            =  186,000 miles/second
                                                    =  300,00 kilometers/second
                                                    = 3.00 X 10⁸ meters/second
                                                    =  0.0020  AU/second

    A light year is the distance that light would travel in a year at the speed shown above.  Its value in kilometers and AU is:
                    1 light year (ly)        =  9.46 X 10¹² km
                                                   =  63,240  AU

        Even the closest stars are several light years  away from the solar system.  The distance to the three closest stars and the distance to some of the brightest stars are shown in the tables below.

        Distances to the three nearest stars:

                    Star                                Distance
                    Alpha Centauri                    4.3 ly
                    Barnard's Star                    5.9 ly
                    Wolf 359                            7.6 ly

        Distances to the brightest stars (as seen from Earth in the Northern Hemisphere):

        Star                    Distance                    Star                Distance

    Sirius                        8.6 ly                    Arcturus                 36.0 ly
    Vega                        26.5 ly                    Capella                    45.0 ly
    Rigel                     900.0 ly                    Procyon                    11.3 ly
    Betelgeuse             520.0 ly                    Altair                      16.5 ly
    Aldebaran              68.0 ly                     Spica                    220.0 ly
    Antares                520.0 ly                    Fomalhaut                22.6 ly
    Pollux                     35.0 ly                    Deneb                 1,600.0 ly

    Convert the following measurements from ly to AU
                Number of AU = number of ly X 63,240 AU/ly

                1.  How many AU is 0.1234 ly? _____________
                2.  How many AU is 4.30 ly? ______________
                3.  How many A U is 0.0105 ly? ____________

    Convert the following measurement from AU to ly
                Number of ly - number AU X 0.00001581 ly/AU

                1.  How many ly is 39.44 AU? ____________________
                2.  How many ly is 20,000 AU? ___________________
                3.  How many ly is 100,000 AU?  _________________

    Use the information the above list of stars and conversion chart to measure the distance of the following stars in kilometers.  Make sure you also use scientific notation as your final solution.

                Rigel
 

                Sirius

                Betelgeuse