Astronomy 113. Dr. Joseph E. Pesce, Ph.D. Dr. Joseph E. Pesce, Ph.D.

Transcription

1 Astronomy 113 Dr. Joseph E. Pesce, Ph.D.

2

3 Introduction

4 1-4 Introduction Astronomy & Astrophysics ASTRON = Star NOMOS = Law PHYSIC = Nature

5 1-5 ³Astronomy: observable properties of objects in the sky (brightness, motion, spectra ³Astrophysics: intrinsic properties of objects (mass, density, temperature, size)

6 1-6 Our understanding is based on laws of physics: Electro-magnetic Gravity Quantum Mechanics

7 The Scientific Method: hypothesis, design observations to falsify hypothesis, improve observations. No proof theory is correct, just accumulation of supporting evidence 2. No definitive answers 3. Sky/universe is ever-changing - a wonderful and violent place 4. Celestial objects evolve: stars are born and die, universe expands 5. Astronomy is a time machine 6. An indirect science

8 1-8 Goals ³Explain Scientific Method ³Discuss Importance of using physical laws & lab measurements in Astronomy to investigate remote objects ³Understand scientific notation ³Define major units used by Astronomers to express distance

9 1-9 Scientific: ³Must assume laws of physics are valid everywhere (space & time) ³Astronomy is a branch of Physics ³Modern Astronomers try to determine physical nature of celestial objects & relationship among the various objects

10 1-10 Philosophical: ³Replacement of geocentric cosmology with heliocentric one difference between modern philosophy, religion, art, and music and medieval counterparts.

11 1-11 Publicity: ³Modern technology arises from understanding laws of nature (Basic Science); less rapid development if all scientists were involved in Applied Science.

12 1-12 ³Astronomy is observational rather than experimental: All direct information about physical conditions of celestial objects must come from an understanding of the nature of atoms & their constituents (i.e., the smallest entities in the universe - how ironic!)

13 1-13 Scientific Method Our ideas must agree with what we observe So Devise a theory (a collection of ideas which appear to explain an observation): Theory must be consistent with observation Theory must make predictions which can be tested Experimental verification Observe, theorize, test Theory is scientific only if it can be potentially disproved We will see later the example of Geo/Heliocentric views.

14 Hypothesis Testing 1. Create Hypothesis 0. Initial Observation 3. Observation 2. Prediction Cycle Definition 0. Initial Observation 1. Create Hypothesis 2. Prediction 3. Observation - Info gathering 4. Hypothesis testing (Intent is to disprove hypothesis)

15 1-15 Scientific Notation 1 million billion = 1,000,000,000,000,000,000,000,000 So Cumbersome!! Scientific Notation: 10 followed by an exponent or superscript = # of zeroes/digits after 1 Powers of Ten

16 1-16 Positive exponents 10 0 = = 10 (10 x 1) 10 2 = 100 (10 x 10) 10 3 = 1,000 (10 x 10 x 10) 10 4 = 10,000 (10 x 10 x 10 x 10) ten to the fourth Distance between Sun and Earth = 150,000,000 km 1.5 x 10 8 km

17 1-17 Negative exponents 10 0 = = 0.1 (1/10) 10-2 = 0.01 (1/100) 10-3 = (1/1000) x 10 6 = 5,678, x 10-9 = Thousand, million, billion, trillion

18 1-18 Math: To multiply: add exponents (5x10 5 )x(2x10 20 ) = 10x10 25 or 1x10 26 To divide: subtract exponents 6x10 23 /2x10 7 = 3x10 16

19 Scales

20 2-1 Distances Numbers are vast ² Quickly make human scales (inches, meters, etc) unruly - or numbers unimaginably large In the Solar System we use the Astronomical Unit (AU) ² Average distance Earth - Sun = 1.5x10 8 km or 93 million miles) ² Sun to Jupiter is 5.2 AU But even AUs are awkward

21 2-2 Light Years light year = distance light travels in 1 year (going 186,000 miles/s or 300,000 km/s) 1 Light year (ly) = 9.46 x km = 6 x miles or about 63,000 AU

22 Parsec Parsec (pc) = the distance at which 1 AU makes an angle of 1/3600 o (= 1 arcsecond) [PARallax SECond] Earth arcsecond 1 AU 1 parsec Sun 1 pc = 3.09 x km = 3.26 ly Proxima Centauri is at 1.3 pc 1 kpc = 10 3 pc = kilo pc Sun to center of Milkyway = 8.6kpc 1 Mpc = 10 6 pc = Mega pc Distance to Virgo Cluster = 20 Mpc

