<p><a href="https://en.wikipedia.org/wiki/Surface_brightness" rel="nofollow noreferrer">Surface brightness</a>
of galaxies and nebulae is typically expressed as visual magnitude per square arcsecond.
<a href="https://en.wikipedia.org/wiki/Skyglow" rel="nofollow noreferrer">Skyglow</a>
is often quantified in the same way.
Given the angular dimensions of the frame and the magnitudes of a few stars in it, photometry software could compute sky brightness by excluding all visible stars and dividing the total brightness of the remainder by its angular area.</p>

<p>Here is an
<a href="http://www.unihedron.com/projects/darksky/magconv.php" rel="nofollow noreferrer">online tool to convert</a>
between $\mathrm{mag} / \mathrm{arcsec}^2$ and $\mathrm{cd} / \mathrm{m}^2$.</p>

CC BY-SA 3.0
2017-02-21T18:13:06.930Z
+1. I've never seen cd/m² used in an astronomical context, but mag/arcsec is quite common (I'm not an observer, though).
pela
CC BY-SA 3.0
2017-02-21T17:45:57.870Z
2017-02-21T18:11:55.787Z
2017-02-21T18:11:55.787Z
Mike G
<p>The brightness of a diffuse area is it's luminance, and can be quoted in units of $cd\,m^{-2}$. Measuring it would require knowledge of the properties of the camera: the sensitivity (at various wavelengths) of the ccd, the aperture, length of exposure and so on. This could probably be found using calibration images, in which a known luminance is imaged and then the brightness of the skyglow is compared.</p>

<p>The brightness of a star is usually given as a magnitude which is related to the amount of electromagnetic energy received from the star per second, in a particular range of wavelengths.</p>

<p>It's unclear what you mean by "brightness due to skyglow". The brightness of stars is not due to skyglow, but excessive skyglow can mask dimmer stars and diffuse objects.</p>

CC BY-SA 3.0
2017-02-21T06:42:17.970Z
2017-02-21T17:13:03.173Z
2017-02-21T17:13:03.173Z
James K
<p>What is the name and unit of the quantity in which star brightness due to skyglow is measured? How can it be measured from RAW image files of stars?</p>

<star><astrophysics><photometry>
What is the unit used for star brightness due to skyglow?
Shashank Acharya
# What is the unit used for star brightness due to skyglow?

(Asked by: username_0 on 2017-02-21)

What is the name and unit of the quantity in which star brightness due to skyglow is measured? How can it be measured from RAW image files of stars?

**Tags:** star, astrophysics, photometry


---

## 2 answers

### Answer by username_1 on 2017-02-21. Score: 3 | Accepted answer

[Surface brightness](https://en.wikipedia.org/wiki/Surface_brightness)
of galaxies and nebulae is typically expressed as visual magnitude per square arcsecond.
[Skyglow](https://en.wikipedia.org/wiki/Skyglow)
is often quantified in the same way.
Given the angular dimensions of the frame and the magnitudes of a few stars in it, photometry software could compute sky brightness by excluding all visible stars and dividing the total brightness of the remainder by its angular area.

Here is an
[online tool to convert](http://www.unihedron.com/projects/darksky/magconv.php)
between $\mathrm{mag} / \mathrm{arcsec}^2$ and $\mathrm{cd} / \mathrm{m}^2$.

> **Comments:**
> **username_2**: +1. I've never seen cd/m² used in an astronomical context, but mag/arcsec is quite common (I'm not an observer, though).  

---

### Answer by username_3 on 2017-02-21. Score: 4

The brightness of a diffuse area is it's luminance, and can be quoted in units of $cd\,m^{-2}$. Measuring it would require knowledge of the properties of the camera: the sensitivity (at various wavelengths) of the ccd, the aperture, length of exposure and so on. This could probably be found using calibration images, in which a known luminance is imaged and then the brightness of the skyglow is compared.

The brightness of a star is usually given as a magnitude which is related to the amount of electromagnetic energy received from the star per second, in a particular range of wavelengths.

It's unclear what you mean by "brightness due to skyglow". The brightness of stars is not due to skyglow, but excessive skyglow can mask dimmer stars and diffuse objects.


---

