Expert: Ed - 4/13/2005

Question
I have used all my star maps and concluded that the star I have been watching over the past few nights is in fact, Sirius. I understand that it is a binary star and knowing this I am still stumped as to why it "twinkles" and with so many different colors, could you help me understand why? Hope you can help, thanks!

Answer
Leslie,

Sirius appears to twinkle, from Earth, but no more than any other normal star.  It has nothing to do with the fact that it's a binary system...  I'm not sure why you're seeing multiple colors.  That should not really happen.  Look more carefully -- could you be making a mistake?

Stars twinkle because of turbulence in the atmosphere of the Earth. The phenomenon s known as "stellar scintillation."  As the air in our atmosphere moves about, due to winds at high altitude, the light from the star is refracted in different directions. This random refraction causes the star's image to change slightly in brightness and position, therefore it "twinkles." This has been one of the main strengths of the Hubble telescope: 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 width, as seen from Earth, even though it is only a fraction of a degree. 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.

I hope that this is helpful to you.  Keep Looking Up!
--Ed