Expert: Courtney Seligman - 11/17/2008

Question
What exactly is a parsec? I don't understand why astronomers would use this
instead of light-years, it seems so much more complicated.

Answer
When measuring the distance of a star, its apparent displacement in direction, due to our motion around the Sun, is used to measure its "parallax". This measurement is then converted to a distance using the following formula:

Distance = (some constant) / parallax

The value of the constant depends upon the units used for the distance, and for introductory lecture purposes, it seems simpler to define a unit (the parsec) for which the constant is one, so the equation can be written as

Distance (in parsecs) = 1 / parallax

There's nothing that says you have to use parsecs. You can use any unit you want for the distance; it's just that then, you have to use a constant corresponding to that unit of distance.

For kilometers, the constant is about 30 trillion.
For miles, since they're larger than kilometers, the constant is smaller -- only about 20 trillion.
For Astronomical Units (the size of the Earth's orbit), which are much larger yet, the constant is 206,000.
For light years, the constant is 3.26.
And for parsecs, since that's the way they're defined, the constant is 1.

In my classes, I discuss the reasoning behind using parsecs, since that's the way the topic is presented in the textbook; but I always express distances in light years when I can, because they're easier to comprehend, and automatically give you the "look-back" time associated with the object you're observing. So, for example, the Eagle Nebula (where the so-called Pillars of Creation are located), is about 7000 light years away, so we see it as it was 7000 years ago. (As it happens, the Pillars were destroyed about a thousand years ago, by the shock wave from a nearby supernova; but we won't see that for another 6000 years.)

Courtney Seligman
Professor of Astronomy
Long Beach City College