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INTRODUCTION Powers of Ten A way to express big and large numbers concisely Forces Important in Astronomy Gravity - every piece of matter attracts every other piece Depends on masses and separation - Expressed in math shorthand as F = GMm/r2 Electricity and Magnetism Depends on charge (+ or -) - Like charges repel; unlike attract The nuclear or strong force THE COSMIC LANDSCAPE The Earth, Our Home A sphere (Known to ancient Greeks) - R about 6000 km Rotates - causes day and night Has an atmosphere - held on by Earth's gravity Has a magnetic field - deflects compass needle The Moon Orbits Earth - held in place by gravity A sphere - about 1/4 radius of Earth Mass is much smaller than Earth's Mass - consequently weaker gravity No atmosphere - result of that weak gravity The Sun a star Gaseous - mix of mostly H and He Held together by gravity Holds Earth and other planets in orbit by its gravity Has a magnetic field - sunspots Hot - heated by nuclear reactions (Fusion of H into He) Tsurf - 6000 K Tcore - 15 million K The Solar System flattened system - held together by gravity The Nine Planets Asteroids and comets The Astronomical Unit - distance between Earth and Sun Other stars Nearest is 4.3 light years If pinheads, separated by about 35 miles The Milky Way Galaxy Assemblage of some 100 billion stars - held together by gravity 100,000 ly across (approx) Rotates - approx once every 220 million years Galaxy Clusters and the Universe The Local Group - small cluster of about 30 galaxies A few million ly across - held together by gravity The Local Supercluster - a group of galaxy clusters- held together by gravity Dark Matter Universe contains large amounts of material that has not yet been directly detected We know it is there from its gravitational attraction on matter (stars and galaxies) that we can see Nature of this material is perhaps the major mystery in astronomy today. Amount of dark matter is huge - roughly 100 times the amount of luminous matter The Universe The largest astronomical structure we know of HISTORY OF ASTRONOMY Why Important? To see how astronomers have used observation and hypothesis to understand the heavens Also introduces importance of astronomy to everyday life All our time-keeping based on astronomical motion Prehistoric Astronomy Classical Astronomy Astronomy in the Renaissance Newton and Astrophysics The Modern Astronomy Early Ideas — Prehistoric Astronomy Why? Time keeper Harvest crops Study the future God!! Out comes Stars, Sun, and Moon are mounted on the Celestial Sphere - a sphere that surrounds Earth( its not the real model) Star patterns are fixed – Constellations ( pattern of Stars, use imagination) Celestial sphere rotates around Earth and causes rising and setting of stars and other objects Motion of the Sun and Stars Daily Motion and Annual Motion Saesons Sun's path across the sky changes with seasons The Ecliptic: The sun path across the background sky. High in Summer, low in winter Stars visible near Sun before dawn and after dusk change during year. Sun's shifts its position on celestial sphere with respect to the stars Sun's path = Ecliptic Solstices and Equinoxes The Direction of the rising and sitting change through the year Sun rises due east and sets due west on Equinoxes Equinoxes (approx. equal hours of day and night) Marks start of spring and autumn Sun rises farthest north along horizon at June solstice Rises farthest south at December solstice Marks start of summer and winter The Planets and the Zodiac Some star-like objects change position with respect to stars Called Planets, from ancient Greek for "wanderer" Their path always near the ecliptic. Band they are found within = Zodiac The Moon Cycle of phases - caused by changing illumination as Moon orbits Earth (rises in the east and sets in the west). Moon shifts position against stars - approx follows Zodiac Eclipses - caused by Moon's shadow falling on Earth, or vice-versa Early Ideas of the Heavens: Classical Astronomy The Shape of the Earth The Size of the Earth Distance and Size of Sun and Moon Models for the Motion of the Planets Earth at center Other objects move around it affixed to "crystalline spheres" Planets sometimes reverse direction with respect to stars Retrograde motion Explained by "epicycles" - small spheres attached to larger ones Ptolemaic Model Astronomy in the Renaissance Copernicus - revives Sun-centered model of Heavens Tycho - made detailed observations of planet positions Kepler - used Tycho's positional data to derive "Laws of Planetary Motion" Orbits are ellipses - Sun off-center at focus Equal area law Orbital period related to orbital size (P2 = a3) Galileo - applied telescope to study of heavens Laws of motion Discovered moons orbiting Jupiter - Earth not center of all motion Discovered phases of Venus - proof that Venus orbits Sun, not Earth Isaac Newton and the Birth of Astrophysics Law of Gravity Mathematical expression of laws of motion GRAVITY AND MOTION Why Important? Solving the Problem of Astronomical Motion Inertia Tendency of an object at rest to remain at rest and an object in motion to keep moving Extended by Newton to First Law of Motion An object at rest remains at rest. An object in motion continues to move in a straight line at a constant speed unless a net force acts on it. Such motion is called "uniform" Non-uniform motion is called an acceleration - a change of either speed or direction Orbital Motion is not uniform Object follows a curved path Therefore, according to First Law, a force must act on it For most astronomical objects that force is Gravity Law of Gravity Described by Newton - F = GMm/r2 Newton's Second Law of Motion F = ma Relates acceleration of an object to the force acting on it Allows shape of orbit and details of motion to be worked out Mass measures amount of material more technically, its inertia Newton's Third Law action = reaction Measuring a Body's Mass Using Orbital Motion Newton's laws of gravity and motion allow mass of object to be deduced from orbital motion of object moving around it Surface Gravity Determines gravitational force of planet on object = weight of object Escape Velocity Speed needed to move away from an object and not fall back Gravitational force of planet, star, and so forth makes it difficult for a mass to move away from object. Higher velocity allows a mass to rise further from object. Important in determining whether planet has atmosphere and for black holes (vescape = c) LIGHT AND ATOMS Why Important? Nearly all our knowledge of heavens comes to us through the "light" we receive from astronomical objects. Understanding nature of light from these objects yields knowledge of their properties Properties of Light