Astro 1, Midterm Study Guide
Astro 1, Midterm Study Guide Astro 1
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This 25 page Study Guide was uploaded by Mariela Ortiz on Tuesday October 18, 2016. The Study Guide 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 21 views. For similar materials see Basic Astronomy in Science at University of California Santa Barbara.
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Date Created: 10/18/16
1.1 - To understand the universe, astronomers use the laws of physics to construct testable theories and models Scientific method: approach based on fundamentally on observation, logic and skepticism Requires that our ideas about the world around us be consistent with what we actually observe 1. Hypothesis: collection of ideas that seems to explain what is observed Must always agree with existing observation unless the scientist thinks existing results are wrong and can give compelling evidence to prove it Scientist then uses logic to work out implications of the hypothesis and make prediction that can be tested Only on firm ground after it has accurately forecast the results of new experiments or observations 2. Models: hypotheses that have withstood observational or experimental tests Tells us about the properties and and behavior of some object or phenomenon Ex. model of atom: electrons orbiting a central nucleus 3. Theory: a body of related hypotheses pieced together in a self-consistent description of nature Ability to make predictions that can be tested by other scientists If predictions are verified, theory might be correct If unable to verify, theory needs to be adjusted or replaced entirely An idea that can’t be tested by experiment or observation does not qualify as a scientific theory Skepticism is an important part of the scientific method The more radical a hypothesis, the more skepticism and critical evaluation it will receive from the scientific community Extraordinary claims require extraordinary evidence Scientists must also be open-minded and willing to discard long-held ideas if they fail to agree with new observations and experiments if the new data survives critical review Astronomers use the laws of physics to interpret and understand their observations of the universe Laws governing light and its relationship to matter are particularly important because the only info we can get about stars and galaxies comes from the light we receive 1.2 - By exploring the planets, astronomers uncover clues about the formation of the solar system Solar system: the Sun and all the celestial bodies that orbit it Many of the planets and their satellites were shaped by collisions with other objects Meteorites: oldest objects found on Earth; chemically distinct bits of interplanetary debris that sometimes falls to our planet’s surface Oldest meteorites date back 4.56 billion years 1.3 - By studying stars and nebulae, astronomers discover how stars are born, grow old, and die The Sun is the nearest of all stars to Earth At the very center, thermonuclear reactions convert hydrogen to helium Releases a vast amount of energy which escapes as light Nuclear reactions consume the original material so the star eventually dies The rate at which stars emit energy in the form of light tells us how rapidly they are consuming their nuclear fuel (hydrogen) and how long they have left More massive stars: more fuel to consume but faster lifespans Nebulae (single: nebula): huge clouds of interstellar gas found scattered across the sky Some stars are born from the material of the nebula itself Others (more massive than the Sun) end their life with a massive detonation called a supernova (plural: supernovae) Pulsars: result of some dead stars; spin rapidly at rates of tens or hundred of rotations per second Black holes: results of some dead stars; inconceivably dense objects where gravity is so powerful nothing can escape Stars return the gas of which they are made to interstellar space Contains heavy elements created during the star’s lifetime by nuclear reactions in its interior 1.4 - By observing galaxies, astronomers learn about the origin and fate of the universe Stars aren’t uniformly spread across the universe, but are grouped together in huge assemblies called galaxies Typical galaxy contains 700 billion stars; some are much smaller, and others devour neighboring galaxies (galactic cannibalism) Quasars: sources of energy that look like neighboring stars Draw their energy from material falling into enormous black holes Big Bang: cosmic explosion that acted as the beginning of the universe 1.5 - Astronomers use angles to denote the positions and apparent sizes of objects in the sky Angle: measures the opening between two lines that meet at a point Measured in degrees Can be used to describe distance and apparent size (angular size and angular diameter) For smaller angles, a degree can be divided into 60 arcminutes 1° = 60 arcminutes = 60 ’ ’ ’’ 1 = 60 arcseconds = 60 If given the angular size and