Astrophysics 1 PDF

PART 1 Our position Earth → Solar System → Orion Arm → Milky Way → Local Group → Local Supercluster → The Universe Picture 1: Standard model of elementary particles Forces Picture 2: History of the Universe Scales in the universe:  Heisenberg limit  Schwarzschild limit  Pl...

PART 1 Our position Earth → Solar System → Orion Arm → Milky Way → Local Group → Local Supercluster → The Universe Picture 1: Standard model of elementary particles Forces Picture 2: History of the Universe Scales in the universe:  Heisenberg limit  Schwarzschild limit  Planck scale Stars and Galaxies - Life cycle - Star:  Degenerate matter: occurs at extremely high densities and low temperatures. Degenerate from “degenerate energy levels” – same energy but quantum numbers are different. refer to dense stellar objects (white dwarfs and neutron stars) where thermal pressure alone is not enough to prevent gravitational collapse. The Pauli exclusion principle significantly alters a state of matter at low temperatures. o At the core: a mixture of He nuclei and electrons – contracts and becomes denser. o Pauli Exclusion Principle: electrons - No two electrons are allowed to have exactly the same state. When squeeze too many electrons into a small space they will repel each other. This repulsion is called Electron Degeneracy Pressure, much stronger than the usual repulsion between charged particles. o Degenerate matter exhibits quantum mechanical properties when a fermion system temperature approaches absolute zero. These properties result from a combination of the Pauli exclusion  principle and quantum confinement. The Pauli principle allows only one fermion in each quantum state and the confinement ensures that energy of these states increases as they are filled. The lowest states fill up and fermions are forced to occupy high energy states even at low temperature. o When degeneracy pressure is stronger than thermal pressure, the gas becomes degenerate. Generate gas is very different from an ideal gas. ▪ Ideal gas’s pressure depends on density and temperature. ▪ Degenerate gas: depends only on density Increasing T doesn’t increase the pressure (as long as T is cool enough that the gas is degenerate) As T is rising, nuclear reactions can take place (degenerate gas doesn’t expand to regulate the temperature rise by expanding) => critical point of T: helium fusion. o In stars: ▪ Low mass: He core becomes degenerate during Red Giant phase ▪ Medium and High mass: not degenerate while Red Giants. ▪ Sun: becomes degenerate when density > 10^6 kg/m3, T ~ 20 million K ▪ Most stars are supported against their own gravitation by normal thermal gas pressure, while in white dwarf stars the supporting force comes from the degeneracy pressure of the electron gas in their interior. In neutron stars, the degenerate particles are neutrons.  The Helium Flash: o Large amount of He fuses into Carbon in a few seconds The Red Giant stage with an inert Helium core continues until the helium is hot enough for ignition through the triple-alpha process o The gas quickly gets hot and returns to the ideal state. Then He burning reactions occur in a hot ideal gas, at a slow, stable rate. Luminosity decreases, outer layers of the star shrink o The period of stable helium fusion in the core is called the Horizontal Branch.  Final Stages of a Low-Mass star:  o The star begins to contract and heat up. Helium in the shell begins to fuse to Carbon o Burning Helium leaves behind Carbon "ashes". The Helium forms a shell around the Carbon; Hydrogen forms a shell around Helium. Carbon ignition requires hotter temperatures than are available in the core, but Oxygen is usually created in some quantities by 12C + 4He → 16O o Burning in the shell causes the star to expand again, becoming bigger & redder → beginning of a second red giant stage: Asymptotic Giant Branch (AGB) ▪ The sun: this phase lasts only ~ 1 million years. The final stage of the Sun's life: the ejection of the outer He and H layers. These layers are hot and glow: the glowing ejected gas is called a planetary nebula.  Summary: Evolution of an intermediate mass Star: o Main-sequence: burn H in core to fuse He o Red Giant Branch: H burns in the shell, He core becomes degenerate, leading to Helium flashes o Horizontal Branch: He burns in the core to fuse C o Asymptotic Giant Branch (AGB): The star experiences double shell burning, H and He burn in shells around a degenerate core. Star becomes bigger and redder. o After AGB: ▪ The outer envelope drifts away. ▪ The surface temperature increases.  ▪ The core becomes so hot that it ionizes the drifting envelope, causing the envelope to glow as a planetary nebula. ▪ At the end of its evolution, the star becomes a white dwarf with a typical mass of 0.6 M⊙  Planetary Nebula: o End of AGB phase o non spherical shapes of the Circumstellar envelope o Bipolar outflow and/or jets and/or equatorial disks or torus o Possible causes: stellar rotation, magnetic field, binarity, ….?  