The Nature of Light -Waves of electric/magnetic energy or Particles (Photons) Light and Color - color set by wavelength (red long, blue short) White Light - mix of all wavelengths The Electromagnetic Spectrum: Beyond Visible Light Ordering by decreasing wavelength gives Radio Waves, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays Energy Carried by Electromagnetic Waves (shorter wavelengths, more energy) Wien's Law: A Color-Temperature Relation Red cool, Blue hot Atoms Structure of Atoms (nucleus and orbiting electrons) The Elements (number of protons in nucleus determines kind of element) Atoms store energy by lifting electrons to higher orbits Return of electron to lower orbit releases energy which appears as light (or more generally, electromagnetic radiation) Each kind of atom has unique set of electron orbits - motion of electrons between orbits leads to unique set of wavelengths of light Thus - Atoms can be identified by the light they emit or absorb Gives composition of stars, gas clouds, and planets Types of Spectra Hot low density gas - emission line spectrum Light only at some wavelengths High density hot gas, liquid, or solid - continuous spectrum Light at all wavelengths Cool gas between observer and light source absorption line spectrum Light missing or dimmer at some wavelengths The Doppler Shift Observer sees the wavelengths emitted or absorbed by a moving source shifted If source and observer move apart, wavelengths increase (shifted to red - redshift) If source and observer approach, wavelengths decrease (shifted to blue - blueshift) Earth's atmosphere absorbs some wavelengths Especially in non-visible wavelengths Atmospheric absorption limits astronomers ability to study heavens from ground (IR, UV, etc) THE EARTH - OUR HOME PLANET The Earth as a Planet Shape is approx sphere - radius about 6400 km Rotation bulges equator slightly Composition of the Earth Crustal rocks directly sampled Interior composition deduced from density = Mass/Volume Density is about 5.5 gm/cm3 Midway between rock (3 gm/cm3) and iron (8 gm/cm3) The Earth's Interior Probed with Earthquake Waves Reveal thin crust, mantle, liquid core, and solid inner core Crust and Mantle are rock (silicates) Liquid and inner core are iron/nickel Heating of the Earth's Core Some heat is probably residual from Earth' formation Some heat supplied by radioactivity in rock - like natural nuclear reactor Age of the Earth Deduced from radioactivity in rock - about 4.5 billion years Heat of interior "stirs" interior - convection Convective motions make large thin areas of crust slide over mantle rock Plate Tectonics Collision of plates buckles crust - mountain ranges Separation of plates opens rifts - some ocean basins Plate motion also triggers volcanic eruptions along some plate edges The Earth's Atmosphere Composition of the Atmosphere - 78% Nitrogen, 21% oxygen smaller amounts of water, carbon dioxide and other gases Origin of the Atmosphere Gas from volcanic eruptions has accumulated over time Gases added include nitrogen, water, and carbon dioxide Some of atmosphere may have come from comets that struck Earth and evaporated Oxygen in atmosphere formed (mostly) by photosynthesis by plant-life The Ozone Layer - absorbs UV, protecting us The Greenhouse Effect Carbon dioxide and water in atmosphere trap heat Makes Earth warmer than if gases not present Earth's Magnetic Field Rotation and convection in liquid iron/nickel core probably generate Earth's Magnetic Field Magnetic field affects motion of gases in upper atmosphere - leads to aurora Motions of the Earth Earth spins on axis - cause of day and night Orbits Sun Sun orbits Milky Way Galaxy The Seasons Earth's rotation axis is tilted about 23.5o with respect to its orbit Earth acts like gyroscope and keeps approx same orientation as it orbits Sun As result, Sun shines most directly on different hemispheres at different times of year - leads to more heating and more hours of daylight Air and Ocean Circulation Rotation of Earth creates the Coriolis Effect A deflection of the path of objects moving over Earth's surface Deflection drives winds and ocean currents Precession Earth's rotation angle does not keep exactly same orientation Slowly "wobbles" - once every 26,000 years Leads to different "North Stars" May cause climate change THE MOON Description of the Moon Barren ball of rock orbiting Earth R about 1/4 REarth M about 1/80 MEarth Surface Features Highlands - low density, light colored rock - heavily cratered Maria - higher density, dark colored rock - relatively smooth Most Craters are impact features - circular pit, raised rim of ejected material Rays - light colored streaks of pulverized rock radiate from some craters Rilles - lunar canyons No folded mountains, few if any volcanic peaks No signs of tectonic activity as on Earth Structure of the Moon Crust and Interior - Outer layers rigid rock Low density implies much smaller iron core than Earth Inner core may be hot, but Small size of Moon let it cool far more than Earth Absence of a Lunar Atmosphere Low gravity means hard to retain an atmosphere - gases readily escape Lack of appreciable inner heat means little or no volcanic activity to generate atmosphere Orbit and Motions of the Moon Moon rotates although it keeps same face to Earth Rotation period = orbital period - rotation synchronous Orbit tilted about 5o with respect to Earth's orbit around Sun Origin and History of the Moon Moon probably formed by giant impact that splashed matter from Earth's crust Material orbited Earth, gradually accumulated to form Moon Bombardment by debris late in formation left craters Huge impacts blasted basins that later flooded with lava to form Maria Moon's small size led to rapid cooling and little subsequent heating Since then Moon essentially inactive Eclipses Rarity of Eclipses results from tilt of Moon's orbit Shadows of Earth and Moon therefore generally miss one another Appearance of Eclipses Tides Moon's gravitation attraction creates two tidal bulges on Earth's oceans Earth rotates under bulges, so at a given location on ocean, water rises twice and falls twice each day. Tidal Braking - rotation of Earth under the tidal bulges creates friction like a brake slows Earth's rotation BASIC FEATURES AND ORIGIN OF THE SOLAR SYSTEM Solar System Consists of 9 planets (MVEMJSUNP), their moons, the Sun, and smaller objects: asteroids and comets. All held in single system by Sun's gravity Sun's mass about 1000 greater than all other objects put together Solar System is essentially a disk All planets orbit Sun in same direction and in nearly same plane. Solar System contains two major types of planets Small rocky (rich in silicates and iron) objects like Earth - Terrestrial planets Large