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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) The Night Sky Historical View of Our Universe geocentric model of the universe • Ptolemaic Model heliocentric model of the universe • Copernican Model Coordinate Systems – how do we find something in the sky? Local Horizon System • altitude, azimuth Celestial Coordinate System • right ascension, declination Key Points of Chapter 12 (including material no longer in textbook) The Structure of Stars The Brightness of Stars The Temperature of Stars The Types of Stars The Life Cycle of Stars Galaxies Hubble classification of galaxies AGNs Key Points of Chapters 12 (including material no longer in textbook) The Big Bang Theories of creation of universe Big Bang vs. Steady-state The Curvature of the Space-Time Continuum The destiny of the universe The density of matter in the universe • Dark matter and dark energy Some Fundamental Units in Astronomy Angular measure 1 degree = 60 minutes = 3600 seconds Hour-angle measure one hour is 15 degrees of arc Light Year distance traveled by light in a year Almost 6 trillion miles • Calculate it Astronomical Unit (AU) mean distance of Earth to Sun Historical Perspective Geocentric Model of the Universe Earth at center Ptolemaic model • Ptolemy then church Heliocentric Model of the Universe Sun at center Copernicus (some early Greeks before), Galileo, Kepler, Tycho, Newton The Night Sky Finding an object in the sky Relative to your location and time Altitude – angular measure above horizon Azimuth – angular measure for direction starting at North and going eastward along the horizon Independent of location on Earth Right Ascension – hour angle from Vernal Equinox Declination – angular measure above or below celestial equator The Stars in the Sky The Brightest Stars Common Name Sun Sirius Canopus Rigil Kentaurus Arcturus Vega Capella Scientific Name Sol Alpha CMa Alpha Car Alpha Cen Distance (light years) 1.5 x 10-5 8.6 74 4.3 Apparent Magnitude -26.72 -1.46 -0.72 -0.27 Absolute Magnitude 4.8 1.4 -2.5 4.4 Spectral Type G2V A1Vm A9II G2V + K1V Alpha Boo Alpha Lyr Alpha Aur 34 25 41 -0.04 0.03 0.08 0.2 0.6 0.4 Rigel Procyon Achernar Betelgeuse Hadar Acrux Beta Ori Alpha CMi Alpha Eri Alpha Ori Beta Cen Alpha Cru ~1400 11.4 69 ~1400 320 510 0.12 0.38 0.46 0.50 (var.) 0.61 (var.) 0.76 -8.1 2.6 -1.3 -7.2 -4.4 -4.6 Altair Aldebaran Antares Spica Pollux Fomalhaut Becrux Deneb Regulus Adhara Alpha Aql Alpha Tau Alpha Sco Alpha Vir Beta Gem Alpha PsA Beta Cru Alpha Cyg Alpha Leo Epsilon CMa Alpha Gem Gamma Cru Lambda Sco 16 60 ~520 220 40 22 460 1500 69 570 0.77 0.85 0.96 0.98 1.14 1.16 1.25 1.25 1.35 1.50 2.3 -0.3 -5.2 -3.2 0.7 2.0 -4.7 -7.2 -0.3 -4.8 K1.5IIIp A0Va G6III + G2III B81ae F5IV-V B3Vnp M2Iab B1III B0.5Iv + B1Vn A7Vn K5III M1.5Iab B1V K0IIIb A3Va B0.5III A2Ia B7Vn B2II 49 120 330 1.57 1.63 (var.) 1.63 (var.) 0.5 -1.2 -3.5 A1V + A2V M3.5III B1.5IV Castor Gacrux Shaula (var.) (var.) (var.) (var.) Interpreting the Table Distance In light years Apparent Magnitude Absolute Magnitude Spectral Type (example for Sun which is G2V) G is spectral class 2 is spectral sub-class With spectral class leads to specific surface temperature V is luminosity class Giant, sub-giant or main sequence • Main sequence is defined as hydrogen core fusion Stellar Structure Stellar Structure Our Sun (and others) Core Radiation zone Convection zone Photosphere A theoretical model of the Sun shows how energy gets from its center to its surface Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius The core is surrounded by a radiative zone extending to about 0.71 solar radius In this zone, energy travels outward through radiative diffusion The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure In this zone, energy travels outward primarily through convection Astronomers probe the solar interior using the Sun’s own vibrations Helioseismology is the study of how the Sun vibrates These vibrations have been used to infer pressures, densities, chemical compositions, and rotation rates within the Sun Neutrinos reveal information about the Sun’s core—and have surprises of their own Neutrinos emitted in thermonuclear reactions in the Sun’s core have been detected, but in smaller numbers than expected Recent neutrino experiments explain why this is so The photosphere is the lowest of three main layers in the Sun’s atmosphere The Sun’s atmosphere has three main layers: the photosphere, the chromosphere, and the corona Everything below the solar atmosphere is called the solar interior 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 Above the photosphere is a layer of less dense but higher temperature gases called the chromosphere Spicules extend upward from the photosphere into the chromosphere along the boundaries of supergranules The outermost layer of the solar atmosphere, the corona, is made of very hightemperature gases at extremely low density The 