<p>The volume isn't well-defined, because the radius isn't. There are practical values for the radius such as scale length/height, half-light radius, <span class="math-container">$R_{200}$</span>, etc. that are commonly used, depending on what you're interested in. But the densities never reach zero, so any value for the radius will be arbitrary. Moreover, you always only observe a 2D projection of a galaxy, so the third dimension is even more uncertain.</p>

<p>At any rate, I can't think of any use of the volume of a galaxy. Various scaling relations exist between the radius and quantities such as mass, concentration, rotation velocity, etc., and of course such scaling relations also correlate with volume, but since volume is more uncertain than radius, I don't see a reason for trying to estimate it.</p>

<p>Sometimes one is interested in the <em>volume filling factor</em> of various components of the galaxy, e.g. the volume fraction occupied by warm gas, hot gas, or molecular clouds. But in this case there's no need to define a precise volume of the total galaxy; one may just consider "a representative part" of it.</p>

CC BY-SA 4.0
2020-03-11T21:51:38.873Z
2020-03-11T21:51:38.873Z
pela
<p>Pretty much awkward question though. I would like to ask how is the volume of a Galaxy X is measured? For example consider milky way, what are the techniques to measure the same? How accurate and precise are the obtained results? What instruments are used for the same? When was this techniques developed? What are he beneficial outcomes of determining the GALAXIES' volume?</p>

CC BY-SA 4.0
2020-03-05T19:52:38.910Z
Is there a particular example of the volume of a galaxy that you have seen, and provoked this question.  I'm not sure if "volume" is well defined for galaxies.
James K
CC BY-SA 4.0
2020-03-05T22:29:52.890Z
If you can't find an example of a galactic volume to support your question, you can change it to ask "How might the volume of a galaxy be defined?" But then people will ask how you plan on using it because you could use a gravitational definition or a density definition.
uhoh
CC BY-SA 4.0
2020-03-06T12:17:18.380Z
People, I don't believe that there should be a edit in the question. However, I do have mentioned the questions concerned....
Pranay
CC BY-SA 4.0
2020-03-09T22:47:57.003Z
To calculate the volume, you first need to define the boundary. Are you including outermost *stars*, or the gas envelope that extends much further out? What about the dark matter halo?
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-03-09T22:51:15.527Z
You might find the answers to [this question](https://astronomy.stackexchange.com/q/18220/14094) relevant.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2020-11-17T16:17:09.010Z
Hi Praney, did the answer below answer your question? Otherwise feel free to ask for clarification :)
pela
<galaxy><distances><milky-way><size><local-group>
Volume of Galaxies
Pranay
# Volume of Galaxies

(Asked by: username_0 on 2020-03-05)

Pretty much awkward question though. I would like to ask how is the volume of a Galaxy X is measured? For example consider milky way, what are the techniques to measure the same? How accurate and precise are the obtained results? What instruments are used for the same? When was this techniques developed? What are he beneficial outcomes of determining the GALAXIES' volume?

**Tags:** galaxy, distances, milky-way, size, local-group
> **Comments:**
> **username_1**: Is there a particular example of the volume of a galaxy that you have seen, and provoked this question.  I'm not sure if "volume" is well defined for galaxies.  
> **username_2**: If you can't find an example of a galactic volume to support your question, you can change it to ask "How might the volume of a galaxy be defined?" But then people will ask how you plan on using it because you could use a gravitational definition or a density definition.  
> **username_0**: People, I don't believe that there should be a edit in the question. However, I do have mentioned the questions concerned....  
> **username_3**: To calculate the volume, you first need to define the boundary. Are you including outermost *stars*, or the gas envelope that extends much further out? What about the dark matter halo?  
> **username_3**: You might find the answers to [this question](https://astronomy.stackexchange.com/q/18220/14094) relevant.  
> **username_4**: Hi Praney, did the answer below answer your question? Otherwise feel free to ask for clarification :)  

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## 1 answers

### Answer by username_4 on 2020-03-11. Score: 3

The volume isn't well-defined, because the radius isn't. There are practical values for the radius such as scale length/height, half-light radius, $R\_{200}$, etc. that are commonly used, depending on what you're interested in. But the densities never reach zero, so any value for the radius will be arbitrary. Moreover, you always only observe a 2D projection of a galaxy, so the third dimension is even more uncertain.

At any rate, I can't think of any use of the volume of a galaxy. Various scaling relations exist between the radius and quantities such as mass, concentration, rotation velocity, etc., and of course such scaling relations also correlate with volume, but since volume is more uncertain than radius, I don't see a reason for trying to estimate it.

Sometimes one is interested in the *volume filling factor* of various components of the galaxy, e.g. the volume fraction occupied by warm gas, hot gas, or molecular clouds. But in this case there's no need to define a precise volume of the total galaxy; one may just consider "a representative part" of it.


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