Except for our sun, no one has ever seen the disc of a star. They are just too far away, no telescope is powerful enough to magnify a stellar disc sufficiently so that it appears to be anything but a point. In optical jargon, stars are point sources. They may appear to be larger or smaller than one another, but that is just an illusion. They are all bright or faint, and their images on photographic emulsion or on CCD devices or the human retina are always dots. The brightest of them may spill over into more emulsion crystals, or CCD pixels, or cells in our eyeballs, but they are all just dimensionless points. We know they have shapes, we have determined this with all sorts of clever techniques such as eclipse light curves and interferometers, but we have never actually resolved a point source.
But there are other things in the sky besides stars. solar system planets and satellites, nebulae, galaxies and other objects are not points, they actually have a shape. They are also quite large: most of the clusters, nebulae and galaxies amateur astronomers see through their backyard scopes have an angular size and a shape, we just can’t see them without a telescope because they’re too faint. The Andromeda galaxy, for example, is a hazy oval almost three degrees across, that’s six times the diameter of the full moon. A good pair of eyes on a dark night can just make it out as a tiny, dim, fuzzy ball, the bright central core. But in photographs or with even the least optical aid we can see its full size and shape. These are extended sources.
Both point and extended sources have brightnesses which astronomers measure in magnitude. The faintest star a sensitive human eye can see on a dark night is about magnitude 6. The brightest stars in the sky are magnitude 1, 0 or even brighter (magnitudes can go negative). A look through any astronomical catalogue will show that many extended sources are brighter than magnitude 6, but only a tiny handful are visible to the unaided eye. How can this be?
Point sources are either visible or invisible, depending on whether their magnitude reaches the detection threshold of the human eye (as I mentioned earlier, about 6). Remember, magnitudes are a power law, each magnitude is 2.5 times brighter than the next, so that a 1st magnitude star is 100 times brighter than a 6th magnitude star.) But for extended sources, the process is more complex, and it depends on environmental conditions and human physiology.
A 6th magnitude nebula SHOULD be visible to the unaided eye, after all, a 6th magnitude star is. But the nebula’s brightness is spread out over an extended area because it is not a dimensionless point. The larger that area is , the fainter each square angular unit area of the object appears. For example, the bright central core of the Andromeda Galaxy exceeds 6th magnitude in brightness, so it is just barely visible to the naked eye. The entire Andromeda Galaxy has an integrated total brightness of about mag 3.5. That is extremely bright, but it is spread out over such a large area that we just can’t see it, except for the brightest central portion.
That’s a geometrical factor, the same magnitude spread out, or extended, over a larger area, is less visible. The result is point sources are always easier to see than extended sources of the same magnitude.
Another factor is more physiological in nature. The human eye is very sensitive to contrast, that is, it can see fainter objects that are contrasted against a dark background. The slightest amount of moonlight, scatter, sky brightness or light pollution can make fainter point or extended sources undetectable. This pollution can be due to human artificial lighting reflected off dust or moisture suspended in the atmosphere, or even a faint electric glow (skyglow) that is perfectly natural. Skyglow is related to the aurora. This skyglow varies a lot, but it never goes away entirely. To get a really black sky, you need to go to a high, dry, cold place like a desert mountaintop. To eliminate it entirely you need to go above the atmosphere altogether. The upshot is that the darker the night, the fainter you can see.
Astronomical telescopes are not designed to make small objects look bigger, they are meant to make faint objects look brighter. But regardless of the quality or size of the telescope there are limits as to how “faint” it can go. First, the sky must be dark, which is why astronomers avoid urban skies and moonlit nights. Second, the eye must be healthy and fully dark-adapted to optimize your view. To view extended objects, low magnifications are preferred–for example, viewing a faint nebula at 50x will make it half the size of its appearance at 100x, this means its area will be 1/4 as much. The same amount of light will be spread out over a much smaller area, so it will look brighter.
For point sources, the magnification should be irrelevant, but in practice it does make a difference. Since at high powers the skyglow is spread out over a larger area, but the stellar image is not, the contrast is increased. The faintest stars show up better at high power. Of course, image quality is compromised at higher magnifications except in the most expensive eyepieces, so this must be taken into consideration as well.
For the visual astronomer, the optimum magnification to view any object will depend on the object’s nature, the transparency and steadiness of the atmosphere, the amount of light pollution, and the visual acuity of the observer. The observer must also determine what he wants to see, faint stars or delicate traces of nebulosity, or some other form of detail. On some nights in some places, some objects just look better in some eyepieces.