Astronomy 1010 Study Guide Midterm Important Terms to Know ● Universe The sum of all matter and energy ● Observable universe The portion of the universe that can be seen from Earth ● Galaxy a big island of stars, containing perhaps trillions of stars ● Galaxy cluster A group of more than a few dozen large galaxies ● Superclusters If you want to learn more check out select progeny of the cross rt/rt×rt/rt.
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Regions of galaxies and galaxy clusters that are tightly packed ● Star Ball of gas, generates heat + light through nuclear fission in its core ● Planet A moderately sized object that orbits a star and shines by reflecting light from its star ● Moon (or satellite) An object that orbits a planet ● Asteroid A small rocky object that orbits a star ● Comet A small icy object that orbits a star ● Nuclear fusion The process in which lightweight atomic nuclei bond to make heavier nuclei ● Constellation A region of the sky with well defined borders ● Zenith The point directly overhead ● Horizon All points 90° away from the zenith ● Meridian Line passing through zenith and connecting north and south points on the horizon ● Angular size Physical size x [360°/ ( x dist.)] 2� ● Latitude Position north or south of equator ● Longitude Position east or west of prime meridian ● Sidereal day 23hrs 56 mins ● Sidereal month 27.3 days (Moon’s orbital period around the Earth) ● Synchronous rotation A rotation with each orbit ● Speed Rate at which objects move ● Velocity Speed & direction ● Acceleration Any change in velocity ● Momentum= mass x velocity ● Net force Changes momentum, which generally means an acceleration ● Angular Momentum The rotational momentum of a spinning or orbiting object ● Mass The amount of matter in an object ● Weight The force that acts on an object ● Thermal Energy The collective energy of many particles ● Ellipse Elongated circle ● Escape velocity If an object gains enough orbital energy, it may escape ● Nebula elements that formed planets were made in stars and then recycled through interstellar space● The Nebular Theory of Solar System Formation Our solar system formed from a giant cloud of interstellar gas ● Lithosphere A planet’s outer layer of cool, rigid rock ● Impact Cratering Impacts by asteroids or comets ● Volcanism Eruption of molten rock onto surface ● Tectonics Disruption of a planet’s surface by internal stresses ● Erosion Surface changes made by wind, water, or ice ● Greenhouse gas molecules with two different types of elements ● Astronomical Unit (AU) Average distance between the earth and the sun (93 million miles) ● Lightyear The distance that light can travel in a year ○ Not a measurement of time ○ 1 lightyear is about 9.46 trillion kilometers The Scale of the Universe On a scale of 1to10 billion ● A lightyear is about one millimeter ● The sun is the size of a large grapefruit ● Earth is the size of a ball point ● The distance from Pluto and the next star is about the distance from the East Coast to the West Coast The Milky Way cannot be put on the 1to10 billion scale The Milky Way has about 100 billion stars Is one of about 100 billion galaxies 10^11 stars/galaxy x 10^11 galaxies = 10^22 stars Sun moves randomly relative to the other stars ● Typical relative speeds of more than 70,000 km/hr ● Sun orbits the galaxy every 230 million years The History of the Universe We know that the universe is expanding because the space between galaxies is increasing. This means that at some point, they were very close together. If you go far back enough, you reach the beginning, which we call the Big Bang. ● 14 billion years ago Stars Are not living organisms, but they still go through “life cycles.” Are born when gravity compresses the material to the point at which the center becomes dense and hot enough to generate energy by nuclear fusion. “Live” as long as they can generate energy. Die and blow much of their content back out into space. Cosmic Calendar The 14 billion year age of the universe compressed into one year. The Big Bang occurred on January 1. The Milky Way formed in February Our solar system did not form until September Recognizable animals came into existence midDecember The entire history of human civilization falls into just the last half minute of the year The ancient Egyptians built the pyramids about 11 seconds ago The average college student was born about 0.05 seconds ago How the earth moves The Earth rotates once each day around its axis Earth rotates from west to east Earth rotates at a speed of more than 1000 km/hr (600 mph) Earth is orbiting the sun at more than 100,00 km/hr (60,000 mph) Earth orbits counterclockwise Earth’s orbital path makes a flat plane called