Dr. Aaron Titus | Department of Physics, High Point University PHY1050      Astronomy of Stars, Galaxies, and the Cosmos home | WebAssign | textbook | course calendar

## unit prefixes

Prefixes are used instead of powers of 10. For example kilo means 1,000. Therefore, a kilogram is the same as 1,000 grams. A kilometer is the same as 1,000 meters.

If you have recently purchased a computer, you probably bought a hard drive with somethine like 40 gigabytes (GB). Well, giga means 1 billion (1,000,000 or 106). Therefore, 1 gigabyte is 1 billion bytes.

Of course, the capacity of hard drives has increased in recent years. Not to long ago, we bought hard drives whose capacity was measured in megabytes (MB). The prefix mega means one million, so 1 megabyte is equal to 1 million bytes.

Here's a list of a few common prefixes. Because we deal with such large numbers in astronomy, we generally will use kilo, mega, and giga.

prefix meaning power of 10
micro (µ) one-millionth 10-6 1/1,000,000
milli (m) one-thousandth 10-3 1/1000
centi (c) one-hundredth 10-2 1/100
kilo (k) one thousand 103 1000
mega (M) one million 106 1,000,000
giga (G) one billion 109 1,000,000,000
tera (T) one trillion 1012 1,000,000,000,000

To convert from a prefix to a number, just substitute the number that corresponds to that prefix.

For example, if you bought a 40 gigabyte (GB) hard drive, then simply substitute "billion" for "giga" to convert the units from GB to bytes. So, 40 gigabytes is equal to 40 billion bytes. I just took out the "giga" and replaced it with "billion." We could also write it in scientific notation by noting that "billion" is 109. Thus, a 40 gigabyte hard drive is 40 x 109 bytes.

The diameter of the Milky Way galaxy, our galactic home in the universe, is 100 kly (kly means kilo light-year). How many light-years is the diameter of the galaxy?

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