Stars

On a clear dark night, a few thousand stars are visible to the naked eye. With binoculars and powerful telescopes, we can see so many stars that we could never hope to count them. Even though each individual star is unique, all stars share much in common. The Sun, which is the source of virtually all light, heat and energy reaching the Earth, is the nearest star. Today, we know that stars are born from interstellar gas clouds, shine by nuclear fusion and then die, sometimes in dramatic ways.

Globular Cluster M3.

Some 500,000 stars are

crammed into an region about

100 light years across in

this globular cluster

Why do stars twinkle?
The songline goes "Twinkle twinkle little star". What is the cause of the "twinkling" of stars? Does light from planets "twinkle" as does light from stars?

Stars twinkle because of turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle." This is one of the reasons the Hubble telescope is so successful: in space, there is no atmosphere to make the stars twinkle, allowing a much better image to be obtained.
Planets do not twinkle the way stars do. In fact, this is a good way of figuring out if a particular object you see in the sky is a planet or a star. The reason is that stars are so far away that they are essentially points of light on the sky, while planets actually have finite size. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, presenting a relatively stable image to the eye.