AST201 Complete Lecture Notes

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University of Toronto St. George
Astronomy & Astrophysics
Stephen Swales

AST201H1 – Lecture #1 January 8 , 2013 • Fritz Zwicky – in the 1930’s looked at groups of galaxies and measured how they moved around; he noticed that they moved faster than they should have been o He thought 90% of the universe was invisible/missing  “Missing mass” • Vera Rubin – PhD in astronomy – observed stars in galaxies and how they moved around o She found similar things to that of Zwicky; things were spinning faster than the law of gravity suggested (prediction) o Further away from the center of the galaxy, things spun slower (observation) o According to Rubin, about 10% was known (“luminous matter”) matter and the remaining was dark matter • Niels Bohr – “Anyone who is not shocked by quantum theory, does not understand it yet” • 4.6% is luminous matter and the remainder is dark matter o Dark matter is everywhere, around us… o 95% of the universe is unknown • Somewhere in all that ‘empty’ space, dark matter and dark energy are lurking • We know there is dark energy because we are expanding at a constant rate; in fact we are expanding faster and faster – otherwise parts of the world would be expanding at different rates o The universe is expanding into itself; it can do that AST201H1 – Lecture #2 th January 10 , 2013 • Star  generates nuclear fusion reaction o Dark matter is what our universe is predominately comprised of • Epicycle  circular rotation on a circular rotation o E.g., how Mars rotates in a circular motion as it orbits in it’s cycle • Occam’s Razor – created by William Occam o Use as the simplest hypothesis • Galileo did not invent the telescope, he borrowed the idea from the Dutch (who were using it for military purposes) o He took the idea and used it as method of looking in the sky • Scientific method was first used by Tycho Brahe and his disciple was Johannes Kepler  utilized Copernicus’ model because it was simpler, and used it to refine to break the assumption that everything moved in circles o He came up with the 3 laws • The more an orbit is stretched, the higher it’s eccentricity becomes; if we stretch it all the way to 1, it becomes a flat line o A circle has an eccentricity of 0 • Anything in the universe with MASS produces gravity AST201H1 – Lecture #3 January 15 , 2013 • Physics term – “solid body rotation” • Kepler was the first to discover the motion of planets – farther away from the sun, the longer the rotational period o If this was an example of solid body rotation, then all planets would terminate their period at 165 years (Earth years) • Keplarian rotation or orbits is the opposite of solid body rotation which is a perfect explanation/example of Kepler’s 3 law • Isaac Newton – discovered the laws of motion which could explain Kepler’s laws 1. Objects at rest remain at rest and objects in motion remain in motion remain in motion in a straight line, unless acted upon by an external force 2. Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object) a. F = M x A • Gravity is unique because it only ever adds up – there is no ‘anti-gravity’ • This property allows gravity to dominate all other forces on large scales, even though it is by far the weakest force GMm F gravity 2 r AST201H1 – Lecture #3 January 17 , 2013 • ANYTHING with a quantity of mass, it produces gravity • If two stars were put together, they would eventually collide because of their acting gravity • The moon is moving perpendicular to the force of gravity it is feeling o Being pulled towards the Earth, but because it’s moving, it won’t hit the Earth o As long as they are in balance the moon will not crash! • Earth exerts a gravitational force on the moon – why doesn’t it crash into Earth? o The moon is moving parallel to the surface of the Earth • Why don’t astronauts fall to the Earth? o They are moving very quickly around the Earth o There is lots of gravity out in space  • V = V orbital o Is the MINIMUM speed it will produce an elliptical orbit o If you throw an object faster than MINIMUM, it will reach a circular orbit • V escape escape speed doesn’t depend on the direction in which the object is thrown o V escape 1.4 x Vcircular • Robert Hooke thought light was a wave &Newton thought it was a stream of tiny particles o Physicists found that it’s a wave and a particle • Light is a wave-particle • What kind of waves does light produce? o Water waves require to wave o Sound waves require to wave o Light is not a wave in something, it’s a wave of something  A wave of electric and magnetic fields  Light is an electromagnetic wave • Electro-magnetism with a strength of 1/137 and an infinite range – makes atoms cling to each other • “Electromagnetic radiation” = light • Speed of light = wavelength x frequency o All light travels with a speed c = 5.3million km/sec • Wavelength determines the color of light o Increasing frequency – “bluer” o Increasing wavelength – “redder” AST201H1 – Lecture #4 January 22 , 2013 • Speed of light is constant in a vacuum • White is the combination of colors in equal proportions, whereas black is the absence of color • Light can have any wavelength – with a few minor exceptions • True color imaging – is pictures taken and applied filters (blue, red, and green) o Then combine them and that is what the true color is • Cat’s eye nebula (false coloring image) – from a red; you take the bluest red and code it as blue, the middle red and code it as green, and finally take the reddest red and code it red – then combine them • Different wavelengths show different physical conditions AST201H1 – Lecture #5 January 24 , 2013 Light and Matter • 1814 – Fraunhofer detects lines in the spectrum of the Sun o The solar spectrum  They didn’t understand what matter was back then to completely understand the graph o Blue light has higher energy, higher frequency but lower wavelength • What establishes the identity of an atom is the number of protons • Niels Bohr – electrons can only orbit at certain fixed distances from the nucleus – orbits are quantized • Electrons in different orbitals have different energies • Hydrogen emits a shut-ton of red light – given that it is the dominant “characteristic” in the universe Low-Density Gases • Cat’s eye nebula – spread out and diffused, each atom behaves independently o Each atom emits or absorbs light independently it is easier to interpret o Hot – lots of light – if you cooled the gas  dimmer – less colors of light AST201H1 – Lecturth#6 January 29 , 2013 • Dense objects emit light in a very characteristic way o So that they no longer absorb or emit light as individual atoms • Continuous spectrum o Emitted by dense objects is called a blackbody spectrum (typically in physics) • Most dense objects emit something very much like the blackbody spectrum • The more surface area, the more light emitted o But still the same set of colors As a blackbody gets hotter it: 1. It emits more light at shorter wavelengths 2. Emits more light overall Red stars are relatively cold, typically 3,000K White stars are intermediate temperature, typically 9,000K Blue stars are higher in temperature, typically 15,000K Stars emit the entire visible spectrum but it will typically emit one color more than another according to it’s temperature • A star that is 15,000K it will be more violet • The sun will emit roughly equal proportions, but a little more yellow • As for a star that is 3,000K will be more red The Doppler Effect: • The frequency increases as it moves towards you and decreases as it moves away from you AST201H1 – Lecture #7 January 31 , 2013 The Sun • Diameter of the Sun is approximately 110x bigger than the diameter of the Earth • Far from the largest star • There is some carbon and nitrogen to some degree in the center of the Sun – but it is predominantly made of light elements • The Sun shines because it is converting its own mass into energy – it is not burning, even though you can say conventionally, it is o If it was burning, it would die off quickly; tens of thousands of years or at most millions • Einstein came up energy mass o E = mc 2 • If you try to bring two particles together, both with a positive electric charge, they will repel electrically, attract gravitationally o Light charges repel one another • To get particles to overcome their mutual repulsion, they have to have a lot of energy / be very hot – meaning the atoms are moving around really fast – allows them to overcome their mutual repulsion • Sun’s core  15 million K o “P-P Chain” or “Proton-Proton” is the main source of the Sun’s energy • Nuclear fusion – smashing nuclei together to form larger nuclei – to create heavy elements • Nuclear fission – splitting atoms to form light elements – easier to do – on Earth • When anti-matter comes into contact with matter it ignites – creates energy (light) • Neutrinos only interact with other matter via the weak nuclear force, and they ignore the electromagnetic force Pressure from heat generation pushes outward   Gravity pulls inward Solar Thermostat: Gravitational Equilibrium A slight drop in core temperature… Leads to a large decrease in the fusion rate… That lowers the core temperature… Causing the core to contract and heat up… 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 AST201H1 – Lecture #8 February 5 , 2013 • Luminosity – how bright something is intrinsically o The amount of energy something produces per second • Apparent brightness – how bright something appears to you • If a bulb has a luminosity of 100 W when viewed from 1 m away, what would its luminosity be when viewed from 2 m away – it would be 100 W o Its luminosity is irrelevant in terms of distance – (fixed property) • To determine a star’s surface temperature, we would need to measure o The wavelength at which it blackbody curve peaks • To determine a star’s chemical composition, we would need to measure o The wavelengths at which its spectrum shows absorption lines • To determine a star’s