Units for Stellar 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 is abbreviated ly. A light year
is the distance that light travels in a
single year. Since light is very fast, the light year is a very large distance. From
our knowledge of light speed in a vacuum,
we can determine the length of a light year in kilometers simply by multiplying by
the number of seconds in a year:
| tropical year = 3.156 x 107 s |
speed of light (vacuum) = 3 x 105 km/s |
| 1 light year (ly) = (3.156 x 107 s) x (3 x 105 km/s)
= 9.46 x 1012 km |
|
Thus a light year is 9.46 x 1012 km.
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.
|
Some Typical Distances Expressed in Light Years
|
| Quantity |
Distance (ly)
|
| Nearest star |
4.2
|
| Diameter of our
galaxy |
100,000
|
| Distance to Virgo
Cluster |
50,000,000
|
| Most distant objects
seen |
18 x 109 |
|
|
|
|
|
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 4.2 light years away. The adjacent table gives a summary of some important distances
expressed in light years. It is useful to remember these approximate numbers, for they help
to give one a feel for typical distance scales in the Universe.
|
Common Distance Units in Astronomy
|
| Quantity |
Abbreviation |
Distance (km)
|
| astronomical unit |
AU |
1.50 x 108
|
| light year |
ly |
9.46 x 1012
|
| parsec |
pc |
3.08 x 1013
|
| kiloparsec |
kpc |
3.08 x 1016
|
| megaparsec |
Mpc |
3.08 x 1019
|
|
|
|
|
|
The Parsec Distance Unit
The method of parallax suggests 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
(see this
animation)
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; the parsec is also equivalent to
3.26 light years. We also commonly use the kiloparsec
(kpc) and the megaparsec (Mpc) as distance units; these correspond to a thousand
and a million parsecs, respectively. The above left table gives some common distance units
in astronomy expressed in kilometers.
It is also useful to remember the following average distance scales
expressed in parsecs and its multiples.
|
Some Typical Distances Expressed in Parsecs
|
| Quantity |
Distance
|
| Average separation between stars
in galaxy |
1 pc
|
| Diameter of a large spiral
galaxy |
100 kpc
|
| Separation between galaxies in a
cluster of galaxies |
several Mpc
|
| Separation between clusters of
galaxies in a supercluster |
10 Mpc
|
| Most distant galaxies
observed |
thousands of Mpc
|
|
|
|
|
|