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Frontiers in astronomy- Test 1 notes bundle

by: Kathryn Notetaker

Frontiers in astronomy- Test 1 notes bundle PHY 21430

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Kathryn Notetaker
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notes from weeks 1-7 all in 1 document. Can use for test 1.
Spyridon Margetis (P)
75 ?





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This 38 page Bundle was uploaded by Kathryn Notetaker on Friday October 14, 2016. The Bundle belongs to PHY 21430 at Kent State University taught by Spyridon Margetis (P) in Fall 2016. Since its upload, it has received 27 views. For similar materials see FRONTIERS IN ASTRONOMY in Physics at Kent State University.


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Date Created: 10/14/16
Frontiers in Astronomy notes combined- weeks 1-7 Intro  Astronomy is a natural science  greek words ‘astro’ + ‘Nomos’ (the law of stars)  initially was the science of heavens but in a narrower sense  mapping the night sky  recording the position of the stars and planets  unusual events : eclipses, supernova explosions, etc  today vastly expanded that initial scope  today it deals with the study of the UNIVERSE  the collection of all that exists a collection of celestial objects (stars, planets, comets, nebulae, galaxies, etc)  universe is everything  almost 99% of all brightness in the sky are stars  the sun is a star too, but it’s so close that its brightness dominates during the day, obscures any object in the sky  the stars seem to be fixed on the background sky but the planets seem to have more relative to fixed stars after a week or so  comets are much smaller than stars, have a tail  The universe is also called cosmos by the ancient greek philosopher Pythagoras  the word cosmos means order but also ornament to denote the order and beauty one finds in the universe  an orderly and harmonious system as opposed to a disorderly one or chaos  appears static to us but it is an extremely active entity in reality  today is primarily concerned with:  evolution  physics laws  chemistry  composition  meteorology  motion of celestial objects  formation and development of the universe  Astronomy is a branch of physics and mathematics but a lot of other fields contribute, like chemistry, geology, biology, etc  represents a synthesis of our state of the art knowledge of nature in general  the most ancient of natural sciences  the night sky was always there for humans to see  humans started wondering about their place in the universe early on  why and how celestial objects moved?  by observing the sky our ancestors were able to make calendars  uses mathematics as its language to describe the laws of the world  science of rules or logic  the physical world interacts with the world of logic very closely when it comes to its laws and their deeper understanding  There is still a philosophical debate on what comes first, what generates what, something that shows their intimate relationship  “the world is the totality of facts, not of things”- tractatus logicophilosophicus  Ancient times plato/Aristotle debate  what comes first? the physical or mathematics?  school of Athens painting  Aristotle paradigm  points downward, towards physical world  saying physical reality is fundamental  platonic argument  points up, multiverse  mathematical structure is the true reality   More on astronomy  astrophysics and cosmology  questions related  what is the cosmos made of?  how it all begun and evolved to this point?  what is our fate and the fate of our universe  how stars are born and die  how things work in the cosmos  what are the physical laws governing celestial motion and phenomena  can we interfere?  ex. avoiding a cosmic collision with an asteroid?  are there other worlds with intelligent life like ours?  Why bother with questions?  understanding- quenching the intrinsic thrist for knowledge, the need to know, the need to comprehend to find meaning in life  use the power of knowledge to better our lives  energy resources, hydrogen fusion (stars) on earth  ability to inhabit other planets  ability to find and communicate with other life forms  safe space travel  etc  Scientific Method  Science of measurement  backbone of all sciences  3 components  facts, measurements (data)  collected via observation of nature or lab experiments  leads to theory explaining it  hypotheses and theory development  laws of physics are extracted  leads to predictions consistent with previous observations  verification  repeatability, predictability and testing  testing is VERY IMPORTANT and a key element of the method  predictions of new phenomena are observed  Scientific theories  must be testable  must be continually tested  should be as simple as possible  should be elegant… we believe that truth should be beautiful  can be proven wrong, but they can never be proven right with 100 percent certainty The scientific method and the fundamental ideas in physics  Key Modern Ideas of physics used in astronomy  Laws of physics are universal  the same everywhere in the universe and at any scale, human or cosmic  the way nature is  every onbject in the universe is made up of same elements  life development might be similar in other places and the search for ET is based on this  why we look for exo-planets that are similar to earth  gravitation and celestial mechanics  the force that makes an apple fall, is the same force that keeps earth revolving around the sun, stars move around the center of galaxy  symmetries and conservation laws  nature has symmetrical behavior-leads to effects in the laws of physics  this indifference of nature leads to many conservation laws (something stays the same over time)  ex. the energy of an object, the way it moves  for every symmetrical behavior, there must be some physical quantity that is conserved  Quantum mechanics and particle  duality (particle-wave) in microcosm  particles behaving like waves and vice versa  the death of determinism and birth of probable  nothing is certain in most cases several possibilities for outcome of identical experiments  most of our modern-life tech is based on it  theory of relativity  things like space and time are relative  they depend on the point of view  their flow can be modified  the tyranny of speed of light  light speed is constant for everyone  light speed is the speed limit in the universe for any object with mass  we believe nothing moves faster than the speed of light  big bang cosmology  the theory about origin of our universe  how it all started  what is the fate of our universe?  are there other or parallel universes out there?  what is dark energy and dark matter?  