distance, linear size can be determined ⍺d Small-angle formula: D = 206,265 D: linear size or width d: distance ⍺: angle 206,265 = number of arcseconds in a 360° circle, divided by 2???? **necessary so that the units of ⍺ can be in arcseconds 1.6 - Powers-of-ten notation is a useful shorthand system for writing numbers 10 = 1 10 = 1,000 10 = 1,000,000,000 10 = 10 10 = 10,000 10 = 1,000,000,000,000 2 6 10 = 100 10 = 1,000,000 -1 -3 3 -6 6 10 = 1/10 = .1 10 = 1/10 = .001 10 = 1/10 = .000001 -2 2 -4 4 -12 12 10 = 1/10 = .01 10 = 1/10 = .0001 10 = 1/10 = .000000000001 1.7 - Astronomical distances are often measured in astronomical units, light-years, or parsecs Astronomers usually create new units using prefixes 1 nanometer = 10 m -9 1 centinmeter = 10 m -2 -6 3 1 microsecond = 10 s 1 kilometer = 10 m 1 milliarcsecond = 10 arcsec-3 1 megaton = 10 tons 6 Astronomical units (AU): used when discussing distances across the solar system 8 1 AU = average distance between Earth and Sun = 1,496 x 10 km = 92.96 million miles Light-year (ly): distance that light travels in one year, not how long it takes Speed of light in an empty space always has the same value 5 5 3.00 x 10 km/s OR 1.86 x 10 mi/s 1 ly = 9.46 x 10 km = 63,240 AU ≈ 6 trillion miles Parsec (pc): the distance at which 1 AU extends over an angle of 1 arcsec 13 1 pc = 3.09 x 10 km = 3.26 ly 1 kiloparsec (kpc) = 1,000 pc = 10 pc 3 6 1 megaparsec (Mpc) = 1,000,000 pc = 10 2.1 - Naked-eye astronomy had an important place in civilizations of the past Positional astronomy: the study of the positions of objects in the sky and how these positions change The astronomical knowledge of ancient civilizations is the foundation of modern astronomy 2.2 - Eighty-eight constellations cover the entire sky Constellation: groupings of stars in the sky The entire sky is divided into 88 regions known as constellations Some cover large areas in the sky (like Ursa Major) and others are smaller (Crux) Most stars seem close together but are actually nowhere near each other 2.3 - The appearance of the sky changes during the course of the night and from one night to the next Diurnal motion: daily motion of stars across the sky The Earth rotates once a day around an axis from the north pole to the south pole and revolves once a year around the sun Stars will rise four minutes earlier every night, and two hours earlier every month Some of the brightest stars are visible in the western hemisphere during winter 2.4 - It is convenient to imagine that the stars are located on a celestial sphere Many ancient societies believed that all stars are the exact same distance from Earth Imagined them all to be bits of fire imbedded in the inner surface of an immense hollow sphere (celestial sphere) with the Earth in a fixed position, no rotation Celestial sphere was thought to rotate around the Earth - no basis in physical reality Projections: made by extending an imaginary line perpendicular to the surface of Earth until it intersects the celestial sphere North celestial pole: projection of Earth’s north pole Polaris is called the North Star or Pole Star because it is less than 1° away from the north celestial pole South celestial pole: projection of Earth’s south pole Celestial equator: projection of Earth’s equator Divides the sky into northern and southern hemisphere Zenith: the point in the sky directly overhead an observer anywhere on Earth Can only ever see one hemisphere at a time The other is below the horizon line Circumpolar: can be seen at any time of night and on any night of the year For observers at most locations on Earth, stars rise in the east and set in the west 2.5 - The seasons are caused by the tilt of Earth’s axis of rotation The seasons are opposite in the northern and southern hemispheres February - midwinter in North America, midsummer in Australia Earth’s axis is not perpendicular to the plane of Earth’s orbit, tilted about 23.5° away Earth maintains this tilt, with the North pole always pointing in the same direction Latitude: denotes how far north or south of the equator you are Analogous to declination: angular distance north or south of the celestial equator Minus sign indicates object of interest is south of the celestial equator Measured in degrees, arcminutes, or arcseconds Longitude: denotes how far east or west of an imaginary circle that runs from the north pole to the south pole Analogous to right ascension: angular distance from the vernal equinox eastward along the celestial equator to the circle used in measuring declination Measured in units of time Ecliptic plane: the plane of Earth’s orbit around the sun Ecliptic: the circular path that the Sun appears to trace out against the stars Plane is the same as the ecliptic plane, but not the same as the plane of the Earth’s equator Equinox: one of two points where the ecliptic and the celestial equator intersect, exactly opposite each other on the celestial plane Also the date on which the Sun passes through one of these points Vernal equinox: