1 Astronomy 101 " " Student contract due by COB tomorrow Student contract due by COB tomorrow (you can leave it in my mailbox) (you can leave it in my mailbox) " " First real OWL homework First real OWL homework up tomorrow; due Sept. 24 before midnight up tomorrow; due Sept. 24 before midnight " " Probably best to work the OWL tutorial before doing the @rst real Probably best to work the OWL tutorial before doing the @rst real homework homework " " Do you PRS?
Maybe not yet Do you PRS? Maybe not yet & & Sept. 11, 2008 Announcements: New Theme for next several lectures: Observing the Sky " " The nature of the Milky Way The nature of the Milky Way " " The celestial sphere, the local sky, and @nding stuff up there The celestial sphere, the local sky, and @nding stuff up there " " The Moon The Moon " " Eclipses Eclipses Discovery-driven vs.
hypothesis-driven science 1 Question 2 Hypothesis 3 a tentative explanation 3 Prediction 4 Test 5 Result 3 confirm, reject, or modify 0 OBSERVATIONS Discovery-driven vs. hypothesis-driven science " Discovery-driven: observations are made first , and then ideas are developed to ... more. less.
understand the observations " The theoretical ideas in this situation can be post dictions rather than pre dictions. The funkiest bit of all: the expansion of the Universe!<br><br> " Mostly all galaxies appear to be moving away from us. " The farther away they are, the faster they are moving. 3 Just like raisins in a raisin cake; they all move apart from each other as the dough (space itself) expands.<br><br> Discovery-driven science! (Luck-driven science?!) Discovery-driven vs. hypothesis-driven science " Discovery-driven: observations are made first , and then ideas are developed to understand the observations Example: Edwin Hubble 9s discovery of the expansion of the universe.<br><br> " Hypothesis-driven: an idea is developed first , and then experiments/observations are made to test the idea Example: Albert Einstein 9s (blown!) prediction of the expansion of the universe. " Science seeks explanations for observed phenomena that rely solely on natural causes. " Science progresses through the creation and testing of models of nature that explain the observations as simply as possible.<br><br> ! Occam 9s Razor " A scientific model must make testable predictions that could force us to revise or abandon the model. Hallmarks of Good Science -- a model which survives repeated testing Theory 2 About Occam 9s razor& " Seek the simplest possible model.<br><br> " Why? " Nothing requires nature to be simple! " However, by beginning with the simplest model, we can gain insight and efficiently make progress.<br><br> " Too many ideas and variables ? difficult to figure out what 9s really happening Scientific Models? Scientific Models?<br><br> Example: models of the atom The Greek philosopher Democritus hypothesized that matter is made of tiny, fundamental particles that he dubbed catoms d (the Greek word for indivisible). But, he also thought that elements came in the forms of fire, water, earth, and air& The modern model of the atom is rather different: cModel d vs. cTheory d: a model Is just a working idea; it may or may not be correct.<br><br> A theory has withstood repeated testing and has more evidence supporting its validity. Models and theories evolve. This model atom is better, but it 9s not fully correct either.<br><br> Democritus. Sound familiar? " " Democritus Democritus ?<br><br> ? democracy democracy " " Many ancient cultures believed that the Milky Way Many ancient cultures believed that the Milky Way is some is some sort of sort of liquid Aowing through the sky liquid Aowing through the sky " " Democritus hypothesized that the Milky Way is composed of Democritus hypothesized that the Milky Way is composed of many distant stars. many distant stars.<br><br> " " Aristotle disagreed Aristotle disagreed & & Galileo: ?rst to observe the Milky Way with a telescope; described as cinnumerable faint stars d (1609) The Milky Way " Our Galaxy is shaped like a disk. " Our solar system is in that disk. " When we look at the Milky Way in the sky, we are looking along that disk, i.e., at the disk edge-on This hypothesis was put forth by Immanuel Kant (philosopher) and Thomas Wright (astronomer) in the 1700s, with some support from Galileo 9s observations in 1610.<br><br> Today, there is abundant evidence supporting this picture. The Milky Way: a disk-like object Left side Right side From this picture, we can conclude that the inner galaxy is on 1. The right side 2.<br><br> The middle 3. The left side 3 The Milky Way: a disk-like object But what are those dark patches and streaks? Answer: dust and gas in space Also, why is it bulged in one direction?<br><br> Answer: because the disk is bulged in that direction. The current model of our galaxy& Scienti?c model : a concept/representation/tool used to think about how something works. Historical evolution of the Milky Way model " 500 BC: Democritus " 1609: Galileo " 1780s: William & Caroline Herschel map the MW with a telescope Textbook reading: Unit 70 Assessing the shape of the Milky Way by counting stars: Suppose that we lived in the middle of a spherical galaxy: In this case, we 9d expect to count roughly the same number of stars looking in any direction.<br><br> Now suppose that we lived at the edge of that spherical object: We 9d observe many stars to the left and very few looking to the right or straight up or down. This is the technique used by the Herschels. Historical evolution of the Milky Way model " 500 BC: Democritus " 1609: Galileo " 1780s: William & Caroline map the MW with a telescope Herschel 9s sketch of the Milky Way (1784): 2500 parsecs " 1920s: Kapteyn used modern techniques & understanding of star brightnesses to remap the Milky Way: Note: a parsec is a unit of distance = 3.3 light years Historical evolution of the Milky Way model " Simultaneously (1920s), Harlow Shapley used the globular clusters to argue that we are not in the center but rather are at the edge of the Milky Way: " Black patch: Herschel 9s map " White dots: globular clusters " Yellow dot: the Sun 100 kpc 4 Historical evolution of the Milky Way model: why did Herschel/Kepteyn get it wrong?<br><br> " Answer: interstellar dust. " Dust fills the interstellar space of the MW and dims the light of stars. " However, there is much more dust in the disk.<br><br> " Globular clusters are in the halo and thus are less affected by dust. Example of an astronomical dust bunny, er, horse& The Herschels could only detect stars in the immediate vicinity of the Sun; beyond this region, dust made the stars to dim for their early telescopes to detect. Kepteyn only used stars in a similar small region because he required cproper motion d to estimate the stellar distances, and proper motions can only be measured for relatively nearby stars.<br><br> Shapley used globulars and thus got the shape/location right, but he failed to correct for dust and ended up with the wrong size.& PRS question. True or false: Constellations are important in modern astronomy . 1.<br><br> TRUE 2. FALSE Constellations. The Problem with Constellations No particular signi?cance; just regions in the sky For example, the cBig Dipper d is a familiar constellaion: Constellations: review from previous lecture We can measure true distances, but it is often quite dif?cult to do.<br><br> More often, astronomers measure angles . Angular separation Angular separation A Constellation " Most official constellation names come from antiquity. Some southern hemisphere constellations were named by European explorers in the 17 th & 18 th centuries.<br><br> " The patterns of stars have no physical significance! Stars that appear close together may lie at very different distances. Modern astronomers mainly use constellations as landmarks, but their usefulness is pretty limited.<br><br> is just a region of the sky Constellations were useful in prehistoric times to keep track of the year, e.g., when to plant or harvest the crops.