On some days, stars can be seen changing rapidly in color and brightness, and this is called twinkling, or, more properly, scintillating. On the right is a 14 second clip of Sirius twinkling.
Twinkling is caused by the star light having to travel through our atmosphere, where patches of light with different densities move around. These differences in density causes the light to refract and, ultimately, change color and brightness for an observer on earth. The longer the light has to travel through the atmosphere, the stronger the effect gets, so stars just above the horizon twinkle the strongest.
The easiest way to capture this twinkling on a photo is to take a long exposure while moving the camera, as was done in the examples below (1 to 2 seconds shutter speed, focal length was 200 mm).
Sirius. (2016-02-28)
Unlike stars, planets are not point sources of light and they don't twinkle for that reason, as can be seen in this photo. This is a combination of two photos taken with the same camera settings. To the left is Sirius and to the right is Jupiter. (2016-02-28)
The three stars from Orion's Belt together. It's clear that they twinkle in different patterns. (2016-02-28)
Two stars with different colors compared. On the left is the colder star Betelgeuse, which, counterintuitively, emits "warm" yellow light. On the right is hot star Bellatrix, which emits "colder" blueish light. In real life, these stars are much further apart, but I moved them closer together for this photo to make the difference in color easier to see. (2016-02-28)
Sirius. (2016-02-28)
Sirius. (2016-02-28)
Two photos combined. To the left is a star at approximately 45 degrees latitude, which gives a much shorter distance for the light to travel through the atmosphere and, as a result, a lot less twinkling. To the right is a star at approximately 70 degrees latitude, which gives an even shorter distance for the light to travel through the atmosphere and this time there is almost no twinkling to be seen. (2016-02-28)
A weird effect when doing the same with the lens slightly out of focus; there is some fine structure in the blurry stripe. (2016-02-28)
Multiple slightly out of focus images of Sirius combined. The structures that are visible are the same as the one in the previous photo. (2017-01-31)
The same principle as the previous photo. (2017-01-31)
Sirius. (2017-01-31)