Scientific Notation
Large numbers are conveniently written in powers-of-ten notation.
Since space is REALLY big, astronomers frequently have to use big numbers. A convenient way of expressing big (or small) numbers is `powers-of-ten' notation, called scientific notation. An explanation is given in section A-2 of the textbook.
The use of big numbers is not confined to astronomers!
Earth Population: 6.08 billion. In standard notation, this is 6,080,000,000. In scientific notation, this is 6.08 x 109.
Stars in our Galaxy: ~200 billion
= 200,000,000,000 = 2 x 10 11.
This number is uncertain for two reasons. First, it is difficult to count the
stars; many of the stars in our Galaxy are hidden from us by
intervening clouds of gas and dust. Second, there is a fundamental
ambiguity in the definition of a star. Stars are being
created, even as you read this, from clouds of interstellar
gas and dust. Other stars are running out of fuel and dwindling
into darkness. The exact instant when a star is `born' or `dies'
is impossible to define precisely.
Don't be intimidated by the large number of stars in our Galaxy. It is smaller than the number of Oreo cookies that have been baked by Nabisco (362 billion according to the Oreo People). It is, however, MUCH smaller than the number of dollars in the U.S. national debt (well over a trillion).
An interesting point of reference:
You are, in a certain sense, midway in size between an atom and a star. The mass of an adult human being is roughly 3 x 1028 times the mass of an atom. The mass of a star is roughly 3 x 1028 times the mass of a human. You are carefully poised midway between the mind-bogglingly small and the mind-bogglingly large.