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The Question

(Submitted February 26, 1998)

I have searched everywhere, and cannot find the measurement of distance in space. What is it? Light years, kilometers, what?

The Answer

The common distance measures we use depend on what we are measuring.

1. For distances within our solar system, or other solar systems, the common unit is the 'Astronomical Unit' (A.U.)

1 A.U. = the average distance between the Earth and the Sun

2. For most everything else, stars, galaxies etc..., the distance unit is the parsec (pc). This is a convenient unit when measuring distances to stars by triangulation (what astronomers call parallax).

1 pc = 3.26 light years = about the distance to the nearest star

1 pc = 60 x 60 x 180/pi A.U. = 206265 A.U. --- by definition.

for distances within our galaxy or other galaxies it is kiloparsecs (kpc):

1 kpc = 1,000 pc

for distances between galaxies, and cosmology it is Megaparsecs (Mpc):

1 Mpc = 1,000,000 pc 3. The exception to these is when one is studying smaller object, such as a star or a planet. Then we might use kilometers. For dust grains, we might use microns (1/1,000,000 of a meter).

4. It is also common to compare objects. For example, if one is studying a star one might say "its radius is 5 solar radii", meaning it is 5 times the size of our sun. Similarly with galaxies, is it bigger or smaller than the Milky Way.

So, all in all, we use many units is astronomy. But, all in all, the parsec is the most common.

Note, astronomers only use light-years when talking to the general public or teaching classes.

Thanks for the question.

Good luck,

Jonathan Keohane
for Ask an Astrophysicist

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