Just wondering. . . . If you combine the magnitudes of all visible stars (down to 6th magnitude), how bright a star will you come up with?

SupernovaCropAbout 9,000 stars are brighter than magnitude 6.5, the traditional criterion for naked-eye visibility. Their combined magnitude is almost exactly –5, which is equal to 100 zero-magnitude stars like Vega. Half these stars lie below the horizon at any moment, and the low-lying stars suffer from heavy atmospheric extinction, so it’s estimated that 3,000 stars are visible on a typical clear, dark night. However, the traditional limit is quite conservative; many people can see 7th-magnitude stars under ideal conditions. The number of stars that are brighter than magnitude 10.5 (a plausible limit for hand-held binoculars) is about 600,000, with a combined brightness equal to 225 zero-magnitude stars. That’s about half the total brightness of all the stars.

The naked-eye planets vary considerably, but they usually add up to anywhere between 50 and 100 zero-magnitude stars — the lion’s share coming from Venus. Curiously, most of the light from sources other than the Sun and the Moon comes from “planets” less than a millimeter across. Their combined glow, called the zodiacal light, is equivalent to 2,500 stars of zero magnitude, yet most stargazers have never seen it. That’s partly because most of it is concentrated near the Sun, where it’s hard to see, and partly because, unlike the Milky Way, the zodiacal light has no sharp edges.

— Tony Flanders


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October 11, 2017 at 5:38 pm

For reference, I metered the very dark night sky the night before the 8/21/2017 eclipse in the middle-of-nowhere Wyoming at 21 3/4 magnitudes per square arc-second. When integrated over the entire half of the sky above the horizon this works out to -7½, or ~3 mags brighter than Venus at its brightest. At least 30-40% of this is said to be due to upper atmosphere skyglow (fluorescence in the ionosphere), with the zodiacal light forming component the next most important. The reading obviously includes both visible and sub-visible stars and galaxies (though I tried to avoid the Milky Way).

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