"If you look at all the objects detectable in Earth's skies, including both naturally occurring bodies as well as artificial satellites, it should come as no surprise that the Sun appears as the brightest object of all. The Sun, after all, produces its own light, sustainably powered by nuclear fusion in its core. That core-generated energy helps keep the Sun from contracting under its own gravitation, but also propagates to the Sun's edge, the photosphere,"
"where the Sun emits radiation over a wide range of wavelengths that correspond to a temperature of around 6000 K. Although the Moon is the second-brightest object in most wavelengths of light, it only appears so bright because of its very close proximity to Earth. From an intrinsic point of view, most of the Moon's light is merely reflected light from the Sun."
"The Sun is the brightest object in Earth's skies, followed distantly by the Moon in second place, as seen from Earth in: All of the other planets, stars, satellites, and galaxies trail far behind both the Sun and Moon in terms of brightness as viewed from Earth. However, when we began looking at the sky at the shortest, highest-energy wavelengths, we realized this picture was no longer universally true."
The Sun produces its own light via sustained nuclear fusion in its core, emitting radiation from the photosphere corresponding to a temperature near 6000 K. The Moon appears bright primarily because it reflects sunlight and lies very close to Earth, making it second in apparent brightness across most wavelengths. Most planets, stars, satellites, and galaxies are far fainter as seen from Earth. At the highest-energy, shortest wavelengths the pattern changes: the Sun emits almost no gamma-rays most of the time, while the Moon emits copious gamma-rays due to interactions that were initially mysterious but later explained by physical processes.
Read at Big Think
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