the ecliptic plane Earth’s axis is tilted by 23.5° The axis points almost directly at Polaris or the North Star Our solar system moves around other stars at about 70,000 km/hr Orbits the Milky Way at about 800,000 km/hrPatterns in the sky We can see more than 2000 stars with the naked eye The brightest stars in a constellation may actually be very far from each other Our view from Earth As Earth orbits the sun, the sun appears to move eastward along the ecliptic Earth rotates west to east, so stars appear to circle from east to west At midnight, the stars on our meridian are opposite the Sun Stars near the north celestial pole are circumpolar and never set We cannot see most of the stars near the south celestial pole Everything else rises in the east and sets in the west Constellations depend on latitude because your position on Earth determines which constellations remain below the horizon Seasons Seasons depend on how Earth’s axis affects the directness of the light Axis tilt changes directions of sunlight throughout the year Sun’s altitude changes with the seasons Why doesn’t distance matter? Variation of Earth Sun distance is small about 3° This small variation is overwhelmed by the effects of axis tilt The Moon The only satellite of Earth Lunar phases are a consequence of the Moon’s 27.3 day orbit around Earth Half the moon is illuminated by the Sun, the other half is dark Synchronous rotation This is why only one side is visible from Earth 41% of the Moon is hiddenMoon phases Eclipses The Earth and the Moon cast shadows When either passes through the other’s shadows, it causes an eclipse Lunar eclipses Can occur only at full moon Can be penumbral, partial, or total Solar eclipses Can occur only at new moon Can be partial, total, or annular No eclipse every month Moon’s orbit is tilted 5° to the ecliptic plane We have two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon Predicting eclipses: eclipses recur with the 18 year, 11.3 day saros cycle but type and location vary Acceleration of Gravity All falling objects accelerate at the same rate (not counting friction of air resistance) On Earth, speed increases 10m/s with each second of falling Galileo showed that g is the same for all falling objects regardless of their mass Mass vs. Weight There is no gravity in space Weightlessness is due to a constant state of freefall Weight can change, mass does not Force causes change in momentum producing acceleration Newton’s Laws of Motion Newton (1642 1727) realized that the same physical laws that operate on Earth also operate in the heavens First Law of Motion: An object moves at constant velocity unless a net force acts upon it Second Law: Force = Mass x Acceleration Third Law: For every force, there is always an equal and opposite reaction force Conservation Laws The total momentum of interacting objects cannot change unless an external force is acting upon them Interacting objects exchange momentum Angular Momentum The angular momentum of an object cannot change unless an external force is acting on it Earth experiences no twisting force as it orbits the Sun, so its rotation and orbit will continue indefinitely The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted Angular momentum conservation explains why objects rotate faster as they shrink in size Where do objects get their energy? Energy makes matter move Energy is conserved but… It can transfer from one object to another It can change in form Energy Basic types of energy Kinetic (motion) Radiative (light) Stored or potential Energy can change type but cannot be destroyed Thermal Energy Related to temperature, but not the same Temperature is the average kinetic energy of the many particles in a substance Measure of the total kinetic energy of all the particles in a substance Gravitational Potential Energy On Earth, depends on… An object’s mass The strength of gravity The distance an object could potentially fall In space, an object or gas cloud has more gravitational energyMass Energy A small amount of mass can release a great deal of energy Concentrated energy can spontaneously turn into particles Conservation of Energy Energy cannot be created or destroyed It can change form or be exchanged between objects A planet keeps rotating because of conservation of angular momentum The Universal Law of Gravitation 1. Every mass attracts every other mass 2. Attraction is directly proportional to the product of their masses 3. Attraction is inversely proportional to the square of the distance between their centers Kepler’s Laws of Planetary Motion First Law: The orbit of each planet around the Sun is an ellipse with the Sun at one focus Second Law: As a planet moves around its orbit it sweeps