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Astro 1, Week 4

by: Mariela Ortiz

Astro 1, Week 4 Astro 1

Mariela Ortiz

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About this Document

These notes cover Chapters 7 and 8 in the Universe text.
Basic Astronomy
Dr. C L Martin
Class Notes
astronomy, astro, Physics
25 ?




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This 5 page Class Notes was uploaded by Mariela Ortiz on Tuesday October 18, 2016. The Class Notes belongs to Astro 1 at University of California Santa Barbara taught by Dr. C L Martin in Fall 2016. Since its upload, it has received 2 views. For similar materials see Basic Astronomy in Science at University of California Santa Barbara.


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Date Created: 10/18/16
7.1 - The solar system has two broad categories of planets: Earthlike and Jupiterlike The four inner planets (Mercury, Venus, Earth and Mars) have smaller orbits and are crowded close to the Sun Next four planets (Jupiter, Saturn, Uranus and Neptune) are widely spaced at great distances from the Sun Most of the planets have orbits that are nearly circular (ellipses) All orbit the Sun in the same counterclockwise direction, on nearly the same plane Terrestrial planets: four small inner planets that all have hard rocky surfaces, resembling Earth Very high average densities Dense iron cores Jovian planets: four large outer planets resembling Jupiter, all mostly gas or liquid Low average densities Composed primarily of light elements No solid surface 7.2 - Seven large satellites almost as big as the terrestrial planets All planets except Mercury and Venus have moons (satellites) Terrestrial: few or no satellites Jovian: so many moons it resembles a miniature solar system Seven of which are as big as planet Mercury 7.3 - Spectroscopy reveals the chemical composition of the planet The most accurate way to determine chemical composition is by directly analyzing samples taken from a planet’s atmosphere If no samples can be taken, astronomers must analyze the sunlight reflected off of distant planets and satellites through spectroscopy Spectrum of the reflected sunlight will have dark absorption lines indicative of a chemical composition Includes the chemical composition of the Sun as well, because light has to pass through the Sun’s atmosphere before it can go anywhere else Light reflecting off of solid surfaces yields broad absorption features, not lines, that can be compared with things found on Earth to infer a chemical composition 7.4 - The Jovian planets are made of lighter elements than the terrestrial planets Outer layers of Jovian planets are composed of the lightest gases, helium and hydrogen Soil samples from terrestrial planets indicate a heavier composition including iron, oxygen, silicon, etc Higher surface temperatures of terrestrial planets help to explain that their atmospheres contain virtually no hydrogen or helium Composed of heavier molecules such as nitrogen, oxygen, and carbon dioxide Higher temperatures mean that if those gases were present, they’d be moving fast enough to escape the relatively weak gravity Jovian planets: low surface temperatures and strong gravity prevent lightweight gases from escaping into space 7.5 - Small chunks of rock and ice also orbit the Sun Asteroids: rocky objects found within the inner solar system Considered minor planets Asteroid belt: region between Mars and Jupiter where most asteroids orbit trans-Neptunian objects: found beyond Neptune in the outer solar system, contain both rock and ice Pluto is considered a trans-Neptunian object Most orbit within a band called the Kuiper belt Centered on the plane of the ecliptic Comets: mixtures of rock and ice that originate in the outer solar system but venture close to the Sun The Sun’s radiation vaporizes some of the comet’s ices when it comes too close, creating long flowing tails of gas and dust particles Composition of comets can be deduced by studying the spectra of their tails Some appear to originate from some locations far beyond the Kuiper belt Source is thought to be a halo of comets called the Oort comet cloud 7.6 - Craters on planets and satellites are the result of impacts from interplanetary debris Many asteroids and comets are in more elongated orbits that can put them on a collision course with a planet or satellite Jovian planets will swallow up the object Terrestrial planets colliding with asteroids or comets result in an impact crater Meteoroids: relatively small objects thought to be the reason behind smaller craters on the Moon Result of collisions between asteroids when their orbits cross The smaller the terrestrial world, the less internal heat it is likely to have retained, and, thus, the less geologic activity it will display on its surface The less geologically active the world, the older, and more heavily cratered its surface 7.7 - A planet with a magnetic field indicates a fluid interior in motion Magnetic field measurements are a powerful way to investigate the internal structure of a world Dynamo: molten material conducts electricity and gives rise to electric currents, which produce a magnetic field that is sustained by the planet’s rotation Can’t take place if a planet or satellite has a mostly solid interior Magnetic fields indicate a liquid presence in the interior of a planet or satellite that generate the magnetic field Spacecrafts often carry magnetometers to measure magnetic fields 8.2 - The cosmic abundances of the chemical elements are the result of how stars evolve Comparison of sizes between terrestrial and Jovian planets suggests that some chemical elements are more common while others are quite rare Hydrogen and helium account for about 98% of the mass of all material in the solar system Elements that make up the Earth and living organisms are relatively rare in the universe as a whole Astronomers believe the universe began in a violent event known as the Big Bang Only the lightest elements could emerge from the enormously high temperatures following the event Heavier elements were later manufactured by stars, either by thermonuclear reactions in their interiors or the violent explosions that mark the end of massive stars Near the end of their lives, stars cast much of their matter back out into space Nebulosity:the ejected material that forms a cloudy region that surrounds the star and is illuminated by it Supernova: stars ejecting their matter in a much more dramatic fashion Interstellar medium: tenuous collection of interstellar gas and dust that pervades the spaces between the stars As different stars die, they increasingly enrich the interstellar medium with heavy Elements New stars then have an adequate supply of heavy elements from which to develop a system of planets, satellites, comets and asteroids Everything is essentially made of recycled cosmic material Stars create different heavy elements in different amounts 8.3 - The abundance of radioactive elements reveal the solar system’s age Radioactive elements: atomic nuclei are unstable because they contain too many protons or too many neutrons Radioactive decay: nucleus rejects particles until it becomes stable, sometimes changes from one element to another Radioactive dating: used to determine how many years ago a rock cooled and solidified Applied to rocks taken from all over the Earth and Moon Meteorites: oldest rocks found anywhere in the solar system, bits of interplanetary debris that survive passing through Earth’s atmosphere and land on the planet’s surface All of them are nearly the same age 8.4 - The Sun and planets formed from a solar nebula Tidal hypothesis: two nearby planets, stars, or galaxies exert tidal forces on each other that cause the objects to elongate Another star happened to pass close by the Sun and the star’s tidal forces drew a long filament out of the Sun which then went into orbit around the Sun in the same direction, on the same plane Planets would then condense from this filament Disproved because the tidal forces strong enough to pull a filament out like that would cause it to disperse before it could condense Nebular hypothesis: the entire solar system, including the Sun, all of the planets, and satellites, were formed from a solar nebula Solar nebula: vast, rotating cloud of gas and dust Each part of the nebula exerted a gravitational attraction on the other parts, and these mutual gravitational pulls tended to make the nebula contract Protosun: relatively dense region in the center of the nebula, formed by the greatest contraction of matter, and eventually developed into the Sun Material falling inward toward the protosun would have gained speed, and the kinetic energy would then convert into thermal energy which caused the temperature inside the protosun to climb until it stabilized at the surface temp of 6000 K Interior continued to get hotter until nuclear reactions began Kelvin-Helmholtz contraction: the gravitational energy of a contracting gas cloud is converted into thermal energy Solar nebula must have been spinning ever so slightly, otherwise there would be no material from which the planets would form Protoplanetary disk: a rotating flattened disk surrounding what will become the protosun


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