Despite the long time a comet takes to orbit the Sun, it is generally only visible with the unaided eye for a fraction of a year. Comets can, however, be seen to move against the background stars in photographs or images taken a day or more apart. This changes with distance from the Sun. The features most familiar to us are those the comet has when it is in the inner solar system. Away from the sun the comet consists only of its tiny, cold nucleus.
Comets are almost always invisible to astronomers until they become active.
They are usually only 1-10 km across, but may be as big as 100 km in diameter. This is the only substantial and solid part of a comet. Together the coma and the nucleus form the head of the comet. The nucleus is very dark, but the coma makes the head the brightest part of the comet. For an explanation of how the tails form, click here.