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High School Teacher Program 2008
Are we really made of stars?
Are we really made of stars?
Recommended for
12-18 yrs olds
Summary
Are we really made of stars? This lesson starts from studying
light emissions from exploding stars, called supernovas.
Those observations and analyses can tell us the origin of the
elements found on Earth and throughout the universe. This
explains how the elements of the periodic system, we find on
Earth, are formed in stars.
Concepts
Hydrogen, Helium, periodic system of the elements, fusion,
pressure, supernova, spectroscopy
Short introduction
Quicktime movie about the periodic system of the elements.
Link to the curriculum
This lesson can cover the following topics:
• The periodic system of the elements
• Matter
• The Earth in the universe
Hints for the teacher
First discuss with your students what they think about this question. Let the students work in
small groups and let them present their ideas. Then watch the little movie.
After the movie you can ask feed-back about how the students feel about this topic. Show
them the table of the elements, explain the different properties of atoms, how they are build in
stars, …
Give the students time to discuss in their groups what they are taught and let them formulate
some questions. Let the students summary what they learned in this lesson.
Are we really made of stars?
Periodic system
All liquids, gases, and solids found on our planet
are made from one or more of 92 naturally
occurring elements. From what they have
observed, scientists have determined that these
same 92 elements are found throughout our
universe. This suggests that a common process
leads to their creation. But how are these
elements created? How did they get so widely
disseminated?
©2008 HST-2008
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High School Teacher Program 2008
Are we really made of stars?
Currently, the most popular theory states that the
nuclei of hydrogen and helium, the lightest and most
abundant elements in the visible universe, were
created in the moments following the Big Bang, 13.7
billion years ago. All other naturally occurring elements
were — and continue to be — generated in the high
temperature and pressure conditions present in stars.
Elements are composed of tiny particles called atoms
that are indivisible under normal conditions. However,
when exposed to high heat and pressure, atoms can
either break apart or fuse together. Under these
conditions, the nucleus of one element can fuse with
the nucleus of a different element, creating the nucleus
of a heavier element. When elements lighter than iron
form, the mass of the new nucleus is less than the
combined mass of the two original nuclei. The
difference in mass between the two is released as
energy. In stars, this kind of reaction is referred to as
stellar nucleosynthesis, but it is more commonly known as nuclear fusion. Nuclear fusion is
used today on Earth in the nuclear explosives called hydrogen bombs. Many people hope that
one day nuclear fusion will be used for peaceful energy production.
Stars are fuelled by nuclear fusion reactions, which take place in their deep interiors, or cores.
Hydrogen nuclei fuse, forming helium nuclei. The energy produced by these fusion reactions
prevents the star from collapsing under its own gravity. Mature stars contain enough hydrogen
nuclei to last billions of years. When a star's hydrogen fuel supply is spent, however, its core
begins to contract. The contraction is so intense that it creates conditions under which helium
nuclei fuse. In this way, helium becomes the star's next fuel source. The fusion of helium nuclei
produces carbon and oxygen nuclei, and in the process sufficient energy is released to
temporarily sustain the star.
Once helium runs out, the nuclei of carbon, oxygen, and other elements begin to fuse. These
new fuel sources are depleted at faster and faster rates. Since the heaviest element created in
a star by nuclear fusion reactions is iron, a large iron core eventually forms at the center of
everything. At this point, gravity becomes overwhelming, the core collapses, and an explosion
occurs, during which outer layers of gas and heavy elements are ejected to space. Such
explosions, called supernovas, occur about once a century in our galaxy. The energy created
by supernovas produces nuclei heavier than iron: silver, gold, platinum, and all the other heavy
elements all the way up through uranium. This process is known as supernova
nucleosynthesis.
Exploding, the star seeds these elements into space. Some of the elements are picked up in
second-generation stars. These will live for a shorter time since they start by bearing heavy
elements within them. Some of the elements the star spews out will collect into planets, which
is where the Earth eventually comes in.
Life in Universe
Observations of many stars and galaxies have shown similar chemical abundances: 98% of
the mass is hydrogen and helium, and all other elements compose the remaining 2%. That 2%
may not seem like much, but it is enough to create all living things on Earth. One of the most
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High School Teacher Program 2008
Are we really made of stars?
common of the remaining elements is carbon. Organic molecules have even been observed in
interstellar clouds and found in comets and meteorites.
While it is still not clear how life on Earth originated from basic organic molecules, the fact is
that life exists. If basic organic molecules were able to create life on Earth, and they are
available elsewhere in the universe, it is not unreasonable to wonder if life has also developed
elsewhere.
If many of the atoms within us are 13.7 billion years old, and as "we" are our bodies, not just
our minds, then, in a way, we also are 13.7 billion years old.
How do we know…
How do we know H en He were present in the Universe from the really
beginning?
