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
- 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:
Here is a concise summary of some distance scales
commonly used in astronomy.
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.