About Stars (some basics) and Describing Sky Glow in a Nut Shell
Light Pollution is not just about being able to see stars, but that is one way we measure it. So a little about stars for the uninitiated seems in order.
Stars are great balls of gas that typically collapse under the pull of gravity inside large clouds of gas and dust in areas sometimes referred to as stellar nurseries. The crushing pull of gravity builds heat and pressure in the core until nuclear fusion occurs. Fusion converts lighter elements, starting with hydrogen, into heavier elements producing prodigious amounts of energy in the process. The energy makes the star shine but also balances the pull of gravity stabilizing the stars contraction. All the elements in the universe heavier than hydrogen are the product of this process that occurred in earlier generations of stars. We are all made of star dust.
Stars come in a great range of sizes, colors, and brightness. Really massive stars are blue in color during most of their relatively short lives. While smaller stars, and all other stars late in their life cycle, are reddish. Our sun is a middling size star glowing with a yellowish light. But, how bright a star appears in the sky is a function of distance as well as intrinsic or “absolute magnitude”. A really bright star may be so because it is close, while a dim star may be a massive blue super giant farther away. When accessing sky glow we are only concerned with the apparent brightness of stars. Under a perfectly dark sky – one without any artificial or moon light – there are about ten thousand stars visible to the naked eye.
How bright stars appear in the sky (apparent magnitude) is described by a magnitude system that, like so much dealing with stars, is rooted in history. Hipparchus created the first substantial catalog of stars in about 129 BC. He classified the brightest stars as first magnitude, another dimmer group, whish judged to be half as bright, second magnitude and so forth down to sixth magnitude, the limit of most people’s vision under a naturally dark sky. It turns out, due to a characteristic of the human eye, each magnitude is actually brighter than the one below it by 2.512 time such that a sixth magnitude star is 100 times dimmer than a first magnitude star, while a seventh magnitude would be 251 times dimmer and an 8th would be 631 times etc.
So how dark is my sky?
The dimmer the star the easier it is to hide it with ambient light in the sky. So a “sixth magnitude sky” is a sky in which stars of a sixth magnitude can be seen near the zenith with the naked eye on a clear moonless night. This is how sky glow is measured in the Globeatnight program and with the Loss of the Night Application described in our section on how to monitor skyglow. This method is often referred to as the Naked Eye Limit Magnitude or NELM. Needless to say, such a method is fairly subjective but useful.
One can also measure skyglow using a meter. In such cases, the sky’s brightness is usually expressed either in terms of candelas (a standard measure of luminance) per square meter of sky, or more commonly, in terms of what magnitude star it would take to equal the measured brightness if each arcsecond of the sky had one star of the stated magnitude.
Another popular way of describing a sky’s brightness, as opposed to measuring it, is the so-called Bortle scale, which ranks a sky in one of nine categories based upon an overall all visual assessment of its characteristics, such as what structures and general features can be observed.
The following table shows how all the principal ways of characterizing light pollution and how they equate to one another. As shown here, a naturally dark sky at sea level equates to about a 21.63 magnitude star per arcsecond of sky, which is just over two ten thousands of a candela per sq. Meter. That falls at the border between Bortle class 2 and 1, where a typical naked eye observer should just be able to make out a 6.8 magnitude star.*
So now that we have walked through the weeds, let’s climb back out before we get confused. If you are using the Globeatnight visual method you will be reporting the naked eye limited magnitude of the sky as shown on the chart you find best represents the stars you can see. If you are using a meter you will likely be reporting measured luminance expressed in stellar magnitude per arcsecond (mpsas).
*Some Bortle charts and Bortle himself assigned a much more ambitious assumption about the faintest stars visible in each Bortle scale level that this chart applies. Such limits may not represent what the average person can see and can be confusing. When accessing the Bortle scale it is best to rely upon the described visual characteristics of the sky or measured luminance and not the state stellar magnitude.