Lecture Notes on Astrophysics PDF

Lecture 21:black holes Black Hole Formation: If the mass of a neutron star exceeds ~2.5 Msun, it will collapse into a black hole. Black hole = Object whose escape velocity exceeds the speed of light. No matter or light can escape a black hole. Black holes have three observable properties, which...

Lecture 21:black holes Black Hole Formation: If the mass of a neutron star exceeds ~2.5 Msun, it will collapse into a black hole. Black hole = Object whose escape velocity exceeds the speed of light. No matter or light can escape a black hole. Black holes have three observable properties, which are... ○ Mass. ○ Charge. ○ Angular momentum. Matter and Spacetime: Mass distorts spacetime much like a bowling ball on a flat sheet, Euclidean geometry no longer works in curved spacetime. Geodesic = The shortest path between two points in curved spacetime. Event Horizon: A black hole has a singularity–all the matter collapsed to one point.An infinitely dense, bottomless well in the fabric of spacetime,Event horizon = The boundary of no return (where the escape velocity is c). The size of the event horizon is determined from the Schwarzschild radius. Hawking radiation Black holes lose energy and particles appear and then are annihilated, eventually the black hole will lose enough energy and explode X-ray Binaries: You can find the total mass of two objects in an X-ray binary by considering the general form of Kepler’s third law, knowing their separation and orbital period. Gravitational Waves: Gravitational waves should move through spacetime like ripples when spacetime is accelerated.Move at the speed of light. Produced when black holes or neutron stars merge. There is a mass gap between neutron stars (2.5 Msun) and black holes (5 Msun) that science has not yet been explained, May be due to accretion of matter from a companion star. Lecture 24: galaxies Galaxies are large bodies of dust and gas Three diff types of galaxies,spiral elliptical and irregular. Elliptical are round ranging from E0 to E7, very little dust and old stellar population Gas in elliptical galaxies typically emit x ray, the gas cannot form new stars Spiral galaxies have arms that lie on a disk and a central bulge, two types sa=bright center tight arms and sc= dim center open arms, stars orbit a disk and are all in same direction, stars in the bulge are in all different directions. Stars can form in the arms of spiral galaxies which makes blue light SO galaxies are like spiral galaxies but they have no arms Irregular galaxies have no shape, they are the product of interaction between two other galaxies, often blue and full of new stars, usually classified as starburst galaxies Dwarf galaxies 1/10th the size of giant galaxies, low surface brightness and the most common type of galaxy Trigonometric parallax and spectroscopic parallax are used to measure the distance of stars in the milky way, Standard candles are objects that have a known luminosity, so that their brightness and luminosity can be combined to calculate a distance. cepheid, la supernova and the tully-fisher relation are all used to calculate distance In order of how far we can measure in distance= radar, trig parallax, spec parallax, chephids, and then la supernovas The redshift of galaxies is caused by them moving away from us. Hubble's constant is distance vs speed graph for galaxies. HO = 70km/s Can calculate mass of galaxies by observing the distribution of luminous matter. Orbital speeds depend on mass, turns out the calculated speed and predicted speed doesn't match up because of dark matter Dark matter is found in outer regions and cannot produce light, takes up 95% of the galaxy. Three predictions on what its made of WIMPs: weakly interacting massive particles ,MaCHOs: massive compact halo objects, such as black holes.Astronomical searches have largely ruled these out. And Exotic elementary particles Quasi-stellar radio sources” (quasars) are bright radio sources with very faint, blue optical counterparts. Quasars are trillions of times brighter than the Sun and are found at hundreds to thousands of megaparsecs ,There are very few in the local universe, More common in the past Quasars are a type of active galactic nucleus (AGN). A “normal” galaxy has an absorption spectrum that is a composite of the light from its billions of stars,AGN have emission lines in their spectra due to accretion onto a central black hole. The luminosity of the AGN can be as large as the rest of the galaxy AGNs can occur in both elliptical and spiral galaxies, seyfert are elliptical galaxies that have an AGN, radio galaxies are elliptical galaxies with a less luminous AGN-m these have alot of radio waves. Lights from radio galaxies ranged from radio to gamma AGN are