gaseous/liquid objects (rich in hydrogen and its compounds) like Jupiter- Jovian Planets Sometimes Inner and Outer planets used to distinguish these two types Differences show strongly in density - about 5 gm/cm3 for terrestrial about 1-2 gm/cm3 for Jovian Solar System contains two major types of small objects Asteroids and comets Asteroids are rocky and orbit between Mars and Jupiter in asteroid belt Comets are icy and most spend most of time far from Sun in Oort Cloud or Kuiper belt. All major objects about same age At least to the extent we can determine. (4.5 Billion yrs) Bode's Law A curious method for generating the distances of the planets from the Sun Start with 0, 3, 6, and so forth, by doubling Add 4, then divide by 10. Gives distance in AU Works accurately for Mercury - Uranus. "Predicted" asteroids Fails for Neptune, but "works" for Pluto Reason not understood Model for Origin of System Solar System formed from collapse of Interstellar cloud Cloud was mostly H and He, about few % heavier elements H and He gaseous, heavy elements in form of tiny solid dust grains rich in iron, silicates, carbon compound, water. Cloud was slowly spinning Collapse result of gravity As cloud shrinks, its rotation speed increased (Cons. of Ang. Mom.) Faster rotation leads cloud to form disk shape = Solar Nebula. Lump forms in center of disk. Becomes young Sun Cloud heated by collapse, but once in form of disk, cools again. Inner disk kept warm by young Sun Higher temperature in inner portion of disk allows only silicates and iron compounds to condense there as new small grains Farther out, disk is cooler, and ices and some gases freeze out as well Disk therefore has two zone structure Inner zone - iron/silicate grains Outer zone - iron/silicate/carbon/water/frozen gases Grains "coagulate" and form pebble-size particles. These stick together into boulder size objects Gravity begins to help clumps form and they rapidly grow to many kilometer size objects = planetesimals Planetesimals of two types Inner ones rocky (silicate/iron) Outer ones water and hydrogen-rich as well Inner planetesimals collide and form inner (terrestrial planets) Ditto for outer planetesimals. They are in region cold enough that they can capture hydrogen and helium (otherwise uncondensed). Turn into outer planets. Some evidence that the 4 big outer planets may have grown directly from "lumps" in the solar nebula. Also evidence that orbital position of outer planets may have drifted Some planetesimals survive: Captured by planets - form moons. Others become asteroids and comets Smaller bodies bombard planets and moons and create numerous craters Sun blows left over gas and dust out of system Extra-Solar Planets Many stars have planets orbiting them. Planets detected by their gravitational tug on parent star That tug moves star slightly, creating a Doppler shift From Doppler shift can deduce (with reasonable assumptions) mass of planet All planets found so far are very massive (Jupiter's mass or more, typically) Many puzzling features in these other planetary systems Orbits of many of these planets are very eccentric. Many of the Jupiter mass planets are very close to star ( some less than an AU) Do these large planets form far out in preplanetary disk and migrate inward? THE TERRESTRIAL PLANETS MERCURY (Dist. from Sun = .4 AU) Radius about .4 REarth Mass about 0.05 MEarth High density (about 5 gm/cm3)implies high iron content. Possibly result of impact stripping much of its rocky mantle No atmosphere - escape velocity so small any gas readily escapes Surface heavily cratered. Unmodified by tectonic forces/volcanic activity. No moon Odd rotation. Locked into 3/2 resonance with its orbital motion around Sun. VENUS (Dist from Sun = .7 AU) Radius about 0.95 REarth Mass about 0.8 MEarth (Thus approximate twin of Earth in size and density [about 5.3 gm/cm3]) Clouds prevent our viewing its surface from Earth with optical telescopes. Spectra show clouds to be sulfuric acid droplets. Spectra also show atmosphere is very dense (approx 100 times density of Earth's) and composed mostly (96%) of Carbon dioxide Dense CO2 atmosphere traps heat by greenhouse effect. Planet very hot as result (T about 900 F) Radar maps show surface is complex Craters, volcanic peaks, lava flows, lava flood plains, Level of volcanic activity implies thin crust and active planet Rotation very slow and retrograde MARS (Dist from Sun = 1.5 AU) Radius about 0.5 REarth Mass about 0.1 MEarth Density about 4 gm/cm3 Atmosphere is thin and contains clouds. Spectra show gas is mostly CO 2. Clouds are water-ice and dry ice Surface has reddish tint from high iron concentration in crustal rocks. Surface features include Craters Canyons Old River beds, sand-bars, islands Inactive volcanic peaks. Polar caps of water-ice and frozen CO2. "Deserts" with sand dunes. Mars has two small moons - Phobos and Deimos. Probably captured asteroids. COMPARISON OF THE TERRESTRIAL PLANETS All have a density of about 5 gm/cm3. Thus, all probably have iron core Small size of Mercury allowed it to lose heat quickly - thus inactive now. Mars too is small, so its interior is probably relatively inactive compared to that of larger Venus and Earth Venus and Earth large enough to have a) retained heat from their formation or b) generated it from radioactive decay Higher core temperature of Earth and Venus make them tectonically active and creates internal motions On Earth, motions lead to plate tectonics. On Venus, for still unknown reasons, little plate motion. Internal activity more like that seen near "hot-spots" on Earth Difference between Earth and Venus atmospheric composition attributed to Lack of liquid water on Venus and in its atmosphere. Liquid H2O absorbs CO2 to form carbonic acid Combines with rock to form carbonates Removes much CO2 from atmosphere. (Amount of CO2 in Earth rock is comparable to that in Venus's atmosphere) OUTER PLANETS (Jupiter, Saturn, Uranus, Neptune, and Pluto) JUPITER (Dist from Sun = 5.2 AU) Radius about 11 REarth Mass about 315 MEarth Huge mass makes Jupiter more massive than all other planets combined Density = Mass/Volume = about 1.3 gm/cm3 This low value compared to the approximately 5 gm/cm3 of terrestrial planets implies very different composition. Must be large quantities of low density elements such as H and its compounds such as water, ammonia, and methane (H2O, NH3, CH4) Spectra support this difference and show presence of helium as well. Atmosphere presumably thickens with depth, changing from gas to liquid as depth increases and compresses material. Below atmosphere a "sea" of liquid hydrogen. Below that, a zone of metallic hydrogen Core of rock and iron compounds likely on the basis of gravitational field and shape of planet