Basic Properties of Stars (continued). Stellar Lives.

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Lecture 38

• Basic Properties of Stars (continued). Stellar
  Lives.

Chapter 17.12 ? 17.16

• Stellar Temperatures
• The Hertzsprung-Russell Diagram
• Groups of Stars and Their Lives

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Surface Temperature
Surface temperature determines a star color.
The coolest stars are red, the hottest ones are
blue.
Only the brightest star colors can be recognized
by the naked eye. The color can be determined
better by comparing a stars brightness in
different filters.
Betelgeuse has a temperature of 3,400 K, Sirius
9,400 K, the hottest stars up to 100,000 K.
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Spectral Type
The surface temperature also determines the line
spectrum of a star.
Hot stars display lines of highly ionized
elements, while cool stars show molecular lines.
Stars are classified by assigning a spectral
type. The hottest stars are called spectral type
O, followed by B, A, F, G, K, M as the surface
temperature declines.
Oh Be A Fine Girl, Kiss Me
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Stellar Masses
It is harder to measure stellar masses. The best
method is to apply Keplers third law in
combination with Newtons law of gravity.
This procedure can only be applied to orbiting
objects Visual binary a resolved pair of stars
(Mizar) Eclipsing binary a pair orbiting in the
plane of our line of sight Spectroscopic binary
an object with regularly moving spectral lines or
with 2 line systems.
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The Hertzsprung-Russell Diagram
Invented by Ejnar Hertzsprung (Denmark) and Henry
Norris Russell (USA) in 1912.
The diagram is a plot of stellar luminosities
against their surface temperatures.
Temperature increases leftward. Luminosity
increases upward.
H-R diagram
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Patterns in the H-R diagram
Main sequence location of the most stars (from
upper left to lower right corner) Luminosity
class V
Supergiant branch along the top (class I) Giant
branch just below the supergiants (class III)
White dwarfs near left corner (small size, high
temperature)
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Groups of Stars by Mass
Low-Mass Stars birth mass lt 2 Msun
Intermediate-Mass Stars 8 lt M/Msun lt 2
High-Mass Stars gt 8 Msun
Low- and Intermediate-mass stars evolve into red
giants and ultimately become white dwarfs.
High-mass stars pass through a supergiant phase
and end their lives in violent explosions.
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Star Birth
A stars life begins in an interstellar
cloud. Star-forming clouds are dense and cold
(10-30 K). These clouds are called molecular
clouds.
The conditions in molecular clouds allow gravity
to overcome thermal pressure and begin the
gravitational collapse.
Gravitational contraction increases the clouds
thermal energy, which is radiated into
interstellar space as long-wavelength infrared
radiation.
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The Protostar Stage
A collapsing cloud fragment starts with some
angular momentum, which increases the spin rate
of the fragment as it collapses.
As a result, a protostellar disk is formed. The
disk slows down the protostars rotation. The
rotation generates magnetic field. The field
lines transfer some of the angular momentum to
the disk. The magnetic field also generates a
strong protostellar wind.
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The Protostar Stage
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A Stars Infancy
A star is born when its core temperature exceeds
10 million K ? hydrogen fusion begins.
The stars interior stabilizes thermal energy
generated by fusion maintains the balance between
gravity and pressure.
The star becomes a main-sequence star.
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Life Tracks
The transitions that occur during star birth can
be shown with a special H-R diagram. Such a
diagram is called an evolutionary track.
A solar-mass protostar path
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Low-Mass Star at Main-Sequence
Low-mass stars produce helium from hydrogen
through the proton-proton chain during their
main-sequence lifetime.
The energy moves outward from the core through
random walk and convection.
The number of particles in the core reduces, the
core keeps shrinking, and the luminosity
increases over time.
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Red Giant Stage
When the core hydrogen depletes, nuclear fusion
ceases in the core. The core with no energy
source shrink faster.
The stars outer layers expand and the luminosity
rises. The stars becomes a red giant through a
subgiant.
The radius increases gt100 times. The luminosity
increases thousands times.
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Red Giant Stage
Why does the star grow bigger when the core is
shrinking?
The core is now made of helium, but the
surrounding layers contain plenty of
hydrogen. Gravity shrinks everything, so fusion
begins around the core (in a shell).
The fusion rate in the shell is higher than in
the core during the main-sequence stage. The
newly produced helium is added to the core.
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Switching Energy Sources
The core and the shell keep shrinking, while
thermal pressure keeps pushing upper layers
outward.
This cycle breaks down when the core reaches a
temperature of 100 million K. At this point
helium starts to fuse together.
Helium atoms have 2 protons and a higher positive
electric charge than hydrogen atoms. Helium
fusion occurs at higher temperatures. The process
converts 3 He nuclei (alpha-particle) into 1 C
nucleus energy according to Emc2.
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Helium Burning
Helium fusion inflates the core, which pushes out
the hydrogen-burning shell the shell burning
rate drops.
The total energy production rate falls from its
red giant phase peak. This reduces the stars
luminosity and decreases the stars radius,
making its surface hotter.
In the H-R diagram, the star goes down and to the
left. All low-mass stars fuse helium into carbon
at nearly the same rate ? they have almost the
same luminosity, but differ in temperature.
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What Will Happen to Earth?
The Sun keeps increasing its luminosity. In 5
billion years from now the hydrogen burning will
stop in its core.
The Sun will then expand to a subgiant. It will
become 2?3 times brighter.
The Earths temperature will rise, the oceans
will be evaporated, the life may not survive. The
Earth may be destroyed, when the Sun becomes
planetary nebula.