out equal areas in equal times This means that a planet travels faster when it is nearer to the Sun and slower when it is farther from the Sun Third Law: More distant planets from the Sun orbit the SUn at slower average speed, obeying the relationship Kepler’s first two laws apply to all orbiting objects, not just planets Gravity & Tides The Moon’s gravity pulls harder on the near side of Earth than on the far side The difference in the Moon’s gravitational pull stretches Earth The Sun also exerts tides on the Earth The net tides are the combined effects of the two Size of tides depends on the phase of the Moon Tidal Friction Tidal Friction gradually slows Earth’s rotation (and makes the Moon get farther from Earth) Two Categories of Planets Terrestrial small, rocky, Small particles of rock and metal were present inside the frost line Planetesimals of rock and metal Jovian large, hydrogenrich Ice could also form small particles outside the frost line Larger planetesimals and planets were able to form The gravity of these larger planets was able to draw in surrounding H + He gasses Moons of jovian planets form in miniature disks ● Swarms of asteroids and comets populate the solar system. ● Notable Exceptions Some planets have unusual axis tilts, unusually large moons, or moons with unusual orbits ● Planets are tiny compared to the distance between them Sun Biggest body in the solar system Over 99.8% of solar system’s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second Mercury Made of metal and rock; large iron core Desolate, cratered Very hot and very cold 425 C C° (day) 170°C (night) Smooth plains Cliffs Moon Craters Smooth plains No water Venus Nearly identical in size of Earth Hellish conditions due to greenhouse effect Volcanoes Few craters Earth Oasis of life Surprisingly large moon Volcanoes Some craters Mountains Riverbeds Mars Looks Earthlike Giant volcanoes, huge canyon, polar caps No atmosphere Riverbeds? Water flowed in distant past Some craters Jupiter Much farther from Sun than inner planets Mostly H/He; no solid surface 300 times more massive than Earth Many moons, rings Io Active volcanoes Europa Possible subsurface ocean Ganymede Largest moon in solar system Callisto Saturn Giant and gaseous Spectacular rings Rings are not solid made of chunks of rock and ice Uranus Smaller than Jupiter/ Saturn but much larger than Earth Made of H/He gas and hydrogen compounds Extreme axis tilt Moons and rings Neptune Similar to Uranus (except for axis tilt) Many moons (including Triton) Pluto Much smaller than other planets Icy cometlike composition Pluto’s moon Charon is similar in size to PlutoFlattening Collisions between particles in the cloud caused it to flatten into a disk Collisions between gas particles in a cloud gradually reduce random motion Collisions between gas particles also reduce up and down motions The spinning cloud flattens as it shrinks Disks around other stars Observations of disks around other stars support the nebular hypothesis Inside the frost line too hot for hydrogen compounds to form ices Outside the frost line cold enough for ices to form Accretion Gravity draws planetesimals together to form planets Many smaller objects collected into just a few large ones Asteroids and Comets Leftovers from the accretion process Rocky asteroids inside frost line Icy comets outside frost line Heavy Bombardment Leftover planetesimals bombarded other objects in the late stages Origin of Earth’s water May have come to Earth by way of icy planetesimals from the outer solar systemCaptured moons The unusual moons of some planets may be captured planetesimals Dating the Solar System We cannot find the age of a planet but we can find the age of the rocks that make it up Age dating of meteorites that are unchanged since they condensed and acreted tells us that the solar system is about 4.6 billion years old Radiometric dating tells us that the oldest moon rocks are 4.4 billion years older The oldest meteorites are 4.55 billion years old Planets probably formed 4.5 billion years ago Radioactive Decay Some isotopes decay into other nuclei A halflife is the time for half the nuclei in a substance to decay Earth’s Interior Core Highest density, nickel, and iron Mantle Moderate density Crust Lowest density, granite, basal, etc. Differentiation Gravity pulls high density material to center Lowerdensity material rises to surface. Material ends up separated by density Lithosphere “Floats” on the warmer softer rock that lies beneath Heat drives Convection Hot rock rises, cool rock falls One convection cycle takes 100 million years on Earth Sources of Internal Heat Gravitational potential energy of accreting planetesimals Differentiation Radioactivity