Unification of particles, forces and space-time  strong evidence that all different forces are nothing but facets of the same fundamental force  what is the nature of the fundamental entity ?  TOE (theories of everything)  is it a string likt he string theory suggests?  is it just an abstract number ?  something else?  what are the fundamental particles of force?  are there more than we know now?  super particles, higgs particle, tachyons, mini-black holes, etc  The philosophical idea of things being ultimately unites is in the core of these theories  Entropy and the arrow of time  some processes are irreversible  aging  the breaking of a piece of glass  what is the nature of time flow as we live it and why the arrow fo time points only to future?  this idea has to do with order and disorder in the cosmos  it relates to the ultimate thermal death of the universe and the struggle of life to create order and diversity  A short tour of the universe  The night sky  at first sight, appears as a sphere  patterns of fixed stars appear stable in time  Celestial sphere and the stars on it are fixed stars  Orion- only visible in the summer  Sun- star closest to us  creates its own energy and light  Moon- satellite that rotates around earth  Planets- move slowly in the sky  Stars and galaxies  milky way  Clusters and super clusters of galaxies  berenices coma galaxy cluster  Universe  possibility of existence of parallel universes Charting the heavens  our place in space  earth is average-we don’t occupy any special place in the universe  earth is just 1 of many planets that exist  The sun is about 100 times larger in diameter than earth  in astronomy miles are too small a unit to measure distances  light year and the AU (astronomical unit)  Light year-the distance a beam of light travels in a calendar year  a light year is equal to 10 trillion kilometers or about 6 trillion miles  light travels 300,000 km or 186 miles a second  the distance between earth and sun is about 93 million miles (1 AU)  1 AU is travelled by light in 8.3 minutes  there are 100-500 billion galaxies in the universe  scales are unfathomable  The universe has 2 parts from our point of view  visible(what we can see) and invisible (resides beyond the limit we can see)  The obvious view from earth  simplest observation: looking at the night sky  about 3000 stars are visible at any 1 time; distributed randomly but human brain tends to find patters  the 3 stars in a line is orion’s belt and the bright spot is his sword  the group of stars in such an image is called a constellation  88 constellations in the whole sky  Taurus example  for a sign to be a zodiacal sign, it has to pass the sun’s path  The “Real” view  stars that appear close in the sky may not actually be close in space  ex. orion seems to be a group of stars, just an illusion due to inability of the human eye to perceive the actual depth of each star  The celestial sphere  for us, stars seem to be fixed on the inner surface of a sphere surround the earth  they are actually all over the space, but lack depth perception  the extension of earth’s rotation axis passes today near Polaris and it defines the North celestial pole  the projection of earth’s equator on the celestial sphere is called the celestial equator  the actual rotation of earth is from west to east from above, which is why it looks like from earth it is east to west  3 movements of earth  more than 20 movements  earth’s rotation around itself  revolves around an imaginary axis that goes through the south and north poles  24 hours to complete full revolution  motion of earth around the sun  earth moves almost in a perfect circle around the sound  completes a revolution in a year  earth’s rotational axis is not parallel to the earth’s rotation around the sun  but have a relative inclination of 23.5 degrees-major consequences for us on earth  wobbling of its axis like a spinning top’s axis is not fix  the axis is slowly moving around  completes a revolution every 25000 years  consequences  in a few thousand years, the the earth’s axis is not going to point to Polaris st thuban was the pole star in 3000 b.c.  1 movement of earth  stars appear to move in circular paths the course of a night  daily motion- the apparent motion from east to west  true motion- the earth rotates around its axis from west to east  the earth’s axis points near Polaris  Daily star motion picture- discussion topic  if the picture was taken for a full day and night, then the trace of each star would be a complete circle (bc earth makes a full rotation in 24 hours)  but in the picture, the traces aren’t complete  by looking at star traces, give an estimate on how many hours this exposure took  Earth’s Days  2 kinds of days  Solar day- about 24 hours  noon to noon  not a true day  how the sun appears to be in the same place at 2 consecutive times  sidereal day  when the earth makes a full revolution  true day  shorter than the solar day- 4 mins shorter  in a week, the sidereal time is about 28 mins shorter  how to get the 4 min difference?  every day, the earth moves about 1 degree around the sun  earth has to cover this extra degree  360 degree in 24 hours  24*60 =1440 minutes  to cover the extra degree it needs 1440/360=4!  Angular measure  full circle contains 360 degrees  each degree contains 60’ arc-minutes  each arc-minute contains 60” arc-seconds  angles are useful when we refer to objects on a circle Earth’s orbital motion around the sun (second movement)  as earth moves around the sun, the parts of the sky that are visible to us at night are also changing  6 months from now, we see different stars in the sky  the apparent yearly motion of the sun is from west to east  sun has 2 motions where it rises in the east and sets in the west, the yearly motion in the opposite direction  seasonal changes to night sky are due to earth’s motion around the sun  the Zodiac  as the sun moves in the sky, goes through 12 constellations  today the sun goes through 13 constellations (Ophiuchus entered before scorpio and before Sagittarius) Seasons, moon-phases, etc-Chapter 1  Sun and seasons  the sun’s apparent path on the sky (ecliptic) is making 23.5 degrees angle with the celestial equator  if there was not a tilt of earth’s axis (0 degrees), the sun would always move on the celestial equator=same or no seasons  with tilt- Sun shines more on the northern hemisphere during the summer and less in winter  there are 2 points where the sun is on both planes  vernal equinox  21 of march (beginning of spring)  autumnal equinox st  21 of September (beginning of autumn)  these seasons are for the north hemisphere  the south hemisphere are opposite  these 2 dates, the length