23 2-4 Earth km Solar System km Stars (nearby) km Galaxy km Local Group km Nearby Clusters km Perceivable Universe km

24 2-5 Time ³Remember, light (information) travels at a fast but finite speed (186,000 miles/sec). ³It takes time for light to travel between objects (light year = distance light travels in one year = 6 trillion miles). ³So, all astronomical objects are observed in the PAST. ²Current value for age of universe is 13.74B yrs

25 2-6 Moon : 1.5 seconds ago Sun : 8.5 minutes ago Pluto : 4-5 hours ago Nearest Star : 4 years ago Center of Galaxy : 25,000 years ago Andromeda Galaxy : 2.6 million years ago Most distant Galaxies : 8-10 billion years ago Quasars : billion years ago Astronomical Time Machine

26 2-7 Time & Large Numbers ³What is a Billion (other than a big number)? In a typical human lifetime of 80 yrs, there are: 3 Billion seconds (If you start counting 1 number every second as soon as you are born, you will only get to 3 billion after 80 years) ³The universe has been around 400 million billion seconds!!!

27 2-8 Size/Distance Example Object Sun Earth Moon Radius 7 x cm 6 x 10 8 cm 2 x 10 8 cm If Sun were 1 meter diameter: Earth s diameter = 1 cm Moon s diameter = 0.3 cm ²Jupiter s = 10 cm (~4 inches) At this scale, 1 AU (1.5 x cm) Becomes 214 meters Proxima Centauri, 4.2 ly (4 x cm) Becomes 35,700 miles!

28 Early Views

29 5-2 Goals ³Compare/Contrast Ptolemaic & Copernican Cosmologies ³State Kepler s 3 laws of planetary motion ³State Why Galileo s telescopic observations are important ³State & Give examples of Newton s 3 laws ³State Newton s law of Universal Gravity

30 5-3 Early Views ³ Early cultures had advanced ideas about astronomy ³ Greeks wanted to understand nature ² Aristarchus (300BC) - proposed Sun-centered (heliocentric) model q q q Shows Sun is distant Measures Moon s diameter relative to Earth s Finds distance to Moon/Sun ² Eratosthenes (3rd century BC) q Measures diameter of Earth (shadow) ² Hipparchus q q q Observed stars Compiled catalogues Deduced precession & its period! ² Change to Geocentric model and circular orbits ( perfection ) ² Ptolemy (150AD) - produces model correctly explaining observations ( Ptolemaic System ) that endures another 1500 years

31 5-4 Cosmologies ³ Greeks observed motions of planets with respect to background stars E W Direct - Eastward motion Retrograde - Eastward, stop, westward, stop, eastward Explanation of this was a challenge for Geocentric view

32 5-5 Cosmologies ³ Models became very complex - relying on epicycles E planet Aristarchus proposed the Heliocentric model to explain

33 5-6 Cosmologies

34 5-7 Copernicus ³ In the 1500 s, Nicolas Copernicus worked out details of a heliocentric cosmogony ² Determined which planets are closer to Sun, etc ² Determined sidereal period of planets (true orbital period) & synodic period (time between two successive configurations as viewed from Earth ² Determined relative distances of planets from Sun ² But, incorrectly assumed circular orbits ³ Heliocentric model is not more accurate than Geocentric model, but simpler (Occam s razor)

35 5-8 A Changing Universe ³ In 1572: Exploding star (supernova) appears ² Causes the questioning of the old view that the heavens are unchanging ³ Tycho Brahe tried to measure distance by parallax, but failed - it was too far away ³ Tycho also measured planetary positions very accurately ³ By about 1600, inaccuracies in predicted planetary positions led Johannes Kepler to abandon circular orbits for elliptical ones Minor axis Major axis Focus Focus Semi-major axis = a Eccentricity e = Elliptical orbits led to accurate orbit predictions

36 5-9 Kepler s Laws (1609) 1. The orbit of a planet around the Sun is an ellipse with the Sun at one Focus. Planets are seen to move more rapidly when they are near the Sun (Perihelion) & more slowly when farthest from the Sun (Aphelion). 2. (Law of Equal Areas) A line joining a planet & the Sun sweeps out equal areas in equal time intervals. 3. (1619) The square of the planet s sidereal period is proportional to the cube of the length of the orbit s semi-major axis: P = period; a = semi-major axis length P 2 = a 3 That is, the closer a planet to the Sun, the more rapidly it orbits & the shorter its year

