Units forStellar Distances

Because the distances to stars are so large, it is useful to introduce some large units of distance measure. The most common are the light year and the parsec.

### The Light Year Distance Unit

In popular discussions, large distances (such as the ones between stars) are often give in units of light years (which we shall abbreviate LY). A light year (which is a unit of distance, NOT time!) is the distance that light travels in a single year. Since light is very fast, the light year is a very large distance. From the knowledge that light travels at a speed of 3 x 1010 cm/s in a vacuum, we can determine the length of a light year in centimeters simply by multiplying by the number of seconds in a year (a tropical year has 3.156 x 107 seconds in it). The result is that a light year is equal to 9.46 x 1017 centimeters, or 9.46 x 1012 kilometers. (Since distances in astronomy often involve large numbers, you may wish to review scientific notation, which is the most useful way to write either very large or very small numbers.)

A light year is often a convenient unit in discussing distances between stars, because the average separation between stars in a galaxy is typically of that order of magnitude. For example, the nearest star to the Sun is Proxima Centauri, which is about 4.4 light years away. Here is a summary of some important distances expressed in light years:

• Light Year: the distance that light travels (through a vacuum) in one year (9.46 x 1017 cm).
• The nearest star (other than the Sun) is 4.4 light years away.
• Our galaxy (the Milky Way) is about 100,000 light years in diameter.
• The distance to the galaxy M87 in the Virgo cluster is 50 million light years.
• The distance to most distant object seen in the universe is about 18 billion light years (18 x 109 light years).

### The Parsec Distance Unit

The method of parallax gives rise to a natural distance unit that astronomers call the parsec (which we shall abbreviate as pc). The parsec is defined to be the distance at which a star would have a parallax angle p equal to one second of arc. From basic trigonometry we find that this distance is equal to 206,265 astronomical units (where the astronomical unit is the average separation of the Earth and the Sun) or 3.26 light years. One also commonly uses the kiloparsec (kpc) and the megaparsec (Mpc) as a distance unit, which correspond to 1000 and 1,000,000 parsecs, respectively. Although the light year is often found in popular level discussions, professional astronomers probably use the parsec, kiloparsec, and megaparsec more commonly as units of large distance. It is useful to remember the following average distance scales:
• The average separation between stars in a galaxy like our own is of order parsecs.
• The diameter of a galaxy like our own is typically of order 100 kpc.
• The separation between galaxies in a cluster of galaxies like our own local cluster is typically several Mpc.
• The separation between clusters of galaxies is typically of order 10 Mpc.
• The most distant galaxies observed are thousands of Mpc away from us.
Here is a concise summary of some distance scales commonly used in astronomy.

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