Properties of stars
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Earth Science, 10e Edward J. Tarbuck & Frederick K. Lutgens Beyond our Solar System Chapter 23 Earth Science, 10e 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 3000 K • Emits longer-wavelength light • Appears red Properties of stars Color and temperature • Between 5000 and 6000 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 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 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 (approximate the size of Earth) Lower-central area on the H-R diagram Not all are white in color Perhaps 10% of all stars Idealized Hertzsprung-Russell diagram 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 close to a very hot star • Two types of bright nebulae • Emission nebula • Reflection nebula The Orion Nebula is a wellknown emission nebula A faint blue reflection nebula in the Pleiades star cluster 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 the cloud 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 year • 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 Evolutionary stages of low mass stars 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 Evolutionary stages of medium mass stars H-R diagram showing stellar evolution 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 Evolutionary stages of massive stars 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 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 Milk Way Galaxy Edge-on view of the Milk Way Galaxy 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 of 20,000 to 125,000 light years • Contains both young and old stars • e.g., Milky Way Great Galaxy, a spiral galaxy, in the constellation Andromeda 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 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 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 sill 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 then the critical density and predict an ever-expanding, or open, universe End of Chapter 23
