Ast201 Exam Notes.docx

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Astronomy & Astrophysics
Michael Reid

Astronomy 201 – Exam Notes - Occam’s Razor tells us that all other factors being equal, simpler explanations are more likely to be true - The universe is the totality of all space, time, energy and matter - Light travels at a constant, finite speed (c = 300 million m/s) - A light year is the distance that light travels in one year (1 ly = 9.46 trillion km) - The universe is about 13.7 billion years old - There is a limit on the amount of universe we can see because the universe is only 13.7 billion years old and the light from other places would not have had enough time to travel to us - What we are able to see is called the observable universe - Looking very far away is like looking back in time because the light that we receive with the images has taken lots of time to get to us and the object has changed/aged within that time - 2.5 million light years away = seeing the object as it was 2.5 million years ago - Stars: enormous spheres of very hot gas which produce energy by nuclear fusion – the sun - Solar system: one or more planets orbiting a star - Galaxy: a collection of billions of stars all orbiting a common centre - The Milky Way galaxy is about 100,000 light years across - Our nearest (large) neighbour galaxy is the Andromeda Galaxy - The milky way and Andromeda are both spiral galaxies - The milky way, Andromeda and a few dozen small galaxies make up the Local Group of galaxies, all of which orbit one another - All of the galaxy clusters in the universe are moving apart from one another - Solar system < galaxy < galaxy (local) group < galaxy cluster < superclusters < wall or filament - Ancient astronomy was often astrological in nature, was geocentric, earth centered - For thousands of years, astronomers insisted on the Pythagorean ideal of circular motion - Objects orbit around a common centre of mass - The scientific method: Kepler’s Laws: Newton’s Laws: - F = GMm/r 2 gravity - an object is in free fall when the only external force acting on it is gravity - at v = v orbitaljects begin to orbit - v – objects break free of the planet’s gravity escape - escape speed doesn’t depend on the direction in which the object is thrown - v > v escape= an unbound orbit - v escape 1.4 x v circular - light is a wave-particle – depending on how you observe it, it may act as either a particle or a wave - wavelength is the distance from one peak to the next (or one trough to the next) - water waves require a medium = water - light is not a wave in something, it’s a wave OF something - light is an electromagnetic wave - electromagnetic radiation = light - wavelength is the distance between adjacent peaks of the electric (and magnetic) field - frequency is the number of times each second that the electric (and magnetic) field vibrates up and down (or side to side) at any point - all light travels with speed c = 5,300,000 km/s - speed of light = wavelength x frequency - wavelength translates into colour - - Light can have any wavelength - - - Different wavelengths show different physical conditions - Emission: the process by which matter emits energy in the form of light – gives out energy (electrons move down in energy shells/orbits - Absorption: the process by which matter absorbs radiative energy – takes in energy so electrons move up in energy shells/orbits, more active - The energy that an atom absorbs or emits translates into energy lines and correspond to certain elements and colours - Spectra lines = chemical composition - Low-density gas = each atom emits or absorbs light independently of the others and produce EMISSION spectra – black line with coloured areas where it picks up light at certain wavelengths - Dense objects (gas, liquid or solid) = emit some light at all wavelengths and is called a CONTINOUS spectrum (all colours) – the particular continuous spectrum emitted by dense objects is called a blackbody spectrum - As a blackbody gets hotter it: Emits more light at shorter wavelengths and emits more light overall - Low-density gas in front of a blackbody = ABSOPTION – some particles of light get scattered and so the spectra is missing some wavelengths of light (full spectra with some black lines) - Doppler Shift – Blueshift: object moving towards us – spectral lines are shifted to shorter wavelengths. Redshift: object moving away from us – spectral lines are shifted to shorter wavelengths - Telescopes have three powers: light gathering power (more light = fainter features), resolution (more resolution = more detail), magnification - Larger diameter = higher resolution - Larger area = higher sensitivity - Ground=based telescopes now use adaptive optics to cancel out the wobbliness the of atmosphere - Adaptive optics typically use a laser guide star and deformable mirrors - The sun – pressure from heat generation pushes outward and gravity pulls inward - The sun is in hydrostatic balance between gravity (in) and thermal pressure (out) 2 - E=mc - Energy (E) = mass (m) - Nuclear fusion = smashing nuclei together to form larger nuclei - Nuclear fission = breaking nuclei apart to form smaller nuclei - Stars can use all sorts of fusion reactions to generate energy - The sun mainly produces energy by fusing hydrogen nuclei (protons) in the proton-proton chain - Hydrogen Fusion by the Proton-Proton Chain - - Neutrinos = antimatter – only interact with other matter via the weak and gravitational forces – no electromagnetism - Sun’s core = 15 million K - Fusion only happens in the core - - - A slight drop in core temperature leads to a large decrease in the fusion rate that lowers the core pressure causing the core to contract and heat up thereby restoring the fusion rate to normal - A slight rise in core temperature leads to a large increase in fusion rate that raises the core pressure causing the core to expand and cool down thereby restoring the fusion rate to normal - The solar cycle: the sun’s activity level varies from quiescent to highly active and back every 11 years - Luminosity – the amount of energy something produces per second – how bright something is intrinsically - Apparent brightness – how bright something appears to you - Spectral line = chemical composition - Doppler shift = motion along the line of sight - Parallax – the apparent shift in the position of a nearby star (relative to distant objects) that occurs as we view the star from different positions in the Earth’s orbit of the sun each year - 1 parsec = 3.3 light years - Of all the light a star emits earth only receives a small fraction – this small fraction determines the star’s apparent brightness or apparent magnitude, m, as seen from earth – the amount of light received from a star falls with the square of our distance from it – this is called the inverse square law of light - Distance (d) + apparent brightness (I) = luminosity (L) - Most stars have thousands of absorption lines - Spectral categories – named OBAFGKM (Oh Be A Fine Guy, Kiss Me) - Each stellar spectrum has absorption lines corresponding to the chemical elements in the star’s atmosphere - The strengths of the lines depend on the temperature of the star. This is a second, more precise way to measure a star’s temperature - Wavelengths of absorption lines = composition - Strength of particular absorption lines = temperature (method 2) - To measure the masses of stars, we rely on binary stars which are pairs of stars orbiting their common centre of mass – most stars are in binaries - Spectroscopic binary – we see the Doppler shift in the light from the system as the two stars orbit one another - On one side of its orbit, star B is approaching us so its spectrum is blueshifted – on the other side, it is receding from us so its spectrum is redshifted - Eclipsing binary – we see the system dim as the other stars eclipse one another - If a system is both a spectroscopic and an eclipsing binary, we can use Kepler’s laws to work out the masses of both stars - We know how to measure: surface temperature (2 ways), apparent brightness, distance, luminosity, composition, speed toward or away from us, rotation rate - H-R diagrams – plot surface temperature vs luminosity - Stars on the main sequence are fusing hydrogen in their cores slash are alive - All main sequence stars are called dwarf stars – can be huge and still called a dwarf - Luminosity class – Stellar Luminosity Classes Class Description
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