of the day and night is the same for every place on earth  summer and winter solstice  2 other points on this travel path  Solstice- meaning the sun stops  summer solstice  the sun reaches the maximum height above the celestial equatot  Winter Solstice  the sun reaches the lowest point  mark the beginning of summer and winter  opposite for the southern hemisphere  Axis  as earth moves around the sun, its axis of rotation stays the same  due to a fundamental law in physics  tilt of axis cause the seasons  Sunlight during the summer solstice hits earth’s north hemisphere almost perpendicularly  why it’s hotter in summer, colder in winter/ hotter in midday and cooler in afternoon  Motion of the moon  phases are due to different amounts of sunlit portion being visible from earth  always start counting from new moon, when the moon is completely hidden since it is in the direction of the sun  waxing (increasing) crescent is the next phase  after the new moon, it starts getting illuminated from the right as we see it from earth until full moon  then gets dark again from right side  synodic month- takes moon about 29.5 days (about a month) to go through whole cycle  sidereal month- 2 days shorter (27.3 days) time it takes for a full revolution around the earth  2 day difference comes from earth’s motion around the sun  Eclipses  Lunar eclipse  earth is between moon and sun, instead of full moon it is a dark moon  partial eclipse-when only part of moon is in earth’s shadow  total eclipse- when all of the moon is in earth’s shadow  Solar eclipse  moon is between earth and sun  partial eclipse- only part of the sun is covered by the moon  total eclipse- when the sun is completely covered by the moon  eclipses occur when earth, moon and sun form a straight line  does not happen in every new/full moon  the moon’s plane of rotation is inclined with respect to sun-earth’s plane (5.2”)  earth’s and moon’s orbits are not in the same plane The Copernican Revolution-Chapter 2  Ancient astronomy  over 6000 years ago  almost every major culture and civilization studied the celestial objects and their move on the sky  religion was one of the driving forces  also navigation, time-keeping, weather changes, animal behavior- things that linked to human survival  only a few select people were allowed to learn about astronomy  people were fascinated with the endless repeating patterns of change in the world around them  practical knowledge  Egyptians relied on astronomical observation to plan the planting and harvesting of crops  knew that the rising of Sirius just before the sun heralded the flooding of the nile river  we believe that megalithic monuments were the first observatories  Stonehenge mark astronomical events the sunrise of the summer solstice when the sun shines through a specific stone, only happens at this time  big horn medicine wheel spokes are aligned with the rising and setting of the sun and other stars thin streak of light aligns perfectly with center of the pattern at noon of summer solstice  Caracol temple built by mayans many windows that align with astronomical events looks like an observatory  Chinese astronomers left behind astronomical events like supernova explosions  main contributors to astronomy as a science were the greeks and muslims  Islamic world provided a link through the dark ages  Most names of sky objects are either greek, muslim or latin greek: orion, helios (sun), planet (wanderer) muslim: zenith, azimuth, rigel, betelguese, vega latin: mars, Jupiter, Neptune  The geocentric universe  ancient astronomers observed:  sun moving daily and yearly on ecliptic  moon doing its phases and move near ecliptic  stars moving only daily, otherwise fixed on sky  5 planets were known:  mercury, venus, mars, Jupiter, Saturn  moved on/near ecliptic, never far from it  Sun, moon and stars have simple movements in sky  planets  move with respect to fixed stars from west to east  change in brightness  change speed  undergo retrograde motion- an apparent change in the direction of motion of a planet a puzzle to early astronomers trying to figure out how heavens work  Inferior and superior planets  inferior-mercury, venus  stay close to sun  have orbits closer to sun than earth’s  they can never be in opposition with the sun  2 conjunctions 1 behind the sun (superior) 1 in front (inferior)  Superior- Mars, Jupiter, Saturn  have orbits that are further away  can be in opposition with the sun (earth in between)  have only 1 conjunction (superior-like)  1 opposition  Geocentric universe  developed by greek philosopher Aristotle-1 theoretical model trying to explain the motion of known celestial objects  called geocentric because earth was at the center of universe the center is where observer is- makes sense  things were supposed to move in perfect circles with constant speed  model employed nested spheres with the stars, planets, etc., attached on them  earth in center, moon, mercury, Venus, sun, mars, Jupiter, Saturn, fixed stars, sphere of the prime mover  Aristotle’s arguments  earth doesn’t feel like it’s moving- why it’s at the center  if it moved, why don’t we feel a head wind  if earth moved, then the stars should have stellar parallax but they don’t- stars should have an apparent movement on the sky  Aristotle’s model  could not account for 2 important things variation in planetary brightness the retrograde motion  1 step towards resolving these issues was to introduce epicycles- a small circle whose center remained on the main path of the planet (deferent)  many more fine adjustments until Ptolemy’s model (~140 AD)  Ptolemy’s (refined) geocentric model  preserved earth at the center, circular motion  interested in accurate calculations, not laws mechanisms or consistency  introduced eccentric (off center) motion, epicycles, deferent and equant to deal with various effects  very accurate in predictions but very complex and inconsistent  violated the law of simplicity- the truth is simple  violated occam’s razor- law of economy  between 2 explanations, the simpler one is probably the correct one Heliocentric model of the solar system  Aristarchus of samos  the sun is the center of the solar system, only moon orbits around earth; planets orbit around the sun  used logic to explain things in a more convincing way  the earth makes 2 motions, 1 around itself and 1 around the sun  still, the Ptolemaic picture survived for almost 14 centuries  Nicolaus Copernicus  challenged ptolemy’s model  re-introduced the heliocentric model  motivation was simplicity  the critical realization that earth is not the center of the universe (Copernican revolution)  explained retrograde motion in a natural way  foundations of the Copernican