Heating of Interior over Time Accretion and differentiation when planets were young Radioactive decay is most important heat source today. Cooling of Interior Convection transports heat as hot materials rise and cool materials fall Conduction transfers heat from hot material to cool material Radiation sends energy into space Role of size Smaller worlds cool off faster and harden earlier The Moon and Mercury are now geologically “dead” Surface AreatoVolume Ration Heat content depends on volume Loss of heat through radiation depends on surface area Time to cool depends on surface area divided by volume Larger objects have a smaller ration and cool more slowlyPlanetary Magnetic Fields Moving charged particles create magnetic fields A planet’s interior can create magnetic fields if its core is electrically conducting, convecting, and rotating Earth’s Magnetosphere Earth magnetic field protects us from charged particles from the Sun The charged particles can create aurorae (Northern Lights) Geological Processes Impact cratering Most cratering happened soon after the solar system was formed Craters are about 10 times wider than the objects that made them Small craters outnumber the larger ones Volcanism Happens when molten rock (magma) finds a path through lithosphere to the surface Molten rock is called lava after it reaches the surface Releases gases from Earth’s interior into the atmosphere Tectonics Convection of the mantle creates stresses in the crust called tectonic forces Compression forces make mountain ranges A valley can form where the crust is pulled apart Erosion A blanket term for weather driven processes that break down or transport rock Processes that cause erosion Glaciers Rivers WindRadiation Protection All xray light is absorbed very high in the atmosphere Ultraviolet light is absorbed by Ozone (O3) The Greenhouse Effect Certain molecules let sunlight through but trap escaping infrared photons A Greenhouse Gas Any gas that absorbs infrared Not a greenhouse gas: molecules with one or two atoms of the same element CO2 etc. absorb IR photons and get excited to a higher level of vibrational state Mercury and the Moon geologically dead Moon Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria that appear to be rather smooth Now geologically dead Mercury Mixture of heavily cratered and smooth regions like the moon The smooth regions are likely ancient lava flows Tectonics of Mercury Long cliffs indicate that Mercury shrank early in its history Mars vs. Earth 50% of Earth’s radius, 10% Earth’s mass Considerably smaller than Earth 1.5 AU from the Sun Axis tilt about the same Similar rotation period Thin CO2 atmosphere: little greenhouse Main difference size Climate Change on Mars Mars has not had widespread surface water for 3 billion years The greenhouse effect probably kept the surface warmer before that Somehow Mars lost most of its atmosphere Magnetic field may have preserved early Martian atmosphere Solar wind may have stripped atmosphere after field decreased because of interior cooling Venus Still geologically active Very hot, volcanoes Impact craters, fewer than Moon, Mercury, Mars Fractured and contorted surface indicate tectonic stresses Little erosion Plate tectonics Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface Venus Temperatures The greenhouse effect keeps its surface at 470℃ Why is the greenhouse effect stronger on Venus than on Earth? Very thick carbon dioxide atmosphere with a surface pressure 90 times than that on Earth Earth escapes this fate because most of its carbon and water are in rocks and oceansAtmosphere on Venus Reflective clouds contain droplets of sulfuric acid The upper atmosphere has fast winds that remain unexplained Runaway Greenhouse Effect Would account for why Venus has so little water Earth’s Unique Features Necessary for Life Surface liquid water Earth’s distance from Sun and moderate greenhouse effect make liquid water possible Atmospheric oxygen Photosynthesis is required to make high concentrations of O2 which produces the protective layer of O3 Plate tectonics Seafloor recycling seafloor is recycled through a process known as subduction Important step in the carbon dioxide cycle 1. Atmospheric CO2 dissolves in rainwater 2. Rain erodes minerals that flow into the ocean 3. Minerals combine with carbon to make rocks on ocean floor 4. Subduction carries carbonate rocks down into the mantle 5. Rock melts in mantle and outgassed CO2 back into atmosphere through volcanoes Climate stability The CO2 cycle acts like a thermostat for Earth’s temperature Longterm Climate change Changes in Earth’s axis tilt might lead to ice ages Widespread ice tends to lower global temperatures by increasing Earth’s