March 21 Autumnal equinox: September 22 Summer solstice: the point on the ecliptic farthest north of the celestial equator Summer begins in the northern hemisphere: June 21 Winter solstice: the point on the ecliptic farthest south of the celestial equator Winter begins in the northern hemisphere: December 21 Days are less than 12 hours long Arctic circle: the circle around Earth at 66.5° N latitude Winter solstice = 24 hours continuous daylight 2.6 - The Moon hekos to cause precession, a low, conical motion of Earth’s axis of rotation Moon slowly changes its position relative to the background stars, but makes a complete trip around the celestial sphere in about a month Path is never far from the ecliptic because the plane of the Moon’s orbit is inclined only slightly from the plane of Earth’s orbit around the Sun Always remains within a band called the zodiac Twelve constellations lie along the zodiac Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces Also passes along a 13th constellation: Ophiuchus Causes a slow change in Earth’s rotation because both the Sun and Moon exert a gravitational pull Earth has an equatorial bulge, and pull from the Sun and Moon causes precession North and South celestial poles change their position relative to the stars Causes Earth’s equatorial plane to change its orientation, and celestial equator precesses as well Also referred to as the precession of equinoxes Positions of the stars change gradually, slowly over time Astronomers always make date of the epoch for which a particular set of coordinates is correct so that star charts and catalogs can be updated 2.7 - Positional astronomy plays an important role in keeping track of time Sundial was invented to keep track of apparent solar time Astronomers use the meridian to obtain more accurate measurements North-south circle on the celestial sphere that passes through the zenith and both celestial poles Local noon: Sun crosses the upper meridian Local midnight: Sun crosses the lower meridian Meridian transit: the crossing of the meridian by any object in the sky Upper meridian transit: crossing occurs above the horizon Not necessarily at the zenith, but at the highest visible point above the horizon Apparent solar day: the interval between two successive upper meridian transits of the Sun From one local noon to the next local noon Length varies from one time of year to another Earth’s orbit is not a perfect circle; actually an ellipse Moves more rapidly in its orbit when it is near the Sun than when it is farther away No comparable foreshortening around the beginning of summer and winter because of the 23.5° angle between the ecliptic and the celestial equator Solar day in March is shorter in March and September than in June or December Mean sun: moves along the celestial equator at a uniform rate Mean solar day: interval between successive upper meridian transits of the mean sun - 24 hours long Sidereal time: based on the apparent motion of the stars 2.8 - Astronomical observations led to the development of the modern calendar Length of a year is approximately 365.25 days Julius Caesar established leap years by adding an extra day to the calendar every four years so that seasonal astronomical events remained on the same day Would be flawless if not for precession Sidereal year: the time required for the Sun to return to the same position with respect to the stars - 365.2564 mean solar days Tropical year: the time needed for the Sun to return to the vernal equinox - 365.3422 mean solar days 3.1 - The phases of the Moon are caused by its orbital motion The Moon takes about four weeks to move around the imaginary celestial sphere People once believed that both the Sun and Moon orbit the Earth Only the Moon orbits it, while the Earth-Moon system orbits the Earth The Sun emits its own light, but the light from the Moon is reflected light Sunlight that has struck the Moon’s surface Reflection: light bouncing off an object Only ever see the half of the Moon that faces the Sun, but not all of this half is necessarily facing Earth Lunar phases: different appearances of the Moon in relation to how much of the illuminated half is visible on Earth New moon: phase during which the Moon is barely visible Can only be located near the Sun, rises around sunrise and sets around sunset Waxing crescent moon: increasing visible surface area First quarter moon: half of the Moon’s illuminated hemisphere and half of the dark hemisphere are visible One quarter of the way around the celestial sphere from the Sun Moon rises around noon and sets around midnight Waxing gibbous moon: more of the illuminated hemisphere is visible Gibbous: swollen Illuminated part is toward the west Full moon: fully illuminated hemisphere is visible on earth Rises at sunset and sets at sunrise Waning gibbous moon: less of the illuminated hemisphere is visible Waning: decreasing Third quarter moon: half of the Moon’s illuminated hemisphere is visible Also called last quarter moon Waning crescent