It is believed that there are three sources of comets. The main source is
the Oort cloud, a vast spherical cloud surrounding the solar system. Figures
vary for the position of the Oort cloud due to the fact that it cannot be
seen. The comets that make up the Oort cloud aren't bright enough to be seen.
The location of the cloud has to be calculated from the orbits of comets entering
the inner solar system. Most people place its location between 30,000 times
and 100,000 times the distance the Earth is from the Sun, which is anywhere
between an eighth of the distance to the nearest star to around a quarter
of the distance to the nearest star. The nearest star to the Sun is 4 light
years away. It is believed that interactions with the Oort cloud and distant
stars, gas clouds or just the arms of our Milky Way galaxy disturbs some of
its comets which change their orbit and start on their long trip toward the
inner solar system.