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STARS
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And stellar processes
Stars
 Stars are the things you see most of in the
night sky.
 You already know all about the Sun, which is
a pretty good example of an average star
 But what exactly is a star???
Stars
 Stars are formed by interstellar dust coming together
through mutual gravitational attraction.
 In a gas cloud (Nebula) all the gas particles are attracted to
one another. This means they have potential energy.
 Over time gas and dust particles will move together. The
potential energy of the gas is ultimately converted to kinetic
energy leading to high temperatures.
 The high temperatures created by the loss of potential
energy can trigger fusion.
 The fusion process releases so much energy that the
pressure created prevents the star from collapsing due to
gravitational pressure.
Stars
 In the middle of the stars the high pressure (due to
gravitational squeezing of the huge mass of the star) and
high temperatures make it too hot for atoms to hold
together, and the interior of a star consists of plasma.
 Plasma is disassociated ions and electrons.
 In these conditions, it is possible for two protons (hydrogen
nuclei) to overcome their electrostatic repulsion and fuse.
 During the fusion process large amounts of radiation (in
terms of Gamma Radiation) are released.
 The star also releases radiation in all other areas of the
electromagnetic spectrum due to its temperature.
Nuclear fusion: Proton
Proton Cycle.
Very high
temperatures
are needed in
order to begin
the fusion
process:
usually 107 K.
Nuclear fusion:
Proton Proton Cycle.
 The proton-proton cycle is the process of fusion
of protons (Hydrogen Nucleii) into Helium.
 Each stage of the cycle involves release of
energy, but most is released in the final stage of
the cycle.
0 
1
1
1
H  H  H  e  ve
1
1
H  H  He  
1
1
3
2
2
1
2
1
3
2
0
0
He  He  He  2 H
3
2
4
2
1
1
Nucleosynthesis
 All stars begin in similar ways, through the fusion
of Hydrogen. Stars of different ages,
temperatures and sizes have different processes,
and different elements are created through
fusion. These different elements gather in the
outer layers of the star.
 The proton proton cycle is one of the processes
that is referred to as “neucleosynthesis” and is
part of the AHL curriculum.
 More on Nucleosyntheis later.
Equilibrium.
 The energy created through fusion travels outwards
creating increasing the Kinetic Energy of the
particles within the star and forcing them outwards
creating an outwards force on the surroundings. The
outward force created by the radiation is called
“radiation pressure”.
 Radiation Pressure pushes outwards on the outer
layers of sun.
 Gravitation pulls the outer layers inwards.
 These create a lovely balance, and the star is in
equilibrium.
 A star is in “Hydrostatic Equilibrium” between radiation
pressure and gravitational forces.
Hydrostatic Equilibrium
Radiation
Pressure
Gravitational
Pressure
The core
supports the
weight of the
whole star!
A star is a big ball of gas,
with fusion going on at its
center, held together by
gravity!
Massive
Star
Sun-like
Star
Low-mass
Star
There are variations between stars, but by and
large they’re really pretty simple things.
What is the most important thing about a
star?
MASS!
Mass
• The mass of a normal star almost completely determines
everything about the star:
• The mass controls the internal pressure of the star.
• The pressure controls its TEMPERATURE!
• The temperature and pressure control the rate of fusion
at the core.
• The temperature and rate of fusion control the
Luminosity of the star.
• The mass controls the lifecycle of the star.
• The mass controls the length of time a star burns for.
• The mass controls what elements are fused within the
star.

