ISES Solar World Congress 1999 Jerusalem

ISES Solar World Congress 1999 Jerusalem


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Sept. 11, 2008 Astronomy 101 Announcements: • Studentcontract due by COB tomorrow (you can leave it in my mailbox) • Firstreal OWL homework up tomorrow; due Sept. 24 before midnight • Probablybest to work the OWL tutorial before doing the first real homework • Doyou PRS?Maybe not yet New Theme for next several lectures: Observing the Sky • Thenature of the Milky Way • Thecelestial sphere, the local sky, and finding stuff up there • TheMoon • Eclipses
Discovery-driven vs. hypothesis-driven science
Discovery-driven:observations are made first, and then ideas are developed to understand the observations • Thetheoretical ideas in this situation can bepostdictions rather than predictions.
Discovery-driven vs. hypothesis-driven science Discovery-driven:observations are made first, and then ideas are developed to understand the observations Example:Edwin Hubbles discovery of the expansion of the universe. Hypothesis-driven:an idea is developed first, and then experiments/observations are made to test the idea Example:Albert Einsteins (blown!) prediction of the expansion of the universe.
Discovery-driven vs. hypothesis-driven science 0 OBSERVATIONS 1 Question 2 Hypothesis – a tentative explanation 3 Prediction 4 Test 5 Result – confirm, reject, or modify
The funkiest bit of all: the expansion of the Universe!
Discovery-driven science! y they (Luck-driven science?!)hey are mov ng. – Just like raisins in a raisin cake; they all move apart from each other as the dough (space itself) expands.
Hallmarks of Good Science • Scienceseeks explanations forobserved phenomena that rely solely onnatural causes. • Scienceprogresses through the creation and testing of models of nature that explain the observations as simply as possible. ! OccamsRazor • Ascientific modelmust make testable predictionsthat could force us to revise or abandon the model. Theory --a model which survives repeated testing
About Occams razor • Seekthe simplest possible model. • Why? • Nothingrequires nature to be simple! • However,by beginning with the simplest model, we can gaininsightand efficiently make progress. • Toomany ideas and variablesdifficult to figure out whats really happening
Scientific Models? Example: models of the atom
The Greek philosopher Democritus hypothesized that matter is made of tiny, fundamental particles that he dubbed “atoms” (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:
“Model” vs. “Theory”:a model Is just a working idea; it may or may not be correct. Atheory has withstood repeated testing and has more evidence supporting its validity. Models and theories evolve.This model atom is better, but its not fully correct either.
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s observations in 1610.
Today, there is abundant evidence supporting this picture.
Scientific Models?
Democritus. Soundfamiliar? • Democritusdemocracy • Manyancient cultures believed that the Milky Way is some sort of liquid owing through the sky • Democritushypothesized that the Milky Way is composed of many distant stars. • Aristotledisagreed
Galileo: first to observe the Milky Way with a telescope; described as “innumerable faint stars” (1609)
The Milky Way: a disk-like object Right side Left side
From this picture, we can conclude that the inner galaxyis on 1. The right side 2. The middle 3. The left side
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? Answer: because the disk is bulged in that direction.
Historical evolution of the Milky Way model • 500BC: Democritus • 1609:Galileo • 1780s:William & Caroline Herschel map the MW with a telescope
Textbook reading:Unit 70
Historical evolution of the Milky Way modelHerschels sketch of the Milky Way (1784): • 500BC: Democritus • 1609:Galileo • 1780s:William & Caroline map the MW 2500 parsecs with a telescope • 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
The currentmodelof our galaxy
Scientificmodel: a concept/representation/tool used to think about how something works.
Assessing the shape of the Milky Way by counting stars: Now suppose that we Suppose that we lived in the lived at the edge of that middle of a spherical galaxy: spherical object:
In this case, wed expect toWed observe many stars count roughly the sameto the left and very few number of stars looking inlooking to the right or any direction.straight up or down. This is the technique used by the Herschels.
Historical evolution of the Milky Way model • Simultaneously(1920s), Harlow Shapley used the globular clustersto argue that we are not in the center but rather are at the edge of the Milky Way: • Blackpatch: Herschels map • Whitedots: globular clusters • Yellowdot: the Sun 100 kpc
Historical evolution of the Milky Way model: why did Herschel/Kepteyn get it Example of an astronomical wrong? dust bunny, er, horse • Answer:interstellar dust. • Dustfills the interstellar space of the MW anddimsthe light of stars. • However,there is much more dust in the disk. • Globularclusters are in the halo and thus areless affected by dust.
PRS question.True or false: Constellations are important in modern astronomy.
We can measure true distances, but it is often quite difficult to do. More often, astronomers measureangles.
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.
Shapley used globulars and thus got the shape/location right, but he failed to correct for dust and ended up with the wrong size.
Kepteyn only used stars in a similar small region because he required “proper motion” to estimate the stellardistances, and proper motions can only be measured for relatively nearby stars.
The Problem with Constellations
No particular significance; just regions in the sky
For example, the “Big Dipper” is a familiar constellaion:
A Constellation is just a region of the sky • Mostofficial constellation names come from antiquity. Some southern th hemisphere constellations were named by European explorers in the 17 th & 18centuries. • Thepatterns of stars have no physical significance! Stars that appear close together may lie at very different distances.
Constellations were useful in prehistoric times to keep track of the year, e.g., when to plant or harvest the crops.
Modern astronomers mainly use constellations as landmarks, but their usefulness is pretty limited.