Stars

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Stars The Sun
If you want to make an apple pie from scratch,
you must first create the Universe - Carl Sagan
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Characteristics of the Sun
The Suns luminosity corresponds to the burning
of 1500 lb of coal every hour on every square
foot of the Suns surface - Hermann von
Helmholtz, 1871
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What keeps the Sun from collapsing
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Answer Energy generated from nuclear reactions
at the Suns Center.
Notice the neutrino.
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Neutrino Experiment
Neutrinos act only very weakly with matter, even
the vast bulk of the Sun offers little
impediment. The detector consists of 100,000
gallons of cleaning fluid (C2Cl4). The neutrino
occasionally strikes a neutron of Cl, converts it
to radioactive Ar (by converting a proton into a
neutron). The amount of Ar is flushed out of the
system and then measured.
Davis Solar Neutrino Experiment (Gold Mind SD)
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Elementary Particles
Only 35 of the solar neutrinos arriving to Earth
are electron neutrinos (2002)
Sudbury Neutrion Observatory
Neutron u d d Proton u u d
Fermions (half integer spin)
Higgs Boson graviton
Neutrino Oscillations Neutrinos can change in
type. They possess mass.
2 miles deep
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Sun Spots
Sun spots are regions of intense magnetic
activity (which inhibits convection), forming
areas of low temperature.
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Sun Spot (UV) from TRACE
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The Solar Cycle
The number of Sun spots varies with a period of
about 11 years. The most recent sunspot maximum
occurred in 2000. Just after sunspot minimums the
sunspots appear poleward of 30 latitude. As the
cycle progresses the spot production migrates
towards the equator. This year (2007) is a
minimum.
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Sunspots are produced by the 22 cycle of the
magnetic field
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Mauna Kea Observatory
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Stars dim with the square of their distance away
from us. If we know the distance from the star,
e.g. from parallax, we then know the luminosity
(the power that they radiate). Many stars are
quite a bit larger than the Sun.
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There exist stars of a range of sizes. What
establishes a stars size is its mass and age.
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When the Sun finishes burning H in its core, it
will collapse, until the core heats up enough to
fuse He. Helium then fuses to carbon in the
core. A shell surrounding the core heats up
enough to fuse H into He. Once He is used up,
the core collapses again. Yet, for small stars,
like the Sun, temperatures never reach the 600
million K needed to burn C. What stops the
collapse? Electrons, which by Fermis law never
occupy the same place. The Sun will shrink to
the size of Earth. The electrons will exert a
pressure that sustains the white dwarf against
further collapse.
H ? He ? C ? O ? Ne ? Mg ? Si ? Fe
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Stars more massive than the Sun achieve higher
temperatures in their cores, and are thus capable
of fusing higher elements.
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As the core collapses, the outer part of the star
cools. This increases the opacity of the stars
atmosphere, thereby hindering the escape of
energy. The star thus expands until it can
radiate the power that it makes. The now
puffed-up star is called a Supergiant (if 10-70
solar masses) or a Red Giant (if solar size).
The Sun, at this stage (in 4.5 Gyr), will be 100
times its present radius and will engulf Mercury,
Venus and Earth.
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We come from stars
Heavy elements are ultimately made from H through
several processes Helium Capture C (3 He), O
(4 He), Ne (5 He), Mg (6 He), Si (7 He).
Photodisintegration (gamma rays break nuclei)
S, Ar, Ca, Fe Slow neutron capture, s-process
copper, silver, lead, gold Rapid neutron
capture, r-process (in super nova) and forms
elements bismuth209 CNO cycle produces N (C
2 protons - e) Note Fe has the highest binding
energy. Energy is needed to fuse Fe into higher
elements. This, we will see, has dire
consequences for the lives of stars.
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The abundances of elements are determined by
their binding energies, their tendencies to
decay, break apart, and capture nucleons.
These processes depend on the temperature
pressure of the elements and their internal
nuclear structure.
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Summary

• Most elements are synthesized in the interior of
  Stars.
• The heaviest, and least abundant, elements are
  synthesized in supernova.
• Our Sun is presently burning H in its core. In
  4.5 billion years it will use up the H in the
  core and collapse. When temperatures are hot
  enough it will burn carbon. When the carbon is
  exhausted it will collapse again. Electrons will
  terminate the collapse, once the Sun reaches
  Earth size. The Sun will become a white dwarf.
• More massive stars are able to achieve
  temperatures hot enough to synthesize heavier
  elements.
• The Suns magnetic field reverses every 11 years,
  producing a periodicity to the sun spots and
  solar activity.

