<p>I think your last sentence summarizes perfectly units in astronomy: <em>it seems like a huge mess</em>.</p>
<p>Your table shows correctly some various units for measuring &quot;things that emit light&quot;, but there are many others. Most of your units are &quot;bolometric&quot;, i.e. they measure that total amount of energy across the full electromagnetic spectrum. But a spectrum gives you the amount of light emitted <em>per wavelength bin</em> (or something equivalent; see below). In your example #2, you have such a spectrum <span class="math-container">$M(\lambda,T)$</span> which, weighted by a response function <span class="math-container">$V(\lambda)$</span>, is then integrated to give you a (semi-)bolometric luminosity (&quot;semi-&quot; because it's only integrated over the visible range).</p>
<p>In your example, <span class="math-container">$M$</span> is a function of wavelength, which is common in the visible range, and also ultraviolet and infrared. In the high-energy end (X-rays and gamma-rays) you would usually use <em>energy</em> (measured in keV or Mev), whereas in microwave and the radio-regime frequency is preferred (measured e.g. in GHz). Sometimes you even measure in &quot;velocity bins&quot; (using <span class="math-container">$\mathrm{km}\,\mathrm{s}^{-1}$</span>) because velocities Doppler shift the spectrum.</p>
<p>Note also that <em>usually</em> (but not always) <a href="https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units" rel="nofollow noreferrer">cgs</a> units is preferred over <a href="https://en.wikipedia.org/wiki/International_System_of_Units" rel="nofollow noreferrer">SI</a> units. That is, luminosity is usually measured in <span class="math-container">$\mathrm{erg}\,\mathrm{s}^{-1}$</span>, not <span class="math-container">$\mathrm{W}$</span>, flux is measured in <span class="math-container">$\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$</span>, not <span class="math-container">$\mathrm{W}\,\mathrm{m}^{-2}$</span>, etc.</p>

CC BY-SA 4.0
2023-10-27T10:11:35.283Z
2023-10-27T10:11:35.283Z
pela
<p>I'm a big fan of astrophysics and astronomy. However i'm quite confused by reading these last years plenty of books on the subject with various definitions (vocabulary) and notations for the same topic. For my personal usage - to not be lost again ... - i have tried summarizing the different bibliographic ressources vocabulary and definitions (cross-checking also with Wikipedia) in the table below:</p>
<p><a href="https://i.sstatic.net/OAE2e.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OAE2e.jpg" alt="enter image description here" /></a></p>
<p>Is it correct or did i summarized something wrong? And by the way do we have in Astronomy an ISO standard that defines worldwide the vocabulary, units and mathematical definitions of all these concepts or not (like we do have in physics?) because actually it seems like a huge mess.</p>
<p>Thanks for your help</p>

<luminosity><apparent-magnitude><magnitude><absolute-magnitude><brightness>
Standard definitions (ISO?) about subjects related to light emitted by stars
Vincent ISOZ
# Standard definitions (ISO?) about subjects related to light emitted by stars

(Asked by: username_0 on 2023-10-27)

I'm a big fan of astrophysics and astronomy. However i'm quite confused by reading these last years plenty of books on the subject with various definitions (vocabulary) and notations for the same topic. For my personal usage - to not be lost again ... - i have tried summarizing the different bibliographic ressources vocabulary and definitions (cross-checking also with Wikipedia) in the table below:

[![enter image description here](https://i.sstatic.net/OAE2e.jpg)](https://i.sstatic.net/OAE2e.jpg)

Is it correct or did i summarized something wrong? And by the way do we have in Astronomy an ISO standard that defines worldwide the vocabulary, units and mathematical definitions of all these concepts or not (like we do have in physics?) because actually it seems like a huge mess.

Thanks for your help

**Tags:** luminosity, apparent-magnitude, magnitude, absolute-magnitude, brightness

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## 1 answers

### Answer by username_1 on 2023-10-27. Score: 3

I think your last sentence summarizes perfectly units in astronomy: *it seems like a huge mess*.

Your table shows correctly some various units for measuring "things that emit light", but there are many others. Most of your units are "bolometric", i.e. they measure that total amount of energy across the full electromagnetic spectrum. But a spectrum gives you the amount of light emitted *per wavelength bin* (or something equivalent; see below). In your example #2, you have such a spectrum $M(\lambda,T)$ which, weighted by a response function $V(\lambda)$, is then integrated to give you a (semi-)bolometric luminosity ("semi-" because it's only integrated over the visible range).

In your example, $M$ is a function of wavelength, which is common in the visible range, and also ultraviolet and infrared. In the high-energy end (X-rays and gamma-rays) you would usually use *energy* (measured in keV or Mev), whereas in microwave and the radio-regime frequency is preferred (measured e.g. in GHz). Sometimes you even measure in "velocity bins" (using $\mathrm{km}\,\mathrm{s}^{-1}$) because velocities Doppler shift the spectrum.

Note also that *usually* (but not always) [cgs](https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units) units is preferred over [SI](https://en.wikipedia.org/wiki/International_System_of_Units) units. That is, luminosity is usually measured in $\mathrm{erg}\,\mathrm{s}^{-1}$, not $\mathrm{W}$, flux is measured in $\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{cm}^{-2}$, not $\mathrm{W}\,\mathrm{m}^{-2}$, etc.


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