Re: why does the star canopus change its colours ?

Dear Siddharth,
     I found this answer to your query at a NASA website h
ttp://starchild.gsfc.nasa.gov/docs/StarChild/questions/question26.html

    "On a clear, dark night, our eyes can see about 6,000 or so stars in 
the sky. They seem to twinkle, or change their brightness, all the time.
In fact, most of the stars are shining with a steady light. The movement of 
air (sometimes called turbulence) in the atmosphere of Earth causes the 
starlight to get slightly bent as it travels from the distant star through 
the atmosphere down to us on the ground. This means that some of the light 
reaches us directly and some gets bent slightly away. To our eyes, this 
makes the star seem to twinkle.

You will notice that stars closer to the horizon will appear to twinkle 
more than other stars. This is because there is a lot more atmosphere 
between you and a star near the horizon than between you and a star higher 
in the sky. Go out some night soon and have a look!"

    From my own experience observing stars, try the following experiment. 
Focus a telescope or binoculars (even low power toy instruments will work) 
on a star near the horizon. Now turn the focusser to that your star image 
is big, blurry and out of focus. Unless India is having exceptionally still 
weather, you will see the image shift, wiggle, shake and shimmy as 
different parts of the atmosphere through which you are looking move and 
undulate.

     Thank you for writing to the Mad Scientist Network.
Jim Foerch
James C. Veen Observatory
Lowell, Michigan
USA

[Moderator's note: Jim is correct that stars near the horizon appear to 
twinkle more, and that may be partly responsible for the color changes that 
you see. This may be more obvious with Canopus than other stars because 
Canopus is so bright: the human eye is not that sensitive to colors of very 
faint objects. Also, the colors of stars also vary a bit with their position in 
the sky. Because the atmosphere absorbs blue light more than red, stars near the 
horizon appear a bit redder- but this is a small effect, not easily detectable 
by the eye.]