•  Cernan Earth and Space Center

    Stellar Extremes

    Grade Level: 7th grade and up 
    Length: 45 minutes 

    From the smallest to the largest, the hottest to the coolest, from the fastest to the most bizarre, all stars have a place in our incredible universe. Stellar Extremes is a multimedia program that combines the planetarium's stars, video segments, panoramic scenes, special effects and astrophotographs to explore the dynamics of stellar interiors, encounter some of the most unusual stars, and find out what it takes to solve stellar mysteries from light years away. The program is presented by an assortment of characters, some fictional (Fred Flower and Uncle Cosmo) and others real (Dr. You-Hua Chu from the University of Illinois at Urbana). 

    The program's first segment features a brief tour of the northernmost portion of the night sky, conducted by Uncle Cosmo. The sky tour identifies Polaris (the North Star), the Big Dipper (Ursa Major), the Little Dipper (Ursa Minor), and the how the Big Dipper can be used to find north on any clear night. Lesser known constellations in this northernmost part of the sky are also mentioned, including Cepheus (the king), Cassiopeia (the queen), Camelopardalis (the giraffe) and Draco (the dragon). 

    The next section reveals that the glittering lights in the night sky are actually giant nuclear furnaces churning out immense amounts of energy. Dr. Chu from the University of Illinois is introduced and she explains that despite the great distances of stars, astronomers can still study them by analyzing their incoming light. The limitation of human vision is explained and leads to a discussion of the electromagnetic spectrum, which includes all forms of radiation, including gamma rays, x-rays, ultraviolet rays, visible light, infrared light, microwaves, and radio waves. The program explains that because our Sun is so close, it serves as a stellar laboratory, revealing to scientists how all stars create such huge amounts of light and heat. 

    The program next explains that although the earth's atmosphere protects us from harmful radiation released by the Sun, it also makes the serious study of the Sun rather difficult. In order to fully understand the physics of stars, scientists must study their output in all wavelengths of the electromagnetic spectrum. Our ability to send instruments into space has revolutionized the study of the Sun, and hence the study of stars in general. The Solar and Heliospheric Observatory (or SOHO, launched in 1995) and Chandra X-ray Observatory (launched in 1999) are two of our most recent ways of studying solar wind, solar structure, and the dynamics of the Sun's interior. The Hubble Space Telescope has similarly provided valuable new insight into the nature of stars other than our Sun. 

    Stellar Extremes next describes how stars form. Uncle Cosmo points out the constellation of Orion and the Great Orion Nebula that is found a short distance below Orion's Belt. It is from these nebulas, Uncle Cosmo explains, that stars and planetary systems form. Gravitational forces draw clouds of hydrogen gas and dust into clumps, which may eventually form protostars. If a critical mass is reached, protostars initiate the process of nuclear fusion, which fuses hydrogen atoms to create helium atoms, releasing large amounts of energy in a process that scientists call the proton-proton cycle. Once this chain reaction begins, the protostar becomes a full-fledged star. For much of its remaining life, the star is balanced by two immense forces -– the star's gravity that tries to collapse the star inward and the star's energetic output, which tries to push the star outward. So long as these forces are balanced, the star maintains its prescribed size and temperature. 

    Stellar Extremes then poses the first (and easiest) of its stellar extreme questions –- which is the closest star? The answer is the Sun, but in answering the question, the program also discusses the relative distances of the Sun, Pluto, Proxima Centauri (the second nearest star to Earth), and other galaxies. Astronomer's preferred unit of length (the light year) is introduced and explained. Uncle Cosmo then points out the Andromeda Galaxy, which can be seen by the unaided eye from a dark location, despite its distance of nearly 3 million light-years. 

    Next, the program describes in detail how astronomers classify stars by their temperature, brightness and size. The Hertzsprung-Russell diagram shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars, and is a valuable tool to understanding the physics of stars. Stars are first classified by their temperature, and are assigned one of the spectral class letters, which (arranged from most hot to least hot) are O, B, A, F, G, K, and M, and (most recently added) L and T. 

    Stellar Extremes then poses the second of its stellar extreme questions -– which is the faintest star? Glowing largely in infrared light, red dwarfs are the faintest of true stars. Red dwarfs are also very low in mass and low in temperature and live very long lives. The least hot of these red dwarf stars is the carbon star, which glows in a deep red color. 

    The program next expands on its earlier discussion of the proton-proton cycle (which converts hydrogen to helium) by introducing the triple-alpha process, which further converts helium nuclei into carbon. The triple-alpha process is used by older stars when most of their original hydrogen has been converted into helium. As the star ages, still heavier elements are created by nuclear fusion. These processes increase the energy output of the star, which causes its outer layers to expand outward. The star becomes a red giant. 

    The next stellar extreme question is "which is the hottest star?" The answer is the white dwarf star. White dwarf stars are produced when red giant stars end their outward expansion and collapse. The resulting white dwarf stars are incredibly hot and dense stars. The hottest such star found to date is 40 times hotter than the Sun. In many cases, the process that creates the white dwarf star also creates an expanding shell of gas that astronomers call a planetary nebula. 

    Next, Uncle Cosmo returns to identify two super giant stars visible to the unaided eye -– Betelgeuse and Rigel -– both located within the constellation of Orion, the hunter. 

    Stellar Extremes next describes the nuclear fusion process that occurs in old, massive stars. After hydrogen fuses to form helium and helium fuses to form carbon, the process continues within these very massive stars, forming heavier elements like neon, magnesium, and silicon. No matter how massive the star is, however, the nuclear fusion process ends with the element iron, since the fusion of iron consumes more energy than the process releases. At this moment in time, the delicate balance between gravity and heat is lost, and the star collapses inward. The rebound of this implosion is the greatest explosion known in the cosmos -– a supernova. 

    The stellar magnitude scale, which measures the brightness of stars, is next discussed. The stellar magnitude scale is somewhat backwards, since the brighter stars have a lower magnitude. First magnitude stars, for example, are brighter than second magnitude stars. The topic of apparent magnitude (how bright an object appears from our vantage point on earth) and absolute magnitude (how bright an object would appear if viewed from a specific distance away) are discussed. 

    The show's final two stellar extreme questions -– "which are the fastest stars?" and "which are the smallest stars" –- lead into a discussion of neutron stars and black holes, each of which results from the death of very massive stars. These tiny remnants of massive stars are incredibly dense and can only be discovered by the effect they have on companion stars that orbit near them.