Nciv Ngal fstar fplanet flife

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Nciv Ngal fstar fplanet flife
Astrophysical considerations (the factor fstar
in the Drake equation. Recall it is the fraction
of stars in a typical galaxy which have the
right chemistry for life. First we need to
make a tentative conclusion concerning what the
right chemistry might be. For example in the
periodic table in the next slide I have circled
the elements which humans need to live.
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We have little idea how many of these elements
would be required for some form of life. However
we can say the following It is unlikely that
the chemistry of just the FIRST TWO elements
(hydrogen and helium) is complex enough to
result in anything at all lifelike. (Some
might even dispute this conclusion. See
the article by Feinberg in your book.) With that
assumption, and some information about how stars
form, we can derive an estimate of fstar
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A brief review of the history of the universe, as
currently understood. About 14 billion years
ago, the space of the Universe was compacted in
an extremely small region and began to rapidly
expand. Roughly one second after the expansion
began, the initial material cooled sufficiently
to leave mainly electromagnetic radiation, two
kinds helium nuclear isotopes, two kinds of
hydrogen nuclei (protons and deuterons) and
electrons. By our assumption, the universe was
not yet chemically complex enough to sustain any
kind of life.
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About 300,000 years after the initial explosion,
the electrons combined with the nuclei to leave
mainly electrically charge neutral atoms of
helium and hydrogen as well as electromagnetic
radiation. After that, the electromagnetic
radiation did not interact strongly with the
matter any more and it remained in the universe
from that time. The discovery by radio
astronomers of this cosmic ray background
electromagnetic radiation is one of the
experimental reasons that we believe this story.
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The universe continued to expand and does so
to this day. The expansion was first discovered
by Hubble and coworkers in the 1920s by
measuring the distances and velocities of distant
galaxies. All the galaxies were found to be
moving away from us and the most distant galaxies
were moving away the fastest. This is what is
expected if the universe Is expanding uniformly
in all directions. Hubble used the Doppler
effect to measure velocities. Since the Doppler
effect appears several times in our story, I will
take a few minutes to explain how it works.
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Star formation. The expansion of the universe is
fascinating but for our purposes, what is more
important is what started happening to the
matter after that 1st 300,000 years. All
massive matter is mutually attracting. Newton
first formulated this idea in his tremendously
successful theory of gravitation, which is still
used to account for the motion of astronomical
bodies and much else. (though for very large
masses and gravitational forces an extension of
Newtons ideas, due to Einstein, is
required.) If the universe were a perfectly
expanding sphere, the mutual attraction would not
matter, but even the slightest unevenness in the
mass distribution will make the mass of the
universe start to clump up.
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The clumping of the mass at first resulted
in dust clouds (of helium and hydrogen) but as it
progressed, the centers of the more
massive clouds and the pressure due to the
gravitational attraction got larger and larger in
them. Eventually, the pressure was so great
that the nuclei of the atoms started to get
pushed together. To do this requires a HUGE
pressure by earthly standards and it only occurs
inside stars and in the (fortunately rare)
explosion of thermonuclear weapons. To get an
idea how this works requires some ideas about two
other fundamental forces of nature, namely
electromagnetic and strong nuclear forces.
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Electrical repulsion


Gravitational attraction
If the gravitational attraction becomes large
enough to overcome the electrical repulsion then
the nuclei can get close enough for the short
range attractive nuclear force to take over. Then
nuclear burning starts in which the nuclei can
fuse with emission of large amounts of kinetic
energy and rise in temperature.
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Thus the stars ignited and the resultant
nuclear burning produces elements of the
periodic table. As you see from the table, this
has been worked out in great detail.
However, from the point of view of chemistry for
building life, there are two problems The
elements formed are inside of high temperature,
high pressure stars. The chain of reactions
stops at iron, short of many elements essential
to life on earth, at least.
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Supernovae The stars die for life. For stars
of the mass of our sun, the nuclear burning
continues to iron and then stops, Then the stars
slowly die and becoming white dwarves and then
cinders. But more massive stars (5 stellar
masses or more) die more spectacularly. When the
burning stops, the gravitational pressure results
in catastrophic implosion followed by a bounce
and emission of massive amounts of material from
the star. Two things happen 1)The elements
formed inside the stars are spread out into the
interstellar medium and 2) Reactions producing
elements beyond Iron take place during the
explosion.
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Supernovae occur about once every 100 years in a
typical galaxy. They are observed regularly in
other galaxies by astronomers and were
observed in our own galaxy in 1006,1054,1572 and
1604. They are the source of the material in the
interstellar medium which is available for
formation of planets with complex chemistry
suitable for life, at least as we know it. In
this sense, stars have died for us. The last
stage of concern is the formation of new, second
generation stars and planets around them, from
this enriched interstellar medium. The earlier,
first generation stars, probably also formed
planets around them as they formed, but those
planets must have been composed only of hydrogen
and helium.
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Now we want to estimate fstar. which is
the fraction of all the stars which are in
this second generation. For that we need just A
few more facts about how stars evolve. By
making detailed mathematical models using known
facts about the nuclear reactions occuring inside
them, you can predict the lifetimes of stars if
you know their masses. From that information We
will use just a couple of facts Stars big
enough to be supernovae live about 107
years. Stars of mass around the mass of our sun
live about 1010 years.
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From the first fact we can estimate the number of
stars which will become supernovae in a typical
galaxy like this Let the number be Nsuper. .
Then since those stars die in about 107 years,
the number dying in a galaxy per year is Nsuper/
107 1/100 So Nsuper is about 100,000 or one in
a million. From the other fact, we can estimate
the Number of second generation stars N2 . The
death rate is equal to the birth rate, giving N2
/1010 5/100, giving N2 about 5x 108
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From this we get fstar N2/Ngal of about
5x108/10115x10-3 The estimate in your book is a
little higher.