Abundances of elements in Sun Theoretical models and measurements of heat loss from atmosphere imply interior is very hot. Heat drives convective motions Planet exhibits strong cloud belts - result of convection inside and Coriolis Effect Cloud motions show rapid rotation of planet. Approx 10 hours Other atmospheric features included Great Red Spot, (a huge atmospheric vortex) and smaller, fainter vortices. Faint, thin, ring system Extensive satellite system Most lie in equatorial plane and orbit in same direction: form mini-Solar System Four largest moons are Galilean satellites: Io, Europa, Ganymede, and Callisto - All rich in silicates (rocky material). All but Io rich in ice Io is heated by strong tidal forces exerted on it by Jupiter. Has erupting sulfur volcanoes Europa also appears to have been heated and partially resurfaced (few impact craters compared to Ganymede and Callisto) Strong magnetic field presumably caused by rapid rotation and convection in deep interior SATURN (Dist from Sun = 9.5 AU) Radius about 9.5 REarth Mass about 95 MEarth Density = Mass/Volume = about 0.7 gm/cm3 - lowest of any planet Low density implies composition similar to Jupiter's Spectra show rich in hydrogen, helium, and hydrogen compounds. Bright, wide ring system Extensive satellite system - similar to Jupiter's Most interesting moon is Titan, nearly size of Mercury. Has an atmosphere rich in nitrogen and perhaps seas of the hydrocarbon ethane URANUS (Dist from Sun = 19 AU) NEPTUNE (Dist from Sun = 30 AU) Radius about 4 REarth Mass about 15 MEarth Radius about 4 REarth Mass about 17 MEarth Uranus and Neptune were discovered in the 18th and 19th centuries respectively. They are very similar in size, mass, and composition. Spectra show their atmospheres to be mostly hydrogen and its compounds Both appear blue because the methane in their atmosphere absorbs red light strongly Both probably have a lot of water in their interior. Uranus's atmosphere shows few features (although infrared observations suggest features may appear and disappear) Neptune's atmosphere shows cloud bands and a Great Dark Spot, reminiscent of Jupiter's Red spot Both have faint rings. Both have extensive satellite systems Uranus has very large tilt of its rotation axis - result of ancient impact? PLUTO (Dist from Sun = 40 AU) Radius about 0.2 REarth Mass about 0.003 MEarth Density is about 2 gm/cm3, implying a rocky/icy structure Spectra show a thin methane atmosphere and methane frost on surface. Orbits the Sun exactly twice in the time Neptune orbits three times – suggesting Pluto has been "captured" into its orbit by Neptune. Many dozen other icy objects orbit at about the same distance. Many astronomers now consider Pluto just the largest of the Kuiper belt objects. METEORS, ASTEROIDS, AND COMETS Meteors We see as very brief bright streak of light in sky Caused by small (typically raisin) size piece of solid material entering atmosphere from space Friction of air molecules with object heats it and makes it glow Most burn up in atmosphere Those that survive to land on ground are called meteorites Source - asteroid and comet fragments Asteroids Rocky and or metallic objects Most orbit in Asteroid belt - between orbits of Mars and Jupiter. Largest is Ceres - Diameter about 1000 km. Most are much smaller (meter to kilometer size) and irregularly shaped Origin of Asteroids - probably left over planetesimals and their fragments Prevented from forming planet by Jupiter's gravitational influence A few cross Earth's orbit - Apollo object Other unusual asteroids Comets Appear as pale, fuzzy, elongated object in night sky Structure of Comets - bright head and long tail Head consists of small (about 10 km) size nucleus - a solid icy and dusty core When near Sun, ices evaporate and frozen gases thaw to create coma Coma is a large diffuse "atmosphere" around nucleus Tail forms near the Sun when Sun's radiation pressure and solar wind blow material from coma out into long plume Light from the Comet comes from reflected sunlight and fluorescence solar UV radiation absorbed and re-emitted as visible light Comet nuclei reside in Oort cloud at extreme outer part of Solar system 40,000 -100,000 AU from Sun. Some nuclei may come from Kuiper belt (flattened region beyond orbit of Neptune) Short Period Comets - such as Halley's Appear periodically (Halley's every 76 years) Fate of Short Period Comets Repeated passage by Sun evaporates ices and drives off gases Leaves trail of rocky and metallic dust and small particles Solids don't evaporate Meteor Showers occur when Earth passes through trail Asteroids and Comets Occasionally strike Earth and other objects in Solar System. Energy released can be huge if impacting body is large Craters found from such events many places A huge impact about 65 million years ago may have exterminated the dinosaurs THE SUN - OUR STAR Basic Properties Incandescent ball of gas Distance 1 AU - determined by radar or triangulation Radius about 100 REarth - determined by measuring angular size Mass - about 300,000 MEarth - determined from modified form of Kepler's Third Law Density - about 1.4 gm/cm3 - suggests composition rich in light elements Composition - about 71% H, 27% He - deduced from density and spectra Tsurface - 6000 K - measured from color with Wien's Law The Solar Interior Studied by calculations (Solar models) and observations of surface Energy is generated in the central core where T is large enough for fusion Energy flows from core by radiation - photons carry energy Near the surface, gas is cooler and absorbs photons Impedes flow of energy Energy carried instead by convection - Convection zone Visible "surface" of Sun = photosphere The Solar Atmosphere The hot solar gases above the photosphere Lower atmosphere = Chromosphere - hot (50,000 K) low density gas Upper atmosphere = Corona - extremely hot - 106 K, very low density gas How the Sun Works Sun is held together by gravity of its matter - Sun is supported by pressure of it gas Hydrostatic Equilibrium = balance of gravitational and pressure forces Pressure in the Sun Pressure in normal gases depends on gas temperature and density Perfect gas law - P = constant x density x Temperature Powering the Sun Sun is powered by nuclear fusion The bonding of light nuclei together to form heavier nuclei Total mass of final nuclei is less than total mass of initial nuclei Mass is converted to energy in this process (E = mc2) Fusion in Sun combines 4 hydrogen nuclei together to form a single helium nucleus Probing the Sun's Core Solar Neutrinos - emitted during hydrogen burning Have immense penetrating power - roughly a light year through lead. Leave Sun freely and thus give us info on core conditions Solar Seismology Just as earthquakes