Ask A Scientist© Astronomy Archive

Question:  This question has probably been asked before but, here goes:
Someone told me that if you see an object in the sky which doesn't twinkle, it's 
probably a planet.  If this is the case, why do stars twinkle?  I have always
noticed this star near the horizon in the winter in Ireland, and it shifts
slowly through the visible spectrum.
Is this due to the Earth's atmosphere/electromagnetic fields, or by gaseous
clouds between Earth and the star?  Someone please have a go at answering as
this is bugging me.   :-)
niall c mccann

Answer:  I suspect that bright, color-changing star is Sirius, the dog star. It
is the brightest star visible from the Northern Hemisphere and is famous for
appearing as different colors. I believe the color changes are due to the
Earth's atmosphere (refraction effect).
And yes it's true that stars twinkle and planets dont. The reason is the immense 
distance of stars compared to planets: there's enough "stuff" (gas and dust) out 
there in interstellar space that causes the twinkle.
OOPS! It's the earth's atmosphere that makes stars twinkle because they are so
distant that they appear as point sources of light that are displaced by
variations in our atmosphere. Planets don't twinkle being much closer, they are
not point sources.
(Thanks to rcwinther for drawing this to my attention)
john