37 5-10 Kepler s Laws (1609) D A Sun C B Focus Focus

38 5-11 Galileo (1610) Makes observations with the first telescope: 1. Phases of Venus - solar system must be heliocentric 2. Four moons of Jupiter (following Kepler s laws) 3. Sunspots All support a changing universe & heliocentrism, but physics was a problem

39 5-12 Galileo (1610): Venus Phases

40 5-13 Newton Finally, toward end of 17th century, Isaac Newton introduces physics & calculus into issue and produces 3 laws: 1. Law of Inertia: a body stays at rest or moves in a straight line at constant speed unless acted on by an unbalanced outside force. Force acting on planets Velocity: speed and direction of motion Acceleration (a): rate at which velocity changes with time (car slows down, speeds up, or changes direction)

41 5-14 Newton 2. Force = mass * acceleration Mass = total amount of material (invariant) Weight is a force with which an object presses on the ground due to gravity (different at different places) 3. Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first body. Angular Momentum: a measure of how much energy is stored in an object due to its rotation or revolution. Depends on: ³ How fast a body rotates ³ Its mass ³ How spread out the mass is The higher the angular motion, mass, or how spread out, the higher the angular momentum. Angular momentum is conserved: ice skater

42 5-15 Gravity Newton did not invent gravity, just described it. From the 1st law, force acting on the planet is toward the Sun. Combing the three laws and Kepler s three laws leads to: Universal Law of Gravity: Two bodies attract each other with a force that is proportional to their masses & inversely proportional to the square of the distance between them ³ Gravitational force decreases with distance like 1/d 2 (used to explain, calculate orbits, etc.) F = G(m 1 m 2 / d 2 )

43 5-16 Proof of Heliocentric Model ³ Aberration of light (Earth revolves around Sun) ³ Parallax (predicted by Greeks, searched for by Brahe, but too small to detect) ² Very small: nearest star only 0.8!) ³ Foucault Pendulum ³ Coriolis Effects Rotation of Earth

44 Nature of Light

45 6-2 Goals ³ List major regions of the spectrum in wavelength order & give examples. ³ List major regions of the spectrum in wavelength order & give examples. ³ Name two classes of telescopes & describe how they work.

46 6-3 Nature of Light ³ White Light is actually a mixture of all colors (Newton - Prism) ² This is not a property of the prism, since the process can be reversed ³ Speed of light is finite, but fast ² In vacuum, c = 300,000 km/s = 186,000 miles/s (ultimate speed limit) ² Light in water, air, glass, etc. travels slower than in vacuum, and other objects can travel faster than light - Cherenkov radiation

47 6-4 History History Isaac Newton s - light is composed of particles too small to detect. Christiaan Huygens light is like a wave Thomas Young experiments showing wavelike properties

48 6-5 Waves What is waving? Electric and Magnetic fields James Clerk Maxwell describes all basic properties of E&M in four easy equations, finding: E & M Forces are two aspects of the same phenomena E & M fields travel through space at the speed of light EM Radiation is thus combined, oscillating E & M fields

49 6-6 Wavelength ³Different colors because wavelength of light is different ² l = angstrom (Å, m, or nanometers, 10-9 m) ²Visible light is Å ( nm)

50 6-7 Particle-Wave Duality ³Light is sometimes like a wave and sometimes like a particle ²Particle nature is seen in the photo-electric effect (Einstein, Nobel prize, 1905) qsome colors of light remove electrons from a metal, but not others. Electrons received different amounts of energy from light packets, or PHOTONS

51 6-8 The Spectrum ³ The shorter a photon s wavelength, the higher its energy: E = (h x c)/l E=energy, h=constant, c=speed of light, l=wavelength ³Visible light is only a small component of EM radiation: Radio Infrared Visible Ultraviolet X-rays g-rays Long l Low E Red Orange Yellow Green Blue Indigo Violet R O Y G. B I V Short l High E Not all transmitted by atmosphere

52 6-9 The Spectrum

53 Electromagnetic Radiation & Spectra

54 ³Know Stefan-Boltzman law and Wien s law ³State Kirchoff s 3 laws of spectral analysis ³Describe Bohr model of the atom; spectral lines ³Know how spectral analysis provides info about chemical composition of celestial objects ³Indicate how protons, neutrons, and electrons are used to define elements 7-2 Goals

