Earth Science, 10e

1
Earth Science, 10e

• Edward J. Tarbuck Frederick K. Lutgens

2
Beyond our Solar SystemChapter 23

• Earth Science, 10e

3
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

4
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

5
Properties of stars

• Stellar brightness
• Controlled by three factors
• Size
• Temperature
• Distance
• Magnitude
• Measure of a star's brightness

6
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

7
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

8
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

9
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

10
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

11
Binary stars orbit each other around their common
center of mass
12
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

13
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

14
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

15
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

16
Idealized Hertzsprung-Russell diagram
17
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

18
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

19
The Orion Nebula is a well-known emission nebula
20
A faint blue reflection nebula in the Pleiades
star cluster
21
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

22
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

23
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

24
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

25
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

26
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

27
Evolutionary stages of low mass stars
28
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

29
Evolutionary stages of medium mass stars
30
H-R diagram showing stellar evolution
31
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

32
Evolutionary stages of massive stars
33
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

34
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

35
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)

36
Crab Nebula in the constellation Taurus
37
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

38
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

39
Face-on view of the Milk Way Galaxy
40
Edge-on view of the Milk Way Galaxy
41
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

42
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

43
Great Galaxy, a spiral galaxy, in the
constellation Andromeda
44
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

45
A barred spiral galaxy
46
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

47
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

48
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

49
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

50
Raisin bread analogy of an expanding universe
51
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

52
Big Bang theory

• Accounts for galaxies moving away from us
• Universe was once confined to a "ball" that was
• Supermassive
• Dense
• Hot

53
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

54
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

55
End of Chapter 23