Star Brightness

Today we measure Star Brightness with a system that was invented by the great scientist, mathematician, and astronomer of Ancient Greece — Hipparchus.

Hipparchus was truly ahead of his time. In my opinion, relative to the level of knowledge, skills, and technology of his time, he is unquestionably among the leading contenders for greatest scientist of all time — where Isaac Newton and Albert Einstein are at the pinnacle by general consensus — one of the GOATs of science.

This is the system of magnitudes. In the second century BCE Hipparchus categorized the stars into groups based on their brightness — in this case the brightness of the star as they appeared to the naked eye from the perspective of an observer on the Earths surface.

Hipparchus divided the stars into 6 magnitudes. The brightest stars are 1st magnitude, and the fainter a star appears the higher its magnitude, all the way from 1st magnitude (the brightest) to 6th magnitude (the dimmest).

Around 2000 years later this system was still in use, and had been refined significantly to make it even more precise. By 1856 Norman Dodgson has made the system of magnitudes even more precise by specifically quantifying star brightness — a 1st magnitude star is the brightest, while a 2nd magnitude star is 2.5x fainter, and a 3rd magnitude star is 2.5x fainter than a 2nd magnitude star, and so on, resulting in a 1st magnitude star being 100x brighter than a 6th magnitude star. [1]

Norman Dodgson defined the stars such as Polaris as 2nd magnitude. Vega (the brightest star in the Summer Triangle) is 0, Sirius (the brightest star in the sky aside from the Sun) is -1.4. The Full Moon (the brightest object you will see regularly at night) is -12.6, while the Sun is -26.8. [1]

Apparent Magnitudes

The downside of the system described above is that it is extremely subjective when it was invented because there wasn’t any other technology available at the time to make measurements more precise — more importantly, it is not actually true.

In Astronomy there is a difference between the Apparent Magnitude and the Real Magnitude of a star. The apparent magnitude is what is being discussed above because it relates the brightness of a star as viewed from the surface of the Earth. Since many of the stars that we see are incredibly far away (and in some cases these objects are not stars at all, but entire galaxies) if they were as close to the Earth as the Sun they would be so bright that they would literally roast the planet.

The Effect Of Distance On Star Magnitude

What it comes down to is the effect of distance on the intensity of a light source. When you check if your flashlight is working by switching it on while staring at the bulb from point-blank range (as we do from time-to-time even though we know better) the light is very bright.

Take that same flashlight a few hundred meters away, and that tiny bulb is but a faint pin-prick of light.

In Astronomy, the implication of this is that for thousands of years we never really knew how bright any star actually was — how luminous — the actual energy output of a star in the form of a numerical value for light-energy.

In order to arrive at that value, we would first need to map out the cosmos, to a degree. Determine the distance to nearby stars so that the relationship between their apparent magnitude and distance from us could be calculated, in order to arrive at a quantity called absolute magnitude.

Absolute Magnitude

Absolute magnitude is the absolute value for star brightness — how much light-energy a star actually outputs.

This value is defined as the apparent magnitude possessed by a star if viewed from exactly 10 parsecs (33 ly) away. To put this number in perspective, the closest star to us is is about 4 light-years (just over 1 parsec) away from us — so the absolute magnitude of α Cent is actually calculated as if it was further away from us than it is.

Resources

1. A Guide To Skywatching. David H. Levy. 1994, 2002.