21:42 ­Earth is a planet in our solar system, which consists of the sun and all the objects that orbit it: the planets and their moons and countless smaller objects including rocky asteroids and icy comets ­We are in the milky way galaxy, a galaxy is a great island of stars in space, containing from a few hundred million to a trillion or more stars ­Groups of galaxies with more than a few dozen members are often called galaxy clusters ­superclusters are the regions in which galaxies and galaxy clusters are most tightly packed ­A star is born when gravity compresses the material in a cloud to the point where the center becomes dense enough and hot enough to generate energy by nuclear fusion ­nuclear fusion is the process in which lightweight atomic nuclei smash together and fuse to make heavier nuclei ­light years: 1 light year is the distance that light can travel in 1 year. This is a unit of distance NOT time ­observable universe is the portion of the entire universe that we can potentially observe. (14 billion light­years) ­earths average orbital distance iscalled an astronomical unit (AU) ­The entire universe is expanding: hubble telescope showed us that virtually every galaxy outside the local group is moving away from us, and the more distant the galaxy the faster it appears to be racing away ­the atoms from which we are made began as hydrogen and helium in the big bang and were later fused into heavier elements by massive stars. Stellar deaths released these atoms into space where our galaxy recycled them into new stars and planets ­Early greeks knew the earth was a sphere because the earths shadow on the moon during a lunar eclipse is circular ­greeks believed earth was the center of the universe and they reasoned that if earth were rotating if you jumped up it would whiz under your feet ­they also believed there were fixed stars in a sphere and that planets slowly drifted and sometimes they drifted backwards (retrograde motion) ­Aristotle believed in this but he also believed made of 4 essences (earth, air, fire and water) 21:42 ­Aristotles model of the planets had them effect eachother somehow so it would produce retrograde motion, however it did not have great predictive power and astronomers wanted to know (i.e. where Jupiter and mars would be) ­later ptolemy made an earth centered model that could predict where the planets were, he thought all planets moved in epicycles centered around earth ­any time there were discrepencies he would add more epicycles, this lasted as the model for a while until Copernicus proposed a new theory ­copernicus made a heliocentric model with the sun in the center, that being so, he could explain retrograde motion: Mars orbits more slowly (already known fact back then) so the Earth catches up and then passes it… Mars undergoes one retrograde motion loop each year as the earth passes by ­copernicus still used perfect circles thus his predictions were not that much better than the previous long­lasting Ptolemic model ­Tycho became famous for discovering a new star (later found to be a supernova) ­Tycho was the first person to repeat his measurements to make sure there were no errors and he also revealed discrepancies in the predictions of both the Ptolemaic and the Copernican models, so he invented his own (which never caught on) ­Kepler worked by Tycho and wanted to analyze his data but did not get the full load of it until Kepler passed away ­Keplers laws were buried in a complicated journal but Newton went through it and found them ­Keplers first law stated that planets orbit in ellipses not perfect circles which is why Tycho’s observations were slightly off ­Keplers second law stated that lines joining a sun and a planet sweep out equal areas at equal times… when a planet is close to the sun the planet moves faster than when far from the sun ­Keplers third law stated that square of a planets orbital period (P) is proportionate to the cube of its semi­major axis (a)… P2=ka3 the more distant a planet is from the sun the longer it takes to complete an orbit 21:42 ­Galileo was the first to discover/use the telescope and he found many stars that were not visible to the naked eye ­He discovered that Venus goes through complete set of phases (like the moon) which matched the Copernicus system not Ptolemaic system ­He found sunspots on the sun, craters/seas on the moon and handles on Saturn—all of these are “imperfections” ­he found 4 moons orbiting Jupiter which proved not all celestial objects orbit earth which was against Aristotle’s thoughts, also since Jupiter was clearly moving and its moons were not left behind, Earth could also move without losing people ­Galileo also discovered that falling objects of different masses accelerate tat the same rate, i.e. on earth a feather and a hammer do not fall at the same rate (air resistance) but on the moon they do (no atmosphere) ­he also said that an external force is responsible for changes in motion, not for the actual motion itself, therfore a natural movement of an object is a constant speed straight line (no acceleration—change in speed or