revolution  earth is not at the center of everything  center of earth is the center of moon’s orbit  all planets revolve around the sun  in circular orbits  the stars are very much farther away than the sun  the apparent motion of the sun and stars is due to earth’s rotation around itself  retrograde motion  Geometry and earth moves faster  Birth of modern astronomy  telescope was invented around 1600  Galileo built his own, he improved the telescope  was inspired by kepler’s work  made detailed observations  published dialogo in 1632  condemned by inquisition  sentenced to house arrest until death  1992 john paul II pardons  Galileo’s observations:  moon has mountains and valleys like earth  sun has sunspots and rotates  Jupiter has moons-smaller objects rotate around larger  venus has phases  Laws of planetary motion  galileo demolished ptolemy’s system and established the Copernican heliocentric system  Copernican approach is a classic example of the scientific method  the laws of gerning the motion of celestial objects were still unknown  the dynamics were missing  Johannes Kepler  kepler’s laws derived data using observations made by tycho brahe  formulated 3 laws that explain everything  Kepler’s laws  the orbits of the planets are ellipses, not circles, with the sun at one focus  the line joining the sun and the planet sweeps out equal areas in equal times  square of period of planet’s orbital motion (P) is proportional to cube of semimajor axis (a)  ratio P^2/a^3 should be constant  properties of planetary orbits  perihelion: closest approach to the sun  Aphelion: farthest distance from sun  Isaac Newton  revolutionized all of science and mathematics  probably the greatest physicist of all time  2 year vacation- there he revolutionized science and math  3 laws of motion and 1 law of gravity  classical mechanics/ newtonian mechanics  Newton’s laws  explain how objects interact with the world and with each other  law of gravity identifies the responsible force behind celestial motion  & provides the formula to estimate it  Principia Mathematica (1686)  explained all motion  most important book in history  Mass-quantity of matter- how much stuff there is (not weight)  force- an action that changes motion (the cause of acceleration  1 law-law of motion  if an object is at rest, it will stay at rest  if an object is in motion, it will not change its motion unless an external force acts on it  2 ndlaw  when force (F) is exerted on an object its acceleration (a) is inversely proportional to its mass (m)  a=f/m  f=ma  3 law-action/reaction law  mutual forces of 2 objects upon each other are equal and directed in opposite directions  Newton’s gravitational law  for 2 massive objects, the gravitational force is proportional to the product of their masses divided byt the square of the distance between them  the farther the objects are, the weaker the force  newton realized the universality of the gravitational force  the same force that makes an apple fall, keeps earth revolving around the sun  Newtonian mechanics  kepler’s laws a r a consequence of newton’s laws  some fine-tuning was done to kepler’s laws due to newton’s laws  einstein’s theory of relativity super-tuned Newtonian celestial mechanics Radiation- Ch. 3  Information from the skies  light was the only connection to space  today, light is nothing else but a form of radiation, electromagnetic waves  besides probe exploration (limited to our solar system), light and electromagnetic radiation is still the main connection to cosmos  other connections are the cosmic rays, neutrino and gravitational waves but the main connection is electromagnetic radiation  Electromagnetic radiation  transmission of electromagnetic energy through space without physical connection through varying electric and magnetic fields  Ex. Light, radio signals  Waves  moving disturbances that transmit energy without the physical transport of material  waves in a pool, waves in a wheat field or waves of people in a football field  waves have wavelength and wave speed  waves DO NOT transport material in order to transport energy waves in a pool just make the water move up and down  Ex. Water wave wave travels and can transmit energy (tsunami)  Properties wavelength: distance between successive crests (troughs) velocity: speed at which crests move period: time between passage of successive crests, a full cycle Frequency: number of waves crests that pass a given point per second Velocity = wavelength/ period Period = 1/Frequency water and sound waves have different speeds electromagnetic waves travel in vacuum with the speed of light  Visible electromagnetic waves  Light white light is a mixture of colors colors are ranges of frequencies in the electromagnetic spectrum the human eye and brain makes us perceive colors white colors split into primary colors when it passes through e.g. a prism or any other transport material ex. a rainbow  Do all waves need a medium?  many kinds of waves need a material, matter , to propagate water waves, sound waves and so on travel in a medium (water, air)  Electromagnetic waves don’t need a medium, they travel in vacuum initially thought to travel through ether, a perfect medium  Created by accelerating charged particles (oscillating currents) and other atomic processes  EM waves: oscillating electric and magnetic fields normal to each other changing electric fields creates magnetic field, vice versa travel in vacuum in the form of quanta, indivisible packets of EM fields the nature of light was something Einstein spent his life to understand  Wave speed (c) c=300,000 Km (186,411 miles) per second speed is large but finite  it can take light millions or billions of years to traverse astronomical distances  Electromagnetic spectrum  Radio- lowest frequencies  Am (long frequencies)>FM(short frequencies)>Microwave  Infrared  visible  ultraviolet-lie beyond violet color  X-rays- soft, hard  Gamma rays-most energetic frequencies(shortest wavelengths)  Red feels ‘warmer’ and blue/purple colder  Red carries less energy than blue  No limit on wavelengths; different ranges have different names  opacity of atmosphere, absorbs most radiation with just a few exceptions like some radio waves and visible light  sectors in other ranges need to be outside the atmosphere, in orbit  examples: microwave, x-ray and gamma ray detectors  Infrared  certain things are transparent in infrared light that are not in visible light  Visible spectrum  the sun is the dominant source of visible light that our eyes perceive  Ultraviolet light  shorter wavelengths than visible light  some insects like bumblebees can see UV  the sun is a source of the full spectrum of UV radiation  X-rays  