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Lecture Outlines PowerPoint Chapter 24 Earth Science, 12e Tarbuck/Lutgens © 2009 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Earth Science, 12e Beyond Our Solar System Chapter 24 Properties of stars Distance • Measuring a star’s distance can be very difficult • Stellar parallax • Used for measuring distance to a star • Apparent shift in a star’s position due to the orbital motion of Earth • Measured as an angle • Near stars have the largest parallax • Largest parallax is less than one second of arc Properties of stars Distance • Distances to the stars are very large • Units of measurement • Kilometers or astronomical units are too cumbersome to use • Light-year is used most often • Distance that light travels in 1 year • One light-year is 9.5 trillion km (5.8 trillion miles) • Other methods for measuring distance are also used Properties of stars Stellar brightness • Controlled by three factors • Size • Temperature • Distance • Magnitude • Measure of a star’s brightness Properties of stars Stellar brightness • Magnitude • Two types of measurement • Apparent magnitude • Brightness when a star is viewed from Earth • Decreases with distance • Numbers are used to designate magnitudes – dim stars have large numbers and negative numbers are also used Properties of stars Stellar brightness • Magnitude • Two types of measurement • Absolute magnitude • “True” or intrinsic brightness of a star • Brightness at a standard distance of 32.6 light-years • Most stars’ absolute magnitudes are between –5 and +15 Properties of stars Color and temperature • Hot star • Temperature above 30,000 K • Emits short-wavelength light • Appears blue • Cool star • Temperature less than 3,000 K • Emits longer-wavelength light • Appears red Properties of stars Color and temperature • Between 5,000 and 6,000 K • Stars appear yellow • e.g., Sun Binary stars and stellar mass • Binary stars • Two stars orbiting one another • Stars are held together by mutual gravitation • Both orbit around a common center of mass Properties of stars Binary stars and stellar mass • Binary stars • Visual binaries are resolved telescopically • More than 50% of the stars in the universe are binary stars • Used to determine stellar mass • Stellar mass • Determined using binary stars – the center of mass is closest to the most massive star Binary stars orbit each other around their common center of mass Figure 24.4 Properties of stars Binary stars and stellar mass • Stellar mass • Mass of most stars is between 1/10 and 50 times the mass of the Sun Hertzsprung-Russell diagram Shows the relation between stellar • Brightness (absolute magnitude) and • Temperature Diagram is made by plotting (graphing) each star’s • Luminosity (brightness) and • Temperature Hertzsprung-Russell diagram Parts of an H-R diagram • Main-sequence stars • 90% of all stars • Band through the center of the H-R diagram • Sun is in the main sequence • Giants (or red giants) • Very luminous • Large • Upper-right on the H-R diagram Hertzsprung-Russell diagram Parts of an H-R diagram • Giants (or red giants) • Very large giants are called supergiants • Only a few percent of all stars • White dwarfs • • • • • Fainter than main-sequence stars Small (approximately the size of Earth) Lower-central area on the H-R diagram Not all are white in color Perhaps 10% of all stars Idealized HertzsprungRussell diagram Figure 24.7 Variable stars Stars that fluctuate in brightness Types of variable stars • Pulsating variables • Fluctuate regularly in brightness • Expand and contract in size • Eruptive variables • Explosive event • Sudden brightening • Called a nova Interstellar matter Between the stars is “the vacuum of space” Nebula • Cloud of dust and gases • Two major types of nebulae • Bright nebula • Glows if it is close to a very hot star • Two types of bright nebulae • Emission nebula • Reflection nebula A faint blue reflection nebula in the Pleiades star cluster Figure 24.9 Interstellar matter Nebula • Two major types of nebulae • Dark nebula • Not close to any bright star • Appear dark • Contains the material that forms stars and planets Stellar evolution Stars exist because of gravity Two opposing forces in a star are • Gravity – contracts • Thermal nuclear energy – expands Stages • Birth • • • • In dark, cool, interstellar clouds Gravity contracts cloud and temperature rises Radiates long-wavelength (red) light Becomes a protostar Stellar evolution Stages • Protostar • Gravitational contraction of gaseous cloud continues • Core reaches 10 million K • Hydrogen nuclei fuse • Become helium nuclei • Process is called hydrogen burning • Energy is released • Outward pressure increases • Outward pressure balanced by gravity pulling in • Star becomes a stable main-sequence star Stellar evolution Stages • Main-sequence stage • Stars age at different rates • Massive stars use fuel faster and exist for only a few million years • Small stars use fuel slowly and exist for perhaps hundreds of billions of years • 90% of a star’s life is in the main sequence Stellar evolution Stages • Red giant stage • Hydrogen burning migrates outward • Star’s outer envelope expands • Surface cools • Surface becomes red • Core is collapsing as helium is converted