moon: decreased visible surface area Last phase before cycle restarts From any location on Earth, about half of the Moon’s orbit is visible at any time Lunar phases are not caused by the shadow of the Earth on the Moon, but by seeing the illuminated half at different angles as it moves 3.2 - The Moon always keeps the same face toward Earth Moon always keeps essentially the same hemisphere facing the Earth because is is rotating Synchronous rotation: it takes exactly as long for the Moon to rotate on its axis as it does to make one orbit around Earth No side of the moon is perpetually in darkness as a result Far side: side that constantly faces away from the Earth because of synchronous rotation Sidereal month: the time it takes the Moon to complete one full orbit of Earth with respect to the Stars With respect to a reference point (usually the stars) What you would observe while hovering in space watching the Moon orbit the Earth Equal to about 27.32 days Synodic month: the time it takes the Moon to complete one cycle of phases measured with respect to the Sun Also known as the lunar month A day on Earth is measured with respect to the Sun, because approximately 24 hours pass between sunrises or sunsets (synodic day) Lunar day: the time from sunrise to sunrise as seen from the Moon’s surface Equal to a synodic month Longer than the sidereal month because Earth is orbiting the Sun while the Moon goes through its phases The Moon must travel more than 360° along its orbit in order to complete a cycle of phases Synodic month is equal to about 29.53 days to account for the extra distance 3.3 Eclipses occur only when the Sun and Moon are both on the line of nodes There are times when the Sun, Moon and Earth all happen to lie along one straight line Eclipses: shadow of Earth can fall on the Moon or the shadow of the Moon can fall on Earth Lunar eclipse: occurs when the Moon passes through the Earth’s shadow Earth falls between the Sun and Moon so that the Moon is at full phase Face of the Moon seen from Earth appears quite dim Solar eclipse: Earth passes through the Moon’s shadow Moon falls between the Sun and the Earth Moon moves in front of the Sun Can only occur at new moon phases Infrequent because the plane of Earth’s orbit and the plane of the Moon’s orbit are not exactly aligned Angle between the two is about 5° New moon and full moon usually occur when the Moon is either above or below the plane of Earth’s orbit The Moon appears from Earth to be on the ecliptic when an eclipse occur Line of nodes: the line along which the planes of Earth’s orbit and the Moon’s orbit intersect Passes through Earth and points in a particular direction in space Eclipses can only occur when pointed toward the Sun and the Moon lies on or very near the line of nodes Nothing to do with solstices or equinoxes Rotates slowly westward because of the gravity of the sun 3.4 - The character of a lunar eclipse depends on the alignment of the Sun, Earth and Moon Shadow of Earth has two distinct parts Umbra: no portion of the Sun’s surface can be seen from the Moon Total lunar eclipse: occurs if the Moon travels completely into the umbra Small amount of light reaches the Moon during a total lunar eclipse through the thin ring of atmosphere around the Earth Most of the sunlight that passes through is red, makes the Moon glow in faintly red hues Maximum possible duration if the Moon travels directly through the center of the umbra Totality: the period when the Moon is completely within Earth’s umbra Can last for as long as 1 hour and 42 minutes because the Moon’s speed through Earth’s shadow is 3600 kph or 2280 mph Partial lunar eclipse: occurs if only part of the Moon passes through the umbra Penumbra: a portion of the Sun’s surface is visible Not quite as dark Penumbral eclipse: Earth blocks only part of the SUn’s light so none of the lunar surface is completely shaded Only looks a little dimmer than usual 3.5 - Solar eclipses also depend on the alignment of the Sun, Earth and Moon Moon fits over the sun during a total solar eclipse because the angular diameter of both is ~.5° Solar corona: thin hot outer atmosphere of the Sun Visible during solar eclipses As the Earth rotates, the tip of the umbra traces an eclipse path across Earth’s surface The Moon’s penumbra covers a large portion of Earth’s surface during a solar eclipse Anyone standing inside can see a partial solar eclipse Perigee: the point in the Moon’s orbit nearest Earth Eclipse path is widest Moon’s umbra does not reach all the way to Earth’s surface in some eclipses Apogee: farthest position from Earth Annular eclipse: Moon appears too small to cover the Sun completely A thin ring of the Sun is seen around the edge of the Moon Slightly more common because the Moon’s shadow often fails to reach Earth even when properly aligned for an eclipse 4.1 - How ancient astronomers attempted to explain the motions of the planets The universe can be described and understood logically Geocentric model: most astronomers