Remember the little rhyme from childhood, “Twinkle, twinkle, little star.  How I wonder what you are”?  Well, many stars do twinkle because their brightness changes with time.

For example, a star called Algol in the constellation Perseus periodically fades to half its normal brightness.  This cycle occurs about every three days.  Algol stays dim for about ten hours.  Then, just as rapidly as it faded, it grows bright again.  The reason for Algol’s strange behaviour is that it’s not one star but two stars, one bright and one dim.  The two stars orbit around one another.  They behave like the flashing light of an ambulance as its metal baffle rotates around a bulb, repeatedly blocking the light beam.  When Algol’s dim star moves in front of the bright star, it seems to become fainter because some of its light is blocked from view.  As the dim star moves away from the bright one, Algol regains its brightness.

Another kind of twinkling star is called a ‘Cepheid variable.’  Delta Cephei, the fourth brightest star in the constellation Cepheus, winks every 5.37 days.  In this case the explanation is that the star is pulsating in and out like a balloon being blown up and then partially deflated.  When the star is at its smallest it’s bright, when it expands, it grows dimmer.

So next time a star seems to wink at you, remember it’s either pulsating or rotating.