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Option D.2 Stellar Characteristics
Stars
 A star is a big ball of gas with fusion going on in its center
that is held together by gravity
 Stars are formed by interstellar dust coming together
through mutual gravitational attraction
 The loss of potential energy is responsible for the initial high
temperature necessary for fusion
 Fusion process releases so much energy that the pressure
created prevents the star from collapsing due to
gravitational pressure
 Called Stellar Equilibrium
 Thermal Pressure out = Gravity in
Nuclear Fusion in Stars
 Stars emit a great deal of energy
 Source for all energy is the fusion of hydrogen into helium
(sometimes referred to hydrogen burning)
 Rxn is nuclear not chemical
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H H H β  υE
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H  H  He    E
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He  He  He  2 H  E
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Nuclear Fusion in Stars
 Mass of the products is less than the mass of the
reactants
 Using ∆E=∆mc2 we can work out that the sun is
losing mass at the rate of 4 × 109 kg s-1
 Takes place in the core of the star
 Eventually all this energy is radiated from the
surface
 Calculate the rate at which energy is being
emitted
Stellar Spectra
 Radiation from stars is not a perfect continuous spectrum
 There are particular wavelengths that are missing
 The missing wavelengths correspond to the absorption
spectrum of a number of elements
 Although is seems sensible to assume that the elements
concerned are in the Earth’s atmosphere, this
assumption is incorrect
 Wavelengths would still be absent if light from the star
was analyzed in space.
 Absorption is taking place in the outer layers of the star
 This means that we have a way of telling what elements
exist in the star- at least in its outer layers
Stellar Spectra
Absorption Lines
and
Classifications
Stellar Spectra
 Wavelengths absorbed are characteristic of atoms present
 Absorption spectrum can be used to identify the
elements present in the outer layers of the star
 Deduce both chemical and physical data about stars
 Chemical composition and surface temperature
 Speed and direction of motion using the Doppler Effect
 If source of waves is moving towards us, their
frequency is shifted upwards
 Stars moving towards us are called blue-shifted
 If source of waves is moving away from us, their
frequency is shifted down
 Stars moving away from us are called red-shifted
Luminosity
 Luminosity is total power radiated by a star (Watts)
 Different than power received by an observer on
Earth (brightness)
 Depends on surface temperature of the star and
radius
 L α T4
Lαr
Apparent Brightness
 Brightness of a star is the power per unit area received by
an observer on Earth (Wm-2)
 If two stars were at the same distance away from the
Earth then the one with the greatest luminosity would be
brighter
 as the distance increases, the brightness decreases
since the light is spread over a bigger area
 But, stars are at different distances from the Earth (d)
L
b
4d 2
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It is thus possible for two very different stars to have
the same apparent brightness
all depends on how far away the stars are
Temperature of Stars
 If the energy emitted from a star is analyzed over a
range of wavelengths, the peak wavelength can be
determined
 Using Wien’s Law, the surface temperature of the
star can be determined
 As temperature increased, total energy increases
(area under curve)
 Luminosity is proportional to T4
 Stefan-Boltzmann law
Binary Stars
 A binary star is a stellar system
consisting of two stars orbiting around
their common center of mass
 Called companion stars
 A large percentage of stars are part of
systems with at least two stars
 Binary star systems are very important
in astrophysics, because observing
their mutual orbits allows their mass to
be determined
 The masses of many single stars can
then be determined by extrapolations
made from the observation of binaries
Hubble image of
the Sirius binary
system, in which
Sirius B can be
distinguished at
the lower left
Single Stars
 The source of energy for our sun is the fusion of hydrogen
into helium
 This is also true for many other stars
 There are however, other types of stars that are known to
exist in the universe
 RED GIANT STARS
 WHITE DWARF STARS
 CEPHEID VARIABLES
Red Giants
 As the name suggests, these stars are large in size and red in
color
 High Luminosity
 Since they are red, they are comparatively cool
 Source of energy is the fusion of some elements other than
hydrogen (He to heavier elements)
 Red Supergiants are like red giants only cooler, more
luminous and bigger
 These stars are the largest structures in the universe,
although they are not the most massive
White Dwarfs & Brown Dwarfs
 These stars are small and white in color
 Low Luminosity
 Since they are white they are comparatively hot
 Fusion is no longer taking place, and a white dwarf is just a
hot remnant that is cooling down
 They are usually composed of oxygen and carbon in an
extremely dense form
 Eventually it will cease to give out light when it becomes
sufficiently cold
 It is then known as a brown or black dwarf
Cepheid
Variables
 Unstable stars that have completed its hydrogen burning
phase
 Have regular variations in brightness and hence luminosity
due to the oscillation in the size of the star
 A stellar layer loses hydrostatic equilibrium
 Cycle of Cepheid Star:
1. Outer layer becomes compressed due to gravity
2. Temperature inside increases, thus increasing outward
pressure
3. Layer pushed out ward by increasing pressure
4. Expansion caused layer to cool and less dense
5. More radiation escapes and pressure decreases
6. Outer layer becomes more compressed due to gravity
Hertzsprung-Russell Diagram
 Hertzsprung and Russell discovered the relationship
between surface temperature and luminosity around 1910
 H-R diagram is an attempt to look for patterns in the stars
like the periodic table was constructed to look for
patterns in the elements
 Each dot on the diagram represents a star
 The scales are NOT LINEAR
 Temperature scale runs backwards (high temp on left)
 90% of stars fall on the diagonal band known as main
sequence
 Ordinary stars still producing energy through fusion (like sun)
Absolute magnitude
Betelgeuse
Rigel
Sirius B
Color index, or spectral class
Hertzsprung-Russell Diagram
 Starting at lower right are the coolest stars, reddish in color
 Further up towards the left are hotter and more luminous
stars that are yellow and white
 Still further up are the more luminous blue stars
 Mass of a star increases moving up the main sequence
 Gravitational pressure increases with mass, so to
maintain equilibrium, fusion reactions in the core must
generate a greater radiation pressure
 Star has to ‘burn’ at a higher temperature, giving it a
greater luminosity
 L α M3.5
Stars in the vicinity of the Sun
L  Mass3.5
Specific segments of the main sequence are occupied
by stars of a specific mass
Majority of stars are here
Hertzsprung-Russell Diagram
 About 1% of stars are red giants and supergiants
 Their high luminosity and low temperature means they
have a very large area
 About 9% of stars are white dwarfs
 White dwarfs are very hot and not luminous
 Low luminosity and high temperature means they are
much smaller than main sequence
 Final group of stars (cepheids) congregate in a great band of
instability that appears between main sequence and red
giants