<p>The D25 diameter is defined in terms of the diameter of a certain isophote: specifically, as you note, the isophote where the <span class="math-container">$B$</span>-magnitude surface brightness has a value of 25 mag arcsec<span class="math-container">$^{-2}$</span>.</p>
<p>The system traditionally used for this is the standard <a href="https://en.wikipedia.org/wiki/UBV_photometric_system" rel="noreferrer">Johnson</a> <span class="math-container">$B$</span>-band, where the magnitude <span class="math-container">$m$</span> is defined as</p>
<p><span class="math-container">$m = -2.5 \log_{10} (F_{\rm object} / F_{0})$</span></p>
<p>where <span class="math-container">$F_{\rm object}$</span> is the flux of the object and <span class="math-container">$F_{0}$</span> is the reference flux; in origin, this is the flux from the star Vega observed with the <span class="math-container">$B$</span> filter. I believe this is <a href="https://lweb.cfa.harvard.edu/%7Edfabricant/huchra/ay145/mags.html" rel="noreferrer">usually defined</a> as <span class="math-container">$F_{0} = 4260$</span> <a href="https://en.wikipedia.org/wiki/Jansky" rel="noreferrer">Janskys</a> (Jy).</p>
<p>So, working out the math, <span class="math-container">$m_{B} = 25$</span> corresponds to a flux of <span class="math-container">$4.26 \times 10^{-7}$</span> Jy.</p>

CC BY-SA 4.0
2024-09-12T10:27:09.693Z
2024-09-12T10:27:09.693Z
Peter Erwin
<p>Every site and AI and Google say that one way of measuring a galaxy's diameter is in terms of its isophotal diameter, using the blue D25 standard, using units of, &quot;25 mag per arcsec2&quot;.</p>
<p>But magnitude in astrophysics is a relative standard, so to speak, so...</p>
<p>How much brightness is '25 magnitudes'?</p>

CC BY-SA 4.0
2024-09-12T05:09:58.870Z
Magnitude is not relative
planetmaker
<observational-astronomy><astrophysics><apparent-magnitude><absolute-magnitude><brightness>
What is the unit for 'magnitude' in terms of the Isophotal diameter of a galaxy?
Kurt Hikes
# What is the unit for 'magnitude' in terms of the Isophotal diameter of a galaxy?

(Asked by: username_0 on 2024-09-12)

Every site and AI and Google say that one way of measuring a galaxy's diameter is in terms of its isophotal diameter, using the blue D25 standard, using units of, "25 mag per arcsec2".

But magnitude in astrophysics is a relative standard, so to speak, so...

How much brightness is '25 magnitudes'?

**Tags:** observational-astronomy, astrophysics, apparent-magnitude, absolute-magnitude, brightness

> **Comments:**
> **username_1**: Magnitude is not relative  

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## 1 answers

### Answer by username_2 on 2024-09-12. Score: 5

The D25 diameter is defined in terms of the diameter of a certain isophote: specifically, as you note, the isophote where the $B$-magnitude surface brightness has a value of 25 mag arcsec$^{-2}$.

The system traditionally used for this is the standard [Johnson](https://en.wikipedia.org/wiki/UBV_photometric_system) $B$-band, where the magnitude $m$ is defined as

$m = -2.5 \log\_{10} (F\_{\rm object} / F\_{0})$

where $F\_{\rm object}$ is the flux of the object and $F\_{0}$ is the reference flux; in origin, this is the flux from the star Vega observed with the $B$ filter. I believe this is [usually defined](https://lweb.cfa.harvard.edu/%7Edfabricant/huchra/ay145/mags.html) as $F\_{0} = 4260$ [Janskys](https://en.wikipedia.org/wiki/Jansky) (Jy).

So, working out the math, $m\_{B} = 25$ corresponds to a flux of $4.26 \times 10^{-7}$ Jy.


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