Transcript Stellar Nucleosynthesis

The Big Bang
• Event that occurred approximately 13.7
BILLION years ago
• All the mass and energy concentrated at a
point
• The universe began expanding and
continues to expand
• After 1 million years matter began to cool
enough to form atoms- Hydrogen- the
building block of stars
In the Beginning
• Hydrogen and helium with small amounts
of lithium, boron, and beryllium were
created when the universe was created in
the Big Bang.
• The rest of the elements were produced
as a result of fusion reactions in the core
of stars- stellar necleosynthesis.
In the Beginning
• These reactions created the heavier elements
from fusing together lighter elements.
• When the outer layers of a star are thrown back
into space (supernovae), the processed material
can be incorporated into gas clouds that will
later form stars and planets.
• The material that formed our solar system
incorporated some of the remains of previous
stars.
• All of the atoms on the Earth except hydrogen
and most of the helium are recycled material--They were created in the stars.
Galaxies and Stars
• Galaxy- huge rotating aggregation of stars,
dust, gas held together by gravity
• Earth, the sun and our solar system is part
of the Milky Way
• Stars are massive spheres of
incandescent gases (hydrogen and
helium)
Stellar Nucleosynthesis
Introduction
• Work on stellar nucleosynthesis in the 1950s has led
to our current realization that most of the chemical
elements are synthesized in stars.
• Helium is made by hydrogen burning in the core
during the main sequence and in a shell above the
core in the red giant phase.
• The energy released from nuclear reactions
accounted for the longevity of the Sun as a source of
heat and light.
• The prime energy producer in the sun is the fusion
of hydrogen to helium, which occurs at a minimum
temperature of 3 million kelvins.
Introduction
• The element carbon is created by helium-burning.
• For massive (more than ten solar masses, > 10 MSun) stars, direct nuclear burning continues with the
production of oxygen, neon, magnesium, silicon and
so on, cumulating in the synthesis of iron, the
heaviest element possible through direct nuclear
burning.
• The other heavy elements, from yttrium and
zirconium to uranium and beyond, are produced by
neutron capture followed by decay.
Introduction
• For the majority of stars (~95%, corresponding to
stars with initial masses of less than 8 M-Sun), direct
nuclear burning does not proceed beyond helium,
and carbon is never ignited.
• Most of the nucleosynthesis occurs through slow
neutron capture during the asymptotic giant branch
(AGB), a brief phase (~106yr) of stellar evolution
where hydrogen and helium burn alternately in a
shell.
• These newly synthesized elements are raised to the
surface through periodic "dredge-up" episodes, and
the observation of short-lived isotopes in stellar
atmospheres provides direct evidence that
nucleosynthesis is occurring in AGB stars.
Passive Evolution
• Stellar evolution is relatively well
understood both observationally and
theoretically
• Massive stars are very hot and blue
• Massive stars are very luminous
• Massive stars have very short lives
Passive Evolution - Single Burst
• Single Burst of Star-formation
• Galaxy starts of very blue as the light is
dominated by the massive hot blue stars
• After the burst the massive stars live only
a short time and soon the light of the
galaxy as a whole is dominated by the red
light of the less massive, longer lived stars
• Galaxy gets redder with age
Supernovae
• A supernova is a massive explosion of a
star that occurs under two possible
scenarios. The first is that a white dwarf
star undergoes a nuclear based explosion
after it reaches its Chandrasekhar limit
from absorbing mass from a neighboring
star (usually a red giant).
• The second, and more common, cause is
when a massive star, usually a red giant,
reaches iron in its nuclear fusion (or
burning) processes.
Supernovae
• Iron has one of the highest binding energies of
all of the elements and is the last element that
can be produced by nuclear fusion,
exothermically.
• All nuclear fusion reactions from here on are
endothermic and so the star loses energy.
• The star's gravity then pulls its outer layers
rapidly inward. The star collapses very quickly,
and then explodes.
Composite image of Kepler's supernova from pictures by the Spitzer Space
Telescope, Hubble Space Telescope, and Chandra X-ray Observatory.
The Solar System
• Our solar system is located away from the
galaxy’s center
• Our sun and the planets originated from a solar
nebula that had been enriched with heavy
elements from nearby supernovae (Stellar
Synthesis)
• Solar system is approximately 5 Billion years old
• Composition is 75% hydrogen, 23% helium and
2% other materials
Formation of a
Protostar
Center
contracts
Center
continues to
heat up
Protostar
radiates more
heat
Fusion
begins in the
stars core
Shockwaves
radiate
outward
releasing
material
Material
coalesces
into planets,
moons or
comets
Other
material is
ejected to the
periphery
Our Solar
System
4 inner
planets
(terrestrial)
4 outer
planets
(gaseous)
Solar nebula
photographed
by Hubble