The sun is 30,000 light years from the center of the Milky Way. What does this mean? Not much, if we do not understand what a light year is.
Measuring a distance in terms of time may at first sound peculiar, but we do it often in everyday life. We say, for example, that New Haven is a two-hour drive from New York, or our house is a five-minute walk from the library. Expressing a distance in this fashion implies that we have a standard velocity. Astronomers, in fact, use a velocity standard: the speed of light in empty space is a constant and equals 299,792,458 meters per second (approximately 186,000 miles per second). Moving at this constant and universal speed, light in one year travels a distance defined by astronomers as one light year (ly), a total of 9.5 trillion kilometers. A light-year is a measure of distance. It is the distance light travels in one year at the rate of 186,000 miles per second.
Alpha Centauri, the nearest star, is about four light-years away. Only about forty of the stars in the sky are within sixteen light-years of the earth. The brightest star, Sirius, is nine light-years away. Betelgeuse, one of the largest known stars, is 270 light-years away. Students looking into space at night are now looking at how far they can see. We should not lose sight of how truly immense such distances are. For example, if we were to count off the miles in a light-year, one every second-it would take us about 185,000 years.
We can now use the light-year for setting the scale of the Milky Way Galaxy. In light-years, our galaxy is about 80,000 light-years across, with the sun orbiting roughly 30,000 light years from the center. Within the Milky Way disk, stars are separated by a few light-years.
Au = 1.495978707 x 1013 cm
Parsec = 206265 Au
3.263 x 1018 cm
Ly = 9.4605 x 1017 cm
6.324 x 104 Au
= 3.14159265 or 3 1/7
Circumference (C) of a circle, diameter (D), and radius, R (R =1/2 D)
The area of a circle, using R and D:
The surface area of a sphere of radius, R is: