HNRS 227 Lecture #16, 17 Chapter 12 The Universe presented by Prof. Geller 21, 26 October 2004 Key Points of Chapter 12 (including material no longer in textbook) zThe Night Sky yHistorical View of Our Universe xgeocentric model of the universe Ptolemaic Model xheliocentric model of the universe Copernican Model yCoordinate Systems how do we find something in the sky? xLocal Horizon System altitude, azimuth xCelestial Coordinate System right ascension, declination Key Points of Chapter 12 (including material no longer in textbook) zThe Structure of Stars zThe Brightness of Stars zThe Temperature of Stars zThe Types of Stars zThe Life Cycle of Stars zGalaxies yHubble classification of galaxies yAGNs Key Points of Chapters 12 (including material no longer in textbook) zThe Big Bang yTheories of creation of universe xBig Bang vs. Steady-state zThe Curvature of the Space-Time Continuum yThe destiny of the universe xThe density of matter in the universe Dark matter and dark energy Some Fundamental Units in Astronomy zAngular measure y1 degree = 60 minutes = 3600 seconds zHour-angle measure yone hour is 15 degrees of arc zLight Year ydistance traveled by light in a year xAlmost 6 trillion miles Calculate it zAstronomical Unit (AU) xmean distance of Earth to Sun Historical Perspective zGeocentric Model of the Universe yEarth at center xPtolemaic model Ptolemy then church zHeliocentric Model of the Universe ySun at center xCopernicus (some early Greeks before), Galileo, Kepler, Tycho, Newton The Night Sky zFinding an object in the sky yRelative to your location and time xAltitude angular measure above horizon xAzimuth angular measure for direction starting at North and going eastward along the horizon yIndependent of location on Earth xRight Ascension hour angle from Vernal Equinox xDeclination angular measure above or below celestial equator The Stars in the Sky The Brightest Stars Interpreting the Table zDistance yIn light years zApparent Magnitude zAbsolute Magnitude zSpectral Type (example for Sun which is G2V) yG is spectral class y2 is spectral sub-class xWith spectral class leads to specific surface temperature yV is luminosity class xGiant, sub-giant or main sequence Main sequence is defined as hydrogen core fusion Stellar Structure zStellar Structure zOur Sun (and others) yCore yRadiation zone yConvection zone yPhotosphere A theoretical model of the Sun shows how energy gets from its center to its surface zHydrogen fusion takes place in a core extending from the Suns center to about 0.25 solar radius zThe core is surrounded by a radiative zone extending to about 0.71 solar radius yIn this zone, energy travels outward through radiative diffusion zThe radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure yIn this zone, energy travels outward primarily through convection Astronomers probe the solar interior using the Suns own vibrations z Helioseismology is the study of how the Sun vibrates z These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun Neutrinos reveal information about the Suns coreand have surprises of their own zNeutrinos emitted in thermonuclear reactions in the Suns core have been detected, but in smaller numbers than expected zRecent neutrino experiments explain why this is so The photosphere is the lowest of three main layers in the Suns atmosphere z The Suns atmosphere has three main layers: the photosphere, the chromosphere, and the corona z Everything below the solar atmosphere is called the solar interior z The visible surface of the Sun, the photosphere, is the lowest layer in the solar atmosphere Convection in the photosphere produces granules Think: How do we know? The chromosphere is characterized by spikes of rising gas zAbove the photosphere is a layer of less dense but higher temperature gases called the chromosphere zSpicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules zThe outermost layer of the solar atmosphere, the corona, is made of very high- temperature gases at extremely low density zThe solar corona blends into the solar wind at great distances from the Sun The corona ejects mass into space to form the solar wind Activity in the corona includes coronal mass ejections and coronal holes Apparent and Absolute Magnitude zHow bright something is in our sky yApparent magnitude zHow bright celestial object is compared to all others yAbsolute magnitude yLuminosity zMagnitude Scale ylog scale ylower value brighter (x 2.5) than higher value yabsolute versus apparent xabsolute is magnitude at 10 parsecs Astronomers often use the magnitude scale to denote brightness zThe apparent magnitude scale is an alternative way to measure a stars apparent brightness zThe absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs From Wiens Law: A stars color depends on its surface temperature The spectra of stars reveal their chemical compositions as well as surface temperatures z Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra z Most brown dwarfs are in even cooler spectral classes called L and T yUnlike true stars, brown dwarfs are too small to sustain thermonuclear fusion Relationship between a stars luminosity, radius, and surface temperature Stars come in a wide variety of sizes Finding Key Properties of Nearby Stars Stellar Temperatures and Classification zTemperature of stars yWiens Law yspectral classes based upon temperature xnot linear scale zH-R Diagram ytemperature versus absolute brightness yfollowing the evolution of stars Understanding the aging of stars requires both observation and application of physical principles zBecause stars shine by thermonuclear reactions, they have a finite life span zThe theory of stellar evolution (the life cycle or aging of stars) describes how stars form and change during their life span The Life Story of Stars Thermal Energy Weight of outer layers Gas Pressure Gravity Surface Center Luminosity zGravity squeezes zPressure forces resist yKinetic pressure of hot gases yRepulsion from Pauli exclusion principle for electrons - white dwarf yRepulsion from Pauli exclusion principle for neutrons - neutron star yNone equal to gravity - black hole zEnergy loss decreases pressure zEnergy generation replaces losses zStar is dead when energy generation stops yWhite dwarf, neutron star, black hole The Spectral Measure of Stars - Wiens and Stefan-Boltzmanns Laws The Hertzsprung- Russell (HR) Diagram Interstellar gas and dust pervade the galaxy z Interstellar gas and dust, which make up the interstellar medium, are concentrated in the disk of the Galaxy z Clouds within the interstellar medium are called nebulae z Dark nebulae are so dense that they are opaque z They appear as dark blots against a background of distant stars z Emission nebulae, or H II regions, are glowing, ionized clouds of gas z Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars z Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow Interlude Up in the Sky Tonight Protostars form in cold, dark nebulae z Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar z As a protostar grows by the gravitational accretion of gases, Kelvin- Helmholtz contraction causes it to heat and begin glowing The more massive the protostar, the more rapidly it evolves Protostars evolve into main-sequence stars z A protostars relatively low temperature and high luminosity place it in the upper right region on an H-R diagram z Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram z When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main sequence star Interlude - Humor zOK stellar recruits, its time to learn whats really in store for you! I know that before you signed up to be a massive star you read the fancy brochures that talked about how brightly youd be shining and how youd be visible from halfway across the galaxy. But you mo-rons must not have bothered to read the fine print that said that youd explode in seven million years! And if you did read it then youre even stupider than you look. Seven million is not a long time! Eric Schulman [A Briefer History of Time] Young star clusters give insight into star formation and evolution z Newborn stars may form an open or galactic cluster z Stars are held together in such a cluster by gravity z Occasionally a star moving more rapidly than average will escape, or leave the cluster z A stellar association is a group of newborn stars that are moving apart so rapidly that their gravitational attraction for one another cannot pull them into orbit about one another z Star-forming regions appear when a giant molecular cloud is compressed z This can be caused by the clouds passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms Supernovae compress the interstellar medium and can trigger star birth A stars lifetime on the main sequence is proportional to its mass divided by its luminosity z The duration of a stars main sequence lifetime depends on the amount of hydrogen in the stars core and the rate at which the hydrogen is consumed z The more massive a star, the shorter is its main- sequence lifetime The Sun has been a main-sequence star for about 4.56 billion years and should remain one for about another 7 billion years During a stars main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity When core hydrogen fusion ceases, a main- sequence star becomes a red giant Red Giants z Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star z This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star z The core shrinks and becomes hotter, while the stars outer layers expand and cool z The result is a red giant star Fusion of helium into carbon and oxygen begins at the center of a red giant z When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core z This process, also called the triple alpha process, converts helium to carbon and oxygen Planetary Nebulae Death of a Solar Mass Star Planetary Nebula - NGC light years away in Aquarius Planetary Nebula - NGC light years away in Cygnus Evolution from Giants to Dwarfs White Dwarf Properties Sirius A Sirius B - WD Stellar Evolution by Mass from the Main Sequence Mass (M Sun = 1) White dwarfs Ns Black holes Main sequence stars Heavy nuclei fusion Supernovae Planetary nebulae C detonation Helium flash Supergiants Giants A Massive Star (~25 M sun ) SN 1987A Outburst Large Magellanic Cloud February 23, 1987 Progenitor star was a blue supergiant of about 20 M sun Crab Nebula A.D. Neutron star NASA JPL GENESIS Education/Public Outreach There are 92 elements found in nature. They were all produced BY THE STARS. Periodic Table of the Elements Los Alamos National Laboratories Copyright Periodic Table of the Elements, Los Alamos National Laboratories From Galaxies to Cosmology zGalaxies your own Milky Way ydifferent types xelliptical, spiral, barred spiral zHubbles Law zCosmology Hubble proved that the spiral nebulae are far beyond the Milky Way z Edwin Hubble used Cepheid variables to show that the nebula were actually immense star systems far beyond our Galaxy Galaxies are classified according to their appearance Galaxies can be grouped into four major categories: spirals, barred spirals, ellipticals, and irregulars The disks of spiral and barred spiral galaxies are sites of active star formation Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited Irregular galaxies have ill-defined, asymmetrical shapes They are often found associated with other galaxies Astronomers use various techniques to determine the distances to remote galaxies Standard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances z The Tully-Fisher relation, which correlates the width of the 21- cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance z A method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxys size to its surface brightness distribution and to the motions of its stars The Distance Ladder Recall the Doppler Shift zA change in measured frequency caused by the motion of the observer or the source yclassical example of pitch of train coming towards you and moving away Hubbles Law zThe further away a galaxy is, the greater its recessional velocity and the greater its spectral red shift Hubbles Conculsion zFrom Hubbles Law we can calculate a time in the past when universe was a point zBig bang occurred about 15 billion years ago ybig bang first proposed by George Gamow based upon such evidence ybig bang named by antagonist Fred Hoyle who preferred the steady-state model Big Bang Summary Keplers Laws of Planetary Motion zKeplers First Law of Planetary Motion yplanets orbit sun in an ellipse with sun at one foci zKeplers Second Law of Planetary Motion yplanets sweep out equal areas in equal times xtravel faster when closer, slower when farther zKeplers Third Law of Planetary Motion yorbital period squared is proportional to semi-major axis cubed P 2 = a 3 Planetary Observations zPlanets formed at same time as Sun zPlanetary and satellite/ring systems are similar to remnants of dusty disks such as that seen about stars being born zPlanet composition dependent upon where it formed in solar system Other Planet Observations zTerrestrial planets are closer to sun yMercury yVenus yEarth yMars zJovian planets furthest from sun yJupiter ySaturn yUranus yNeptune Other Observations zRadioactive dating of solar system rocks yEarth ~ 4 billion years yMoon ~4.5 billion years yMeteorites ~4.6 billion years zMost orbital and rotation planes confined to ecliptic plane with counterclockwise motion zExtensive satellite and rings around Jovians zPlanets have more of the heavier elements than the sun A Linear View of Abundance Log Abundance of Elements Planetary Summary Nebular Condensation (protoplanet) Model zMost remnant heat from collapse retained near center zAfter sun ignites, remaining dust reaches an equilibrium temperature zDifferent densities of the planets are explained by condensation temperatures zNebular dust temperature increases to center of nebula Nebular Condensation Physics zEnergy absorbed per unit area from sun = energy emitted as thermal radiator zSolar Flux = Lum (Sun) / 4 x distance 2 zFlux emitted = constant x T 4 [Stefan-Boltzmann] zConcluding from above yields yT = constant / distance 0.5 Nebular Condensation Chemistry