higher energy, shorter wavelengths  refer in terms of energy rather than wavelength  have very small wavelengths between .03 and 3 nanometers  Gamma rays  smallest wavelengths  most energy  produced by hottest and most energetic objects in the universe  gamma waves are generated by nuclear explosions, lightning and the less dramatic activity of radioactive decay  Behavior of light waves across the electromagnetic spectrum  when a light wave encounters an object, they are either:  transmitted (like through a piece of glass)  reflected (like in a mirror)  absorbed  refracted (from a prism)  diffracted  scattered  Reflection  waves can change direction if they hit a mirrored surface  topography of the moon  a laser light was reflected on the moon’s surface, different heights are shown as different colors  Absorption  waves can be absorbed by a surface, depositing its energy into the surface  image of Atlanta  dark surfaces are hotter since asphalt absorb the energy of light and heat up  Diffraction  bending and spreading of waves around an obstacle  when a light waves strikes an object with a size comparable to its own wavelength  Scatter  when light bounces off an object in a variety of directions  scattered amount depends on the wavelength of the light and the size and structure of the object  Kelvin (k) temperature scale  All thermal motion ceases at 0 k (absolute zero)  water freezes at 273 k and boils at 373  Rule of thumb  at high enough temperatures kelvin/ Celsius scales have the same rating  to convert them to F just multiply by 2  Thermal radiation  blackbody spectrum: radiation emitted by an object depending only on its temperature (T)  intensity: amount of radiation, strength of radiation  intensity is maximum at a characteristic frequency for given T  Radiation laws  Peak frequency is directly proportional to temperature  or equivalently peak wavelength inversely proportional  as temperature goes up, the peak goes to the right  Total energy emitted is proportional to fourth power of a temperature  picture in slide  the low temperature is best viewed in radio waves (R)  the high temperature one peaks in the UV are (U)  The Doppler Effect  if one is moving toward a source of radiation, the wavelength seem shorter (higher frequency)  if moving away, they seem longer (lower frequency)  same effect that makes the sound of a passing truck change as it passes in front of you  depends only on the relative motion of source and observer  it doesn’t matter who is moving  the faster the relative motion the more dramatic the effect  Relationship between frequency and speed:  New freq. = old freq. * (wave + object speed)/Wave speed  OR  change in frequency = object’s velocity/wave velocity  EX  for a speed of 30 km/s (earth’s speed around the sun) the Doppler shift if we use a laser gun  30 km/ 300,000 kms= 0.0001 or .01%  Measuring velocities with the Doppler effect  a radar gun that police use Spectroscopy-Ch. 4  Light and spectroscopy  Light- a wave that sometimes behaves like a particle when it interacts at atomic levels.  leads to the development/ revolution of quantum mechanics  Spectroscopy- the collection of observational and theoretical techniques that enables us to determine the nature of distant atoms by the way they emit and absorb radiation  By looking at the light they absorb and emit we can identify the atoms in the composition of stars, nebula, etc.  Spectral lines  spectroscope: instrument used in astronomy  splits light into component colors and analyzes its composition  Emission lines  a hot bulb emits a full continuous spectrum (white light)  a lamp that contains a hot gas like hydrogen or mercury (street lamp) emits specific intense colors specific frequencies called emission lines  emission lines- single frequencies emitted by particular atoms  characteristic for each hot gas  emission spectrum can be used to identify elements  Absorption spectrum  A hot gas emits characteristic light. a cold gas that is between a source of white light and the observer does the opposite, it absorbs the same parts of the spectrum  if a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit  can be used to identify elements  a hot sodium gas will emit the 2 characteristic yellow lines whereas if white light passes through a cold sodium gas it will absorb exactly the same 2 lines  Fraunhofer lines- collection of the suns absorption lines in a visible spectrum  sun’s visible spectrum-most of the spectrum is a white light emission spectrum, also has a lot absorption lines  produced by the sun’s own atmosphere  or by earth’s atmosphere  Important detail  A cold gas absorbs light in its characteristic lines but what happens to this light/energy?  it is re-emitted right back  Why do we see a dark absorption line instead of the full spectrum again?  the re-emitted light is emitted in all directions at random, most of it (>99%) will go in directions other than the one where the observer is  so relative to the rest of the spectrum, this part will be very, very faint, it will appear as a dark line  Kirchhoff’s laws- 3 spectroscopic rules  luminous solid, liquid or dense gas produces continuous spectrum  low density hot gas produces emission spectrum  continuous spectrum incident on cool, thin gas produces absorption spectrum  using these laws, astronomers begun to first analyze the sun’s spectrum and identify the elements in it  they found a mysterious element with unknown spectral lines  called helium (Helios-sun)  later found on earth as well  Atoms and radiation  existence of spectral lines required new model of atom so that only certain amounts of energy could be emitted or absorbed  was one of the effects that led to the development of quantum physics  Bohr model had certain allowed orbits for electron  the electron can orbit at a certain radius around the proton in a hydrogen atom (the ground state)  part b: orbit is well defined, if the electron wants to make a jump from the ground state to this state, the jump is discontinuous, cannot be in both places  Emission energies  correspond to energy differences between allowed levels since levels are discreet, energies are quantized not continuous  modern model has electron cloud rather than orbit  EXAMPLE  hydrogen atom  energy levels of the hydrogen atom showing 2 series of emission lines  1 series is for transitions to ground state ( Lyman series in the UV light)  the other is for transitions to excited state (balmer series in the visible part)  hydrogen alpha line- the energy emitted between the ground state to the first excited state  The photoelectric effect  when light shines on metal, electrons can be emitted  frequency must be higher than minimum characteristic of