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

AI Thread Summary
To measure the distance to a galaxy, simply knowing the observed time and diameter is insufficient; additional variables are required. Standard methods for measuring galaxy distances include using Cepheid variable stars and redshift measurements from supernova brightness. Angular size alone is not reliable due to the vast variation in galaxy sizes, making it difficult to determine proximity. Parallax can be used for closer objects, but galaxies are typically too far for this method to be effective. Advanced techniques, such as launching probes for precise measurements, are currently impractical due to cost and technological limitations.
StillLearningToronto
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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.
 
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StillLearningToronto said:
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?
 
russ_watters said:
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 I've 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?
 
StillLearningToronto said:
Im trying to figure out if its possible to measure the diameter of an object (in MPC)
With the variables time (say I've 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.
 
StillLearningToronto said:
Just trying to figure out the most simple way to go about doing something like that.

hi, welcome to PF

to specifically answer your thread title ...

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
 
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For Galaxies that are far away, red shifts and IA supernovae brightness.
 
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?
 
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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.
 
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LURCH said:
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.

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.
 
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stefan r said:
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.

That got me thinking about a somewhat unrelated question. I wonder how far away we could triangulate with the current voyager baseline (not that they have the perfect telescopes for this purpose, but say they did). V1 is 1.8e13 m from earth, so a 1 arcsec quality parallax measurement would let you triangulate about 400 light years. Not very far, really. You would need 1/50 arcsecond measurements to even be able to measure the distance to the galactic center.
 

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