55 7-3 Blackbody - I Heat an Iron Bar 1. As it heats becomes brighter because it emits more EM radiation 2. The color (l of emitted radiation) changes with temperature Cool Hot IR, red UV, blue First noted by Thomas Wedgewood in 1792 Blackbody is an object which absorbs all EM radiation which strikes it and is heated. Energy is reemitted. Amount at each wavelength depends on temperature Temperature Energy

56 7-4 Blackbody - II Blackbody curves: temperature profiles of intensity of blackbody at different wavelengths Stefan-Boltzman Law (Intensity-temperature relationship for blackbodies): An object emit energy at a rate proportional to the 4th power of its temperature (in Kelvin, absolute scale) E = s T 4

57 7-5 Wien s Law Relationship between color peak & temperature found by Wien in 1893 Wien s Law: λmax = 0.29(cm) T(K) The hotter an object, the shorter l max Very useful for determining temperatures of star s surface - since brightness & size don t need to be known Peak of Sun about 5800Å (5000K), so why not bluegreen? (scattering)

58 7-6 Spectra - I Fraunhofer: solar spectrum has dark lines (spectral lines) Kirchoff-Bunsen: spectra of each element has characteristic pattern of spectral lines Element: a fundamental substance which can t be broken into more basic chemicals Spectral analysis led to discovery of new elements (e.g., cesium & rubidium) 1868, solar eclipse, saw helium on Sun 27 years before detected on Earth

59 7-7 Spectra - II Each element has characteristic spectrum so by observing a spectrum of an astronomical object, we can determine types of elements present We use instruments - spectrometers and spectrographs - to observe spectra (like a prism) Kirchoff noted dark lines (absorption) and bright lines (emission) in spectra from different conditions of source

60 7-8 Kirchoff s Laws 1. A hot object, or hot dense gas produces a continuous spectrum (no lines, a blackbody spectrum 2. A hot rarified (low density) gas produces emission lines (bright features) 3. A cool gas in front of a continuous source of light produces absorption (dark) lines [absorption if background is hotter than foreground gas Emission if background is cooler]

61 7-9 Kirchoff s Laws

62 7-10 Spectra

63 7-11 Solar Spectrum

64 7-12 Why Do Spectra Occur? Rutherford (1910): Atoms consist of positively charged, massive nucleus, orbited by tiny, negatively charged electrons Nucleus: protons (+) and neutrons (x) Attract electrons (-) # of protons determines element: H = 1p He = 2p U = 92p # of neutrons can vary: O has 8p but can have 8, 9, or 10 neutrons leading to slightly different types of O (isotopes) Atoms usually have same # of p and e - Ion if different # of p & e - Ionization: process which removes e -, creating ion (knock away e - with high energy photon = photoionization) Molecules: atoms bound together which share e -

65 7-13 The Bohr Model H has 1 e - and 1 p: spectrum has pattern of lines from 656nm to 364nm, called the Balmer series (after the person who discovered formula for calculating (1885). Niels Bohr understood mathematically/physically e - can have specific orbits (n=1,2,3,4.). To move from 1 level to another, an e - must lose or gain a specific amount of energy. Outer - inner (4-1): e - must lose energy Inner - outer (1-3): e - must gain energy proton n=

66 7-14 Energy-Level Diagram

67 7-15 Doppler Shift Spectral lines shifted due to motion Doppler shift for sound and light (because light is a wave) Motion towards source (or source towards you) compresses wavelength shorter wavelength = bluer light (blueshift) Motion away from source (or source away from you) stretches wavelength longer wavelength = redder light (redshift) ( λ obs λ λrest rest ) = Δλ λrest = vr c

68 7-16 Doppler Shift

69 Thank You!

70 The Night Sky

71 3-2 Goals ³Describe Nature and value of constellations ³Define elements of equatorial coordinate system ³Define two solstices & 2 equinoxes ³Describe how the orientation of the ecliptic on the celestial sphere produces seasons

72 3-3 The Night Sky ³Constellations - apparent 2-D groupings of stars (Big Dipper, Leo, Orion) ²Formed mostly by Greeks (100 BC AD) ²Some older (Babylonian) ²Can tell seasons by which are visible ³ Celestial Sphere - the hollow shell on which stars are attached ²Constellations divide sphere into 88 regions

73 3-4 The Night Sky ³ Stars are seen in projection ² Stars in constellations are note related - are at varying distances (3-D) ² Stars move, but are so distant it is difficult to detect

74 3-5 The Night Sky ³ Project Earth s geographical features onto sphere to establish directions ²Like latitude and longitude on Earth, need 2 coordinates to locate and object: North Celestial Pole Declination: N & S of Celestial Equator Right Ascension: E & W around Celestial Equator Right Ascension North Pole Declination Equator Celestial Equator