material  increased frequency- more energetic electrons  increased intensity-more electrons, same energy  can only be understood if light behaves like particles- a quantum effect  Einstein explained it and got the Nobel prize for it  Formation of spectral lines  absorption can boost an electron to the second (or higher) excited state  2 ways to decay  direct decay-directly to ground state-a single photon that is highly energetic  cascade-one orbital at a time-more than 1 photon emitted, 1 for each cascade, lower energy  absorption spectrum: created when atoms absorb photons of right energy for excitation  multi-electron atoms: much more complicated spectra, many more possible states  helium atom- nucleus that has 4 particles, 2 protons and 2 neutrons and 2 electrons orbiting around the nucleus  as the complexity of the atom increases, so does the complexity of its emission and absorption spectra  Ionization  we have when the photon is very energetic so that the electron can completely escape the atom  the degree of ionization ( how many electrons are stripped off) depends on the temperature and other conditions  Will change its energy levels. the new energy levels are different than in the neutral atom  new set of lines can appear  this set is different for different degrees of ionization  Spectral lines  occur in all parts of the electromagnetic spectra  visual and UV in light elements like hydrogen transition  infrared and radio from very high level transitions to levels near by  x-rays can be produced from heavy element transitions to/from ground states  Molecules  groups of atoms bound together with chemical bonds  can vibrate and rotate, besides having energy levels like atoms  electron transitions (a)-produce visible and ultraviolet lines  vibrational transitions (b)- produce infrared lines  rotational transitions (c)-produce radio-wave lines  Molecular spectra  Much more complex than atomic spectra, even for hydrogen but... still they can be used to identify elements  molecular hydrogen-more lines  atomic hydrogen-less lines  Spectral-line analysis  information that can be obtained about a star, nebula, etc. from its spectral lines  chemical composition-(looking at characteristic absorption/emission lines) and relative abundance (relative strength)  Temperature (what lines are present and their strength)  radial velocity (Doppler shift of spectral lines tells us about star approaching or receding)  Doppler shift example  the star’s spectrum is red shifted or blue shifted relative to un-shifted spectrum  the amount of shifting is proportional to the radial velocity and it is twice for the approach than the recession  by looking at the characteristic shift we can determine the radial velocity  Line broadening  line profile: by tracing the changing brightness across a typical emission line and expanding the scale, we obtain a graph of the line’s intensity versus its frequency  line broadening- a broader line, can be due to a variety of causes  thermal broadening: atoms move randomly produce broadened spectral lines as their individual redshifted and blue shifted emission lines merge in our detector  the hotter the gas, the greater the degree of thermal broadening  the Doppler shift may cause thermal broadening of spectral lines  rotation of spectral lines through the Doppler effect  as a star rotates one side moves towards us while the other moves away  the more rapid the rotation, the greater the broadening  The starlight’s message  Spectral information derived from starlight Observed spectral characteristic Information provided Peak frequency or wavelength Temperature (Wien’s law) (continuous spectra only) Lines present Composition, temperature Line intensities Composition, temperature Line width Temperature, turbulence, rotation speed, density, magnetic field Doppler shift Line-of-sight velocity  Summary of chapter 4  spectroscope splits light beam into component frequencies for spectral studies  continuous spectrum is emitted by solid, liquid and dense gas  hot gas has characteristic emission spectrum  continuous spectrum incident on cool thin gas gives characteristic absorption spectrum  spectra can be explained using atomic models, with electrons occupying specific orbitals  emission and absorption lines result from transitions between orbitals  molecules can also emit and absorb radiation when making transitions between vibrational or rotational states Telescopes- Ch. 5  Intro  instruments have developed dramatically in the past 50 years or so  only optical telescopes were available up to that point designed to best operate in the visible part of the spectrum  Optical telescope  assists the human eye to collect light  2 kinds  refracting- light passes through a transparent medium like a lens tube like, relatively narrow and long  reflecting-light reflects on a polished surface like a mirror  thicker, tube-like and more compact  Refracting  progressive bending of light rays towards a common focal point, the focus  Reflecting  the light does not go through from one medium to another, just reflects or changes direction when hits a polished surface  Types of telescopes  used to gather and focus electromagnetic radiation (visible light)  can be seen by the human eye, or recorded in photographs  modern telescopes are all reflectors  chromatic aberration-light travels through lens is refracted differently depending on wavelength (reflectors don’t have this effect)  some light traveling through lens is absorbed  mirror reflectivity can easily be better than 99%  large lenses can be very heavy, and can only be supported at edge  a lens needs 2 optically acceptable surfaces; mirror only needs 1  4 types of reflecting telescopes  prime focus – 1 mirror, put eye or camera in the prime focus  used for astrophotography  Newtonian focus-the focus is deflected on the side where the eyepiece is  so light isn’t blocked  very common amateur telescope, usually longer than other and easy to use  cassegrain focus-a small mirror is placed, diverts back and through the primary mirror to the back of the telescope, makes focal length much longer  used by serious amateurs and professionals  Compact and easy to attach on moving mechanisms. high magnification in a compact design  ex. Hubble space telescope (HST)  the largest, most complex and sensitive instrument ever deployed in space  most expensive $9 billion total  released in 1990  main mirror is 2.4 m in diameter  designed for visible, infrared and ultraviolet radiation  revolves around earth at 600 km in 95 m  nasmyth/coude focus- combines both properties of the cassegrain and Newtonian focus  a secondary lens that reflects back towards primary mirror, another small mirror