Transcript StellarNucleo

Stellar Nucleosynthesis
Formation of heavier elements inside
stars
How old are your atoms?
• Almost as old as the universe
92 Natural Elements
• The periodic chart contains 92 naturallyoccurring elements in the universe
• Elements 1 (H) to 26 (FE, iron) are
synthesized in the core of normal stars like
the Sun
Most Abundant Element in the
Universe
• Hydrogen, H, comprises 99% of the visible
universe
• Hydrogen is the lightest element
• H has one proton
Elements Heavier than 26
• The heavier elements are much less
abundant than H
• Elements heavier than 26 are formed in
supernova explosions
• Supernovae are vast explosions in which
a whole star is blown up. They are mostly
seen in distant galaxies as `new' stars
appearing close to the galaxy of which
they are members. They are extremely
bright, rivaling, for a few days, the
combined light output of all the rest of
the stars in the galaxy.
• A supernova is one of the most
energetic explosive events known to
man. Supernovas occur when a stars
nuclear fuel is exhausted and thus
no longer being supported by the
pressure from the release of nuclear
energy.
• If the star is really massive, then its
middle will collapse and release a lot
of energy. Many supernovae have
been seen in galaxies close by. They
are considered rare occurrences in
our own galaxy.
• If the star is particularly massive, then its
core will collapse and in so doing will
release a huge amount of energy. This will
cause a blast wave that ejects the star's
envelope into interstellar space.
• The result of the collapse may be, in some
cases, a rapidly rotating neutron star that
can be observed many years later as a radio
pulsar.
• Supernovae emit huge amounts of various
types of radiation: X-rays, ultraviolet,
infrared, gamma rays, neutrinos, cosmic
rays and radio waves.
Type II Supernova Explosions
• A progenitor star that will explode must
have 8 or more solar masses
• Sun can never go supernova because it has
only 1 solar mass and is thus not massive
enough
Frequency of Supernovae
• About once per second in the universe
• Last one visible to the naked eye was in
1987 in the LMC (Large Magellanic Cloud)
• No supernova has been seen in our Galaxy
since Kepler's (1604).
Force of a Supernova
• The tremendous force of the explosion
when a star goes supernova forces protons
together and also neutron capture
• Protons have a like charge (+)(+) and do not
want to be next to one another
• Once the protons are forced together, the
strong nuclear force (SNF) takes over and
glues the protons together with gluons.
Balance of Gravity and Nuclear
Pressure
• A star like the sun assumes a natural
spherical shape due to the 2 main forces that
act upon it.
• The gravitational force due to the 8 solar
masses tries to collapse the star while:
• The tremendous pressure from the nuclear
reaction occurring at the core of the star
tries to expand the star
Start of a Supernova
•
•
•
•
•
•
Fuel in the core becomes Iron.
Iron cannot produce fusion energy
Fusion stops (Proton-proton chain)
Nuclear pressure fails
Gravity wins and star collapses
Mass falls into the core crushing the
electrons into the protons forming neutrons
Iron Core Collapse
• It takes about 1 second for the iron core to
collapse
Rebound
• When the mass reaches the center and after
it forms the neutron star, the mass rebounds
and the supernova explosion ensues
• A nebula forms around the neutron star
– Crab nebula from the 1054 supernova
• A teaspoon of neutron star weighs a billion
tons!
If the Star is a Super Massive
Star
• Then a black hole forms instead of the
neutron star
Elements in your Body
• Of the elements in your body, the only one
not formed by stellar nucleosynthesis is H
• Don’t really want a supernova to occur too
close to earth
• Radiation equivalent to 1 billion suns
Type I Supernova
• Mass transfer to a white dwarf from a giant
primary
• http://id.mind.net/~zona/mstm/physics/wave
s/standingWaves/standingWaves1/Standing
Waves1.html