Physics of Astronomy :Physics of Astronomy Paul Urban Astrosoc Computer Rep 2001


Overview :Overview We’ll cover the following broad topics The Philosophy of Physics Classical Mechanics Chaos Theory Stellar Evolution The Expanding Universe Relativity


The Philosophy of Physics :The Philosophy of Physics The most fundamental idea behind Physics is probably that “everything around us is governed by a very small number of laws that we can in principle understand” Physicists try to figure out these laws by following the Scientific method


The Heliocentric Solar System :The Heliocentric Solar System Classical Mechanics


The Laws of Planetary Motion :The Laws of Planetary Motion Kepler studied the observations of planetary motion and found that he could explain them with a much simpler model His model is expressed as three laws Law of Ellipses Law of Equal Areas Harmonic Law Classical Mechanics


1. Law of Ellipses :1. Law of Ellipses Classical Mechanics The orbits of planets are ellipses with the Sun at one focus


2. Law of Equal Areas :2. Law of Equal Areas Classical Mechanics A line joining a planet to the Sun sweeps out equal areas in equal intervals of time


3. Harmonic Law :3. Harmonic Law Classical Mechanics The ratio of the square of a planet's orbital period to the cube of its average orbital radius is constant


Newtonian Mechanics :Newtonian Mechanics Newton introduced the concept of a ‘force’, something that acts to change the motion of matter Classical Mechanics Newton’s gravitational force explained the motions of the planets, and agreed completely with Kepler’s laws


Determinism :Determinism Classical Mechanics “If we know the state of the Universe, we can predict its entire future”


Determinism Dies :Determinism Dies Chaos Theory Determinism has ben overthrown by two important branches of Science Chaos Theory Quantum Mechanics


Chaos Theory :Chaos Theory Chaos Theory holds that complexity arises in even the simplest systems If we don’t know the current state of the system accurately, our predictions of it’s future state will grow progressively inaccurate Quantum Mechanics shows that we can never know it’s current state accurately Chaos Theory


Fractals :Fractals Chaos Theory


Planetary Orbits :Planetary Orbits Chaos Theory


Star Qualities :Star Qualities Stellar Evolution


Stellar Structure :Stellar Structure Stellar Evolution Pressure causes fusion Weight balanced by energy released during fusion Different elements formed in layers Evolution determined only by the star’s mass


The Life and Times of a Star :The Life and Times of a Star Stellar Evolution


The Universe, its Expanding! :The Universe, its Expanding!


Evidence for Inflation :Evidence for Inflation Olber’s Paradox: “If the universe is infinite in size and age, why isn’t the sky uniformly bright?” In 1929 Edwin Hubble discovered the Red-Shift Law: The more distant the galaxy, the faster it is retreating from us The Expanding Universe


Red-Shift Explained :Red-Shift Explained Red-Shift is like the Doppler Effect for sound, except applied to light Astronomers locate chemical ‘fingerprints’ in a galaxy’s spectrum and can deduce it’s velocity relative to us The Expanding Universe


Measuring Distance :Measuring Distance Parallax Type-I Supernovae The Expanding Universe Cepheids Tully-Fisher Correlation


Parallax :Parallax The Expanding Universe


Cepheids :Cepheids The Expanding Universe


Supernovae :Supernovae The Expanding Universe


Tully-Fisher :Tully-Fisher The Expanding Universe


The Big Bang :The Big Bang We think it is the space between the galaxies, the very fabric of the universe, that is expanding There is much evidence that at some point in the past everything was squashed up into a tiny volume, and has been expanding ever since The Expanding Universe


It All Started With a Singularity :It All Started With a Singularity The Expanding Universe


Relativity :Relativity


Relativity Has Two Branches :Relativity Has Two Branches Special Relativity is concerned with how we measure things in fast-moving frames of reference General Relativity is concerned with accelerating frames of reference, and gravitational fields Relativity


Special Relativity :Special Relativity Relativity is not “everything is relative”! The Principle of Relativity holds that the laws of nature are the same for every observer Light in a vacuum always has the same speed when measured, no matter what the observer’s velocity is (a confirmed fact) Relativity


Special Relativity (cont) :Special Relativity (cont) Special Relativity holds that, to all observers, the speed of light is absolute and that measured lengths and time intervals are not! The faster two observers move relative to each other, the more they disagree about lengths and times Relativity


Special Relativity (cont) :Special Relativity (cont) It takes more and more energy to increase one’s speed, and an infinite amount to reach light speed What would it really be like travelling at near light speed? Relativity


Warp 0.92 (0.75c) :Warp 0.92 (0.75c) Relativity


General Relativity :General Relativity If you were in a closed lift and you felt a force pressing you towards the floor, you wouldn’t be able to tell if the lift was accelerating, or if you were really feeling gravity’s effects General relativity holds that gravity affects matter and energy, by warping the fabric of space-time! Relativity


General Relativity (cont) :General Relativity (cont) Relativity


Black Holes :Black Holes After a supernova explosion the core of the star may become so dense, and space-time so warped that even light itself cannot escape! Relativity


Black Holes (cont) :Black Holes (cont) Astronomers detect black holes by their gravitational tug on nearby stars and by the radiation that the matter spiraling into them gives off Relativity


Q&A :Q&A Are you still there? Have you got any questions?