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 How bright something is in our sky Apparent magnitude How bright celestial object is compared to all others Absolute magnitude Luminosity Magnitude Scale log scale lower value brighter (x 2.5) than higher value absolute versus apparent absolute is magnitude at 10 parsecs Astronomers often use the magnitude scale to denote brightness The apparent magnitude scale is an alternative way to measure a star’s apparent brightness The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs From Wien’s Law: A star’s color depends on its surface temperature The spectra of stars reveal their chemical compositions as well as surface temperatures 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 Most brown dwarfs are in even cooler spectral classes called L and T Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion Relationship between a star’s luminosity, radius, and surface temperature Stars come in a wide variety of sizes Finding Key Properties of Nearby Stars Stellar Temperatures and Classification Temperature of stars Wien’s Law spectral classes based upon temperature not linear scale H-R Diagram temperature versus absolute brightness following the evolution of stars Understanding the aging of stars requires both observation and application of physical principles Because stars shine by thermonuclear reactions, they have a finite life span The 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 Gravity squeezes Pressure forces resist Kinetic pressure of hot gases Repulsion from Pauli exclusion principle for electrons - white dwarf Repulsion from Pauli exclusion principle for neutrons - neutron star None equal to gravity - black hole Energy loss decreases pressure Energy generation replaces losses Star is “dead” when energy generation stops White dwarf, neutron star, black hole Luminosity Surface Gravity Weight of outer layers Gas Pressure Thermal Energy Center The Spectral Measure of Stars Wien’s and Stefan-Boltzmann’s Laws The HertzsprungRussell (HR) Diagram Interstellar gas and dust pervade the galaxy Interstellar gas and dust, which make up the interstellar medium, are concentrated in the disk of the Galaxy Clouds within the interstellar medium are called nebulae Dark nebulae are so dense that they are opaque They appear as dark blots against a background of distant stars Emission nebulae, or H II regions, are glowing, ionized clouds of gas Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars 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 Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar As a protostar grows by the gravitational accretion of gases, KelvinHelmholtz contraction causes it to heat and begin glowing The more massive the protostar, the more rapidly it evolves Protostars evolve into main-sequence stars A protostar’s relatively low temperature and high luminosity place it in the upper right region on an H-R diagram Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main sequence star Interlude - Humor “OK stellar recruits, it’s time to learn what’s 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 you’d be shining and how you’d be visible from halfway across the galaxy. But you mo-rons must not have bothered to read the fine print that said that you’d explode in seven million years! And if you did read it then you’re 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 Newborn stars may form an open or galactic cluster Stars are held together in such a cluster by gravity Occasionally a star moving more rapidly than average will escape, or leave the cluster 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 Star-forming regions appear when a giant molecular cloud is compressed This can be caused by the cloud’s 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 star’s lifetime on the main sequence is proportional to its mass divided by its luminosity The duration of a star’s main sequence lifetime depends on the amount of hydrogen in the star’s core and the rate at which the hydrogen is consumed The more massive a star, the shorter is its mainsequence 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 star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity When core hydrogen fusion ceases, a mainsequence star becomes a red giant Red Giants Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star The core shrinks and becomes hotter, while the star’s outer layers expand and cool The result is a red giant star Fusion of helium into carbon and oxygen begins at the center of a red giant When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core 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 7293 450 light years away in Aquarius Planetary Nebula - NGC 7027 3000 light years away in Cygnus Evolution from Giants to Dwarfs Dwarf Properties Sirius A Sirius B - WD Property Earth Sirius B Su n Mass (Msun) 3x10 -6 0 .9 4 1 .0 0 0 .0 0 9 0 .0 0 8 1 .0 0 Luminosity (Lsun) 0 .0 0.0028 1 .0 0 