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The Death of Stars

• The Bigger They are the Harder They Fall

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The Fate of Our Sun

• Our Sun is large enough to burn hydrogen into
  helium and helium into carbon, but the nuclear
  reactions will go no further.
• All its fuel will be spent in about 5 Gyr. It
  will spend some time as a red giant, but
  eventually end as a white dwarf.
• It is remarkable and important (for us) that the
  Sun is relatively stable with constant output for
  most of its 10 Byr life.

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Electron Degeneracy, Planetary Nebulae, and White
Dwarfs
Once the fuel runs out, solar mass stars collapse
violently, expelling the outer layers of gas and
creating a planetary nebulae, shown to the right
(Planetary nebulae have nothing to do with
planets). Further collapse is prevented not by
the temperature of the star, but by the pressure
caused by electrons. According to the Pauli
exclusion principle, it is impossible for 2
electrons to occupy the same state thus, there
is a limit to how tightly electrons can be
packed. Electrons packed this tightly are called
degenerate. The electrons in white dwarf stars
are degenerate.
White Dwarf
Planetary Nebula
In white dwarf stars there is a balance between
gravity and electron pressure. No nuclear
reactions are occurring and white dwarfs cool
very slowly over time.
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The White Dwarf Sirius B

• Temperature 24,790 K 4.29 ?
  Solar
• Radius 5838 km 0.0084 ? Solar
  1.15 ? Earth
• Mass 2.06?1030 kg 1.034 ? Solar
• Density 2.5?103 g/cc 2000 ? Solar
• Luminosity .0025? Solar

1 teaspoon of Sirius B weighs 5 tons
In the constellation of Orions dog
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Supernova and Neutron Stars

• A different fate awaits stars with masses greater
  than 8 Solar Masses.
• If the force of gravity is strong enough,
  electrons and protons combine, creating
  neutrons e p ? n neutrino
• Quickly, all the electrons and protons in a star
  are converted to neutrons. Enormous amounts of
  energy are released in a supernova explosion.
• The stellar remnant left behind is composed
  completely of neutrons, i.e. a neutron star.

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Supernova 1987A
Supernova have been important historically. Tycho
and Kepler both observed supernova. The only
supernova in modern time, visible to the naked
eye, was detected on Feb. 23, 1987 and is known
as SN1987A. A tremendous amount of energy is
released in a supernova. SN1987A emitted more
than 100 billion times as much visible light as
the Sun for over one month! Temperatures as high
as 2?1011 K were reached.
Sanduleak
Images of the star Sanduleak before and after it
went supernova. Something to think about
Sanduleak is 169,000 light years from Earth.
This means that SN1987A actually occurred in
167,000 BC.
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Neutrinos from Supervona

• Neutrinos are emitted when electrons and protons
  combine to form neutrons.
• Most of the energy of a supernova is carried off
  by neutrinos, for SN1987A this was 1046 Watts.
• Roughly 1013 neutrinos from this supernova passed
  through your body on Feb 24, 1987.
• Neutrinos interact so weakly with matter that
  only about one dozen neutrinos were measured at
  the worlds largest neutrino detectors.

Davis Solar Neutrino Experiment (Gold Mind SD)
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Supernova Remnants
The image to the right shows the remnant of
SN1987A several years after the explosion. The
two bright stars are far from SN1987A and have no
relation to it, they just happen to be in the
field of view. The bright ring is hot gas and
dust expelled in the explosion and now expanding
into space. The two larger, fainter rings were
unexpected and remain a mystery. An, as yet
undetected neutron, star lies at the center of
the expanding ring.
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Neutron Stars

• Collapse of massive stars stops when the
  gravitational force is balanced by the pressure
  of neutrons.
• Neutrons, like electrons, obey the Pauli
  exclusion principle.
• Neutron stars are essentially a massive nucleus.