give info about interior of Earth, so waves traveling through Sun give info on its interior Waves create surface disturbances that can be observed. Solar Magnetic Activity Sun has magnetic field whose level of activity changes over time Sunspots - dark region on face of Sun (photosphere) - often huge compared to Earth Regions are dark because cool Cool because magnetic field inhibits heat rising freely in convection zone Magnetic effects directly measurable with Zeeman effect Spots often in pairs or groupings Number of spots changes with time. Reaches peak approx. every 11 years - The Solar Cycle Prominences and Flares Prominences are plumes, arches, or other shapes of luminous gas. Relatively long-lived, days to weeks Prominence supported by magnetic field Flares are brief intense brightenings of gas, generally near sunspots Flares triggered by rearrangement of local magnetic field Heating of the Chromosphere and Corona Magnetic activity probably heats chromosphere and corona - recall hotter than photosphere Waves travel out along magnetic field and agitate gas - heat it The Solar Wind The hot coronal gas has such high pressure that Sun's gravity cannot confine. (recall P increases as T increases) Coronal gas expands into space as solar wind The Solar Cycle The varying level of magnetic activity (spots, prominences, flares) Cause of the Solar Cycle - winding up of the Sun's magnetic field below its surface Field begins weak but grows stronger Field breaks through surface, creating spot pairs Links between the Solar Cycle and Terrestrial Climate Solar cycle not always same level of activity - many fewer spots at some maxima than at others Some evidence that low levels correspond to climatic cooling on Earth Maunder minimum - period of few spots 1645 - 1715 Coincides with "Little Ice age" - period of abnormal cold in Europe North America. Reason for such a link - if true - not understood MEASURING THE PROPERTIES OF STARS Measuring a Star's Distance Measuring distance by Triangulation and Parallax Distance that cannot be measured directly may be measured using triangulation Method involves Construct a base-line of known length Measure angles from ends of base-line to object whose distant you seek Construct scale model of triangle with same angles Measure unknown distance on scale model (or use Trig) Astronomers use similar method to find distance to stars Use Earth's motion around Sun to observe star from two different locations Radius of Earth's orbit then becomes base-line Star appears to lie in different positions as seen from ends of baseline That change in position is called parallax More remote stars show smaller change in position - smaller parallax Distance is thus inversely related to parallax By choosing units of distance cleverly, d = 1/p where d is measured in parsecs and p in arc seconds. 1 parsec (pc) = 3.26 light years Even nearby stars have only very tiny parallax, implying large distance Measuring distance by the Standard Candle Method Parallax method fails for very distant stars Their change in position is too tiny to measure Standard candle method uses fact that the distance to a light source of known brightness can be deduced from its apparent brightness For example, headlights of on-coming cars give clues to their distance Measuring the Properties of Stars from their Light Temperature - use Wien's Law - relates star's temperature to its color Luminosity - defined as power output of star the amount of energy it emits per second Luminosity (L) is related to star's distance (d) and its apparent brightness (B) - how bright it looks to us - by Inverse-square law - B = L/(4d2) . Procedure - Measure apparent brightness (B). Measure distance (d) using parallax Solve inverse square law for L = 4d2B and insert values for B and d Radius - Use Stefan-Boltzmann Law - L = 4R2T4 L = total energy emitted by star = energy emitted per square meter from star's surface (T4) multiplied by the number of square meters of star's surface, its surface area 4R2 Procedure - Measure T - by Wien's law for example Measure L as described above Solve for R The Magnitude System Astronomers sometimes use magnitude system to describe star brightness System based historically Stars that look brightest called magnitude 1 Stars just barely visible called magnitude 6 (Note this give scale working backwards: brighter stars have smaller magnitudes) Scale is set so stars of magnitude 1 are 100 times brighter than stars of magnitude 6. Thus 5 magnitudes corresponds to a brightness ratio of 100 Spectra of Stars Used to measure MANY properties of stars Composition - from which lines appear in spectrum Temperature - Affects a Star's Spectrum Different Temperatures lead to different appearing spectra Spectra classified on basis of appearance - which lines are strong Primary Classes are O, B, A, F, G, K, M O stars are hot and blue, M stars are cooler and red Sun is a G star Subclasses denoted by number. (G5 has T about mid-way between G0 star and A0 star) Motion - use Doppler shift of spectrum lines Binary Stars Some stars have companions held in orbit about them by their mutual gravity Percentage of stars that are binary is unknown for sure. Probably at least 40% - Maybe 80% Binary stars used to find masses and radii of stars Types of Binary Stars Visual binaries - two separate stars visible telescopically Spectroscopic Binaries - stars too close to see as separate stars Detected by doubling of spectrum lines Eclipsing Binaries - stars' orbits oriented so one passes between us and the other - light is blocked - eclipsed We see brightness of pair change Sometimes we see light from both stars, sometimes only one Measuring Stellar Masses with Binary Stars Use modified form of Kepler's third law m+M = a3/P2 m and M are in solar mass units, a, their separation, is in AU, and P, their orbital period, is in years Method works because stars held together by gravitational force and gravitational force depends on mass The H-R Diagram The Hertzsprung-Russell or H-R diagram is a graphical way to depict the properties of stars Stars are plotted in the diagram according to their Luminosity and surface temperature The temperature scale runs backwards - that is, hotter stars are found on the left, cooler stars on the right Equivalently, bluer stars are on the left, redder stars are on the right The plot shows Most stars (about 90%) lie within a band from upper right to lower left (From luminous and blue to dim and red) This band is called the MAIN-SEQUENCE A few stars lie in the upper right (luminous and cool) A few stars lie in the lower center (dim and hot) Thus, stars form THREE MAIN CLASSES Analyzing the H-R Diagram The Stefan-Boltzmann law allows us to interpret these classes A star can be luminous by