reflectivity CO2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again Habitable Planets Located at an optimal distance from the Sun for liquid water to exist Large enough for geological activity to release and retain water and atmosphere Jovian Planet Formation Beyond the frost line, planetesimals could accumulate ice Hydrogen compounds are more abundant than rocks and metals so jovian planets got bigger The jovian cores are very similar Mass of 10 Earths The jovian planets differ in the amount of H/He compounds Differences Timing: The planet that forms earliest captures the most hydrogen + helium gas. Capture ceases after the initial solar wind blows the leftover gas away Location: The planet that forms in a denser part of the nebula forms its core first Density Uranus and Neptune are denser than Saturn because they have less H/He proportionally But that explanation does not work for Jupiter Size of Jovian planets Adding mass to a jovian planet compresses the underlying gas layers Greater compression is why Jupiter is not much larger than Saturn in size, even though it is three times more massive Jovian planets with even more mass can be smaller than Jupiter Interiors of Jovian Planets No solid surface Layers under high pressure and temperatures Cores (10 earth masses) made of hydrogen compounds, metals, and rock The layers are different or the different planets Jupiter Interior High pressure causes the phase of hydrogen to change with depth Hydrogen acts like a metal at great depths because its electrons move freely Comparing Jovian Interiors Models suggest that cores of jovian planets have similar composition Lower pressures inside Uranus and Neptune mean no metallic hydrogen Jupiter’s Magnetosphere Jupiter’s strong magnetic field gives it an enormous magnetosphere Gases escaping Io feed the donutshaped Io torus around Jupiter Aurora on Jupiter Jupiter’s aurorae are permanent their intensities vary The satellite spots where the magnetic field lines connect to Jupiter’s biggest moonsJupiter’s Atmosphere Hydrogen compounds in Jupiter form clouds Different cloud layers correspond to freezing points of different hydrogen compounds Other jovian planets have similar cloud layers Jupiter’s Colors Ammonium sulfide clouds reflect red/brown Ammonia, the highest, coldest layer, reflects white Saturn’s Colors Saturn’s layers are similar but are deeper in and further out from the Sun Methane on Uranus and Neptune Methane gas on Neptune and Uranus absorbs red light but transmit blue light Blue light reflects off methane clouds, making those planets look blue Jupiter’s Great Red Spot A storm twice as wide as Earth Has existed for at least three centuries Weather on Jovian planets All have strong winds and storms Satellites Size of Moons Small Moons (<300 km) No geological activity Far more numerous than the medium and large moons Not enough gravity to be spherical Medium + Large Moons (>300 km) Enough selfgravity to be spherical Have substantial amounts of ice Circular orbits in same direction as planet rotation Jupiter’s Galilean Moons Tidal Heating Io is squished and stretched as it orbits Jupiter Tidal heating arises because Io’s elliptical orbit causes varying tides Io’s orbit is elliptical because of the orbital resonance Io shares Europa Interior warmed by tidal heating Ganymede Largest moon in the solar system Clear evidence of geological activity Tidal heating plus heat from radioactive decay Callisto Classic cratered iceball No tidal heating, no orbital resonances But it might have a magnetic field Saturn’s Moons Titan Largest moon of Saturn Only moon in solar system that has a thick atmosphere It consists mostly of nitrogen with some argon, methane, and ethane Surface has liquid methane, “rocks” made of ice Radar imaging of Titan's surface has revealed dark, smooth regions that may be lakes of liquid methane. Medium Moons Almost all show evidence of past volcanism Fountains of ice particles and water vapor Jovian Planet Rings Saturn’s Rings Made up of numerous, tiny individual particles Orbit over Saturn’s equator They are very thin Gap Moons Some small moons create gaps within rings Ring Systems All four jovian planets have ring systems Others have ring particles that are smaller and darker than Saturn’s They formed from dust created in impacts on moons orbiting those planets Rings aren't’ leftover from planet formation because the particles are too small There must be a continuous replacement of tiny particles The most likely source is impacts within the jovian moons Ring Formation Jovian planets all have rings because they possess many small moons closein Impacts on these moons are random Saturn’s incredible rings may be an “accident” of our time