75 3-6 The Celestial Sphere

76 3-7 Angular Measures Angle, or Angle of arc 360 o in a circle 90 o in a right triangle 1 o has 60 (arcminutes) 1 has 60 (arcseconds) Moon subtends 1/2 o (angular diameter) Need distance to tell real size

77 3-8 Earth s Rotation ³Spin on Axis - counterclockwise from N-pole ²Causes apparent motion of stars ²Causes day & night (24 hours) ³Earth also Revolves around the sun ²1 revolution = 1 year = 365 1/4 days q Causing different constellations to be seen q Individual stars rise 4 minutes earlier each night ³Noon when Sun is highest in sky (different for different places - longitude) ²Timezones (every 15 o of longitude)

78 3-9 Earth s Rotation

79 3-10 Earth s Rotation ³ Seasons - caused by tilt of Earth s axis of rotation compared to plane of orbit ² 23 1/2 o ² Summer - Sun is highest in sky leading to longer heating - NOT because Earth is closer to Sun!! ³ Ecliptic - plane of apparent motion of Sun across sky - actually Earth s orbital plane ³ Equinoxes - when Sun crosses Celestial Equator ² Equal Night - 12 hour day 12 hour night q VERNAL Equinox (Spring) - 21 March (Sun crosses Cel. Eq. going N) q AUTUMNAL Equinox (Fall) - 21 September ( going S) q Summer SOLSTICE - 21 June - Sun highest in N, longest daylight q Winter SOLSTICE - 21 December - Sun lowest in N, shortest daylight

80 3-11 Seasons

81 3-12 Seasons

82 Precession & Eclipses

83 4-2 Goals ³Describe precession, its effect on our observations of the stars, and why it occurs ³Explain by diagram how lunar phases are controlled by positions of the Sun & Moon ³Explain why & when solar & lunar eclipses occur & why they aren t every month

84 4-3 Precession ³ Gravitational attraction of Earth s bulge by Moon ³ Earth responds by wobbling - changing direction in which rotation axis points on the Celestial Sphere ³ Currently, N. Polar axis points to North star (Polaris) - the Celestial North Pole - but wobble traces out circle ³ Rate of precession is slow - 26,000 years to complete circle ³ North star varies ³ Precession also changes location of equatorial plane, so Celestial equator precesses, as do equinoxes ² Precession of the equinoxes

85 4-4 Precession

86 4-5 Phases of the Moon Caused as Moon orbits Earth - phase depends on how much of sunlit side we see: C 1st Quarter D B Waxing Gibbous E Full Waning Gibbous F midnight 6 pm 6 am noon H Waxing Crescent A New Waning Crescent Sun 29 1/2 days to go through phases - Can correlate position in sky (time of day) & position moonth G 3rd Quarter A B C D E F G H

87 4-6 Phases of the Moon

88 4-7 Eclipses ³ Lunar eclipse - Moon passes through Earth s shadow Sun Earth Moon Moon is full ³ Solar eclipse - Moon s shadow moves across Earth Sun Moon is new Moon Earth ³ Need proper alignment of Sun-Earth-Moon ³ Should happen every month, but

89 4-8 Lunar Eclipses Useful for studying Earth s & Sun s atmosphere. Remember: Moon is full, and seen anywhere Moon is visible

90 4-9 Eclipses Moon s orbit is inclined 5 o to ecliptic Moon 5 o Earth line of nodes Sun Eclipses take place only when new or full Moon occurs as moon crosses ecliptic at line of nodes (the intersection of the 2 planes) ² There are 2-5 eclipses per year (max = 7)

91 4-10 Solar Eclipses By coincidence, Sun & Moon have same angular diameter as seen from Earth Penumbra Umbra Sun Moon Earth Three types of Solar eclipse: 1. Total Solar Eclipse - observer is in umbra of Moon s shadow (moves rapidly over Earth with a short min - duration 2. Partial Solar Eclipse - observer in penumbra only 3. Annular Eclipse - umbra doesn t reach Earth - Moon appears too small to cover Sun & we see ring of Sun around edge of Moon Remember: Moon is new, and seen only on certain areas of Earth because shadow is large

92 4-11 Solar Eclipses Remember: Moon is new, and seen only on certain areas of Earth because shadow is large

93 4-12 Eclipses DEMONSTRATION: Display umbra/penumbra with overhead projector