Subject: Science/Astronomy
Ages: Late Elementary - High School
Length: Snippet: Recommended: 12 minutes.
Lesson: One class period.
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Learner Outcomes/Objectives: Students will get a feeling for the relative sizes and distances of
stars and of galaxies and of the vastness of space as compared to our tiny little world on planet
Earth.
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Rationale: Cosmic Voyage presents in a very attractive way the huge distances involved in
astronomical research. Students usually know that stars are in galaxies and that there are huge
numbers of galaxies in the universe. However it is usually not so easy to convey a feeling of the
relative scales and distance of stars and galaxies: whereas galaxies populate very densely the
universe, stars are very far away from each other within a galaxy.
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Description of the snippet: While the Oscar nominated documentary Cosmic Voyage is
certainly worth watching in its entirety (36 minutes), this snippet lesson plan focuses on the
sequence running from minutes 9:45 to 12:02 (two minutes and 17 seconds). In this sequence
viewers are taken through increasingly larger perspectives on space, starting with a view of the
sun from a distance where soon the nearest stars, the triple system Alpha Centauri, come into
view, all the way to the largest scales of the universe that can be mapped. However, in order
to properly understand the rationale of the documentary, we recommend that students are
shown the film from the beginning up to minute 12:02 (twelve minutes and two seconds).
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Helpful Background:
This cosmic voyage with “landmarks” for us to keep in mind the scale of things in the universe
in terms of powers of ten has two major highlights: the scale at which stars begin to be seen in
groups and at their typical distances from other stars, at 1015 meters, and when the same
happens with galaxies, several powers of ten later, at 1023 meters.
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There is a fundamental difference between these two scales: the distances between stars are
huge with respect to their sizes, so that it can be said that space within galaxies is virtually
empty, except, of course in their very dense centers, where there is evidence for supermassive
black holes attracting huge amounts of matter on themselves. Galaxies on the other hand are
proportionally very close to each other.
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It is therefore frequent to see galaxies interacting and colliding, while stars within one of them
hardly ever do so, not even when two galaxies smash into each other: the stars fly by each
other like two swarms of mosquitoes.
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See an excellent explanation of this with beautiful images from the Hubble Space Telescope:
http://hubblesite.org/explore_astronomy/hubbles_universe_unfiltered/1
There are more visual presentations of galaxy interactions at:
http://astronomy.swin.edu.au/cosmos/I/Interacting+Galaxies
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Interacting Stars
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That there are hardly any collisions between stars does not mean that stars do not interact.
When star formation occurs in a gas cloud, the formed stars are inevitably gravitationally
bound to each other. Sometimes a large number of them stay together as a cluster of stars,
and as many as 85% stars of the Milky Way are in fact binary stars, triplets or multiple systems.
There is a minimum mass for a lump of gas to become a star. It has been estimated to be
between 40 and 80 times the mass contained in Jupiter. In other words, if Jupiter had
accumulated about that many times its actual mass when the solar system was formed, we
would now live in a binary system. Such a situation was imagined in the Star Wars Universe,
where Luke Skywalker’s planet Tatooine has two suns. But would a binary be able to have
planets in stable orbits, and therefore living beings to enjoy a double sunset? It seems to have
been proven theoretically and observed in practice. Learn more about it at
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980122c.html .
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A frequent and well studied case is that of two interacting stars where one of them is a
compact white dwarf star or the collapsed remnant of a star that has ceased to emit energy in
the form of heat and light through thermonuclear reactions. The other is a still active star. The
collapsed companion has a very strong gravity and is absorbing huge amounts of gas from the
active star. In the case of a white dwarf being the compact companion, the infalling material
can trigger a supernova of the type known as Ia (roman numeral “I” and the letter “a”). These
supernovae have been very important for astrophysicists over the last decade, because they
are known to have a very precise luminosity, and can therefore be used as “standard candles”
to determine the distances of the galaxies in which they occur: as we know how bright they
always are, the fainter we see them, the further away they are. This in turn has allowed
astronomers to discover that the expansion of the universe is not only not slowing down (as
would be expected due to the mutual gravitational attraction of all objects in it) but in fact
speeding up! The still mysterious origin of this acceleration of the universe’s expansion has
been named “dark energy”. Find out more about supernovae Ia and dark energy at
http://hubblesite.org/hubble_discoveries/dark_energy/de-type_ia_supernovae.php and the
links therein.
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Galaxies and the Large Scale Structure of the Universe
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As mentioned above, galaxies do interact and collide. Quite often, in fact. Galaxies come in
different shapes, the two main types being “spiral” galaxies and “elliptical” galaxies. The
“spiral” shape is usually interpreted as a whirlpool of stars, gas and dust falling into a
supermassive black hole at the center. The “elliptical” shape is believed to be related to the
common occurrence of galaxy collisions: after two (spiral) galaxies smash into each other, the
gas, dust and stars are mixed together and stripped off their spiral shape to finally clump into a
featureless and more or less “elliptical” agglomeration.
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Their distances are relatively small, at a scale of 1:25 with respect to their size. This means that
our galaxy being about 100,000 light years across, typical distances to other galaxies are 2.5
million light years. A huge distance, but relatively close in astronomical scales.
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Like stars, galaxies can be found in clusters, but astronomers surveying the universe are finding
that these clusters are not evenly distributed over space. Instead, they build a large scale
structure of the universe, with large “filaments” made of millions of galaxies and clusters of
galaxies. The formation of this structure from a smooth distribution of matter at early stages of
the universe is being studied and simulated with computers by astronomers. Videos and stills
of these simulations can be found at: http://cosmicweb.uchicago.edu/filaments.html
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A breathtaking video with the whole known universe has been made by the American Natural
History Museum. Everything in it is based in real data, it is not a filmmaker’s rendering of what
is imagined out there: every object in the sequence has been detected and catalogued by
astronomers
http://www.metacafe.com/watch/5027750/the_known_universe_by_amnh/
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Using the snippet in class:
Preparation
Be familiar with the location of the segment: If the recommended snippet of 12 minutes will
be shown, play the movie from the beginning until minute 12:02, just before the movie
changes location to an aerial view of the Dutch town of Delft.
Step by Step
1. This
lesson
plan
is
best
if
preceded
by
TWM’s
<a
href=”http://www.teachwithmovies.com/members/snippets/sn-math-powers-of-tencosmic-voyage.html”>Snippet Lesson Plan on the Powers of Ten **</a> <br><br>
2. Show the snippet in class
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3. Comment on the snippet directing the discussion towards a full comprehension of the
difference in the relative sizes and distances for stars and galaxies as introduced in the
Helpful Background Section.
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4. Re-run the sequence starting from minute 9:45 and ask the students to count how
many powers of ten (represented as circles in the movie) have to be traveled after the
last view of the solar system, before the nearest star system, Alpha Centauri, comes
into view. Ask them to do the same for the Milky Way galaxy, counting how many
powers of ten have to be passed to see the nearest galaxy.
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In the first case, three powers of ten separate the sun from its nearest neighboring
star, while in the case of galaxies, there are nearby galaxies in full view even before we
reach the next power of ten. These galaxies (the Magellanic Clouds) are considered
“satellite” galaxies of our Milky Way. But even discarding these as representative of
typical galaxy distances, a large independent galaxy, Andromeda, comes into view
from the right shortly after we have crossed just one power of ten.
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This comparison shows the concept of relative distances between objects.
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