Note: when we say “normal” star, we mean a star that’s fusing Hydrogen
into Helium in its center (we say “hydrogen burning”).
Luminosity.
 The LUMINOSITY of a star is
how much ENERGY it gives off
every second.
 You can think of luminosity as the
total “power output” of the star.
 Of course, the energy as star emits is
generated by the fusion in its core…
This light bulb has
a luminosity of 60
Watts
The core pressure determines
the rate of fusion…
MASS
PRESSURE &
TEMPERATURE
RATE OF
FUSION
…which in turn determines
the star’s
luminosity!
Luminosity
 Main Sequence (you’ll learn this term in a few
slides) stars have a relationship between
luminosity and mass:
𝐿 ∝ 𝑀3.5
Luminosity
The Luminosity of a star is the total energy that it
releases per second.
Sun has a luminosity of 3.90x1026 W (often written as
L): it emits 3.90x1026 joules per second.
The energy that arrives at
the Earth is only a very small
amount when compared will
the total energy released by
the Sun.
Luminosity.
 So the Luminosity (energy given off by a star) is
controlled by the internal Fusion of the star.
 The amount of internal fusion is controlled by the
mass of a star.
 The Stefan-Boltzmann law relates the energy given
off by a body (really a black body) is related to its
temperature and its surface area..
L  AT
4
 Where: σ =5.67x10-8 Wm-2K-4
 T is the temperature (in Kelvin)
 A is the surface area of the object.
Luminosity
 It is worth noting that the surface area of a star
(a sphere) is dependent on its radius…
A  4r
 So…
2
L   4r T
2
4
 (This eqn is less important than StefanBoltzmann)
Luminosity.
 But remember, energy from the star spreads out in all directions.
By the time it reaches us we can only measure the energy arriving
at us… we only measure “Apparent Luminosity.” (The amount of
energy we receive at Earth is spread out over a vast area…
remember from our Climate Change Unit???)
 Apparent Luminosity (the power we receive at Earth per meter
squared) is called “Brightness” (b).

(This is the energy per metre squared arriving a the Earth every second
from a star).
L
AT
b

2
2
4d
4d
4
Where d is the distance to the star.
Information from Light.
 We can figure out distance (d) to a star,
(but we will discuss how in a later slide
show).
 So if we can figure out temperature, we
can determine the size of the star.
Black Body Radiation
• The intensity of light
emitted by a black body
is distributed over an
infinite range of
wavelengths.
• The peak wavelength
emitted is dependent on
temperature.
• Notice also that different
temperatures have
different shape curves.
Black body radiation
 Remember:
 A black body is a perfect emitter. A good model for a black
body is a filament light bulb: the light bulb emits in a very
large region of the electromagnetic spectrum.
 The interior of a star is generally considered to be a “black
body”
 There is a clear relationship between the temperature of an
object and the wavelength for which the emission is
maximum. That relationship is known as Wien’s law:
max T  constant
max T  2.9x10 m K
-3
Wien Displacement law
By analysing a star’s spectrum, we
can know in what wavelength the
star emits more energy.
The Sun emits more energy at
λ=500 nm.
According to Wien’s law, the
temperature at the Sun’s surface is
inversely proportional to the
maximum wavelength.
So:
T
2.9x10 -3
max
2.9x10 -3

 5800 K
-9
500x10
Black body radiation and Wien Law
Star’s Colour and
Temperature
Why is this important?
 The spectrum of stars is similar to the
spectrum emitted by a black body.
 We can therefore use Wien Law to find the
temperature of a star from its spectrum.
 If we know its temperature and its luminosity
then its radius can be found from StephanBoltzmann law.
Real spectra
Blackbody
Spectrum
Emission and
Absorption
Lines
 Real spectra are more complicated than this (remember
emission and absorption lines?).
 The elements in the outer layer of stars absorb some of the
energy emitted and you get some wavelengths missing.
Stellar
Classification

It has been noticed that stars of
certain temperatures also have
the same absorption spectra.
Suggesting that the same
mechanisms are going on inside
stars of similar temperatures.

Based on the “colour” (ie the
temperature) of stars, and their
emission/absorbtion spectra. We
have classified stars into 7 (ok
there are more, but you need to
know 7…) classifications.