Scientists study the universe by studying the light they catch in their telescopes. By analysing
the spectra of different stars, the American astronomer Edwin Hubble saw that galaxies were
moving away from each other at a rate constant to the distance between them. He determined
that the greater the distance between a galaxy and Earth, the faster that galaxy was moving
away from us — a phenomenon now known as Hubble's law. These findings signalled that the
universe is expanding and laid the foundations for the Big Bang theory, which states that the
universe exploded into existence from a single point or a very small region in time and space
and has been expanding ever since.
Just after the Big Bang, temperatures were so high that particle pairs could be created purely
out of the heat energy present. During its early phases the Universe was “γ radiation
dominated”, that is the photons dominated the energy and pressure of the Universe. As the
Universe expanded, it cooled, T ∼ 1/R, where R is some measure of the "scale of the
Universe". During this cooling process, particles were able to form out of energy: first quarks,
then neutrons, protons and electrons, and then nuclei and atoms. After 1 million years the
temperature was decreased to 4000 K, the temperature where atoms can survive.
The strongest evidence that something like the Big Bang really happened is the “Cosmic
Background Radiation” predicted by Cosmologist George Gamov in 1948 and discovered by
Arno Penzias & Robert Wilson of Bell Labs in 1965.
All those γ-rays described above are part of the
thermal radiation present in the early Universe
because it was hot. As the Universe expanded and
cooled, the radiation field cooled along with it. Gamow
predicted that the Universe should be filled with this
"relic radiation left over" from the Big Bang. Using the
peculiar horn-shaped antenna shown in the picture to
the right, Penzias & Wilson made the first glimpses of
the Cosmic Background Light quite unexpectedly.
Since their discovery the evidence has become
stronger and stronger that we are seeing the light
from the Big Bang.
Penzias & Wilson received the Nobel Prize in Physics in
1978.
By analysing this light, scientists have found evidence
that from the very beginning of our Universe hydrogen
and helium were present, in the comparable rates as we
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Are we really made of stars?
find them in today’s universe: helium is about 25% by mass and hydrogen about 73% with all
other elements constituting less than 2%.
How do we know H fuses to He in stars en He fuses to Be…?
We can’t “see'' further into a star than its outer layers, its photosphere, so how do we know
what goes on inside?
The structure of a star (i.e., how the quantities temperature, density, and pressure vary through
the star) is determined by setting the force of gravity at each layer equal to the available
pressure distributed over the surface area of that layer. The sources of pressure inside normal
stars are gas (proportional to density and temperature) and radiation (proportional to T4),
though the latter is important only in stars much hotter inside than our Sun. Gravity bears
inward, pressure pushes outward, and in this struggle an equilibrium arises, setting up the
structure of the star. Well, except for one thing....
The resulting conditions at the very centres of stars are such that the fusion of lighter elements
into heavier ones occurs naturally, releasing energy through Einstein's most famous equation
E = mc2. This energy must be transported to the surface, either by radiation (photons) or
convection (upward bulk motion of buoyant gas), and when the energy produced in the core is
exactly balanced by the energy lost at the surface as light, then the star can come into full
equilibrium.
What is the evidence that this is what's really happening inside a star?''
1. Until recently, we had only indirect evidence. That is, we could build a computer model
of a star (or a whole series of model stars for a star cluster) and compare their outward
characteristics (luminosity, surface temperature, radius, spectrum) with what we could
derive from measurements of a star of a given mass and composition. We could also
let our models progress in time, as their cores consumed one fuel and fused it into
another, and in this way we could track the evolution of a star, or a whole cluster of
stars, and compare the resulting distributions of the model stars with what is observed.
All such comparisons between model and observation have met with spectacular
success.
2. The observed abundances of the elements on the periodic table are reproduced to
good accuracy by models of galactic evolution, which describe how multiple
generations of billions of stars produce and recycle the elements heavier than helium.
That is, we know WHY carbon, nitrogen, oxygen and iron (for example) are found in
concentrations of 3.5 x 10-4, 9.3 x 10-5, 7.4 x10-4, and 3.2 x 10-5 by number relative to
hydrogen in our Sun, in stars with ages similar to our Sun's, and in gas clouds in this
part of our Milky Way Galaxy.
Recently, improvements in experiment, observation, and technology have allowed us a closer,
more direct peek at the inside of at least 1 star - our Sun. In two words: helioseismology and
neutrinos.
3. Helioseismology - Due to the convective motions of gases in the outer 30% of the Sun's
radius, observed as bubbling granulation cells in the Sun's photosphere our Sun rings
like a bell - though very gently. Acoustic (sound) waves of 10 million modes of
oscillation propagate through our Sun, with periods of typically several minutes.
Detailed analyses of the measurements of these ultra-low amplitude oscillations (with
surface velocities of less than 10 cm/s) allow astronomers to determine how
temperature, density, and composition vary through the Sun's interior. These vibrations
are detected as tiny Doppler shifts in the light emanating at the photosphere. This is
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Are we really made of stars?
similar to how geologists determine the Earth's interior structure and state through the
study of earthquake waves, or loosely analogous to an ultrasound sonogram revealing
the interior structure of a human.
