Star magnitudes

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In addition to their myriad numbers, stars also come in a wide range of brightnesses. It is very often much easier to find a particular star or star group if you can determine beforehand how bright it is. To do so, we need some standard scale of brightnesses to use as a gauge.

In the second century B.C., the ancient Greek astronomer Hipparcos became the first to develop a qualitative scale for determining the brightness of a star. In the Hipparcos system, naked-eye stars were sorted into six different classes of brightnesses, or magnitudes. Twenty of the brightest stars were given first-magnitude status. The faintest stars that could be discerned were designated magnitude 6. Stars that fell between these two extremes were given intermediate magnitudes -2, 3, 4, and 5.

With the advent of the telescope, however, countless stars fainter than magnitude 6 were observed and their brightnesses had to be measured more precisely. By the middle of the nineteenth century, astronomers agreed to refine the magnitude scheme in order to make it a quantitative rather than a qualitative scale. A difference of a single magnitude now corresponds to a brightness ratio of 2.5. Hence, a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2, and 100 times brighter than a star of magnitude 6.

The original 20 first-magnitude stars ranged so widely in brightness that the very brightest, by virtue of the revised magnitude system, were assigned negative and zero values. In order of diminishing brightness, these are: Sirius, -1.46; Canopus, -0.72; Arcturus, -0.04; Rigel Kentaurus (Alpha Centauri), 0.00; Vega, 0.03; Capella, 0.08; Rigel, 0.12; Procyon, 0.38; Achernar, 0.46; Betelgeuse, 0.50; Hadar, 0.61; Altair, 0.77; Aldebaran, 0.85; and Antares, 0.96. The star Spica, at 0.98, is almost exactly a magnitude 1 star.

If it shines or reflects light in the sky, it has an apparent magnitude, not only the planets, the Moon, and the Sun, but meteors and artificial satellites as well. Venus, at its brightest, is magnitude -4.7; the Moon is magnitude -13; and the Sun is magnitude -26.5. On this scale, the Sun is about 9 billion times brighter in light output than Sirius, which is 25 magnitudes fainter. Sirius, on the other hand, is about a trillion times brighter than the faintest star (magnitude 28.5) visible with the Hubble Space Telescope.

Occasionally, I'll describe objects that can best be seen in binoculars or a small telescope. Whereas the naked eye can see only to magnitude 6 or so, a good pair of binoculars in dark skies can see stars as faint as magnitude 10; a 4-inch telescope, magnitude 12; an 8-inch, magnitude 14. Even if objects can be glimpsed with the naked eye, like the Andromeda Galaxy or the Orion Nebula, more features can be seen in those objects when you are able to gather more of their light. More light translates into more detail or, to invoke a technical term, greater 'resolution.' That's where even slight optical aid can help enhance the beauty of objects in the night sky.

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