The Sun and other
Stars
Chapters 11, 12, 13 and 14
The importance of your text!
 As you can see we will be combining bits
from several different chapters in your
book. Make sure that you read each
chapter!
 I will indicate which sections of the
chapter you need to reread for the test.
 You will also get a review sheet before
this test, BUT you must be prepared and
have read ALL of the four chapters!!!!
Various laws used to explain
solar phenomena
– Used determine the
Sun’s mass. From this we deduce its
surface gravity.
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– Used to determine the
Sun’s surface temperature by its color.
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– Used to determine the
amount of energy released based on
temperature.
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The Sun
The Sun
– The Sun’s outer atmosphere.
Temperature is about 5 million K
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– The Sun’s lower
atmosphere. (4,500 K up to 50,000 K)
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– The Sun’s visible surface.
Temperature is about 6,000 K
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– Center of the Sun,
Temperature is about 15 MILLION K.
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The Sun
 Composed of: ____________
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27%___________
2% vaporized elements such as Fe and C
 The Sun is ______ AU from the Earth
 Burns 600 million tons of Hydrogen EVERY second!
 Produces 4x1026 Watts of energy
 Is actually brighter than 85% of the stars in the galaxy.
 Is in spectral class G2 which means it produces “white
light”. BECAUSE of atmospheric scattering the Sun
appears yellow.
Solar Eclipses
 We can learn a lot about the Sun and actually
see the corona during a Solar eclipse.
 List of upcoming Solar eclipses:
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Total Solar Eclipse of 2008 August 01
Total Solar Eclipse of 2009 July 22
Total Solar Eclipse of 2010 July 11
 These will be visible from Asia or S. America
 The next total eclipse visible from the United
States won’t happen until August 21, 2017.
Energy Transfer
 The core of the Sun is extremely hot. The heat
radiates out from the core by the movement of
photons.
 This area is called the
. The
photons of light slowly move through the dense
core.
 Just below the photosphere the Sun is so
dense that movement of photons is so slow that
convection currents begin to circulate the Sun’s
energy. This is called the
zone.
Granulations
 Textures seen in the Sun’s photosphere.
 They are created when hot gas rises to
the surface of the sun. They appear
brighter because they are hotter than the
surrounding area.
 When they cool they look darker and
sink back into the interior of the Sun.
 Gases rise to the surface about
1km/second.
Chromosphere
 Usually invisible
 Can only be seen during a solar eclipse.
 Emits bright red light because of the
High H content
 ___________ – Thin columns of hot
gases that jet out of sun.
Fueling the Solar Fires
 Hydrostatic equilibrium -
(See figures 11.8 & 11.9)
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Prevents the sun from collapsing or separating
 Hydrostatic equilibrium explains the Sun or any other
star’s structure, but it does not explain what keeps it
glowing
Fueling the Fire
 Nuclear reactions were first suggested to fuel the Sun in
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1899, but could not be proven.
In 1905 when Einstein developed E=mc2 astronomers
were able to provide evidence for their nuclear theory.
E=mc2 states that mass can become energy.
C = the speed of light, so it only takes a minute amount
of mass to generate a large quantity of energy.
This lead the way for two astrophysicists to determine
that the Sun was powered by the fusion of Hydrogen
atoms.
Nuclear Fusion
 When 2 or more nuclei are bonded together
to form a single, heavier nucleus.
 The process of fusing H into He takes three
steps. It is called the proton-proton chain.
Three steps to He formation by
the Proton-proton chain
 Two H atoms collide and form an isotope of H
called deuterium.
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This releases subatomic particles called positrons
and neutrinos. Neutrinos leave, but the positrons
hang around and will be important later.
 The Deuterium then collides with another H atom
to produce an isotope of He called He3
 Two molecules of He3 collide to form He. In the
process, two protons are ejected
 Each step releases ENERGY
Solar and Stellar Magnetism
– A dark cooled region of
the Sun’s surface created by magnetic
activity.
 The sun rotates and as a result of its
large amount of charged particles has a
strong magnetic field.
 This strong magnetic field pulls some
electrons more than others and results in
a more rapid cooling (sun spots)
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Other magnetic disturbances
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– A cloud of hot gas in
the Sun’s outer atmosphere. This cloud
is often shaped like an arc (fig 11.17
and 11.18)
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– A sudden increase in
brightness of a small region in the Sun.
Solar wind
 The outflow of low-density, hot gas from
the Sun (or star)
 Caused by the gradual loss of particles
from the Sun because they have enough
energy to escape the gravity of the Sun
Life cycle of the Sun
The Sun as a star
 Remember the Sun is an average star,
much like many the other stars in the
night sky.
 When we discuss what fuels the Sun we
are also discussing what fuels other
stars.
 Before we go into Stellar evolution we
first need to understand how we group
stars. (Chapter 12)
Star size and color
 Most stars are similar to the Sun in size,
composition, and color.
 Some are 30 times more massive
 Some are blue because of increased
temperature
 Some are red because they are cooler
 All stars are very far away, and their
distance affects how we see them
Luminosity
 The amount of energy radiated per
second by a body.
 When we discuss the luminosity of a star
it is measured in units of the Sun’s
luminosity
 The Sun puts out about 4 x 10 26 watts
Inverse Square law
 The apparent brightness of an object decreases
inversely as the square of its distance.