These are :

O.B.A.F.G.K.M
Stellar classification
 Each classification has a unique temperature (Class O
being the hottest, and Class M being the coolest).
 You will need to know the order of these.
 I suggest another Mnemonic.
 Oloomis Banjo Annoys Friends and Gives Kids
Migraines.
 Or use the ones most commonly used by others…
 (I did not make the next ones up…)
OBAFGKM - Mnemonics
O Be A Fine Girl/Guy Kiss Me
Only Boring Astronomers Find Gratification in
Knowing Mnemonics!
The Spectral Sequence
Class
Spectrum
Color
Temperature
O
ionized and neutral helium,
weakened hydrogen
bluish
31,000-49,000 K
B
A
F
G
neutral helium, stronger
hydrogen
blue-white
10,000-31,000 K
strong hydrogen, ionized
metals
white
7400-10,000 K
weaker hydrogen, ionized
metals
yellowish white
6000-7400 K
still weaker hydrogen, ionized
and neutral metals
yellowish
5300-6000 K
K
M
weak hydrogen, neutral
metals
orange
3900-5300 K
little or no hydrogen, neutral
metals, molecules
reddish
2200-3900 K
L
no hydrogen, metallic
hydrides, alkalai metals
red-infrared
1200-2200 K
T
methane bands
infrared
under 1200 K
Spectra.
 Spectra tell us about the physics of the star and
how those physics affect the atoms in it
The Hertzsprung-Russell diagram
 When all the observed
stars were plotted in a
graph of Luminosity vs
Spectral Class (surface
temperature), we start to
see some “grouping” of
different types of stars.
The Hertzsprung-Russell diagram
• This diagram shows a
correlation between the
luminosity of a star and its
temperature.
• The scale on the axes is not
linear as the temperature
varies from 3000 to 25000 K
whereas the luminosity varies
from 10-4 to 106, 10 orders of
magnitude.
• It’s also important to realize
that temperature (along the X
axis) is hottest on the LEFT
and Coldest on the right.
H-R diagram
 The stars are not randomly distributed on
the diagram.
 There are 3 features that emerge from the
H-R diagram:
 Most stars fall on a strip extending
diagonally across the diagram from top
left to bottom right. This is called the
MAIN SEQUENCE.
 Some large stars, reddish in colour
occupy the top right – these are red
giants (large, cool stars).
 The bottom left is a region of small stars
known as white dwarfs (small and hot)
Hint…
 Know your way around the H/R diagram.
Look.
Types of Stars
 Main Sequence Stars
 These are “ordinary”
stars that produce
energy from fusion of
hydrogen and other light
nuclei such as carbon
and helium.
 Most (90%) of all stars
are main sequence stars.
 The sun is a main
sequence star (G class).
Types of Stars
 Red Giants

Red Giant stars are stars (of similar
mass to our Sun) approaching the end
of their life cycles. In these stars,
there is less hydrogen fusion
occurring in the core. The reduced
fusion leads to reduced radiation
pressure from the core and leads to
gravitational collapse (within the
core). The core then starts to fuse
Helium, and fusion that is still
occurring happens in a shell
surrounding the core. This leads to an
increase in the luminosity of the star,
and causes the outer layers to
expand. These outer layers cool
down, and the star appears red (due
to lower temperature).
 Larger than the sun.
 Cooler than the sun.
 Similar or Slightly greater Luminosity
than the sun.
Types of Stars
 Supergiants
 MUCH Larger than the sun
(radii ~30-500 times greater
radii than the sun, or more.)
 Cooler than the sun (red!).
 Much greater Luminosity (
up to >1,000,000 times) than
the sun.
 These stars are later stages
of the life cycle of mainsequence stars that are
considerably more massive
than the sun.
Supergiants
Types of Stars
 White dwarf stars
 Small and very hot stars.
 Since they are so hot,
they appear white.
 These stars are last stage
of the life cycle of
smaller mass main
sequence stars.
 Fusion no longer occurs
in white dwarf stars, they
are stellar remnants that
are cooling.
 As they cool and give out
less light, they become
“brown dwarfs”
Types of Stars

