Earth Science, 12e
Beyond Our Solar SystemChapter 22
After reading, studying, and discussing the chapter, students should be able to:•Discuss the principle of parallax and explain how it is used to measure the distance to a star. •List and describe the major intrinsic properties of stars. •Describe the different types of nebulae. •Describe the most plausible model for stellar evolution and list the stages in the life cycle of a star. •Describe the possible final states that a star may assume after it consumes its nuclear fuel and collapses. •List and describe the major types of galaxies. •Describe the big bang theory of the origin of the universe.
Learning Objectives
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 Hertzsprung-Russell diagram
Figure 24.7
Variable stars
Stars that fluctuate in brightnessTypes 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 gravityTwo 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 usUniverse 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
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