"Twinkle, twinkle little star" is an enchanting recitation for children, but it contains two misconceptions that attest to the fallibility of Earth-based observation. First, stars aren't little. Some may be Earth-sized, but most are bigger than Jupiter, the largest planet in the solar system. Second, they don't twinkle; they shine, and the brightness and color of the light from each star yields information about its temperature, size and even its age and likely fate.
The Filter of the Atmosphere
Viewing the stars from Earth is a bit like looking at them through a a water filter because, compared to the emptiness of space, the atmosphere is dense. Moreover, air is always moving, so starlight appears to be shifting and shimmering. The atmosphere also makes stars appear dimmer than they would be if we could see them from space. Popular depictions of stars with points or rays extending from a bright center notwithstanding, stars appear from space as round points of light; the reason they twinkle in photographic images is because light diffracts in lenses and mirrors.
Stars Have Different Colors
If you stop and examine the sky on a dark, moonless night, it's easy to spot color differences among stars. The color of a star is a visual indication of its surface temperature. The hottest stars are blue, and the next hottest are white. Yellow stars like the sun are next, while red stars are the coolest of the visible stars. Many red stars are so dim that people can't see them at all, and some stars, called brown dwarfs, hardly emit any light at all. Some stars don't emit light -- they trap it. These are black holes, the leftovers of hot, giant stars that have exploded as supernovae.
Stars Are Different Sizes
One reason that stars vary in brightness is that hotter stars emit more energy than cooler ones, but another important reason is that some are much bigger than others. For example, Betelgeuse -- a star in the constellation Orion -- shines with a red light, but it appears bright to us because it's simply huge. If it took the place of the sun, its surface would extend to the orbit of Jupiter. White dwarfs, on the other end of the scale, are Earth-sized, but they are among the hottest objects in the sky. They are the remnants of dying stars and are often surrounded by a ghostly gas formation known as a planetary nebula.
Apparent and Absolute Magnitude
Some stars appear brighter to Earthlings simply because they're closer. Astronomers rank the brightness of stars -- as seen from Earth -- by assigning them a number known as apparent magnitude -- the smaller the magnitude, the brighter the object. They have also devised a measure that ranks stars according to how bright they are when compared to each other. This number, called absolute magnitude, describes how bright a star would appear if it were 10 parsecs (about 32.6 light-years) distant. With an apparent magnitude of minus 26.7, the sun is the brightest object in the sky. Its absolute magnitude, however, is only 4.7. If that were its apparent magnitude, it wouldn't even be visible to the naked eye of a person in an urban center.