direction) ­as earth moves we move with it which is why we do not feel it moving (heliocentric model) ­Newton invented the reflecting telescope (same type that is used today) ­Velocity: both speed and direction ­Acceleration: rate at which velocity changes (in speed, direction or both) ­To accelerate: to move around a curve at a constant speed or to move in a constant line at a faster or slower speed ­Newtons first law: objects move uniformely unless acted upon them…an object at rest tends to stay at rest, an object in motion tends to stay in motion rarely see this bc usually there are frictional forces acting upon objects inertial motion is motion with constant velocity thus a net force always produces a change in velocity or an acceleration (a net force is an unbalanced force) -Newtons second law: 21:42 ­Newton invented the reflecting telescope (same type that is used today) ­Velocity: both speed and direction ­Acceleration: rate at which velocity changes (in speed, direction or both) ­To accelerate: to move around a curve at a constant speed or to move in a constant line at a faster or slower speed ­inertia: resistance to change in motion ­Newtons first law: objects move uniformely unless acted upon them…an object at rest tends to stay at rest, an object in motion tends to stay in motion rarely see this bc usually there are frictional forces acting upon objects inertial motion is motion with constant velocity thus a net force always produces a change in velocity or an acceleration (a net force is an unbalanced force) ­Newtons second law: states when a force acts on body, it accelerates the body in the direction of the force, at a rate proportional to the strength of the force and inversely proportional to the mass of the body F=ma when you kick a heavy box and a light box with the same amount of force, the light box goes farther mass is the source of inertia (how an object resists change to to its state of motion… i.e. resists acceleration) ­Newtons third law: when two bodies interact they exert equal forces on eachother in opposite directions when you jump off a chair you exert a force onto earth equal to its force exerted onto you…however from the second law we know that your force (your mass is smaller) accelerates earth far less than Earth’s force (bigger mass) accelerates you ­Law of conservation of momentum: in an absense of a net external force, linear momentum is conserved… i.e. when marble A is moving and marble B is at rest, if marble A hits marble B, marble B goes off with the velocity that marble A had and marble A stops. ­A change in velocity and/or a change in mass can cause a change in momentum… i.e. rockets or balloons when the air is let out ­Newtons equation of gravity, F=gmM/d^2 (G is the same at all times and everywhere in the universe) 21:42 ­Angular momentum (m*v*r) measure of the spin an object has. (Figure skater turning example…leg and arm out=bigger radius=slower turn, tucked in=smaller radius=faster turn) ­Particles near poles of spinning cloud not supported against pull of gravity, so they collapse ­Particles in plane experience an outward centrifugal force (felt by a particle in a rotating frame of reference) that balances the inward pull of gravity, forming a disk 21:42 ­Energy cannot appear/disappear out of no where. Objects can only gain or lose energy by exchanging energy with other objects ­potential energy is the same as stored energy ­when you lift a heavy object you exert energy which later will become kinetic energy when the object is dropped ­kinetic energy is the energy of motion. The greater the mass and the speed of an object are the more kinetic energy there will be (ke=1/2mv^2) ­as a train accelerates down a hill the potential energy is converted to kinetic energy (there is very little potential energy at the bottom of the hill) ­Earths tides are mainly due to gravitational influence of the moon. The side of earth facing the moon feels a stronger gravitational pull than other side ­we experience 2 high tides and 2 low tides every day/25 hrs ­the suns gravitational pull should be substantial but it is secondary to the moon because although the sun is bigger and has a larger pull, it is farther from the Earth than the moon and so the Earth’s far side from the sun is not much farther than the Earth’s close side so although the magnitude of the suns pull on both sides of the earth is bigger, the differential force is half that of the moons ­When the sun, moon and earth align, the tides are extreme bc both forces are acting along the same line ­When the sun and moon are perpendicular to the earth, they are pulling in different directions but the moon is greater so it wins but the tides are smaller because of the suns mitigating influences ­the earth also produces tides on the moon… earth is 80x the size so the tides produced are also 80x the size (moon has no oceans so its rocks that get stretched) ­the moon has become locked into a synchronous rotation (we see the same side of the moon at all times) ­Light is extremely important in astronomy because it is an observational science ­can get