In this picture showing a model
representing the observed oscillations in
our Sun, the blue areas are approaching,
while red areas are receding. The density,
temperature, and even changes in
composition may all be measured through
an analysis of the millions of modes of
oscillation. The latest results show that the
best theoretical models predict a structure
of our Sun (pressure, temperature,
density, relative fractions of hydrogen,
helium, heavy elements as functions of
distance from the Sun's centre) that differs by 0.1%-0.3% from that determined from
helioseismology observations.
4. Solar Neutrinos - Many of the thermonuclear fusion reactions predicted to occur in the
core of our Sun produce a sub-atomic particle, known as a neutrino. As its name
implies, it has no charge, and its main property is that it does not interact with other
matter very much (doing so through the weak nuclear force). Until very recently, we did
not know if it had any mass (without mass, like a photon, it would travel at the speed of
light). Because it interacts very little with other matter, it was known that most neutrinos
should "fly" out of the Sun, and that with special detectors we might be able to detect a
fraction of them here on Earth. The total number of neutrinos, recently observed,
agrees with the number calculated using the standard computer model of the Sun. Yhis
shows that scientists now understand how the Sun shines, the original question that
initiated the field of solar neutrino research.
Why the fusions stop at Fe?
During most of their lives, stars fuse hydrogen into helium in their cores, but the fusion process
rarely stops at this point. Massive stars become much hotter internally than stars like the Sun,
and additional reactions occur after all the hydrogen in the core has been converted to helium.
At this point, massive stars begin a series of nuclear burning, or reaction, stages: carbon
burning, neon burning, oxygen burning, and silicon burning. In the carbon burning stage,
carbon undergoes fusion reactions to produce oxygen, neon, sodium, and magnesium. During
the neon burning stage, neon fuses into oxygen and magnesium. During the oxygen burning
stage, oxygen forms silicon and other elements that lie between magnesium and sulphur in the
periodic table. These elements, during the silicon burning stage, then produce elements near
iron on the periodic table.
Massive stars produce iron and the lighter elements by the fusion reactions described above,
as well as by the subsequent radioactive decay of unstable isotopes. Elements heavier than
iron are more difficult to make, however. Unlike nuclear fusion of elements lighter than iron, in
which energy is released, nuclear fusion of elements heavier than iron requires energy.
Because there is no system that can deliver this energy, the reactions in a star's core stop
once the process reaches the formation of iron.
©2008 HST-2008
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High School Teacher Program 2008
Are we really made of stars?
How do we know supernovae create elements heavier than Fe?
A supernova is a stellar explosion. They are extremely luminous and cause a burst of radiation
that often briefly outshines an entire galaxy before fading from view over several weeks or
months. During this short interval, a supernova can radiate as much energy as the Sun could
emit over its life span.
The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of
light, driving a shock wave into the surrounding
interstellar medium. This shock wave sweeps up
an expanding shell of gas and dust called a
supernova remnant.
Long ago, scientists learned that they could heat
atoms of an element to a glow and direct the light
through a prism to produce a set of coloured
lines. These spectral lines are unique for each
element: No two elements produce the same
colours and line positions along a spectrum. By
using instrumentation that reads light signatures
from far away — a technique known as
spectroscopy — scientists today know with great
certainty which elements a planet or a star, or
even a star's dispersed remnants, contains.
How do we know we are 13.7 billion years old?
The universe is expanding with the velocity v =
Hubble suggested a linear relationship: v = H d
⇒H =
v [km / s ]
.
d [MegaPar sec]
d
.
t
H…Hubble Constant,
d…distance of an object
You can get the velocity because of the redshift (Doppler effect), but it was very difficult to
measure the distance. When 2 stars have the same luminosity and are at the same distance
they will appear like 2 equal lanterns. When one is farer away that one will seem darker with
Luminosity L ∝
1
.
d2
Finally they found kind of a “standard candle”: a binary system of 2 stars:
when a white dwarf and a red giant are orbiting around their common centre of mass then it
may occur that material from the companion red giant is attracted by the white dwarf until the
white dwarf can no longer support its own weight and burns its nuclear fuel so suddenly that it
explodes. These explosions always release about the same amount of energy and have
almost the same peak of brightness. These explosions emit x-rays through the bremsstrahlung
process and are called “Type Ia Supernovae”. They are these standard candles which allow to
measure the true distance.
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Are we really made of stars?
So they could measure the Hubble Constant with H = 70
As v =
d
= H ⋅d
t
[km / s ]
[MegaPar sec]
.
1
t
d….distance ⇒ H = …….time that has passed
⇒ age of the universe
Bibliography
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http://www.teachersdomain.org/resources/phy03/sci/phys/matter/origin/index.html
http://www.amateurspectroscopy.com/Spectroscope.htm
http://www.db.dk/bh/lifeboat_ko/SPECIFIC%20DOMAINS/Periodic_system.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hydhel.html
http://cass.ucsd.edu/public/tutorial/BB.html
http://homepages.wmich.edu/~korista/starstruct.html
http://en.wikipedia.org/wiki/Binary_star
http://universe-review.ca/R02-07-candle.htm
©2008 HST-2008
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