<p>You would need to find the distance from one to the other.  That is a mathematical exercise in using the cosine rule.</p>
<p>You then use magnitude formulae.  For convenience you could calculate the absolute magnitude of each galaxy:</p>
<p><span class="math-container">$$M_{\text{abs}} = m_{\text{app}} -5(\log_{10}(d_{\text{parsec}}) -1)$$</span></p>
<p>And then the same formula, rearranged to find the apparent magnitude of Andromeda from Triangulum.</p>
<p>One factor that is not accounted for is extinction due to dust and gas in the galaxy.  If you view a galaxy from the edge, much of its light is absorbed by dust in the galaxy, so the absolute magnitude of a galaxy will vary according to the direction from which it is viewed. I've made no attempt to adjust for that.</p>

CC BY-SA 4.0
2021-11-20T16:01:38.720Z
2021-11-20T16:01:38.720Z
James K
<p>The given apparent magnitude of M31 galaxy is 3.4<span class="math-container">$^m$</span>, M33 - 5.7<span class="math-container">$^m$</span>. I wonder how can I calculate the apparent magnitude of one of them (ex. M33) if I imagine myself being in the other (ex. M31)?</p>

<galaxy><apparent-magnitude>
How to find magnitude of one galaxy being a hypothetical observer in the other?
ALiCe P.
# How to find magnitude of one galaxy being a hypothetical observer in the other?

(Asked by: username_0 on 2021-11-20)

The given apparent magnitude of M31 galaxy is 3.4$^m$, M33 - 5.7$^m$. I wonder how can I calculate the apparent magnitude of one of them (ex. M33) if I imagine myself being in the other (ex. M31)?

**Tags:** galaxy, apparent-magnitude

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## 1 answers

### Answer by username_1 on 2021-11-20. Score: 1 | Accepted answer

You would need to find the distance from one to the other. That is a mathematical exercise in using the cosine rule.

You then use magnitude formulae. For convenience you could calculate the absolute magnitude of each galaxy:

$$M\_{\text{abs}} = m\_{\text{app}} -5(\log\_{10}(d\_{\text{parsec}}) -1)$$

And then the same formula, rearranged to find the apparent magnitude of Andromeda from Triangulum.

One factor that is not accounted for is extinction due to dust and gas in the galaxy. If you view a galaxy from the edge, much of its light is absorbed by dust in the galaxy, so the absolute magnitude of a galaxy will vary according to the direction from which it is viewed. I've made no attempt to adjust for that.


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