# Why can't astronomers use parallax to measure distances to other galaxies?

Nov 19, 2015

Parallax only works for relatively close stars in our own galaxy. Other galaxies are simply too far away.

#### Explanation:

Parallax works by measuring the apparent shift of an object against its background from two different vantage points. Astronomers make observations from Earth on either side of the sun.

The parallax formula gives the distance, $d$ to an object given the parallax angle, $p$. The distance is measured in parsecs, and the parallax angle is in arc-seconds. $1 \text{ parsec}$ is equal to about $3.3 \text{ light years}$.

$d = \frac{1}{p}$

The Andromeda Galaxy, M31, is the nearest major galaxy to the Milky Way. The distance to M31 has been measured using other techniques to be $2.5 \cdot {10}^{6} \text{ light years}$, or $7.6 \cdot {10}^{5} \text{ parsecs}$.

Using the slightly modified parallax formula, we can find the necessary parallax angle to measure the distance to Andromeda.

p = 1/d = 1/(7.6*10^5" parsecs") = 1.3*10^-6 " arc-seconds"

This is an incredibly small angle. For comparison, the resolution of the Hubble Space Telescope is $.05 \text{ arc-seconds}$, so even Hubble would not be able to detect the necessary angular shift of the nearest galaxy to effectively use parallax as a measure of its distance.