White Dwarf:  o The C and O core left behind is supported by degeneracy pressure. (no nuclear fusion needed to keep it from collapsing) o The left-over Carbon core is called a White Dwarf. o Initial surface temperature could be 200K K so it will glow blue- white. It only shines by left-over heat (no energy source, no fusion, no contraction). It cools off and fades away slowly, eventually becomes a Black Dwarf. o Typical mass ~ sun (1.4Ms) but radius is ~Earth’s. o Famous white dwarfs: Sirius B and Sirus A. (binary system)  Evolution of High Mass Stars: o Large core, so when the fuel is burnt out, they cannot be stabilized even by degenerate pressure, and collapse further. o Stages: 1. fusion of an element in a core and lighter elements in shells around the core 2. exhaustion of the element 3. core collapse and heating 4. fusion of heavier elements o The star's surface becomes cooler and becomes a blue giant and later a red supergiant. There are several oscillations from these 2 phases, correspondent to ignition of the next, heavier, fuel in the core. o Loses significant mass during super giant stage. Super Giant stage: (M ~ 10 – 25 Ms) ▪ Onion skin structure: inert Fe core, surrounded by shells of heavier elements ▪ Fusion stops at iron because it is the most stable element (it has the highest binding energy per nucleon). Fusing iron into heavier elements or splitting it into lighter ones requires energy instead of releasing it. o For M = 25 Ms: ▪ H burning: 7 million years ▪ He burning: 500 000 years ▪ Carbon burning: 600 years ▪ Neon Burning: 1 year ▪ Oxygen burning: 6 months ▪ Si burning: 1 day  Supernova explosion o Supernova ▪ When a star develops an iron core, an energy crises occurs, the core is also degenerate, so no heat source is needed to keep it stable. ➔ the core is an iron white dwarf o For WD stars with mass > 1.4Ms (Chandrasekhar limit) the electrons would have to move faster than the speed of light to create enough pressure to halt the collapse. ➔ the WD > 1.4 Ms will collapse. o Main sequence stars with M> ~8 MSun eventually form white dwarf stars with masses larger than the Chandrasekhar limit and collapse. This is the beginning of a Core Collapse Supernova also known as a Type II Supernova ▪ Supernova of a star 8Ms < M < 20Ms:  When the supernova begins the iron core collapses rapidly and becomes denser.  When the density is very high: Proton + electron → neutron + neutrino   The neutrinos escape easily and carry off energy. The neutron gas collapses until the density is extremely high. When the neutron density becomes > ~1014 g/cm3, neutron degeneracy pressure provides an outward pressure, which suddenly halts the gravitational collapse. → neutron star  The outer layers are still collapsing inwards and collide with the hard surface of the newly formed neutron star → causes a violent rebound and a shock wave bounces outwards, colliding with the outer layers of the star.  The explanding wave carries a lot of energy, which is the fuel to create high mass elements, like Uranium. The supernovae are responsible for all the elements with masses > Fe found on Earth. ▪ Supernovae with a star with M > 20 Ms  Formation of a neutron star at the core in a supernova  Neutron stars have a maximum mass near ~2.2 MSun, over which neutron degeneracy pressure can't balance gravity.  In the very high mass stars, the neutron core goes over the critical mass, and the neutron star collapses. If no other sources of pressure: the collapsing material forms a black hole  The massive stars can collapse and create a blackhole if they are massive enough, without a supernova explosion. o Neutron stars: ▪ Remnant cores of a massive stars (8 – 20 Ms) ▪ Leftover core of a supernova. Held up by Neutron degeneracy pressure (protons and electrons combine into neutrons) ▪ Initially, a neutron star is very hot, ~ 10^11 K (100 billion K). It glows mainly in the X-ray part of the spectrum. Over its first few hundred years of life, the neutron star's surface cools down to ~ 10^6 K (1 million K) and continues to glow in the X-ray. ▪ Properties: mass: 1.4 – 2 Ms (?); radius ~ 10 km; density ~ 10^14g/cc  ▪ Pulsars: (not all neutron stars are pulsars) Pulsars are a type of neutron star that are rapidly spinning and emit beams of electromagnetic radiation from their magnetic poles. o Gramma Ray Burst: Collapse of a very massive star into a black hole flashes of gamma r ays, brightest electromagnetic events o Stellar black holes ▪ Many stellar black holes (mostly members of binaries) are known today. ▪ Most massive: in M33, 16 solar masses, 1 Mpc away and orbits its companion (70 solar masses!) in 3.5 days. ▪ Least massive: 3.8 solar masses, 3 kpc away ▪ Cyg X1 – most famous: One of the most intense X-ray sources in the sky, 8.7 solar masses, 2 kpc away, orbits its companion variable blue supergiant in 5.6 days, sometimes called microquasar. o White Dwarfs in Binary System can also go Supernova: ▪ A WD receiving mass from a companion star (filling up the Roche lobe of larger star) its mass will slowly increase. When the mass reach the Chandrasekhar limit, it will collapse. The Collapse cause the degenerate Carbon gas in the core fuse together explosively => supernova Type Ia.  Conservation of mass, angular momentum and magnetic flux in stars, white dwarf and neutron stars  Stellar Nucleosynthesis: o Nuclear fusion: until Ca ▪ Pp cycles ▪ CNO bi-cycle ▪ He burning ▪ C burning ▪ O burning ▪ Si burning o Photodisintegration rearrangement: intense gamma ray radiation drives nuclear rearrangement until Fe o Most nuclei heavier than Fe are due to neutron capture: ▪ S-process (slow compared to beta-decay) ▪ R-process (rapid compared to beta-decay)