earth science, 13e - katy independent school...

© 2012 Pearson Education, Inc.

Earth Science, 13e

Tarbuck & Lutgens


Beyond Our Solar System

Earth Science, 13e

Chapter 24

Stanley C. Hatfield

Southwestern Illinois College


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)

• 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


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


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


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



Parts of an 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



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 percent 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


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 percent 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



• 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


Stellar evolution

Stages



• Possibilities

• Massive star

• Over three 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


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

Types of galaxies

• Existence was first proposed in mid-1700s

by Immanuel Kant

• Four basic types of galaxies

• Spiral galaxy

• Arms extending from nucleus

• About 30 percent of all galaxies

• Large diameter up to 125,000 light years

• Contains both young and old stars

• e.g., Milky Way


Spiral Galaxy Messier 83


Galaxies

Other galaxies


• Barred spiral galaxy

• Stars arranged in the shape of a bar

• Generally quite large


• Elliptical galaxy

• Ellipsoidal shape


• Most are smaller than spiral galaxies; however,

they are also the largest known galaxies


A barred spiral galaxy


Galaxies

Other galaxies


• Irregular galaxy

• Lacks symmetry


• 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 other 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 then the critical

density and predict an ever-expanding, or

open, universe


End of Chapter 24

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