being either hot or big or both A star can be dim by being either cool or small or both Thus, luminous cool stars must be big - GIANTS and dim hot stars must be tiny - DWARFS The Mass-Luminosity Relation Measurement of main-sequence stars in binary systems shows that the more massive a star, the more luminous it is L is approx M3, if we measure both L and M in solar units Thus, the main-sequence is a mass-sequence. Hot, luminous stars are more massive than cool, dim stars Variable Stars Some stars change in luminosity - illustrated by plotting a light curve (Brightness versus time) For many variable stars, the changes are repetitive, that is, the stars cycle from bright to dim and back again repeatedly Many variables pulsate - that is, they change size, expanding and contracting Finding a Star's Distance by the method of Standard Candles Method uses inverse square law - B = L/(4d2) Choose a type of star whose luminosity (L) we know Perhaps from spectrum, perhaps from light variability properties Measure its apparent brightness (B) Solve equation for d2 by cross-multiplying by 4d2 to get 4d2 B = L Divide by 4B to get d2 = L/(4B). Take square root to get d. Evaluate d by plugging in measured numbers for L and B. STELLAR EVOLUTION Star Formation Stars form from Interstellar Gas Clouds Clouds are big (several light years diameter) Cold (T about 10 - 100 K), low density Gravity draws material into clumps that turn into protostars Protostars - earliest stage of a star's life - star not yet stabilized - still shrinking Protostar surrounded by disk of gas and dust Similar to disk that formed planets in Solar System Gas flows away from star and disk in two narrow cones - Bipolar Flows Main Sequence Stars Star's size stabilized and burning hydrogen into helium in its core Star's Mass Determines its Core Temperature More mass leads to more gravity More gravity requires more pressure to support More pressure achieved by being hotter Hotter leads to more luminosity Thus, along main sequence in H-R diagram, hotter and more luminous stars must be more massive Mass also determines speed with which star burns up its core hydrogen Limits star's lifetime - Main Sequence Lifetime Massive stars are more luminous More luminosity implies faster burning of fuel Faster burning of fuel leads to shorter lifetime Thus, massive stars do not live as long as low-mass stars Stellar Mass Limits If mass too small, core not hot enough to burn hydrogen (M greater than about 0.08 MSun for hydrogen to burn) If M too large, star so hot as it forms that it drives away gas near it Mass cannot accumulate on star and so its mass remains less than about 100 MSun Consumption of all the hydrogen in a star's core forces star to adjust structure Core contracts and heats. Hydrogen burns in zone around core - shell source Outer layers of star swell up (expand) and cool Star becomes Red Giant - leaves main sequence Giant Stars - tiny, dense, hot core with huge cool "envelope" Nuclear Fuels Heavier than Hydrogen T in core may be hot enough to fuse helium into carbon (3 4He ---> 12C) - requires T about 108 K Ignition of helium in core of low-mass star heats core and eventually leads to core expansion and envelope contraction Star changes from red giant to yellow giant (In H-R diagram, shifts to left and down toward main sequence) Pulsating Stars Some stars swell and shrink in radius. This causes their luminosity to change with time - pulsating variable stars. Time to complete pulsation cycle = period Several important classes - Cepheids and RR Lyrae variables - will be used later to measure distances Why Do Stars Pulsate? Atmosphere traps radiation. Star swells, allowing radiation to escape The Period-Luminosity Relation The more luminous the star, the slower it pulsates (longer its period) Law allows determination of star's luminosity by timing its variations Inverse-square law then allows distance to be found Measure apparent brightness; comparison with luminosity determined from period gives distance Death of Stars Like the Sun Helium burning in core eventually consumes all the helium and forms carbon core Carbon core shrinks and heats Outer layers of star expand to make star red giant again Carbon and silicon atoms in cool outer layers condense to form grains Grains "pushed-on" by radiation pressure from luminous core of star Grains in turn push on gas, stripping off outer layers and forming huge shell around star Shell expands and thins out - hot core of star is revealed Ultraviolet radiation from core heats shell and makes it glow Planetary Nebula Old Age of Massive Stars Formation of Heavy Elements: Nucleosynthesis High-mass stars have hotter cores than lowmass stars Tcore in very massive stars hot enough to fuse elements heavier than helium and carbon, such as O ---> Neon ---> Silicon ---> Iron Reactions produce energy to keep star supported Core eventually becomes iron, but iron cannot fuse and release energy Iron core thus is end for high-mass star Iron core collapses - density rises Protons and electrons in core forced so close they "merge" ---> neutrons Core becomes tiny (10 km) ball of neutrons Outer parts of star fall in on tiny core - triggers massive explosion Supernova Explosions Explosion blows off outer layers at 10,000 km/sec Ejected debris called Supernova Remnant Remnant core becomes neutron star (if core less than about 3 MSun) or black hole (if mass of core is more than about 3 MSun) Testing Stellar Evolution Theory We can test correctness of basic theory of stellar evolution by comparing H-R diagram of a real star cluster with H-R diagram generated by a computer for group of model stars. The procedure is as follows: Construct computer models for stars with a number of different masses Let each model star with its different mass "evolve" on the computer Plot the resulting evolutionary track of each model star on an H-R diagram Draw a line that connects the points on each track that have the same age For example, at t = 0, all stars will lie on the main-sequence At some later time, the high-mass stars will have evolved slightly off the main-sequence At a still later time, some of the most massive stars will have become red giants Lines so determined = "isochrones", that is, lines of equal age In a real star cluster, we assume all stars form at approximately the same time At birth, all stars lie on the main-sequence As real stars age, plotted positions on the H-R diagram shift We test to see whether our calculated H-R diagrams look like real H-R diagrams We find that they do, implying our theory is correct. Measuring the Age of a Group of Stars Construct an H-R diagram for the group whose age we seek Compare the real H-R diagram to the computer generated H-R diagrams Look for a computer H-R