"The only major difference between the sun and the stars we see at night is that the sun happens to be close to uswhich is advantageous, assuming you enjoy being alive. Astronomers enjoy this as well but have another reason for rejoicing in the sun's proximity: this allows us to see it as a disk. The sun is, of course, three-dimensional."
"The stars in the night sky are a bit farther away; the nearest one to us, Proxima Centauri, is roughly 280,000 times more distant than the sun! This makes it appear correspondingly smaller through a telescopeinfinitesimally smaller, in fact, appearing as only a point of light. When an object appears this way, we say it's unresolved; when it's apparently big enough to exhibit an actual shape, then it's resolved."
The Sun appears as a resolved disk because it is close enough to Earth to subtend a measurable angular size, allowing detailed observation of surface features such as sunspots, faculae, and granules. Most other stars lie so far away that even the nearest, Proxima Centauri, appears as an unresolved point of light. Telescopic resolving power depends primarily on aperture size, so many stars are theoretically large enough to be resolved by the largest telescopes. Atmospheric turbulence blurs fine details and limits ground-based resolution. Techniques such as adaptive optics dynamically reshape telescope mirrors to compensate for atmospheric motion and restore sharpness.
Read at www.scientificamerican.com
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