that sends it out to the side  used for heavy instruments  Telescope size  light gathering power- how much light is collected  light-gathering power: improves detail  brightness proportional to square of radius of mirror  resolving power: can distinguish objects that are closer together  fuzzy objects become much clearer  resolution: gets better with telescope size and smaller wavelengths  gets worse as wavelength gets bigger  better in blue light, worse in infrared  the limiting factor is the diffraction of light, the scattering of light around the edges of the optics  when optics are perfect when we say that resolution is diffraction-limited  ex. Andromeda galaxy  a. 10’ (arc seconds)  b 1’  c. 5”  d.1”  the smaller the number, the better, means smaller angular distances can be resolved  Images and detectors  image acquisition: charge- coupled devices (ccd’s) are electronic devices which can be quickly read out and reset (same chip is in a digital camera)  record the image in tiny pieces called pixels. the more pixels you have the better (up to diffraction limit)  more sensitive than film and better to control and integrate  High resolution astronomy  atmospheric blurring- due to air movements/turbulence  star “twinkling”-the apparent movement of the image of the star around as time goes by  solutions  put telescopes on mountaintops, especially in deserts (chile, Hawaii)  put telescopes in space (Hubble)  active optics- control mirrors based on temperature and orientation  adaptive optics: track atmospheric changes with laser; adjust mirrors in real time  Radio Astronomy  radio telescopes (RT): to see radio-wave frequencies  radio waves also penetrate earth’s atmosphere like visible light, so they can be ground based  similar to optical telescopes in operation  less sensitive to imperfections (due to longer wavelength) can be made very large  must be tuned to see different radio frequencies (like a radio)  must be large since star radio energy is little  the earth receives just a trillionth of a watt (compare with 100 million watts in visible and infrared light from night-sky stars)  largest radio telescope is the 300-m dish in Arecibo Puerto rico (not steerable)  Advantages of radio astronomy  longer wavelength means poor angular resolution  can observe 24 hours a day  clouds rain, snow, light, dark don’t interfere  observations at an entirely different frequency; different info  visible and radio emitters are not the same  nebulae are transparent to radio signals not light  things like the galactic center can only be seen in radio waves  Interferometry  combines information from several widely spread radio telescopes as if it came from a single dish  resolution will be that of dish whose diameter = largest separation between dishes  involves combining signals from two receivers; the amount of interference depends on the direction of the signal  info can be used to increase the angular resolution of the radio telescopes  we can get radio images whose resolution I s close to optical  interferometry can also be done with visible light but is much more difficult due to shorter wavelengths  Space based astronomy- infrared  most infrared radiation is absorbed by atmosphere but it is important to astronomy  ex. interstellar space star formation, planet-star system research  infrared radiation- can produce an image where visible radiation is blocked; generally can use optical telescope mirrors and lenses  infrared telescopes can also be in space  the infrared image is viewed in false colors where white is hottest then orange and then black  spritzer space telescope- an infrared telescope that orbits around the sun trailing behind earth to avoid thermal radiation  Space absorbed astronomy-UV  UV light must be done in space, as the atmosphere absorbs almost all ultraviolet rays  X-Rays  will not reflect off mirrors as other wavelengths do  will reflect at a very shallow angle and can therefore be focused  ex. supernova remnant  Cassiopeia A  10,000 light years above earth  white= brightest, red= faintest  visible in optical part of the spectrum  Gamma rays  cannot be focused at all  images are therefore coarse  Full- spectrum coverage  much can be learned from observing the same astronomical object at many wavelengths  milky way ex  a- radio wave  b-infrared  c-visible  d- x-ray  e- gamma ray  Summary  refracting telescopes make images with a lens  reflecting telescopes make images with a mirror  modern research telescopes are all reflectors  CCDs are used for data collection  data can be formed into image, analyzed, spectroscopically or used to measure intensity  large telescopes gather much more light, allowing study of very faint sources  large telescopes also have better resolution  resolution of ground-based optical telescopes is limited by atmospheric effects  resolution of radio or space- based telescopes is limited by diffraction  active and adaptive optics can minimize atmospheric effects  radio telescopes need large collection area; diffraction limited  interferometry can greatly improve resolution  infrared and ultraviolet telescopes are similar to optical  ultraviolet telescopes must be above atmosphere  x rays can be focused but very differently than visible light  gamma rays can be detected but not imaged The solar system- Ch. 6  An inventory of the solar system  solar system- the region of space the falls within the gravitational influence of the sun, a 5 billion year old star  early astronomers knew  moon  planets  comets  meteors  not until Galileo’s/telescope time that:  heliocentric system established  detailed observations of planets, moons, etc.  Uranus, Neptune, many moons, first asteroids  20 century knowledge of solar system expanded  started with detailed space exploration via big telescopes, later with probes  Solar system is now known to have:  166 moons  1 star  eight planets  eight asteroids  more than 100 Kuiper belt ( beyond Neptune) objects more than 300 km in diameter (smaller asteroids, comets and meteoroids)  Pluto is no longer a planet  Comparative planetology  compare properties of different worlds, systems, in order to understand origin and evolution of our solar system  tries to explain vast differences of planet’s fates and features  more than 400 extrasolar planets have been found  many more are added yearly  understanding planetary formation in our own solar system helps understand its formation as well as formation of other systems  Measuring the planets  tools used to get properties  distance form sun- known by Kepler’s laws (and scale fixed by some means, e.g. radar ranging on Venus)  orbital period- can be observed (taking into account earth’s own motion)  radius- known from