to carbon • Eventually all nuclear fuel is used • Gravity squeezes the star Stellar evolution Stages • Burnout and death • Final stage depends on mass • Possibilities • Low-mass star • 0.5 solar mass • Red giant collapses • Becomes a white dwarf Stellar evolution Stages • Burnout and death • Final stage depends on mass • Possibilities • Medium-mass star • Between 0.5 and 3 solar masses • Red giant collapses • Planetary nebula forms • Becomes a white dwarf H-R diagram showing stellar evolution Figure 24.11 Stellar evolution Stages • Burnout and death • Final stage depends on mass • Possibilities • Massive star • Over 3 solar masses • Short life span • Terminates in a brilliant explosion called a supernova • Interior condenses • May produce a hot, dense object that is either a neutron star or a black hole Stellar remnants White dwarf • Small (some no larger than Earth) • Dense • Can be more massive than the Sun • Spoonful weighs several tons • Atoms take up less space • Electrons displaced inward • Called degenerate matter • Hot surface • Cools to become a black dwarf Stellar remnants Neutron star • Forms from a more massive star • Star has more gravity • Squeezes itself smaller • Remnant of a supernova • Gravitational force collapses atoms • Electrons combine with protons to produce neutrons • Small size Stellar remnants Neutron star • Pea-size sample • Weighs 100 million tons • Same density as an atomic nucleus • Strong magnetic field • First one discovered in early 1970s • Pulsar (pulsating radio source) • Found in the Crab nebula (remnant of an A.D. 1054 supernova) Crab Nebula in the constellation Taurus Figure 24.14 Stellar remnants Black hole • More dense than a neutron star • Intense surface gravity lets no light escape • As matter is pulled into it • Becomes very hot • Emits X-rays • Likely candidate is Cygnus X-1, a strong X-ray source Galaxies Milky Way Galaxy • Structure • Determined by using radio telescopes • Large spiral galaxy • About 100,000 light-years wide • Thickness at the galactic nucleus is about 10,000 light-years • Three spiral arms of stars • Sun is 30,000 light-years from the center Face-on view of the Milky Way Galaxy Figure 24.18 A Edge-on view of the Milky Way Galaxy Figure 24.18 B Galaxies Milky Way Galaxy • Rotation • Around the galactic nucleus • Outermost stars move the slowest • Sun rotates around the galactic nucleus once about every 200 million years • Halo surrounds the galactic disk • Spherical • Very tenuous gas • Numerous globular clusters Galaxies Other galaxies • Existence was first proposed in mid-1700s by Immanuel Kant • Four basic types of galaxies • Spiral galaxy • Arms extending from nucleus • About 30% of all galaxies • Large diameter up to 125,000 light-years • Contains both young and old stars • e.g., Milky Way The Andromeda Galaxy is an example of a large spiral galaxy Figure 24.20 Galaxies Other galaxies • Four basic types of galaxies • Barred spiral galaxy • Stars arranged in the shape of a bar • Generally quite large • About 10% of all galaxies • Elliptical galaxy • Ellipsoidal shape • About 60% of all galaxies • Most are smaller than spiral galaxies; however, they are also the largest known galaxies A barred spiral galaxy Figure 24.22 Galaxies Other galaxies • Four basic types of galaxies • Irregular galaxy • Lacks symmetry • About 10% of all galaxies • Contains mostly young stars • e.g., Magellanic Clouds Galaxies Galactic cluster • Group of galaxies • Some contain thousands of galaxies • Local Group • Our own group of galaxies • Contains at least 28 galaxies • Supercluster • Huge swarm of galaxies • May be the largest entity in the universe Red shifts Doppler effect • Change in the wavelength of light emitted by an object due to its motion • Movement away stretches the wavelength • Longer wavelength • Light appears redder • Movement toward “squeezes” the wavelength • Shorter wavelength • Light shifted toward the blue Red shifts Doppler effect • Amount of the Doppler shift indicates the rate of movement • Large Doppler shift indicates a high velocity • Small Doppler shift indicates a lower velocity Expanding universe • Most galaxies exhibit a red Doppler shift • Moving away Raisin bread analogy of an expanding universe Figure 24.24 Red shifts Expanding universe • Most galaxies exhibit a red Doppler shift • Far galaxies • Exhibit the greatest shift • Greater velocity • Discovered in 1929 by Edwin Hubble • Hubble’s Law – the recessional speed of galaxies is proportional to their distance • Accounts for red shifts Big Bang theory Accounts for galaxies moving away from us Universe was once confined to a “ball” that was • Supermassive • Dense • Hot Big Bang theory Big Bang marks the inception of the universe • Occurred about 15 billion years ago • All matter and space was created Matter is moving outward Fate of the universe • Two possibilities • Universe will last forever • Outward expansion will stop and gravitational contraction will follow Big Bang theory Fate of the universe • Final fate depends on the average density of the universe • If the density is more than the critical density, then the universe would contract • Current estimates point to less than the critical density and predict an ever-expanding, or open, universe End of Chapter 24