believed that the Earth was at the center of the universe The Sun, Moon, stars and planets all revolved around the Earth Now recognize that the stars are not merely points of light on an immense celestial sphere All planets rise in the east and set in the west once a day Slowly move on the celestial sphere with respect to the background of stars but character of motion for planets is slightly different Sun and Moon always move from west to east on the celestial sphere, opposite from which the celestial sphere appears to rotate Sun follows the ecliptic while the Moon follows a path that is slightly inclined to the ecliptic Each move at relatively constant speeds around the celestial sphere with the Moon’s speed slightly faster than the Sun Planets also appear to move along paths close to the ecliptic, but each of the planets appears to wander back and forth on the celestial sphere with varying speed Direct motion: planets move slowly eastward relative to the stars Retrograde motion: planets occasionally seem to stop and back up for several weeks or months All go through retrograde at different intervals Ptolemaic system: each planet is assumed to move in a small circle (epicycle) whose center moves in turn with a larger circle (deferent) which is centered approximately on Earth Both rotate counterclockwise When the planet is on the part of the epicycle closest to Earth, the motion of the planet along the epicycle is opposite to the motion of the epicycle along the deferent Planet therefore appears to slow down and proceed in retrograde Occam’s razor: when two hypotheses can explain the available data, the hypothesis requiring the fewest new assumptions should be favored 4.2 - Nicholaus Copernicus devised the first comprehensive heliocentric model Heliocentric model: all planets revolve around the Sun Different planets take different lengths of time to complete an orbit The occasional retrograde motion of a planet is merely the result of Earth’s fast motion Could be used to determine the arrangement of the planets without ambiguity Inferior planets: planets with smaller orbits, closer to the sun Mercury and Venus Greatest eastern elongation: as far east of the Sun as possible, visible after sunset Elongation: the angle between the Sun and a planet as viewed from Earth Greatest western elongation: as far west of the Sun as possible Inferior conjunction: between Earth and the Sun Superior conjunction: on the opposite side of the Sun Superior planets: orbits larger than Earth’s, further from the sun Best seen in the night sky when it is in opposition The plant is in the part of the sky opposite the Sun and highest in the sky at midnight Appears brightest because it is closest to Earth Synodic period: the time that elapses between two successive identical configurations as seen plane Objective lens: large diameter, long-focal-length lens at the front of the telescope Forms the image Eyepiece lens: smaller, shorter-focal-length lens at the rear of the telescope Magnifies the image Light-gathering power of a telescope depends on the lens diameter Magnification/magnifying power = focal length of objective lens focal length of eyepiece lens A lens bends different colors of light through different angles Different colors do not focus at the same point, so stars viewed through a simple lens telescope have a fuzzy, rainbow-colored halo around them (chromatic aberration) Can be corrected through the use of an objective lens that is not just a single piece of glass Many negative aspects to refractors 1. Glass must be made totally free of defects, extremely expensive 2. Glass is opaque to certain kinds of light UV light is absorbed almost completely and visible light is considerably dimmed 3. Producing a large lens entirely free of chromatic aberration is impossible 4. A large lens tends to sag and distort under its own weight Distortion has negative effects on image clarity 6.2 - A reflecting telescope uses a mirror to concentrate incoming light at a focus Reflecting telescope: uses a curved mirror to make an image of a distant object Objective/primary mirror: forms the image Light reflects off the surface of the mirror rather than pass through it Defects within the glass would have no effect on the optical quality of the telescope as a whole Don’t suffer from chromatic aberration Reflection is not affected by the wavelength of the incoming light, only the angle at which it hits the mirror All wavelengths are reflected in the same focus Mirror can be mounted with braces to avoid warping from gravity Must be designed to minimize spherical aberration Spherical aberration: different parts of a spherical mirror focus light onto slightly different spots, resulting in a fuzzy image Can be avoided by polishing the mirror’s surface to a parabolic shape, but this eliminates a wide angle view Coma: defect of parabolic mirrors where star images far from the 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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