The universe is not limited by our imagination. It stretches for a dozen more orders of magnitude in either direction, towards the immense as well as towards the infinitesimal. Even to record such a range of size requires us to fall back on exponential or power-of-ten notation, in which 1,000,000,000 or one billion becomes 10, and one billionth or 0.000000001 becomes 1/10 or 10. The diameter of the Earth is 1,300,000,000 cm, or 1.3 x 10 cm, and the diameter of the observable universe has been estimated to be 1.7 x 10 cm. The Earth fits into this universe like a single small bacterium fits into the entire solar system. At the other extreme, the diameter of an atomic nucleus is only 0.0000000000001 cm, or 10 cm. Such vast size ranges are nearly beyond our comprehension.

The pages of this chapter show a gallery of objects of different sizes: one typical  object  for  each  order  of  magnitude  or  ten-fold  change   in  length, from 10 cm (the universe) down to 10 cm (an atomic nucleus). These are the boundaries of reality as we know it.

Each object is 10 times the length of the object just below it, and 100 times the length of the object two places below. The first eleven orders of magnitude, from 10 cm to 10 cm, describe astronomical bodies outside our solar system: stars, galaxies, and galactic clusters.