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