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

1 Answer
Nov 19, 2015

Answer:

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.

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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" parsec"# is equal to about #3.3 " light years"#.

#d = 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 *10^6" light years"#, or #7.6*10^5 " parsecs"#.

http://www.space.com/29378-andromeda-galaxy-larger-than-thought.html

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 " 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.