made from super massive black holes, material from accretion disks power the AGNS Quasars consume 1.2 solar masses a year turning it into energy Lecture 26 the milky way Milky way resembles a spiral galaxy, Galactic plane tilted at an angle of 63 degrees to the celestial equator, Brightest sections and core visible during the summer months in Northern Hemisphere, astronomers use the measure velocities of spiral arms to determine how far away a cloud is Emissions from h2 clouds. Co molecules emit light more readily than h2 molecules Spiral arms exist because material in the center of the galaxy orbits faster than material farther out. Bulge causes spiral pattern Spiral density waves are disk disturbances, greater mass and density and pressure in the ISM. do not orbit same rate Most stars form in the spiral arms but move out of them, spiral structures are in galaxies because of disturbances in the disk as it rotates Globular clusters found in the halo, up to 3 billion years old and very low in metal content.star formation could have occurred first in the halo which was followed by disk formation as new stars from more metal will be there, can be used to measure distance because of known RR lyrae variables Chemical compositions and enrichment, Stars form heavy elements in their interiors through nuclear fusion When stars die, heavy elements are releases to the interstellar Medium. New stars will form from this enriched gas. Therefore, older stars have fewer metals while younger stars have more Milky way disk. Has a thin and thick disk. The thin one contains younger stars and dust, metal abundance increases in the outskirts. Thick disk has older stars with heavy elements. The halo.The halo is divided into three parts: inner halo (15kpc), outer halo, and gas halo. Stars in the halo have random orbits and can cross through the disk. Halo stars crossing the disk can be identified by their high velocities. Stars in the outer halo have lower fractions of heavy elements. They may have been stripped from dwarf galaxies in previous mergers The milky way's magnetic field is weaker than earths and cosmic rays are charged particles moving close to the speed of light, these are trapped by magnetic field. When they spiral around magnetic field lines, they produce synchrotron radiation that is detectable on Earth. Cosmic rays can have very high energies. They are likely initially accelerated by supernova explosions Neutral hydrogen gas pervades the Milky Way, and it emits radio waves with λ = 21 cm.Once the orbit and distance of an object is known, astronomers can calculate the amount of mass Enclosed. Milky way rotation curve. When graphed the speed measurements show a flat rotation curve. Our super massive black hole.The center of the Milky Way is crowded with stars and hot gas. The motions of the stars near the center (< 0.1 ly) reveal that there is a black hole at the Milky Way’s center.Gas orbiting or falling onto the black hole produces X-rays, but the black hole is not an AGN. The black hole may have been active in the past, as shown by the gamma-ray bubbles. The local group- galaxies are bound gravitationally to other galaxies. In our local group there are 2 giant barred spiral a smaller spiral and a bunch of dwarf galaxies Galactic mergers- mergers and collisions continue in local groups, very unlikely but the milky way is colliding with a sagittarius dwarf galaxy creating a stream of stars. We are also going to collide with andromeda in 4 billion years, most likely creating a giant elliptical galaxy. Lecture 27 PeVatrons: “sources capable of accelerating particles up to at least PeV (1015 eV) energies” Large Hadron Collider accelerates particles up to ~ 10 TeV Cosmic Ray Abundance: 2% electrons, 98% hadrons Hadrons: 89% H, 10% He, 1% heavier elements Rare elements and radioactive isotopes are over abundant due to spallation The Cosmic Ray Spectrum Knee (~ 3 ∗ 1015 eV): power law spectrum steepens from q ~ 2.7 to q ~ 3.1 Change in cosmic ray composition from proton-rich to heavier particles Ankle (~ 5 ∗ 1018 eV): flattens back to q ~ 2.7 All cosmic rays start to have extragalactic origin Gyro-radius of a Ankle 1018 eV proton is hundreds of parsecs Cosmic rays interact with other particles to produce energetic gamma rays Hadronic interaction Pion production and decay, Neutral pions decay into two gamma rays p + p→ π0 +𝑋; π0→2𝛾 Cherenkov radiation is contained in a cone around the direction of motion with an opening angle o cos = 𝑐/𝑛 𝑣 ▪ Where n is the index of refraction of the medium, v is velocity, c is the speed of light Radiation is produced only when the particle moves at relativistic speeds v > c/n. Lecture 28 Cosmology is the study of the university and its structure,history, origins and future Cosmological