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Nuclear Astrophysics (a Cosmic Cookbook) Dr Chris Ruiz - TRIUMF Nuclear Astrophysics Atomic nucleus 1 x 10-15 m The everyday star ~1 x 109 m 1,000,000,000,000,000,000,000,000 x smaller! We are interested in…. X-ray bursts Classical novae We can never touch these stellar events, only observe them from afar. But, we can do experiments on Earth which can help us to understand them… supernovae We’ll find out how… Helium + Hydrogen (Don’t try this at home!) = Periodic table of the chemical elements Need some physics.. No equations! What is a star? A big ball of hot gas, right? Stars in history…. Babylon, 1,800 B.C Astronomical records for agriculture Babylonians invented 360º in a circle 60 minutes in an hour, 60 seconds in a minute, the sexagecimal (base 60) system! Also, days of the week, signs of the zodiac,…. …but thought Sun was a planet! Aristarchus 310-230 B.C Heliocentric theory Eratosthenes 276-194 B.C Calculated circumference of Earth, distance to Moon and Sun, using only trigonometry and careful observations Within 1%! Burning of Alexandria, 391 C.E The Dark Ages • Science of the Greeks ‘heretical’ • Only Monks could read & write • All literature dedicated to glory of God and the literal interpretation of the gospels • Motions in the heavens unquestionable facts of creation • Enquiring minds had to do so in secret (Alchemists) However, Arabic kingdom of Al-Andaluz (Andalucia) in Spain carried on much Greek mathematics and astronomy Christian monks were known to have visited this kingdom and accessed Greek texts…undercurrent of things to come The Renaissance (rebirth) and heliocentrism Copernicus: heliocentric theory (again!) Galileo: telescope + more support for heliocentric theory! Kepler: laws of planetary orbital periods Isaac Newton 1643-1727 1686 • Universal Gravitation • Derived Kepler’s laws • Last doubts of Heliocentrism removed Josef von Fraunhofer 17871826 • 1814 invented spectroscope and discovered hundreds of dark lines in solar spectrum Explained in 1859 by Kirschoff and Bunsen as Atomic Absorption Lines Planck’s radiation law, 1900 • 1895 Helium isolated and shown to be in solar spectrum • Also Silicon, Carbon, Sodium, Oxygen,… • Kelvin-Helmholtz mechanism: Sun is a hot liquid body, cooling, contracting, and generating heat from gravitational energy. Problem: • 20 million year lifetime! • 1920 Arthur Eddington shows that treated as a hot gas, Sun’s gravity must balance outward internal pressure…Sun must be millions of degrees! • Also proposes Sun converts hydrogen to helium with a net release of energy! How can this be? 1908 Ernest Rutherford Nobel prize for radioactive decay experiments, discovery of the atomic nucleus - proposes Sun has an internal heat source…radioactivity! 4 hydrogen atoms are heavier than 1 helium atom, so if you can somehow ‘join’ all the hydrogen together, perhaps the difference in mass will come out as energy…..? He H HH H Albert says Yeeeeees!! E = mc2 (ok, I lied about no equations!) So now all we need is a preponderance of hydrogen in the sun… check: Cecilia Payne 1925 Now for the Nuclear physics….. Electrostatic forces between charged particles Thus for the constituents of an atom: Like charges repel + + Positively charged protons repel each other + - Opposite charges attract A negatively charged electron feels an attractive force towards a proton A neutron has no net electric charge Conditions inside a star • It’s Hot! distribution of speeds The hotter a gas is, the greater the average speed (energy) of the particles is, and the greater the range of speeds In addition: electrons are ripped off nuclei because of collisions - this is called a plasma So have protons of many different energies in a sea of electrons So can protons fuse together? • According to classical physics, we would expect the proton to fuse with another if it has enough energy to overcome the electrostatic barrier (imagine a marble ‘rolling’ up the curve) • But the average energy of a proton in the sun is much lower than the barrier • But according to quantum physics, there is a small probability that the particle ‘tunnels’ through the barrier • Although this probability is miniscule, just think how many protons are in the sun! • Need to know how to make helium (4 particles) - probability of 4 protons coming together are same time is almost zero • So what happens when just 2 protons fuse…….? + + Two protons tunnel and fuse + + At the moment of fusion, one proton turns into a neutron via the emission of a positively charged electron (positron) and a neutrino, a small, very light particle