diagram of the same shape as the real one Read off its computer-determined age. That will be the age of the real star group STELLAR REMNANTS The end stages in a star's life White Dwarfs Formed when stars like Sun reach end of life R about 0.01 RSun = about REarth M about MSun No nuclear burning - shine by residual heat Will cool and grow dimmer Structure of White Dwarfs Density about 106 gm/cm3 - roughly 16 tons/cubic inch At such compressions, material does not behave like ordinary gas degenerate Degenerate gas is much less compressible Pressure does not depend on Temp Adding material raises gravitational attraction of body on itself - BUT Interior pressure supporting star rises less. Thus P insufficient to support star if mass made too large Maximum mass = Chandrasekhar Limit = 1.4 MSun White Dwarfs in Binary Systems: Novas and Supernovas of Type I White Dwarf with a companion star may attract matter from companion New matter is rich in hydrogen Matter accumulates on White Dwarf and eventually burns explosively Nova Explosion Nova outbursts may recur If too much mass accumulates, mass of white dwarf exceeds Chandrasekhar Limit and star collapses Collapse compresses and heats interior of White dwarf Triggers nuclear burning of carbon and oxygen - star explodes Supernova Explosion - Type I Neutron Stars Form when massive star's iron core collapses and triggers supernova explosion Radius - 10 km Mass about 1 to 3 MSun Collapse to tiny radius increases rotation rate (by Conservation of Angular Momentum) Collapse also amplifies any magnetic field in star Combination of rotation and magnetic field - radiation beams from poles Beams sweep cross sky - if one points at Earth, we see burst of radiation each time star spins - pulses Spinning neutron star detectable as pulsar X-ray Binary Stars results if neutron star in binary system Mass captured from companion falls toward neutron star, spins around it as accretion disk before falling onto star Gas falling on to star and gas in accretion disk are very hot - may emit xrays Black Holes Core of very massive stars may collapse into black hole when star explodes as Supernova if core mass greater than about 5 MSun Why "black?" Escape velocity equals speed of light = c No radiation or matter can escape from such object Size of "black hole" measured by its Schwarzschild radius (about 3 km for star of 1 MSun) Observing Black Holes Black holes in binary star systems may capture mass from companion Gas forms accretion disk around black hole before falling in Orbital motion heats gas to 10 million K - emits x-rays Gravitational Waves from Double Compact Stars Hawking Radiation Black holes may emit tiny amounts of radiation "Quantum" process may permit hole to convert some of its mass into radiation Loss of mass leads to evaporation of hole Process extremely slow - 1067 years or so for solar mass black hole THE MILKY WAY Refers to the band of light made up of millions of faint stars that we see in the night sky Band is our galaxy Basic Structure of Milky Way Disk and spiral arms winding out from central regions Bulge of older stars clustered around center of disk Halo - swarm of much older stars completely surrounding disk Dark Matter Halo - vastly larger and more massive - composition unknown Nucleus - the innermost core Sun about 1/2 to 2/3 way out in disk between two spiral arms Basic Properties of Milky Way Diameter - about 100,000 ly for visible disk Mass - greater than a few times 1011 MSun Disk rotates - near Sun takes about 240 million years Velocity of rotation is about 220 km/sec Contents Stars and Interstellar matter Interstellar matter is gas (mostly H and He) plus tiny dust particles Entire assemblage held together by gravity - each star and gas cloud follows own orbit Age of the Milky Way About 15 billion years (deduced from age of oldest stars) How do We Know the Shape, Size, and Mass of the Milky Way? Shape - appearance on the sky as a narrow band of stars. Band implies disk shape not sphere shape. Mass - Treat Milky Way as giant binary star with Sun as one star and rest of Milky Way as other star. Use modified form of Kepler's Third Law Size and Sun's Location Worked out by Harlow Shapley in 1920's He observed globular clusters: Noted they outline Milky Way on sky. Measured distances to clusters using variable stars in them Plotted Clusters in a scale drawing of system Noted that Sun lies off center Measured size using scale of drawing. Two Stellar Populations: Pop I and Pop II Pop I tend to occur in disk and move along roughly circular orbits Pop II are very old, tend to be red, occur in halo, move along more elongated orbits Pop I contain higher proportion of heavy elements than Pop II Star Clusters Two main types - open and globular Open - few hundred stars, loosely clumped - youngest stars found in them Globular - 105 - 106 - densely clumped - oldest stars found in them Gas and Dust in the Milky Way Interstellar Dust: Obscuration and Reddening - limits view of galaxy In disk can only see a few thousand light years Clouds contain much dust - block light of background stars (Dark nebulas) Interstellar Gas Visible Emission from Interstellar Gas Gas near young luminous, hot stars heated and glows (Emission nebulas) Radio Waves From Cold Interstellar Gas Useful for mapping the structure of Milky Way because radio waves penetrate dust The Galactic Center - The Nucleus of Milky Way Invisible at optical wavelengths - Too much dust Study at radio, infrared, and gamma ray wavelengths Large number of stars packed densely - about 1/1000 separation near Sun Rapid orbital motion of stars and gas around core deduced from spectra Black hole? - If so, M about million solar masses How Formed? Starts small - few solar masses Grows by collecting gas Eventually gets big enough to "eat" stars History and Formation of Milky Way (Offers not only picture of galaxy's birth but origin of two stellar populations) Intergalactic gas cloud - pure H and He, slow spin Collapses under its gravity - some stars of pure H and He form in collapsing gas Collapse is slow; takes several 100 million years Massive stars that form evolve, generate heavy elements in core and die Explode as supernova and add heavy elements to gas Additional stars form in collapsing gas, but now contain some heavy elements These become Pop II stars "Polluted" gas collects in disk - result of cloud's rotation Spiral arms form as "density wave" Clouds in arms collapse and form Second generation of stars in disk - Pop I Model requires modification now because of evidence that Milky Way has "swallowed" a number of smaller galaxies. OTHER GALAXIES Galaxies are immense star systems held together by the mutual gravity of the member stars Galaxy Types Spirals (S) - like Milky