The Question
(Submitted April 20, 1998)
What is the error factor in the distances to galaxies? For instance, the
Andromeda galaxy (M31) is 2.8 million light years. How accurate is that?
Is the error plus or minus 1 million light years or what? And does that
error factor get much worse the farther out we go?
The Answer
Nearby galaxies, where you can see individual stars, probably have about a
10% distance uncertainty.
For galaxies at distances where you can't see individual stars, the
distance is found by multiplying the redshift by a number called the Hubble
Constant H0. The value of H0 is a contentious issue, but the two extreme
camps are arguing for ~55 or ~70 km/s/Mpc, which means that there is about
a 30% range in how far away people think any given galaxy is. The accuracy
with which the redshift is measured doesn't depend much on the distance to
the galaxy, so this is a constant factor: you know that one galaxy is 3.0
times as far away as another, and not 3.1 or 2.9 times.
At distances which are a good fraction of the age of the universe away, the
question is how constant the Hubble constant is. Depending on how much
mass there is in the universe (and thus how much the universe has been
slowed down by gravityso how much faster it was expanding in the early
days) this can add an additional uncertainty range of 50%.
David Palmer
for Ask an Astrophysicist
