Transcript 1. accretion disk -

1. accretion disk • flat disk of matter spiraling down onto the
surface of a star. Often from a companion
star.
2. alpha process • Two step process in the center of stars
which have silicon-28 in their cores.
• Photodisintergration breaks nuclei into
helium nuclei (alpha particles) which then
combine into heavier elements.
3. carbon detonation supernova • a type-I supernova.
• White dwarf in a binary system accretes
enough mass that electron degeneracy
pressure can no longer support the star.
• The star collapses and the temperatures
reach a level that causes carbon fusion in all
parts of the star simultaneously and an
explosion results.
4. Chandrasekhar mass • Maximum mass of a white dwarf if electron
degeneracy pressure is to prevent
gravitational collapse.
• Once it is exceeded a type-I supernova
results.
5. helium capture • The formation of heavier elements by the
capture of a helium nucleus.
• This requires less energy than the
combining of like nuclei so it happens more
readily.
6. neutron degeneracy pressure • Pressure that results when neutrons are
pushed together to the point of contact.
• The neutrons resist being compressed.
7. neutronization • When the collapsing core of a high mass
star is compressed to the point that protons
and electrons are crushed together to form
neutrons and neutrinos.
• This is one of the major occurrences in the
formation of a type-II supernova.
8. nova • A star that suddenly increases in brightness,
then slowly fades back to its original
luminosity.
• The result of an explosion on the surface of
a white dwarf, cause by the accumulation of
matter from a binary companion.
9. photodisintegration • Photons at high temperature breaking heavy
elements into lighter nuclei, and eventually
to protons and neutrons.
• Prior to a supernova, photodisintegration
“undoes” all the previous 10 billion years of
nuclear fusion.
10. progenitor • A star that generates a supernova explosion.
11. recurrent nova • A star that “goes nova” a number of times
over the course of several decades.
12. r-process • Creation of heavy elements by neutron
capture during supernova explosions.
• Free neutrons streaming from an exploding
supernova collide with heavy elements and
produce heavier elements. The heaviest
elements in the universe are produced by
the r-process.
13. s-process • Neutrons captured by nuclei in a star until
an unstable isotope is created.
• The nucleus then decays to a new stable
nucleus; this continues until no heavier
stable nuclei exist.
• The “s” means “slow” ; the time between
captures is long compared to the half-lives
of the radioactive elements produced.
14. standard candle • Any object with a recognizable appearance
and a known luminosity such that it can be
used to establish distance.
• Supernovae are good standard candles.
15. stellar nucleosynthesis • Formation of heavy elements by the fusion
of lighter nuclei in the cores of stars.
• All elements except for H and He are
formed by stellar nucleoynthesis.
16. supernova • Explosive death of a star, caused by sudden
nuclear burning (type-I), or enormously
energetic shock waves (type-II).
17. supernova remnant • Scattered glowing remains from a
supernova that occurred in the past.
• Crab Nebula is one example.
18. type-I supernova • A carbon detonation supernova.
• (see #3).
19. type-II supernova
• Highly evolved stellar core rapidly
implodes and then explodes, destroying the
surrounding star.
1. What makes a nova?
• A white dwarf in a binary system collects
material from its companion. This collected
gas gets hotter and denser until the
hydrogen ignites and produces helium in an
intense surface burn.
2. What makes a light curve?
• The magnitude of the nova or supernova
changes over time; a graph of this change is
called a light curve.
3. What is a supernova?
• A massive stellar explosion which destroys
the original star.
4. How often can we expect to
see a supernova?
• We should expect to see a supernova in a
visible part of our galaxy every 100 years or
so.
• We are long overdue (since 1604).
5. What evidence is there that
many supernova have occurred?
• We can detect the glowing supernova
remnants.
6. According to historical
accounts, how did the explosion
creating the Crab Nebula appear
to observers on Earth?
• Its brightness exceeded that of Venus.
• Perhaps was brighter than the Moon.
• Could be seen in the daytime for a month.
7. How do supernovae work as
standard candles?
• We know the absolute brightness of all
supernovae is the same, so we can compare
this to the apparent brightness and find the
distance.
8. Which elements existed in the
early universe?
• hydrogen and helium
9. How were all of the other
elements in the universe formed?
• They were formed by stellar nucleosynthesis;
formed by nuclear fusion in the core of stars.
10. Why do star’s cores evolve
into iron, but not into larger
elements?
• Nuclear fusion involving iron does not
produce energy. Iron nuclei are so compact
that energy cannot be removed by
combining them into heavier elements.
This loss of energy causes a loss of
pressure which stops fusion (temporarily).
• Iron formation is a ‘fire extinguisher.’
11. How are nuclei heavier than
iron formed?
• 1. The ‘s-process’ (slow). Iron captures a
single neutron, and then another, and then
another. Eventually an unstable form of iron
is formed, and it decays into a heavier stable
element.
• 2. The ‘r-process’(rapid). The intense
pressures involved in a supernova explosion
force heavier elements to gain free neutrons
produced by the explosion. This occurs too
rapidly for the nuclei to decay and therefore
produce elements that cannot be formed by
the s-process.
12. What makes a massive star
collapse?
• Gravitational pull that exceeds the heat and
pressure that holds a star at its present
volume.
• The heat decreases with the fusing of iron
whish results in a decrease of pressure.