Which astronomical objects that appear as points of light to the unaided eye appear as disks with the help of a telescope?
Astronomical objects that appear as points of light to the unaided eye but appear as disks with the help of a telescope include planets and some larger moons in our solar system, as well as a few bright star clusters.
To understand why these objects appear as disks when viewed through a telescope, we need to consider the concept of resolving power. Resolving power refers to the ability of a telescope to distinguish between two closely spaced objects. It depends on the diameter of the telescope's primary lens or mirror.
Planets in our solar system are relatively close compared to other astronomical objects. When observed through a telescope, they appear larger and reveal more details than what can be seen with the naked eye. The resolving power of the telescope allows us to see the disk of a planet, which is especially true for planets like Jupiter and Saturn with their prominent features such as cloud bands and rings.
Similarly, certain larger moons in our solar system, such as the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), can also appear as disks when observed through a telescope. These moons are close enough and large enough that their individual shapes and surface details can be resolved by telescopes.
Lastly, some bright star clusters, such as the Pleiades, can also appear as disks through a telescope. Although individual stars in the cluster are still too distant to be resolved as disks, the combined light from all the stars can create a glowing disk-like appearance when observed through a telescope.
It's important to note that most other astronomical objects, such as stars, galaxies, and nebulae, still appear as points of light even when observed through a telescope because they are located much farther away, and their individual details cannot be resolved with the available telescope resolving power.