Way - Disk systems with Spiral arms Ellipticals (E) - no arms - smooth distribution of stars Irregulars (Irr) - no obvious order or structural pattern Barred Spirals (SB) - like S type, but with arms winding out from end of bar S0 - disk systems with no spiral structure Properties of Galaxy Types S - mix of stars (about 85%) and interstellar matter (about 15%) Stars are mix of Pop I ("young") and Pop II (old) E - mostly Pop II - generally no "cold" interstellar matter. Thus, nothing to make new stars from Why different Types? Answer not known for sure Possibilities include Amount of rotation of gas cloud from which galaxy formed (rapid rotation leads to S's; slow rotation to E's) Rate of star formation in parent gas cloud (rapid formation leads to E's; slow to spirals) Amount of random motion in gas (Lots leads to S's; little to E's) Rate of rotation of dark matter halo. (rapid rotation leads to S's; slow rotation to E's) Measuring Distances to Galaxies and the Hubble Law In early 1900's galaxies found to be receding from Milky Way Deduced from spectra which show redshift, implying motion away from observer Redshift observed in all but a very few nearby galaxies Redshift larger for dimmer galaxies Recession is faster for more distant galaxies To derive law for velocity, need distance of galaxy To find distance to galaxy For nearby galaxies - use inverse-square law and method of standard candles Pick an object of known brightness (Cepheid variable, planetary nebula, supernova). Measure how bright it looks. Compare to known brightness to get distance Recession velocity related to distance by simple law Vel = constant x distance The Hubble law - now written as V = HD H determined from nearby galaxies with D and V measured independently Value of H is uncertain. If V measured in km/sec and D in Mpc, H = 50 km/sec/Mpc or as much as 100 km/sec/Mpc Law implies Universe is expanding Distance to galaxy can be found from its velocity Ex: Suppose measured V = 3000 km/sec and H is 75 km/sec/Mpc V=HD. Therefore, D= V/H = 3000/75 = 40 Mpc. ACTIVE GALAXIES Small, luminous nucleus, often variable, spectra imply core filled with hot gas moving at high velocity Types Seyferts - spirals - small luminous nucleus Radio Galaxies - E's strong radio emission at core and outside galaxy in "lobes" Often narrow intense jet point from core to lobes Quasars - appear star like - hard to tell type of galaxy Immense distance inferred from large redshift of spectrum (some as much as 10 billion ly) High luminosity (about 1000 L Milky Way) inferred from visibility at their large distance Model to Explain Activity Model must explain high energy output from small region Hypothesis - Massive (108 solar mass) black hole in core Gas collects around black hole in accretion disk and is heated Gas "boils off" disk to form jets Disk may help channel gas into narrow streams Evidence supporting model High velocity of gas and stars in core implies large mass Mass so inferred too large to be anything but black hole Galaxy Clusters Groups of galaxies held together by mutual gravity Types Rich and poor Poor contain dozen to hundred or so members - members often spirals Milky Way belongs to a Poor cluster - the Local Group Rich contain hundreds to thousands of member galaxies - typically ellipticals Present evidence is that galaxies form in small groups which then are drawn together by their mutual gravity to form larger, richer systems. Galaxy clusters show evidence of dark matter - member galaxies move faster than explainable on the basis of their luminous material Galaxy Clusters contain huge amounts of hot (million K) gas - detected by its x-ray emission - in fact, clusters contains more mass in the form of hot gas than stars. COSMOLOGY The Structure and Evolution of the Universe Basic Observations of the Universe Distribution of Galaxies - spread approximately uniformly Motion of Galaxies Receding from one another as expressed by the Hubble Law Note: Galaxies are carried apart by the expansion of space itself. They are NOT flying away from each other the way fragments of a bomb do. The Cosmic Background Radiation Weak radiation coming from all directions in space Temp of radiation approx 3K above absolute zero Uniformity of hydrogen and helium abundance (about 71%, 27% respectively) Conclusions Deduced from the Basic Observations of the Universe Galaxies were once closer together than now. That is, Universe smaller in past than now Universe was therefore denser and thus compressed Universe was once very hot - compression heats T ≈ 10 Trillion K Whole Universe packed into region approx size of Solar System Universe expands from this hot dense state in Big Bang We are not at the Center of the Universe All points move away from all other points Big Bang occurred about 15 billion years ago Deduced from time for galaxies to move apart. Expansion cools Universe and leaves only low temperature radiation that we see as the CMB. Evolution of the Universe: Expansion forever or Recollapse? Fate of Universe determined by whether its total mass is sufficient to exert enough gravity to stop expansion. Observations suggest too little mass to stop expansion. Thus, Universe probably will expand forever. Observation of distant galaxies suggests they are receding more slowly for their distance than predicted by the Hubble Law - implies expansion is accelerating. (Recall that looking out in space = looking back in time) Acceleration may be evidence for Cosmological constant - a term introduced by Einstein into his equations for the structure of the Universe. Energy contributed by the cosmological constant may dominate the mass/energy content of the Universe. Observations of irregularities (lumps) in the CMB suggest Universe is flat, as predicted by certain models of the Universe (see below). The Origin of the Universe History of Matter and Radiation in the Early Universe At Birth (Big Bang) Universe mainly radiation. Some radiation changes to matter via E = mc2 Forms quarks, anti-quarks, electrons and positrons. Expansion cools and some matter stabilizes as quarks turn into protons and neutrons. Cooling continues with expansion. Some protons fuse into helium, creating uniformity of H/He ratio. More cooling allows electrons to attach to nuclei to form atoms. Still more cooling allows gas to form clouds which turn into galaxies. Expansion continues - NOW The Inflationary Universe and Grand Unified Theories Universe formed from "false vacuum" All forces (gravity, electromagnetism, strong) act as single force Forces "Unified" - thus Grand Unified Theory (GUT's) Young Universe expanded by enormous factor Evidence mentioned above about size distribution of lumps in the CMB support a flat Universe, as predicted by inflationary models.