This new nucleus is called a deuteron. It is a type of hydrogen (because it has one proton). Each type of chemical element (hydrogen, helium etc), has several versions of itself that contain more or less neutrons than usual. These are called isotopes. Isotopes Number of protons 3 6Li 3He 4He 1H 2H 3H 0 1 2 2 1 0 3 4 Number of neutrons Fusion in the sun, gradually converts hydrogen to helium! The are thousands of nuclear isotopes! uranium gold iron carbon • Hydrogen burns to helium • When run out of hydrogen, star shrinks a bit, getting dense enough to fuse three helium nuclei together to make carbon • carbon can fuse with helium to make oxygen, and so on…. Red Supergiant A more massive star than our sun Massive stars will have several burning stages, resulting in an ‘onion-skin’ structure… Red Giant (helium burning) Star like our sun Supernovae • When the evolved star on the previous slide can burn no more, it collapses • The core compresses until it becomes the most dense substance known: a neutron star • The rest of the star violently explodes, inducing even more nuclear burning and spreading all the elements into space • This material can then form a new solar system, like ours! Energy released = 1046 Joules (in a matter of seconds) = 30,000 trillion, trillion x Annual U.S Energy Consumption Can outshine the entire Galaxy it occurs in! More energy than our sun will generate in its lifetime! (cup of tea ~600 Joules) Classical novae Stellar Binary system Red Giant White Dwarf • In binary systems, one star can evolve to a compact white dwarf while the other can become a red giant • hydrogen-rich material is transferred (or accreted ) onto the white dwarf surface • the temperature and density get so high that hydrogen starts to fuse with carbon, nitrogen, oxygen and heavier stuff • this thermonuclear runaway blasts the material off the star into space at very high speeds! X-ray bursts Neutron star in binary system • neutron star has very strong gravitational field • temperatures and densities reached much higher than novae • different set of nuclear reactions occur in thermonuclear runaway • material cannot escape! X-ray bursts are very interesting: the observations don’t quite agree with the theory yet; they are extremely regular; no-one quite knows what the composition of the neutron star is! nucleosynthesis Slow-neutron process (Proton-fusion up, beta+ decay diagonal) Neutron fusion across, beta- decay diagonal (cold neutron fusion) Rapid-proton process (hot hydrogen fusion) 56Fe Rapid neutron process (hot neutron fusion) Static H,He,C,O,Ne, Si burning in massive star So we want to study the nuclear reactions… Most involve radioactive nuclei, with short lifetimes can’t make a ‘sample’ of them So create nuclei (using nuclear reactions) continuously, i.e. create a beam of them Then either: Quickly study how they decay, or how they respond to magnetic & electric fields …..(tells you about their structure, how heavy they are etc) Accelerate the nuclei up to stellar speeds and fuse them with hydrogen or helium (or other stuff) At the TRIUMF facility ‘ISAC’, we do exactly this: create exotic, short-lived radioactive nuclei only found in stars, and we either watch them decay, trap them and measure how heavy they are, or accelerate them and fuse them with other nuclei…… Measuring fusion reactions: producing exotic nuclei experiment World’s largest cyclotron Beam of exotic nuclei at same energy as in star Particle accelerator Decay or mass measurement experiments Magnetic Isotope selector Solid target nuclear reactions Ionizer (strips electrons off) Proton beam 75% speed of light! Exotic atoms Measuring fusion reactions: The DRAGON • With this instrument we measure the strength of the fusion reactions 27Si hit a hydrogen (or helium) gas • The exotic nuclei recoils target • If fusion occurs, the product nuclei will be heavier than the original nuclei • We use electric and magnetic fields to separate out these heavy nuclei and count, one-by-one, how many we make! • We give the results to other astrophysicists who make computer models of exploding stars Summary The laws of physics allow the existence of stars via the transformation of lighter elements to heavier ones All we need is hydrogen, and a little helium then gravity and the other forces of nature take care of the rest Massive stars make massive elements Explosions of dying stars spread synthesized material around the galaxy This star-enriched material can form a new star system Then the chemists, geologists, and evolutionary biologists can take over “We are all made of star stuff” - Carl Sagan 1934-1996