A typical Neutron star is 1.5 x as massive as the
Sun, but has a diameter of 10 km. The density of
a neutron star is 1014 g/cc one teaspoon weighs
one billion tons. Gravity on the surface of a
neutron star is 30,000 times stronger than on the
surface of the Earth.
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The Crab Nebula and Pulsars
In 1054, Chinese astronomers recorded a supernova
in the region now known as the crab nebula. In
the 1st year of the period Chih-ho, the 5th moon,
the day chi-ch'ou, a guest star appeared... After
more than a year it gradually became
invisible..."
More than 900 years later, a pulsar was detected
at the center of the nebula. Pulsars are
objects that emit radiation at radio wavelengths
with a very regular frequency, as shown to the
left.
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Pulsars are Believed to be Rotating Neutron Stars
As a star collapses in a supernova its magnetic
field is preserved, but intensified as it is
squeezed into a smaller object. Similarly the
neutron star will rotate, as did the original
star, but much faster (think about a twirling ice
skater). Charged particles trapped by the
magnetic field radiate energy at radio
wavelengths. Most of this radiation comes out
along the poles. The radio emissions are like a
searchlight and we only detect them when the
searchlight passes over the Earth. There must
be many more pulsars than we observe, since most
radio beacons will miss the Earth.
Pulsar rotation periods can be as small as a
fraction of a second. Pulsars have been detected
in x-rays as well as radio wavelengths.
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• What happens when the mass of the collapsing star
  is great enough to overwhelm the neutron
  degeneracy pressure?

X-ray image of Cygnus X-1 from NASAs Marshall
Flight Center.
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Escape Velocity
Lets re-think Newtons experiment. If you
launch something from Earth with a high enough
velocity, it goes into orbit. If the velocity is
increased further it can escape. The escape
velocity depends of the mass and radius of Earth.
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Schwartzchild Radius
What if vesc c (300,000 km/s) ? This happens
at a distance from mass M R 2GM/c2, known
as the Schwartzschild radius. Both matter and
light within this distance to a black hole
(inside the Schwartzschild radius) can not
escape.
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Black Holes in Binary Systems
The most straightforward way to search for a
black hole is to Keplers third law. The best
place to apply this technique is an x-ray binary.
In these systems one of the stars is seen in
visible light and the other is a copious source
of x-rays. The x-rays show the position of the
(possible) black hole. How do x-rays escape from
a black hole? They dont. The x-rays are
emitted by matter from the visible star that
falls into the black hole accelerating to
velocities near the speed of light as it
falls. If we can determine the orbital period of
the binary system, we can then use Keplers 3rd
law to calculate the mass.
If the mass of the unseen companion is large,
this and the presence of x-rayssuggest that it
is a black hole. Currently, the best candidate
is Cygnus X-1.
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Super Massive Black Holes
An x-ray image of the center of the Milky Way
The center of our galaxy emits a copious source
of x-rays and appears to be extremely massive.
Stars in the Milky Way orbit around an unseen
central object. Analysis of the orbital
velocities of the stars about the center of the
galaxy (using Keplers 3rd law) imply a mass of
2.6?106 solar masses inside a volume 0.03 light
years in diameter. It is impossible to pack
stars together that tightly. It is likely that
the object at the center of our galaxy is a super
massive black hole. The same is likely the case
for other galaxies.
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Summary

• Stars die by expelling catastrophically the outer
  layers. The inner layers contract to a very
  dense amber.
• Massive stars (8 x Ms) explode into supernova,
  while solar-type stars explode as less energetic
  planetary nebula.
• The remnant of the Sun will be a white dwarf,
  supported by electron degeneracy.
• The remnant of a massive star is a neutron star,
  supported by neutron degeneracy.
• A stellar core more than 3 Ms has enough gravity
  to overwhelm the neutron degeneracy pressure. No
  known force can support gravity and collapse
  continues. The result a black hole.
• The Schwarzschild radius is the distance from a
  black hole where even light can not escape.