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

STELLAR
d = 1/p
Parallax angle = .77 arc sec
PARALLAXd = 1/.77 = 1.3 pc
Parsec (pc) = 3.26 ly
PROPERTIES OF STARS
•
•
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 (5.87 trillion
miles)
•
PROPERTIES OF STARS
  Stellar
•
brightness
Controlled by three factors
Size
  Temperature
  Distance

PROPERTIES OF STARS
•
Magnitude
•

Measure of a star’s brightness
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
•  (the more negative the number the
brighter the object is)
- 26.7
- 11
-  4.4
Bright 
APPARENT MAGNITUDE
Sun
Full Moon
Venus (at its brightest)
- 1.5
Sirius (brightest star)
+6
Limit of naked eye
+ 10
Binocular limit
+ 15
Pluto
+ 30
Dim 
+ 21
Limit of large telescope with naked eye
Limit of large telescope using photography
PROPERTIES OF STARS
•  Absolute
magnitude
•  "True"
or intrinsic brightness of a star
•  Brightness at a standard distance of 32.6
light-years (10 parsecs)
•  Most stars' absolute magnitudes are
between -5 and +15
•
Parsec = 3.26 ly
PROPERTIES OF STARS
  Color
•
and temperature
Hot star
Temperature above 30,000 K (53,540 °F)
  Emits short-wavelength light
  Appears blue

•
Cool star
Temperature less than 3000 K (4,940.3 °F)
  Emits longer-wavelength light
  Appears red

PROPERTIES OF STARS
•
Between 5000 and 6000 K (8,540.3 °F – 10,340 °F)
Stars appear yellow but it’s really white, our atm. causes this
  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

Two stars orbiting one another
Stars are held together by mutual
gravitation
•  Both orbit around a common center of mass
•
Visual binaries are resolved telescopically
  More than 50% of the stars in the universe are binary stars
  Used to determine stellar mass


This means that the Sun is not like most stars we don’t have a
partner
BINARY STARS ORBIT
EACH OTHER
AROUND THEIR
COMMON CENTER OF
MASS
Figure 24.4
PROPERTIES OF STARS
•
Stellar mass
Determined using binary stars – the center of mass is closest
to the most massive star
  Mass of most stars is between one-tenth and fifty times the
mass of the Sun


The Sun is just average when it comes to mass
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
  Very large giants are called supergiants
  Only a few percent of all stars

HERTZSPRUNG-RUSSELL
DIAGRAM
•
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

IDEALIZED HERTZSPRUNGRUSSELL DIAGRAM
Figure 24.5
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
•
THE ORION NEBULA IS A WELLKNOWN EMISSION NEBULA
Figure 24.8
A FAINT BLUE REFLECTION NEBULA IN
THE PLEIADES STAR CLUSTER
Figure 24.9
INTERSTELLAR MATTER

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 the cloud
  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
EVOLUTIONARY STAGES OF
LOW MASS STARS
Figure 24.12 A
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
EVOLUTIONARY STAGES OF
MEDIUM MASS STARS
Figure 24.12 B
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
EVOLUTIONARY STAGES OF
MASSIVE STARS
Figure 24.12 C
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
MILK WAY GALAXY
Figure 24.18 A
EDGE-ON VIEW OF THE
MILK 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 of 20,000 to 125,000 light
years
•  Contains both young and old stars
•  e.g., Milky Way
•
GREAT GALAXY, A SPIRAL GALAXY, IN
THE CONSTELLATION ANDROMEDA
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 sill 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