Interstellar and Intergalactic Distances Using Cosmic Voyage
Subject: Science & Technology; Astronomy — Interstellar and Intergalactic Distances
Ages: 10+; Late Elementary to High School
Length: Film Clip: approximately 12 minutes or a shorter clip of approximately five minutes will also work; Lesson: One 50 minute class period.
Cosmic Voyage describes in a very attractive way the huge distances of deep space. 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 densely populate the universe, stars are very far away from each other within a galaxy.
DESCRIPTION OF SNIPPET
Viewers are taken on a journey through the distances of space changing the magnification of one scene by a factor of ten at each step. 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 distances in space, starting with a view of the sun and then widening the viewer’s perspective until the nearest stars, the triple system Alpha Centauri, come into view. By the end of the clip, the viewer’s perspective has been widened to include the largest scales of the universe that can be mapped. However, in order to properly understand the rationale of the documentary, TWM recommends that students be shown the film from the end of the opening credits (about 40 seconds into the film) up to the scene that shows the Dutch town of Delft (minute 12:22, about twelve minutes).
The “Cosmic Voyage” taken in the film contains “landmarks” to enable the viewer to keep in mind the scale of things in the universe in terms of powers of ten. The journey has two major thresholds: first, the point at which stars begin to be seen in groups, at 1015 meters, and second, when the same thing happens with galaxies, several powers of ten later, at 1023 meters.
There is a fundamental difference between these two scales: the distances between stars are huge with respect to their sizes. There are, for example, about one million diameters of the Sun between the Sun and its closest stellar neighbor, Alpha Centauri. 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 to themselves. Galaxies, on the other hand, are proportionally very close to each other, on a scale of 25 to 1, where 1 is an average size of a galaxy. It is therefore frequent to see galaxies interacting and colliding, while stars within the galaxies hardly ever do so, not even when two galaxies smash into each other: the stars fly by each other like two swarms of mosquitoes. What smashes, in a collision between galaxies, is their interstellar dust and gas.
See an excellent explanation of this with beautiful images from the Hubble Space Telescope: Hubble’s Universe Unfiltered — Show One: Interacting Galaxies Galore. There are more visual presentations of galaxy interactions at: Interacting Galaxies from COSMOS – The SAO Encyclopedia of Astronomy.
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 gravitationally bound to each other. Sometimes a large number of them stay together as a cluster of stars, and as many as 85% of the stars in the Milky Way Galaxy are in fact binary stars, triplets or multiple systems.
There is a minimum mass required before a lump of gas coalesces to become a star. This 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, it would now be 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 star system be able to have planets in stable orbits, and would living beings be able to experience a double sunset? It seems to have been proven theoretically and observed in practice. Learn more about it at Ask an Astrophysicist.
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 very strong gravity and absorbs 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: since it is known how bright they always are, the fainter they become, 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, it is 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 Dark Energy at Hubble Discoveries and the links therein.
Galaxies and the Large Scale Structure of the Universe
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.
Their distances are relatively small, at a scale of 1:25 with respect to their size. This means that the Milky Way Galaxy is 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.
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: Formation of the large-scale structure in the Universe: filaments from the University of Chicago.
The Known Universe Scientifically Rendered For All to See is 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 cataloged by astronomers.
1. Become familiar with the location of the snippets and cue the DVD to the first snippet.
The minute and second calculations may differ from what is set out below. Check your disc for exact locations before using the film in class.
If the recommended snippet of approximately 12 minutes will be shown, play the movie from the end of the opening credits (40 seconds into the film) to minute 12:22, just before the movie changes location to an aerial view of the Dutch town of Delft (about 12 minutes). A shorter clip that works almost as well begins at minute 7:17 (the beginning of the scene at Venice’s St. Marks Square) and lasts until minute 12:22.
STEP BY STEP
1. If the recommended snippet of 12 minutes will be shown, play the movie from the end of the opening credits (about 40 seconds into the film) until minute 12:22, just before the movie changes location to an aerial view of the Dutch town of Delft
2. 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.
3. 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.
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 the next power of ten is reached. These galaxies (the Magellanic Clouds) are considered “satellite” galaxies of the 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 reaching just one power of ten. This comparison shows the concept of relative distances between objects.