Astronomy 201 - Homework #1				Sullivan, Winter 1993

	Answer the following questions presented as part of the text.  
Answer in a neat format on your own paper.  This assignment is due Friday,
February 19 at lecture time.  

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	This assignment will follow the adventures of Peppy the proton
as she chronicles the history of the universe.  Peppy was formed as part
of the Universe during the Big Bang and was left over after the first
three minutes of nucleosynthesis as a lone Hydrogen nucleus.  About
100,000 years later, Peppy finally settled down with an electron and
began her journey.

	After about a half billion years of aimlessly floating around in
space, Peppy found herself drawn to an unstable and collapsing region of
the Universe.  This gigantic cloud of gas eventually collapsed to form
a large spiral galaxy known as the Milky Way.  Peppy got cards and letters
from some of her friends in the less fashionable places of the Milky Way
(the outer regions of the disk and halo).  She learned from them that there
were fewer heavy elements to talk to in those regions that she was used
to having around closer to the center of the galaxy, where she lived.

(1)  Why do the outer parts of galaxies tend to have fewer heavy elements 
     heavier than Helium) than the inner parts?  Answer in two or three 
     sentences.

	After a few billion years, Peppy got bored, so she invited about a
trillion trillion trillion trillion billion of her closest friends over
to form a very high mass star.  They formed a star of much higher mass
than our Sun (about ten solar masses), so it burned itself out very
quickly, eventually blowing itself up as a supernova.  Peppy managed to
keep her shape as a Hydrogen atom, but a lot of her friends were joined
together to form heavier elements like Carbon, Oxygen, Silicon, Iron, and
even heavier elements.  Peppy's cousins, the Planck twins, could only
get about a hundredth as many friends to form a star together, so they
were still part of their star long after Peppy's party ended in a supernova.

(2)  Why do only the most massive stars (greater than about 6 solar masses)
     eventually erupt into supernovae?  Again, explain in two or three 
     sentences.

(3)  Massive stars (say, greater than a solar mass) have more fuel to 
     burn than stars like the Sun.  Why, then, do they not live as long?
     Give a numerical example from your notes or the book to support
     your argument.

	After the supernova, Peppy and some of her friends got together
and decided to try it again.  This time, they got fewer friends to come
to the star formation, and so they ended up with a collapsing cloud of
about the same mass as our Sun.  Peppy decided to stay near the edges
with a few of her heavier friends (elements) and eventually got swept
out into a spinning disk.  Peppy was pretty far out on the edges of this
forming solar system, and it was fairly cold out there, cold enough so
that another Hydrogen atom and an Oxygen atom could join with her to
form a water molecule.

(4)  During the condensation of the solar system from the initial disk of
     material, it was too hot in the Earth's vicinity for any water to
     condense out of the nebula and form as part of the Earth.  Where, then,
     did all the water in the Earth's oceans come from?  Answer in a
     sentence or two.

	Peppy, now as part of a water molecule, used the path you suggested 
in your answer to question #4 to find her way to the Earth about a half
billion years after the Earth formed.  Eventually, Peppy and her two
pals found themselves in a small primordial pond.  When she started
writing to her long lost cousins on Venus and Mars, she found that a
lot of her friends that used to be water in the outer solar system
were either "hiding out" or weren't water molecules any more; whereas,
most of the water on Earth remained intact in liquid form.

(5)  We would assume that all the inner planets were equally successful
     at "acquiring" water in the early history of the solar system.  For
     Venus and then Mars, describe what happened to that water.  Answer
     separately for each planet with two or three sentences, explaining
     what happened and why.

	As Peppy and her friends lounged around in their small pond in a
seedy section of the South American tectonic plate, they watched over the
billions of years of the Earth's history as the atmosphere slowly changed. 
Early on, there was a lot of Carbon Dioxide in the atmosphere, but something
drastic happened to it after about the first billion years of the Earth's
existence.  The largely Carbon Dioxide atmosphere was altered into more
like what we have today.  Peppy's friends on Venus didn't understand this
change on Earth, since Venus always kept most of its Carbon Dioxide in
its atmosphere.

(6)  What happened on the Earth to change this "primitive" Carbon Dioxide
     atmosphere into something like what we have today?  Describe the 
     major process that removed Carbon Dioxide from the atmosphere.

(7)  Why didn't this same process occur on Venus?

	On Mars, it was even worse!  Peppy's friends soon discovered that
most of the lighter gases, like water vapor and Hydrogen, left the planet
soon after they arrived.  Carbon Dioxide was left behind, but not much.

(8)  Why did some of the gases escape Mars as opposed to Earth and Venus,
     and why did Carbon Dioxide remain behind?  

(9)  It turns out that Hydrogen gas quickly escaped from _all_ of the inner
     planets.  Why?  Which planets in the solar system, if any, managed
     to hold on to their Hydrogen and why?

	As the eons wore on, Peppy often gazed up at the Moon overhead
and wondered what role it played in the grand scheme of things as her
little pond eventually expanded to form a small lake in the foothills of
the Andes about 50 million years ago.

(10)  How did the presence of the Moon help the development of life
      on Earth?  

	Finally, all her questions answered, Peppy settled down for a 
long wait, to see what would finally happen to the Earth after her
friends in the Sun were finished with their party.  Before she had too
much time to think about this, though, a llama came along and slurped
her up.