Stars twinkle for the same reason that the air
shimmies above a radiator or a fire or hot pavement; because
of warm air rising in the atmosphere.
Heat can move in one of three ways. The first is
conductance. In conductance, the heat passes through
something solid, such as wood or metal. This process is pretty
slow.
Another way heat moves is radiation. In radiation,
the heat passes directly through space in the form of photons,
tiny packets of energy travelling at the speed of light.
Radiation is the way that the sun's warmth reaches Earth.
Finally there's convection. In convection, heat
warms the air. The warm air becomes less dense (and thus
lighter) than the cool air around it, so it rises. Convection
is the reason stars twinkle.
When air heated by convection rises, it tumbles
and swirls. When light passes from the cool, dense air through
the warm swirling, not-so-dense air and back again, it gets
bent this way and that. That's why the air shimmies over a
fire or a radiator.
As air warmed by the earth rises through the atmosphere, it
breaks into bubbles of warm air. As light from the stars
passes through the bubbles, it's bent back and forth. This is
what makes the stars seem to twinkle.
If there weren't any atmosphere, as on the moon, the stars
would shine steadily. |