Naked Eye Limiting Magnitude: Assessing Sky Brightness

There are a variety of ways of measuring your night sky quality, and one of the most effective ways is by looking for the faintest star you can find with your naked eye, and noting its brightness, or magnitude. This provides what is known as Naked Eye Limiting Magnitude, NELM.

Of course just randomly casting about the sky for faint stars can lead you on a merry chase, and so a very useful method is to use one specific constellation – one you can always see, no matter what time of year – and look only at stars within that one constellation. This narrows the field somewhat, and makes your task that much easier.

For observers in Europe and North America the constellation of Ursa Minor, the Little Bear, provides an excellent choice for estimating NELM.

The overall shape of Ursa Minor is made up of seven bright-ish stars, but around and amongst these are many more fainter stars.

Ursa Minor

Bright Star Name (Bayer Designation)	Magnitude
Polaris (α)	1.95
Kocab (β)	2.05
Phercab (γ)	3.00
Yildun (δ)	4.35
Urodelus (ε)	4.20
Ahfa al Farkadain (ζ)	4.25
Anwar al Farkadain (η)	4.95

Even some of these “brighter” stars might not be visible from city centres. For example, if you are observing from a site with Bortle Class 8 you would not see η-UMi, while those unhappy stargazers under a Bortle Class 9 sky would only be able to pick out the three brightest stars, α-, β-, and γ-UMi. Only at Bortle Class 7 and darker will you make out all seven of the main stars of Ursa Minor.

But what if you’re at a good dark sky site? Well, you’re going to need a longer list of magnitudes, and a more detailed map of Ursa Minor.

Star Number on Above Map	Star Name	Visual Magnitude	Bortle Class
1	α UMi	1.95	9
2	β UMi	2.05	9
3	γ UMi	3.00	9
4	ε UMi	4.20	8
5	5 UMi	4.25	8
6	ζ UMi	4.25	8
7	δ UMi	4.35	8
8	4 UMi	4.85	7
9	η UMi	4.96	7
10	θ UMi	5.00	7
11	11 UMi	5.02	6
12	19 UMi	5.45	6
13	24 UMi	5.75	5
14	λ UMi	6.30	4
15	20 UMi	6.35	4
16	3 UMi	6.40	4
17	π1 UMi	6.55	3
18	HIP74818	6.65	3
19	14 UMi	7.35	2

The stars in the map and table above have been numbered (by me – these aren’t official designations) from 1 to 19, with 1 (Polaris) being the brightest, and 19 (14 UMi) being the dimmest. You will only be able to see all 19 numbered stars from exceptionally dark places, virtually free of light pollution, what Bortle called “typical truly dark sky sites”. From my garden in the outskirts of a major city I can see numbers 11 and 12, but not number 13, giving me an NELM of 5.45.

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