angular size (geometry)  masses- from newton’s laws, esp. easy if the planet has moons  rotation period- from observations, look at features on surface and how long it takes for them to re-appear in the same position  Density (=mass/volume) - knowing radius and mass. air is less dense than water. water less dense than iron  Summarstof properties  1 column- distance from sun normalize to earth sun distance  as we go farther, the distance between planets progressively increases  2 ndcolumn- orbital period, how many years it takes for the object to make a frdl revolution around the sun  3 column- mass, Jupiter the largest planet  number of known satellites- Jupiter, Saturn, Uranus and Neptune have many more satellites than the inner planets  rotation period- how many days it takes for a planet to make a full revolution around itself, the planet’s day  Venus and Uranus have negative numbers, means they rotate in the opposite direction than other planets  densities- inner planets are more dense than outer planets  Overall layout of solar system  size is about 1 million times the earth’s radius- about 50 AU or 1/1000 of a light year  nearest star is several light years away  4 inner planets, mercury, Venus, earth , mars  asteroid belt  Jupiter, Saturn, Uranus, Neptune  Pluto is just an object in the Kuiper belt  all orbits are close to the same plane- the planets move on a plane  only exception is mercury  all planets rotate in the same direction- counter clockwise, if looked at above earth’s north pole  most of them rotate counter clockwise around themselves  photograph:  it is possible for planets to appear in a straight line because the planet’s orbits are close to being in a plane  sun and new moon are also on that line (the ecliptic) but below the horizon  Terrestrial and Jovian planets  the 8 planets are grouped into 2 groups  1 group Terrestrial- small sized planets: mercury, Venus, earth, mars  small and rocky (solid)  close to the sun  rotate slowly  weak magnetic fields  few moons  no rings nd  2 group Jovian- larger planets: Jupiter, Saturn, Uranus, Neptune  large  gaseous surfaces (hydrogen and helium)  far from sun  rotate quickly  strong magnetic fields  many moons and rings  differences  all have atmospheres but surface conditions vary  only earth has oxygen and liquid water on its surface  earth and mars spin at about the same rate; mercury is slower, Venus is slow and retrograde  only earth and mars have moons and magnetic fields  Interplanetary matter  cosmic debris of various sizes orbiting the sun  4 kinds: asteroids, comets, meteoroids and interplanetary dust  asteroids and meteoroids- rocky composition, asteroids are bigger, anything more than 100 meters, hardly evolved since birth of our solar system  comets- icy with some rocky parts  size 1-10 km range  vaporize and break up as they approach the sun, creating their tail  Kuiper belt- an outer asteroid belt  lies beyond Neptune’s orbit  Pluto is a dwarf planet now  spacecraft exploration of the solar system  started in the 1960’s  many missions have probed all the planets of our system  Gravitational Slingshot  can change direction of spacecraft, an also accelerate it  typically used with probes exploring outer solar system  Mercury: so far only 2 missions, one in progress  flyby mariner-10- showed heavy cratered like our moon  Messenger- changed leading theories of planet’s structure and history  Venus: more than 20 missions since 1970  soviet venera program explored atmosphere and surface  US and European programs later mapped planet’s surface  Magellan orbiter- most recent Venus expedition from the US 1990-94  Mars: US mariner orbiter program provided rich, unexpected data  most explored planet, people believe it may harbor life  2 us Viking 1-2 missions landed on planet and conducted geological and biological experiments  Pathfinder: robot sojourner explored surface doing chemical analysis of soil and rocks st  Pioneer and voyager- 1 time flew through outer solar system  they are now on their way to escape solar system  Galileo and Cassini  2 most recent ones to outer planets  Formation of solar system  nebular contraction- cloud of interstellar gas and dust contracts due to its own gravitational field or due to a shockwave coming from a nearby exploding star  conservation of angular momentum means it spins faster and faster as it contracts becoming a flat disk  planets form via the same mechanism in smaller scale  Angular momentum- product of radius and rotation rate must be constant, as a body contracts it speeds up  Nebular contraction theory is supplemented by condensation around dust grains, known to exist in interstellar clouds  dust helps cool the cloud and introduces temperature differences between inner and outer system  helps through the accretion to matter to make larger clumps- protoplanets form  gravitational attraction takes over and planets form  Summary  solar system consists of sun and everything orbiting it  asteroids are rocky, most orbit between orbits of mars and Jupiter  comets are icy and are believed to have formed early in the solar system’s life  major planets orbit sun in same sense, and all but Venus rotate in that sense as well  planetary orbits lie almost in the same plane  four inner planets- terrestrial planets- rocky, small and dense  4 outer planets- Jovian- gaseous and large  nebular theory of solar system formation- cloud of gas and dust gradually collapsed under its own gravity, spinning faster as it shrank  condensation theory says dust grains acted as condensation nuclei, beginning formation of larger objects Earth  Overall structure  2 part core  solid inner core- 1300 km in radius  liquid outer core- 3500 km in size  Mantle- topped by a thin crust  Hydrosphere (oceans)  atmosphere  magnetosphere  Earth’s Atmosphere  made up 80% of nitrogen and 20% of oxygen  small amounts of noble gasses : carbon dioxide and vapor  starts with troposphere (contains life)  stratosphere: ozone layer  mesosphere  ionosphere  exosphere  most of the atmosphere except troposphere is at the freezing or below freezing temperatures  troposphere contains weather and all phenomenon important to life forms  Convection  hot lighter air rising resulting I circulation currents- as air gets warm near the surface tends to get lighter and therefore rise, making space for cool air to come done  depends on warming of the ground by the sun  circulating air creates winds and weather in general  Ionosphere  (ion) contains ionized atoms-electrons removed from atoms- by high energy part of solar radiation bc it is the first thing the high energy part of the solar radiation encounters  good


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