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Beyond the Solar System Chapters 23 and 24 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Stellar parallax © 2011 Pearson Education, Inc. 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 kilometers (5.8 trillion miles) © 2011 Pearson Education, Inc. Properties of stars • Stellar brightness • Controlled by three factors • Size • Temperature • Distance • Magnitude • Measure of a star’s brightness © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Properties of stars • Color and temperature • Hot star • Temperature above 30,000 K • Emits short-wavelength light • Appears blue • Cool star • Temperature less than 3000 K • Emits longer-wavelength light • Appears red © 2011 Pearson Education, Inc. Properties of stars • Color and temperature • Between 5000 and 6000 K • Stars appear yellow • 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 © 2011 Pearson Education, Inc. Properties of stars • Binary stars and stellar mass • Binary stars • Visual binaries are resolved telescopically • More than 50 percent 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 © 2011 Pearson Education, Inc. Binary stars orbit each other around their common center of mass © 2011 Pearson Education, Inc. Properties of stars • Binary stars and stellar mass • Stellar mass • Mass of most stars is between one-tenth and fifty times the mass of the Sun © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Idealized HertzsprungRussell diagram © 2011 Pearson Education, Inc. Hertzsprung-Russell diagram • Parts of an Hertzsprung-Russell (H-R) diagram • Main-sequence stars • 90 percent 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 © 2011 Pearson Education, Inc. Hertzsprung-Russell diagram • Parts of an Hertzsprung-Russell (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 (approximate the size of Earth) Lower-central area on the H-R diagram Not all are white in color Perhaps 10% of all stars © 2011 Pearson Education, Inc. 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 close to a very hot star • Two types of bright nebulae • Emission nebula • Reflection nebula © 2011 Pearson Education, Inc. The Trifid Nebula, in the constellation Sagittarius © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 the cloud Temperature rises Becomes a protostar © 2011 Pearson Education, Inc. 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 balanced by gravity pulling in – Star becomes a stable main-sequence star © 2011 Pearson Education, Inc. 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 percent of a star’s life is in the main-sequence © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Evolutionary stages of low-mass stars © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Evolutionary stages of medium-mass stars © 2011 Pearson Education, Inc. H-R diagram showing stellar evolution © 2011 Pearson Education, Inc. Stellar evolution • Stages • Burnout and death • Final stage depends on mass • Possibilities • Massive star • Over 3 solar masses • 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 © 2011 Pearson Education, Inc. Evolutionary stages of massive stars © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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) © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Face-on view of the Milky Way galaxy © 2011 Pearson Education, Inc. Edge-on view of the Milky Way galaxy © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. Galaxies • Three basic types of galaxies • Spiral galaxy • • • • • Arms extending from nucleus About 30 percent of all galaxies Large diameter of 20,000 to 125,000 light years Contains both young and old stars e.g., Milky Way © 2011 Pearson Education, Inc. Andromeda Galaxy, a large spiral galaxy © 2011 Pearson Education, Inc. A barred spiral galaxy © 2011 Pearson Education, Inc. Galaxies • Three basic types of galaxies • Elliptical galaxy • Ellipsoidal shape • About 60 percent of all galaxies • Most are smaller than spiral galaxies; however, they are also the largest known galaxies © 2011 Pearson Education, Inc. Galaxies • Three basic types of galaxies • Irregular galaxy • • • • Lacks symmetry About 10 percent of all galaxies Contains mostly young stars e.g., Magellanic Clouds © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc. Big Bang theory • Accounts for galaxies moving away from us • Universe was once confined to a “ball” that was • Supermassive • Dense • Hot © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc.