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Chapter 24
Earth Science, 12e
Tarbuck/Lutgens
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Earth Science,
12e
Beyond Our
Solar System
Chapter 24
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 HertzsprungRussell diagram
Figure 24.7
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 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 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% 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 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 than the critical
density and predict an ever-expanding, or
open, universe
End of Chapter 24