"The Sun is playing a secret melody, hidden inside itself, that
produces a widespread throbbing motion of its surface. The sounds
are coursing through the Sun's interior, causing the entire globe, or parts
of it, to move in and out, slowly and rhythmically like the
regular rise and fall of tides in a bay or of a beating heart."
(Kenneth R. Lang)
Have you ever wondered what the Sun would sound like if you could hear
it?
Our Sun lies 93,000,000 miles away, surrounded by the vacuum of space.
Sound won't travel through space, of course.
But with the right instrument,
scientists can "hear" pulsations from the Sun.
The entire Sun vibrates from
a complex pattern of acoustical waves, much like a bell.
If your eyes were sharp enough, you could see a bell's surface
jiggle in complex patterns as the waves bounced around within it.
Likewise, astronomers at Stanford University can
record acoustical pressure waves in the Sun by carefully tracking
movements on the Sun's surface.
To do this, they use an instrument called a
Michelson Doppler Imager (MDI),
mounted on the SOHO spacecraft,
circling the Sun 1,000,000 miles from Earth.
The Sun's acoustical waves bounce from
one side of the Sun to the other in about two hours, causing
the Sun's surface to oscillate, or wiggle up and down.
Because these sound waves travel underneath the Sun's surface,
they are influenced by conditions inside the Sun.
So scientists can use the oscillations to learn
more about how the structure of the Sun's interior shapes its surface.
The Sun's sound waves are normally at frequencies too low
for the human ear to hear. To be able to hear them,
the scientists sped up the waves 42,000 times -- and
compressed 40 days of vibrations into
a few seconds.
What you'll be hearing are just a few dozen of the
10 million resonances echoing inside the Sun.
The Sun vibrates up and down, in and out, much like a pot of fudge boiling
on the stove. When you make fudge, you can see large
bubbles of chocolate covering the pan. And, in each of those
big bubbles you can also see a set of tiny bubbles growing and popping
with gusto. The Sun is the same.
When you look at the following movies, you are viewing
the large scale (big bubble) oscillations (the small ones would
be too tiny to see).
It is these oscillations which generate the sound waves you heard!
Solar oscillations (5 hours of medium-l data, from Sept. 1, 1996)
(mpeg format; 4 meg)
Scientists can learn much about the Sun's complex interior
by tracking and deciphering it's sound waves.
But, since we can't "hear" the Sun directly, scientists have
to watch for the effects of those sounds waves, the Sun's oscillations,
to be able to decipher the sounds.
So, like seismologists who study earthquakes,
helioseismologists study "quakes" on the Sun.
Their job is a bit like figuring out how a piano was put together
by listening to it fall down the stairs!
Would you like to learn more about helioseismology?
If you like to play with Slinky® toys,
there is a wonderful, hands-on activity
to demonstrate the nature of sound waves.
Teachers -- You can easily do this in your classroom.
