Presentation on theme: "1. accretion disk - flat disk of matter spiraling down onto the surface of a star. Often from a companion star."— Presentation transcript:
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.