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Basically: it explains in mathematical terms that the
closer you are to an object the brighter it appears.
The farther away from an object you are the less bright it
appears.
 Physical explanation: When you are close to a light
source the light has had less time and space to
spread out. But as you move away from a light
there is more time and space for the light rays to
spread in all directions.
 Can also be explained by fewer photons per area
Star spectra
The spectra of a star depicts the energy it
emits at each wavelength.
The spectra tells us the star’s:
-
Absorption lines
 Absorption lines are the wavelengths of
energy that particular atoms absorb.
Appear as dark lines in the star’s
spectra.
 Particular atoms absorb particular
wavelengths. – Allows us to determine
stellar composition.
Spectral Classification
 Spectral Classes are arranged by temperature.
 The spectral classes in the order hottest to
coolest is: ______________________
 A star’s spectral class is determined by the
lines in its spectrum
 Hot objects are blue and cool objects are red.
 Class O & B stars are bluish, K & M stars are
reddish.
Hertzsprung-Russell Diagram –
H-R Diagrams
 Named after two astronomers that developed it at
the same time, but independently of each other.
 H-R Diagram – A graph on which stars are located
according to their temperature and luminosity.
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Most stars lie along a diagonal line called the main
sequence.
 The main sequence runs from cool dim stars in the
lower right to hot luminous stars in the upper left.
Main sequence stars fuse H to He in their cores.
H-R Diagrams and Giants
 A star’s luminosity depends on its Surface area and
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temperature.
If two stars are the same temperature but differ in luminosity,
then they must be different in size.
Bright cool stars are called red giants. Red giants are large
stars.
They are very bright because they are very big, but are also
relatively cool.
They appear red because of their low temperature. They are in
the upper right corner of the H-R diagram. (page 379)
Gas giants have relatively low densities
H-R diagrams and Dwarfs
 Hot stars that are large would be the most
luminous stars in the sky, but small stars that
are hot also produce white light, but appear
dim because of their small size.
 White Dwarf is a dense star with a radius
approximately the same as the Earth.
 They do not generate heat via fusion, rather
glow from residual heat.
 They are the last stage of stellar evolution.
Luminosity classes
 Astronomers have grouped stars into 5 classes based
on their luminosity and width of the absorption
spectral lines.
 The five Luminosity Classes (I,II,III,IV,V):
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I = the brightest
V = the dimmest
 Luminosity class is often added to a stars spectral
class.
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The Sun is a G2 star (spectral class) and a V (luminosity
class). Together, the Sun is a G2V star
Stellar luminosity classes
Class Description
Example
Ia
Super-giants
Betelgeuse, Rigel
Ib
Dimmer super-giants
Polaris
II
Bright giants
Mintaka (in Orions belt)
III
Ordinary giants
Arcturus
IV
Sub-giants
Achernar
V
Main sequence stars
The Sun, Sirius
Variable Star
 Not all stars have constant luminosity.
 A star whose luminosity changes is
called a variable star.
 Stars can vary in luminosity because of a
change in temperature or a change in
size
Stellar Evolution
 Add the outline/flow diagram from page 391 to
your notes. (fig 13.1)
 Stars begin as interstellar clouds – A mix of gas.
 When stars like the Sun begin to fuse H to He
they fall into the Main sequence stars.
 The Sun will remain a main sequence star until
uses about 90% of its fuel in the core.
 This is the beginning of the End
Development of a Red Giant
 As a star like the sun uses its last bit of fuel, it
begins to burn the fuel faster, generating more
heat.
 The heat pushes the outer surfaces of the Sun
farther away.
 As these outer surfaces get further from the
heat source they cool and turn a red color.
 The resulting large, red, cool star is called a red
giant.
Red Giant to Yellow Giant
 As more and more H is used the core gets hotter and
hotter. The star gets smaller until He becomes the
nuclear fuel.
 The amount of He is also increasing until H is
expended and is no longer the fuel source for the star.
He begins to fuse together.
 The star begins to be a pulsating Yellow Giant.
 The Star is extremely large and bright.
 Once the He is gone the star remains large but glows
a cooler red. Becoming a red giant again
Red Giant to White Dwarf
 As the large red star emits energy and radiation it
begins to drive its gaseous contents out into space.
 This exposes just the core of the star.
 The core has no other energy source and emits its
stored heat as a tiny white dwarf.
Large stars can form neutron
stars or black holes
 Instead of cooling to form white dwarfs, high mass stars
explode!
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______________ – Any star with a mass 10 times that of the sun.
 Because high mass stars have such an intense gravitational
force, their cores are much hotter.
 This results in the core’s ability to fuse heavier elements
than H and He. In fact high mass stars can fuse C, O and
Even Silicon, but they are not hot enough to fuse Fe.
 The Gravitational pull is so great that the core collapses and
causes a HUGE explosion
The Explosion of an Iron Core
 The core becomes a compressed ball of neutrons
– neutron star, OR
 A black hole, the most dense body known.
 SEE FIGURES 13.2 and 13.3
 We can trace the evolution of a star on an H-R
diagram (see page 407 in your book)
 ? for review – pg 415 1, 2, 10,11,14,15,17