A light-year is a unit
of distance. It is the distance that light can travel in one year. Light moves
at a velocity of about 300,000 kilometers (km) each second. So in one
year, it can travel about 10 trillion km. More p recisely, one light-year is
equal to 9,500,000,000,000 kilometers.
Why would you want such
a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a
few hundred kilometers from New York City to Washington, DC; it is a few
thousand kilometers from California to Maine. In the universe, the
kilometer is just too small to be useful. For example, the distance to the next
nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km.
That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes
hard to write and hard to interpret. So astronomers use other units
of distance.
In our solar
system, we tend to describe distances in terms of the Astronomical
Unit (AU). The AU is defined as the average distance between the Earth and
the Sun. It is approximately 150 million km (93 million miles). Mercury can be
said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from
the Sun. The AU, however, is not big enough of a unit when we start talking
about distances to objects outside our solar system.
For distances to other
parts of the Milky Way Galaxy (or even further), astronomers use units of the
light-year or the parsec . The light-year we have already defined.
The parsec is equal to 3.3 light-years.
Using the light-year, we can say that :
- The Crab supernova remnant
is about 4,000 light-years away.
- The Milky Way Galaxy is about
150,000 light-years across.
- The Andromeda Galaxy is 2.3 million light-years away.
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