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Some notes on the apparent brightness of stars |
Inveterate space tourists in the Civilised Galaxy have the opportunity to compare the brightness of various stars from the surface of any number of terraformed planets; those planets which receive approximately the same amount of energy from their sun as Old Earth (the so-called 'comfort zone') are the most easily terraformed, but the appearance of the central star is often very different.
Planets in orbit around a red dwarf have to be much closer to the cooler star than those around sun-like stars; these planets are usually tidally or harmonically locked to the star, so the movement of the star in the sky is unusual. Being cooler, a red or orange dwarf star has to be larger in the sky than the Sun is in the skies of Old Earth; even Tau Ceti is 15% larger in the sky of Nova Terra than Sol would appear. Similarly those planets around hotter stars (such as Daedalus) have to be much further from the central star; Pi 3 Orionis is 20% smaller in the sky on this world.
However the sunlight is actually brightest on G-type stars (at least to those clades with traditional baseline human spectral perception) due to the large amount of non-visual radiation from hotter or cooler stars. Habitual tourists become aware that the visual magnitude is different to the luminosity, and the light from a sunlike star is rich in the visual.
A red dwarf gives off more energy in the infrared than our sun, so if you had a star which had the same visual magnitude as our sun the amount of energy received in total would be greater, although we would not be able to see it all; in the same way a blue star would give more energy in the UV end of the spectrum, and so a star that appeared to be the same visual magnitude as the sun would in fact be giving a lot more invisible energy...the total luminosity would once again be greater.
Putting it another way, any planet around another star which receives enough starlight to have Earth-like temperatures will get more of that energy in the form of I-R or UV radiation - (unless the star is a very sunlike yellow G-type dwarf) - this will lead to the conclusion that for a planet with Earth-like temperatures, visible sunlight will be dimmer for both a hotter or a cooler star.
Tourists to very sunlike stars like 18 Scorpii are often surprised at the brilliance of the light from the sky, and traditional retrostyled 'raybans' are a popular accessory on these worlds.