# General Q: How do you measure a Galaxy's distance?

If I am given the variable of time (observed), and diameter is it possible to measure the distance of a galaxy? Or do I need more variables?

Just trying to figure out the most simple way to go about doing something like that.

russ_watters
Mentor
If I am given the variable of time (observed), and diameter is it possible to measure the distance of a galaxy? Or do I need more variables?

Just trying to figure out the most simple way to go about doing something like that.
Time and diameter of what?

Time and diameter of what?

Im trying to figure out if its possible to measure the diameter of an object (in MPC)
With the variables time (say ive observed something for 10 years)
And after the 10 years I resolved that the diameter of the object is some angle in arc seconds.

Or would I need more information to figure out distance to that object?

russ_watters
Mentor
Im trying to figure out if its possible to measure the diameter of an object (in MPC)
With the variables time (say ive observed something for 10 years)
And after the 10 years I resolved that the diameter of the object is some angle in arc seconds.

Or would I need more information to figure out distance to that object?
Well, it doesn't take 10 years to measure the diameter, so that time doesn't really tell you anything. I don't see how this can lead to a distance measuremeng.

davenn
Gold Member
2021 Award
Just trying to figure out the most simple way to go about doing something like that.

hi, welcome to PF

General Q: How do you measure a Galaxy's distance?

The standard is to use Cepheid variable stars

do a google search on that and you will come up with lots of info

Dave

russ_watters and Nik_2213
mathman
For Galaxies that are far away, red shifts and IA supernovae brightness.

phyzguy
Calculating the distance from the angular size doesn't really work, because galaxies vary so much in physical size. For example, the Small Magellanic Cloud is a nearby galaxy that is about 7000 light-years across, and a large elliptical like M87 is over 200,00 light-years across. If you measure the angular size, how could you tell if it is physically small and nearby or physically large and further away?

davenn
LURCH
Are you asking about parallax ? It is possible to use time and the diameter of Earth’s orbit to measure the distance to far away objects. But this method is not used for determining the distance to other galaxies; those distances are much too far.

russ_watters and stefan r
stefan r
We could measure the distance to the galaxies with parallax. Launch 2 Gaia like probes out of the solar system in opposite directions. The probes would require a very large power supply and would need to function for a long time. Would also need a more powerful transmitter. Gaia cost over a $billion so not likely to happen soon. Parallax was used to determine that galaxies are in fact to far away to measure. We could measure the distance to the galaxies with parallax. Launch 2 Gaia like probes out of the solar system in opposite directions. The probes would require a very large power supply and would need to function for a long time. Would also need a more powerful transmitter. Gaia cost over a$billion so not likely to happen soon.