The Challenge Astronomers claim to know the reactions going on in the core of the sun and their rates Did my explanations convince you If not can we test our claims directly Two Problems ID: 537487
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Slide1
Testing Our Understanding
Slide2
The Challenge
Astronomers claim to know the reactions going on in the core of the sun, and their rates.
Did my explanations convince you?
If not, can we test our claims directly? Slide3
Two Problems
We can
’
t see directly into the center of the sun
.
The brightness of the sun
right now
only tells us how vigorous the reactions were
some time ago. Slide4
Analogy
When the oven
’
s heating elements are turned off, how quickly does it cool down? (It stays hot for a while!)
Similarly, how do we know the sun is undergoing any reactions
right now?
Maybe it
’s “turned off!”Slide5
Size of the Sun
2.3 light-secondsSlide6
Information Delayed
If the sun were transparent, the photons leaving the core would escape in
about 2 seconds
(and reach Earth 8 minutes later
)
We would see an
intensely hot small central core
In fact it takes about half a million years for the photons to percolate out from the center!Slide7
Diffusion: The
‘
Random Walk
’Slide8
…a.k.a. the
Drunkard
’
s WalkSlide9
Test This Yourself!
Flip a coin umpteen times. (Say, N=100.)
If it
’
s a head, take one step to the
right.
If it
’s a tail, take one step to the left.Slide10
Statistical Analysis Proves:
On average,
after N
steps, you will be about √N steps from your starting point – sometimes to the left, sometimes right.
In our example, after N = 100 coin flips, you will find yourself (typically) about 10 steps from where you started. (√100 = 10)Slide11
Ten Thousand Steps!Slide12
Smell the Lilacs
How does the fragrance reach you
?
Molecules of the scent drift into your
nose, having made their way across
t
he intervening space.
Not by diffusion! Much too slow! W
e
s
mell the flower because of air
currents
that carry the scent.Slide13
Let
’
s Consider Some Numbers
[don
’
t memorize!]
Inside the sun, light goes about 1 mm before running into an electron and
‘bouncing’ in some new direction. Call each of these one ‘step.’The sun’s radius is ~700,000
km or about
700,000,000,000
mm
that is, N = 700 billion steps.
To cover that distance like a drunkard, the light must take N
2
steps ( ~50 x 10
22
) in total – up, down, left, right,…
Travelling at the speed of light, this takes
several hundred thousand years.Slide14
Old Light!
The light energy reaching us
today
was generated by nuclear reactions the best part of a million years ago!Slide15
So Forget About the
Visible Light!
It
tells us essentially nothing about the
current
central state of the
Sun’s ‘engine’.
How can we learn about deep interior in some other way?Slide16
To Probe the Sun
’
s Deep Interior
Find something that comes straight out from the deep interior (
not a photon,
therefore);
orUse a technique analogous to the seismic studies on Earth. That is, measure the ways in which the sun is vibrating, to learn about its internal structure.