Neutron Star




Neutron stars are the remnants of
large mass stars (supergiants) after
they go “supernova”.
Neutron stars are very dense due to
the gravitational collapse of these
large mass stars.
Neutron stars are composed mainly
of neutrons.
Pulsars
Pulsars are highly magnetized
rotating neutron stars which emit a
beam of detectable electromagnetic
radiation in the form of radio waves.
Periods of rotation vary from a few
milliseconds to seconds.
They emits a beam of radiation from
their poles. This acts a little like a
lighthouse, it appears to “pulse” when
the pole points at us.
Schematic view of a pulsar. The sphere in the
middle represents the neutron star, the curves
indicate the magnetic field lines and the protruding
cones represent the emission beams
Types of Stars
 Black hole
 Ok not really a “star”.
 (some) Black holes are the remnants of the very




largest mass stars, after they have gone
supernova.
The gravitational collapse cannot be stopped by
any known process.
The gravitational pull is so great that nothing
(including Electromagnetic Radiation) can get
away from them, and so we cannot directly
observe them.
Physicists call anything we can’t see “black”, since
the colour black is the result of no EM radiation.
We know they exist due to modelling, and due to
the fact that they bend light that travels past
them.
Types of Stars

Cepheid variables





Cepheid variables are stars of variable
luminosity. The luminosity increases
sharply and falls of gently with a welldefined period.
The period is related to the absolute
luminosity of the star and so can be
used to estimate the distance to the
star.
A Cepheid is usually a giant yellow star,
pulsing regularly by expanding and
contracting, resulting in a regular
oscillation of its luminosity. The
luminosity of Cepheid stars range from
103 to 104 times that of the Sun.
By studying the period of brightness of a
Cepheid variable, you can determine it’s
luminosity.
Once you know its luminosity you can
know how far away it is (more on this
later).
Types of stars
 Cepheid Variables, are stars
which brighten and dim
periodically. The time period
of variation is proportional to
the Luminosity of the star.
Types of Stars
 Binary stars
A binary star is a stellar system consisting
of two stars orbiting around their centre
of mass. For each star, the other is its
companion star. 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
clearly
distinguished
(lower left).
Binary Stars
 Binary Stars are stars are
systems of two stars that
orbit a common centre (the
centre of mass of the two
stars combined!).
 We classify three types of
binary stars:
 Visual Binaries.
 Eclipsing Binaries.
 Spectroscopic Binaries.
Binary stars
 Binary stars are useful because we can figure out their
masses (or their combined mass) by measuring how
long they take to complete one orbit around their
centre of mass (the orbital period)
 Their orbital period (T) is related to their masses by:
2 3
4

d
2
T 
G(M1  M 2 )
 Where d is the distance between the two stars.
 We can measure T directly (through observation), so
this allows us to calculate the mass of the stars.
3 Ways of knowing: Binary
stars
 Some binary stars, you can just see.
 Visual Binaries.
 These appear as two separate stars when viewed
through a telescope.
Binary stars
 Eclipsing binaries – some binaries are too far away from us to be
resolved visually as two separate stars.
 At big distances two stars may seem to be one.
But if the plane of the orbit of
the two stars is suitably
oriented relative to that of the
Earth, the light of one of the
stars in the binary may be
blocked by the other, resulting
in an eclipse of the star,
dimming the light from the
system for a while. This
allows us to determine the
period.
Binary stars
 Spectroscopic binaries
– this system is
detected by analysing
the light from one or
both of its members
and observing that
there is a periodic
Doppler shifting of the
lines in the spectrum.
 As a moves away it it’s
light is slightly more
redshifted.
 As it moves towards us
it is slightly more blueshifted.
 This gives us period.
Binary stars
A blue shift is expected as the star
approaches the Earth and a red shift as it
moves away from the Earth in its orbit
around its companion.
If λ0 is the wavelength of a spectral line
and λ the wavelength received on earth,
the shift, z, is defined as:
  0
z
0
If the speed of the source is small compared with the speed
of light, it can be shown that:
v
z
c
The speed is proportional to the shift