A5 Notation

One million can be written as 106 — 1,000,000, i.e., a 1 followed by six zeros. For numbers smaller than unity the notation is similar, e.g., 10"2 = 0.01, or one hundredth. A Jight year is about 6 x 1012 miles. More useful to remember is that a light-year is nearly exactly 1018 cm, or 1013 km.

This superscript notation is also used in another way. If a gas has a density of a million atoms per cubic centimeter, astronomers write that as 106 cm"3.

A common tradition in astronomy is to refer to the mass of astronomical objects in terms of the mass of the sun. One solar mass is about 2 x 1033 grams, a number far too great for us to comprehend. It is easier to describe star's mass in terms of solar mass, which gives us something to sink our imaginations into because now we can relate astronomical mass to something closer to home.

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