Topic 2: HOW IT ALL BEGAN -- MAYBE!


How far away is a star?  Figure it by taking the speed of light, and multiplying
it by the duration of time the light has travelled.  So, for how
long has the light travelled?  Astronomers try to solve this by using
the spectral shift of light.  The spectral shift from distant galaxies 
suggests that the universe is expanding, out of a very small volume.
Extrapolating back in time suggests this occurred about 15 billion years ago.

Some physicists have tried to calculate this "original size."
Some have proposed that the original volume was the size of the solar system;
others claim it may have been much smaller -- smaller than a golfball!
 
As the universe expanded from a condition of nearly "pure energy", 
hydrogen atoms evolved through a process called "nucleosynthesis".  
At a later time, gas clouds of Hydrogen condensed to form main 
sequence stars.  Fusion of hydrogen within the stars then led to the 
formation of helium.  When stars the size of our sun (i.e., stars with 
one solar mass) burned toward extinction, after about 10 billion years, 
their center collapsed while the outer gas layers expanded to form a 
"red giant".  The pressure and temperature in the center of a red giant 
was large enough to allow the fusion of alpha particles (helium atoms
without electrons) to form heavier elements including oxygen, and 
carbon.  Note -- we will discuss later under the topic of radioactivity --
that an alpha particle has two protons and two neutrons, 
so that the major products of fusion all had even atomic weights (that is, 
an even number of protons).  Pressure and temperature were not large 
enough in the red giants to produce the heavier elements.  

On rare occasions a star would form with more than about 20 times the mass 
of our sun.  These stars would burn faster and collapse with the outer 
gas cloud expanding into a "super red giant".  The pressure in the center 
of these super stars was high enough to produce the heavier elements 
including silicon, magnesium, and iron.   Life of a super red giant ends 
after a nuclear "burn", after about 10 million years.  The end is marked 
by a "supernova" explosion, with the heavier elements scattered in space 
once again.  Supernova have actually been observed by astronomers with
telescopes! 

Our solar system, including heavier elements, condensed from the gas cloud
produced by a supernova.  Processes within a super red giant manufactures 
much oxygen, so that after hydrogen and helium, oxygen (atomic weight 8)
is the third most common element in the universe.  Carbon, the element so 
critical for life on earth, is also among the most abundant elements in the 
universe.

The above info (from TE) sets background for the discussion in your text, 
which deals mainly with the solar system. The Sun, mostly Hydrogen and Helium,
is surrounded by two groups of planets, plus Pluto, an exception to some
rules.  The 4 closest planets, from Mercury to Mars and including Earth
(what did I leave out?) are dense bodies of iron and silicates (about 90%
Fe, O, Si, Mg).  Volatile gases were carried to greater distances and
make up the Giant Planets, from Jupiter to Neptune.  The spacing of the
planets is remarkably consistent, as discovered by Kepler, and recognising
the laws of spacing led to discovery of new planets (searching where the
planets "should be").  The physics texts deal with this story, which 
is, I think, a brilliant tale.

The original Earth was, after initial condensation, much more homogeneous
than at present.  Compression led to heating, as did radioactivity,
and the melting point of Iron was exceeded.  Iron, a very dense element,
migrated by gravity to the earth's center (Core), displacing silicate materials
that moved outward, forming the Mantle.  The lightest stuff formed a rind
at the surface that we call Crust.  The Earth formed around 4.7 billion
years ago.  No rocks at the crust are older than 4 billion, so that the
layering of the Earth (its "differentiation"), occurred somewhere between 
these two dates.