speed toward or away from you we would need to measure o The Doppler shift of its spectral lines • Measuring distances is one of the most important things in astronomy o If there are two stars in the sky we are focusing on and one seems brighter than the other, we can’t now for sure because we don’t know their distance  Very difficult • For nearby stars, we use parallax  placing thumb to eye to determine the distance • Light also travels at a constant, finite speed: c = 300 million m/s o In an astronomical scale, we see that it’s pretty slow because it takes time to travel • A light year is the distance light travels in one year: 1LY = 9.46 trillion km • 1 parsec is roughly 3.3 LY • If two stars have different parallax angles, the one with the smaller angle is: o Farther  By doing the eye test with the thumb – holding it close and then moving it as far in front of your eye as possible, doing the same thing would show minimal movement • Of all the light a star emits, Earth receives only a small fraction – all the rest is going in different parts of the universe o This small fraction determines the star’s apparent brightness as seen from Earth • The amount of light received from a star falls with the square of our distance from it – not directly with the distance 1 • This is called the inverse square law of light 2 d AST201H1 – Lecture #9 February 12 , 2013 Spectral Types • Edward Pickering of the Harvard College Observatory and his “computers” – were women o Back then, women weren’t allowed to get degrees, but Pickering got his maid to help him with his analysis of stars because his male workers were pretty bad  That further lead to a team of women working in Astronomy Annie Jump Cannon • The “computer” who finally made sense of the vast catalog of stellar spectra o She holds the record for seeing all the spectra in a way that nobody else could • Cannon realized that you could get rid of most of the spectral categories, keeping only A, B, F, G, K, M, and O o The only one to pick out is M – which has a crazy amount of lines OBAFGKM – Oh Be A Fine Girl Kiss Me • The strengths of the lines depend on the temperature of the star. o Different spectral types of stars are different temperatures Wavelengths of absorption lines  composition Strength of particular absorption lines  temperature (method 2) To measure the masses of stars, we rely on binary stars • Binary stars are pairs of stars orbiting their common center of mass o Most stars are in binaries  E.g., Alpha Centauri A & B is a trinary star system • In a visual binary, we can see both members of the system – this only works for nearby binaries • Spectroscopic Binary – we see the Doppler Shift in the light from the system • Eclipsing Binary – you just measure it’s brightness over a period of time o Even though you can’t see both of them, they are blocking each other If a system is BOTH a spectroscopic and eclipsing binary, we can use Newton’s version of Kepler’s third laws to work out the masses of both stars ________________________________________________________________ ________ We know how to measure: • Surface temperature • Apparent brightness • Distance • Luminosity • Composition • Rotation rate • Speed toward or away from us Scientists plot surface temperature vs. luminosity to get the H-R Diagram AST201H1 – Lecture #10 February 14 , 2013 • The main feature in the H-R diagram is the main sequence of stars o They are in some sense alive, they are burning hydrogen in their cores – that’s what differentiates it or distinguishes it from others  When they stop burning hydrogen, bad things typically happen  If its not on the main sequence it dead or dying Terminology Dwarf Star – it is a star on the main sequence • Diagonal lines – all stars on the main sequence – even if they’re huge White Dwarfs – in terms of size, they are roughly the size of Earth (pretty small) • They vary from A class to O class Luminosity Class – they don’t tell you how bright a star is, they technically tell you the pressure at a star’s surface – correlated with a star’s evolutionary state • I – Supergiants • II – Bright giants • III – Giants • IV – Sub-giants • V – Main sequence stars • Generally higher pressure is smaller and less pressure is larger The Sun is classified as a G2 V Spectral Luminosity Class Class Mass Lifetime ~ Luminosity The bigger a star, the faster it uses its fuel – because of the pressure from gravity, they crush their cores more and burn hotter At the bottom of the main sequence, the stars tend to be smaller – the smaller (red dwarfs) are overwhelmingly feeble (they put out so little light); will live for tens of trillions of years • Near the sun, you get 10s of billions of years • Near the top its near the 10s of millions of years • At the very top, they’re born and they die… almost immediately The way to tell how old a cluster of stars is by looking at the H-R diagram and determining where it starts to peel away AST201H1 – Lecture
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