Under typical conditions and with Earth-based telescopes, stars appear as point sources of light due to their immense distances, making it challenging to resolve their discs. Most visible stars in the night sky, even through powerful telescopes, appear as points because their angular size is too small to detect. However, advancements in telescope technology have allowed us to resolve a few stars as discs.
The most notable star observed as a disc is Betelgeuse, a red supergiant in the constellation Orion. Its size and relatively close proximity (approximately 642 light years away) make it one of the few stars whose disc can be resolved by telescopes equipped with interferometry technology. The Hubble Space Telescope and other observatories using adaptive optics have successfully imaged the disc of Betelgeuse, revealing details about its surface and the surrounding atmosphere.
Another example is the Sun, which is the closest star to Earth and is regularly observed as a disc, showing various features such as sunspots, flares, and prominences. Beyond Betelgeuse and the Sun, other stars like Antares and Aldebaran have also been imaged as discs using similar advanced techniques. Interferometers, which rely on combining the light collected from separate telescopes to increase the effective resolution, have made imaging stellar discs possible, although it remains a complex process that requires precise instrumentation and favorable conditions.