images to reveal brightness, shape and position relative to other objects and can pass light through a prism ­it bend and reflects the light back at different angles ­William Gilbert showed that every magnet has a N and S pole and like poles repel and opposites attract he said that Earth has a magnetic field like that of a giant magnet, (dipole) ­he also found that some substances become electrically “charged” when rubbed with fur or silk, like charges repel and opposite charges attract ­benjamin franklin thought that electric charge is a fluid (we now know as electrons) and that all uncharged substances have some fixed amount of this fluid ­coulomb determined the exact law between 2 charged objects F=kqQ/d^2 this is a lot like gravity ­Field concept describes electric and magnetic interactions in terms of invisible fields that exert forces on charges and magnets ­a static electric field exists around a stationary charge, such as a proton, field lines are radial and another proton would be radially repulsed ­A static magnetic field exists around a stationary magnet and magnetic field lines loop and always have a N and S pole even if you break magnet in half ­with electric charge the lines go out radially (to infinity) with a magnet the lines loop around and come back to the object that emitted them ­therefore these two things seem very different but all that changed when a prof hooked up a coil of wire to a battery and passed a current through the coil and the compass would move… discovered moving electricity induces a magnetic field… also moving magnets induces electricity ­electric and magnetic forces are now known as the electromagnetic force ­Maxwell says that in a vacuum gamma, x, uv, visible, infared, radio all have to travel at the speed of light ­a wave is a pattern that carries energy or signals but not matter (like water) ­bigger wavelength smaller frequency… wavelength*frequency=speed ­proof that light is a wave: pattern how it spreads out is exactly like water ­each element has a unique emission/absorption line pattern… like a fingerprint ­spectra are used to deduce the chemical compositions of distant stars/clouds this is how we know they have the same elements as the sun and earth overall changing an electric field induces changing a magnetic field which gives us an electromagnetic field­­ light! ­light is like when you drop a pebble into water and the waves travel to the shore but not the pebble, light travels in a vacuum (empty space) so what is moving up and down as the wave passes? The electromagnetic field. 21:42 ­charged particles like protons and electrons exert electromagnetic forces on eachother: 21:42 electromagnetic radiation (light) is caused by a moving charge ­atoms are made of a nucleus (protons and neutrons) surrounded by electrons…there are many types of nuclei, only one type of electron ­number of protons and number of neutrons determines element type protons=atomic number, p+n=atomic weight ­electromagnetic radiation behaves both as waves and particles (photons) ­particles of light are called photons, the smallest packets/quanta of light energy… therefore you cannot (by definition ^^^ have half a photon) ­electrons do not move on a fixed path like planets around the sun, instead they form a smeared out cloud that surrounds the nucleus ­einsteins idea­ quantum of light, viewed as particles photons can have different energies. Viewed as waves the energies depend on the frequency of electromagnetic radiation. ­photons energy is E=hf (h is planks constant which measures the size (tiny) of those packets of energy)… photons of a higher energy have a higher frequency and a shorter wavelength ­photon energy is only controlled by frequency so the only way to increase the energy of the electrons is to increase the frequency of the light ­each atom has a lowest energy state (ground state) and highest (excited) ­when an electron moves to a lower state it emits a photon 21:42 ­an energetic enough photon can dislodge an electron from an atom, this is known as ionization and can occur with any photon with an amount of energy more than the minimum required ­atoms are mainly empty space so when put together sometimes they collide but usually it is just the electrons that interact ­the temperature determines how much energy the atoms have/how they are moving around. Higher tempmore energy/moving faster ­in diffuse gas the atoms are so spread out that they don’t really influence eachother ­in hot diffuse gases the frequent collisions between atoms cause most of them to be in excited states ­emmision spectrum­ when hot diffuse gas is dispersed through a prism we see bright lines at specific wavelengths ­2 types of spectra: line spectrum: low density, transparent gases. Thermal spectrum: dense, opaque bodies. ­ a smooth continuum/continuous spectrum is caused by all electrons and atoms interacting with photons inside of a star (or sun) ­in a hot opaque star photons tend to bounce around a lot which randomizes their energy producing a continuous spectrum 21:42 ­wavelength at the peak determines the color of the star