Surface temperature (K) 287 27,000 5770 6 1 .4 1 Radius (Rsun) 3 Mean density (g/cm ) Central temp (K) 3 Central density (g/cm ) 5 .5 2.8x10 4200 2.2x10 9 .6 3.3x10 7 7 7 1.6x10 160 Stellar Evolution by Mass from the Main Sequence Main sequence stars Supergiants Giants Helium flash C detonation Heavy nuclei fusion Supernovae Planetary nebulae Black holes Ns White dwarfs 100 40 10 4.0 Mass (MSun = 1) 1.0 0.4 0.1 A Massive Star (~25 Msun) SN 1987A Outburst Large Magellanic Cloud February 23, 1987 Progenitor star was a blue supergiant of about 20 Msun Crab Nebula - 1054 A.D. Neutron star NASA JPL GENESIS Education/Public Outreach Copyright © Periodic Table of the Elements, Los Alamos National Laboratories © Periodic Table of the Elements Los Alamos National Laboratories There are 92 elements found in nature. They were all produced BY THE STARS. From Galaxies to Cosmology Galaxies our own Milky Way different types elliptical, spiral, barred spiral Hubble’s Law Cosmology Hubble proved that the spiral nebulae are far beyond the Milky Way 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 The Distance Ladder The Tully-Fisher relation, which correlates the width of the 21cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance A method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxy’s size to its surface brightness distribution and to the motions of its stars Recall the Doppler Shift A change in measured frequency caused by the motion of the observer or the source classical example of pitch of train coming towards you and moving away Hubble’s Law The further away a galaxy is, the greater its recessional velocity and the greater its spectral red shift Hubble’s Conculsion From Hubble’s Law we can calculate a time in the past when universe was a point Big bang occurred about 15 billion years ago big bang first proposed by George Gamow based upon such evidence big bang named by antagonist Fred Hoyle who preferred the steady-state model Big Bang Summary Era The Vacuum Era Epochs Main Event Planck Epoch Quantum Inflationary Epoch fluctuation Inflation Time after bang <10-43 sec. <10-10 sec. The Radiation Era Electroweak Epoch Formation of Strong Epoch leptons, bosons, Decoupling hydrogen, helium and deuterium The Matter Era Galaxy Epoch Galaxy formation Stellar Epoch Stellar birth 10-10 sec. 10-4 sec. 1 sec. - 1 month The Degenerate Dark Era 20-100 billion yrs. 100 billion - ???? Dead Star Epoch Black Hole Epoch Death of stars Black holes engulf? 1-2 billion years 2-15 billion years Kepler’s Laws of Planetary Motion Kepler’s First Law of Planetary Motion planets orbit sun in an ellipse with sun at one foci Kepler’s Second Law of Planetary Motion planets sweep out equal areas in equal times travel faster when closer, slower when farther Kepler’s Third Law of Planetary Motion orbital period squared is proportional to semi-major axis cubed • P2 = a3 Planetary Observations Planets formed at same time as Sun Planetary and satellite/ring systems are similar to remnants of dusty disks such as that seen about stars being born Planet composition dependent upon where it formed in solar system Other Planet Observations Terrestrial planets are closer to sun Mercury Venus Earth Mars Jovian planets furthest from sun Jupiter Saturn Uranus Neptune Other Observations Radioactive dating of solar system rocks Earth ~ 4 billion years Moon ~4.5 billion years Meteorites ~4.6 billion years Most orbital and rotation planes confined to ecliptic plane with counterclockwise motion Extensive satellite and rings around Jovians Planets have more of the heavier elements than the sun A Linear View of Abundance Linear Plot of Chemical Abundance 100000 90000 80000 Relative abundance 70000 60000 50000 40000 30000 20000 10000 0 H He C N O Ne Chemical Species Mg Si Si Fe Log Abundance of Elements Logarithmic Plot of Chemical Abundance of Elements 100000 Relative Abundance 10000 1000 100 10 1 H He C N O Ne Chemical Species Mg Si Si Fe Planetary Summary Major Constituents Mass (Earth=1) Density (g/cm3) Mercury Venus Earth Mars 0.06 0.82 1.00 0.11 5.4 5.2 5.5 3.9 Jupiter Saturn 318 95 1.3 0.7 H, He H, He Uranus Neptune 14 17 1.3 1.7 Ices, H, He Ices, H, He Planet Rock, Rock, Rock, Rock, Iron Iron Iron Iron Nebular Condensation (protoplanet) Model Most remnant heat from collapse retained near center After sun ignites, remaining dust reaches an equilibrium temperature Different densities of the planets are explained by condensation temperatures Nebular dust temperature increases to center of nebula Nebular Condensation Physics Energy absorbed per unit area from sun = energy emitted as thermal radiator Solar Flux = Lum (Sun) / 4 x distance2 Flux emitted = constant x T4 [Stefan-Boltzmann] Concluding from above yields T = constant / distance0.5 Nebular Condensation Chemistry Molecule H2 H2O CH4 NH3 FeSO4 SiO4 Freezing Point Distance from Center >100 AU 10 K >10 AU 273 K >35 AU 35 K >8 AU 190 K >1 AU 700 K >0.5 AU 1000 K