principle. Homogeneous = properties of the universe are the same everywhere Isotropic in all directions Hubble flow- motion of galaxies due to expansion of universe universe is expanding galaxies are moving away from us and their speeds are proportional to their distances, further=faster All galaxies are moving away from us making it look like we are at the center of the universe. Homogeneity says not true cuz no center Can be used to find distance and time, further away the further back in time. z=0.1 Age of universe. Galaxies were closer in past but moves away Hubble time is when galaxy separation was 0 Big bang- 13.8 million years ago, created space and time. Space itself is expanding from big bang. Gravity holds together galaxies Scale factor RU is a measure of how much universe has expanded, scale get smaller more look back in time Redshifts of galaxies are not due to doppler shifts but because the light is stretched out and it expands, the wavelength gets longer. More red the more stretched out Z= vr/c no galaxies z> 1 Cosmological redshifts can be expressed as ru = 1/ 1+ z If all matter was in a small volume, conditions would be very hot. This hot, dense gas would have a blackbody spectrum, which should be detectable today. As the universe expanded, the light of the hot gas would redshift and the measured temperature cool. Cosmic microwave background radiation- the observed temp of emissions was 3k- evidence of big bang, leftover radiation from explosion When universe was ht and ionized the photons were trapped with matter can not observe anything in this era, as universe expanded the fog cleared an began to cool/ less dense so we can see At an age of several hundred thousand years, the temperature cooled enough that protons and electrons could form neutral H atoms: this event is called recombination. At that time, light was no longer blocked from its travel by matter. This is what produced the cosmic microwave background. The CMB is the earliest point in the universe that we can observe COBE was able to take measurements of the CMB, learned it was 2.73k and very consistent, slight variations were due to gravity and led to formations of galaxies. Lecture 30 structures in the universe Galaxy groups- collections of dozens of galaxies, local group= milky way, andromeda and triangulum = 50 dwarf galaxies Galaxy clusters are thousands of galaxies containing lots of dwarf galaxies. More massive meaning more elliptical galaxies(coma clusters) Galaxy supercluster contains hundreds of thousands of galaxies, where the milky way resides The universe has a porous structure like a sponge and filamentary with voids in between Peculiar velocity is the velocity of the galaxy relative to the hubble flow; the local group has weird velocity because of the gravitational pull of a supercluster, homogeneous. Dark matter: Theres is 8x more dark matter in galaxy groups then normal space, also filled with hot gas caused by dark matters added gravity Gravitational lensing- reveals the amount of dark matter though bright arcs of deflected light Dwarfs, galaxies, clusters, superclusters, filaments are all formed in order in a process called hierarchical clustering, for this to happen the universe must not have been entirely uniform but must have had denser regions. Matter cannot clump in CMB but since dark matter is immune to radiation and pressure the density fluctuates. Provides structure for things to build from Dark matter has to collapse first to create clumps, but can only collapse so far but the gravitational pull of dark matter pulls in normal matter which created more energy to collapse further, normal matter then loses energy inside the clump and forms stars Cold dark matter - moves slow made from possibly axions or photinos , forms smaller satellite galaxies Hot dark matter moves fast because of possibly neutrinos from the big bang Dark ages- recombination occurred 380000 years after big bang the dark age followed because there was no light from objects Population III stars- the first stars formed in dark matter mini halos which concentrated a lot of neutral hydrogen, gas clouds couldn't cool right meaning these stars were hot and short lives creating a second generation of stars First galaxies- formed from the merging of minihalos of second gen or small first gen stars, shaped by radiation and by first gens elements High-redshift galaxies- very small and very young There were a lot of funky galaxies in the early universe. Triggering supermassive black holes- hierarchical merging triggered the growth of black holes it also triggered star formation Galaxy mergers happen a lot in dense clusters, elliptical galaxies can be made by the merger of spiral galaxies. Cluster mergers merge and also grow hierarchically