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TWINKLE TWINKLE LITTLE STAR
Why Do Stars Twinkle?
BY MARK MANCINI
All things considered, our
atmosphere is pretty great.
This blanket of nitrogen,
oxygen and other gases keeps the world's temperature nice and habitable while
protecting us from harmful UV radiation — to say nothing of the space
debris it vaporizes.
Oh yeah, and without all that
oxygen in our atmosphere, animal life couldn't survive on planet earth. Not a
bad resume.
But despite its many good
qualities, the atmosphere can be a nuisance to astronomy buffs. That's because
it distorts light.
At night, the atmosphere
makes some heavenly bodies appear to flicker and shimmer. The technical term
for this phenomenon is "astronomical scintillation." You probably
know it by a different name: twinkling.
Like an onion, the atmosphere
is made up of layers. At the bottom is the
troposphere, which starts right here at ground level on the planet's surface.
Standing about 5 to 9 miles
(8 to 14.5 kilometers) tall, it's where most of Earth's weather events take
place.
The other layers are — in
ascending order — the stratosphere, mesosphere, thermosphere and exosphere.
(There's also a region called the ionosphere, which encompasses parts of the mesosphere and
thermosphere.)
These layers have different
temperatures.
In addition, the air's
density varies from level to level.
When starlight enters our
atmosphere, it runs into pockets of cool and warm air.
The pockets act as big
lenses, causing the light to change direction — or "refract" — as it passes through them.
Yet the lenses are not fixed
in place; they move around and change shape. As they shift, so does starlight
refraction. That's why the stars appear to twinkle.
Scintillation affects
planets, too. Mercury, Venus, Mars and other planets in our solar system do
twinkle when viewed from Earth on a clear night. (So does our moon.)
However, the planets twinkle
to a barely noticeable degree.
Distance is the main reason
stars twinkle more conspicuously than the planets in our solar system.
Because the former are so far
away, each star looks like a single pinpoint of light. It's a different story
for Earth's moon and our neighboring planets.
Being a lot closer, they're
less affected by the atmosphere.
Planets and moons appear as tiny
disks up in the sky. The
light they emanate comes not from a single point but from many individual
points all clustered together.
These rarely scintillate in
unison, which is why planets and moons don't twinkle as dramatically as the
stars.
Twinkling only can happen
when an atmosphere is present.
It's for this reason that
photos taken by the Hubble Telescope look
so clear; there aren't any atmospheric air pockets to refract the starlight.
Earthbound astronomers use
telescopes with adaptive optics systems to compensate for twinkling, making the
stars look more stable.
NOW THAT'S INTERESTING
Our
closest planetary neighbor is Venus, which is 25
million miles, or 41 million kilometers, away from us at the nearest point in
its orbit.
On
the other hand, you'd need to travel more than four light-years to reach the
closest foreign star system (that of
Alpha Centauri). That's a long way to go.
Just one light-year is equal to 5,878,625,373,183.6 miles or
9,460,730,472,580.8 kilometers.
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There's no shortage
of stars to take in from this lovely spot on top of Hawaii's Mauna Kea
Observatories.
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