Transcript Stars

Stellar Evolution &
Planet Formation
If you want to make an apple pie from scratch, you must first
create the Universe
- Carl Sagan
Review of Planetary Formation
Summary of Planetary Characteristics
• Planets orbit in nearly the same plane (the ecliptic plane), inclinations are
small.
• Planets orbit in the same direction with small eccentricities. The direction is
that which the sun rotates.
• Most of the planets spin in the same direction that they orbit. Venus,
Uranus and Pluto are exceptions.
• Disk of planets, rotating in same direction
• Rocky planets near Sun, icy planetary cores and moons further out
• Little planets near Sun, big guys further out.
• Big planets have big hydrogen atmospheres
The clues towards understanding planetary formation
The Nebula Hypothesis
The solar system (planets, satellites, asteroids, comets, etc.) formed along with the
Sun 4.5 Byr ago from the gravitational collapse of an interstellar cloud of gas and
dust. The planets and Sun formed from the same reservoir of interstellar matter
and are therefore composed of primarily the same elements. As the cloud
collapsed under the force of gravity it began to spin rapidly and then flattened into
a plane. This explains why the solar system is a relatively flat plane and why the
planets orbit in the same direction and tend to rotate in the direction that they
orbit. The collapsing cloud of gas and dust was hottest near the Sun and coolest
far from the Sun. The local temperature determined which compounds solidified
from the gas phase as a function of distance from the Sun. This explains why the
inner planets are composed mostly of rock and the outer planets have large
complements of ice. Ice forming elements are more abundant than rock forming
elements so planets in the outer solar system are larger. In fact they are so large
that their gravitational fields were able to capture the H and He in the cloud. The
gravity of the inner planets is too weak to hold on H and He. This explains why
the outer planets are gaseous and the inner planets are rocky.
Accretion of the Planets
Condensation in the Solar Nebula
High Density
Low Density
Characteristics of the Sun
SUN
Radius
696,000 km (109 RE)
Mass
2x1030 kg (3x105 ME)
Composition
H2, He, other
Temperature
5800 K (surface)
1.55x107 K (center)
Distance (Galactic Center)
26,000 lyr
Distance from Earth
8.32 lmin
Orbital period
220 million years
Luminosity
4x1026 W
The Sun’s luminosity corresponds to the burning of 1500 lb of coal every hour
on every square foot of the Sun’s surface
- Hermann von Helmholtz, 1871
What keeps the Sun from collapsing
Answer: Energy generated from
nuclear reactions at the Sun’s Center.
Notice the neutrino.
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)
Sun Spots
Sun spots are low temperature regions of the photosphere.
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.
Sunspots are produced by the 22
cycle of the magnetic field
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.
There exist
stars of a range
of sizes. What
establishes a
star’s size is its
mass and age.
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 Fermi’s
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.
Stars more
massive than
the Sun
achieve
higher
temperatures
in their cores,
and are
capable of
fusing higher
elements as a
result.
As the core collapses,
the outer part of the star
cools. This increases
the opacity of the star’s
atmosphere, thereby
hindering the escape of
energy. The star thus
expands until it can
radiate the power that it
makes. The now puffedup star is called a
supergiant. The Sun, at
this stage, will be 70
times its present radius
and will engulf Mercury.
We come from the 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 ( rays break nuclei) + He capture: 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
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.
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.
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 Sun’s magnetic field reverses every 11 years, producing a
periodicity to the sun spots and solar activity.
The Death of Stars
The Bigger They are the Harder
They Fall
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 byr. 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 11 Byr life.
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.
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 Orion’s dog
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.
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 lightyears
from Earth. This means that
SN1987A actually occurred in
167,000 BC.
Neutrinos from Supernova
• 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 world’s
largest neutrino detectors.
Davis Solar Neutrino Experiment (Gold Mind SD)
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.
We believe that an as yet
undetected neutron star lies at the
center of the expanding ring.
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
essential a massive
nucleus.
A typical Neutron star is as
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.
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.
Pulsars are thought 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 will radiate energy at
radio wavelengths, but most of this
radiation comes out along the poles
of the pulsar. 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 of the
radio beacons will miss the Earth.
Pulsar rotation periods can be as
small as a fraction of a second.
Pulsars have been detected in xrays
as well as radio wavelengths.
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 NASA’s Marshall Flight Center.
Escape Velocity
Let’s re-think Newton’s
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.
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.
Black Holes in Binary Systems
The most straightforward way to
search for a black hole is to Kepler’s
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 don’t. 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
Kepler’s 3rd law to calculate the mass.
If the mass of the unseen companion is large, this and the presence of x-rays
suggest that it is a black hole. Currently, the best candidate is Cygnus X-1.
Super Massive Black Holes
An x-ray image of the center of the Milky Way
The center of our galaxy is also 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
Kepler’s 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 – they would
collide, destroying each other very quickly. It is likely that the object at the center of
our galaxy is a super massive black hole. The same is believed to be true of many
other galaxies.
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.