astro 113 week1 - Physics & Astronomy

© Dr. Joseph E. Pesce, Ph.D.

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


Introduction


IntroductionAstronomy & Astrophysics

ASTRON = StarNOMOS = Law

PHYSIC = Nature

1-4


³Astronomy: observable properties of objects in the sky (brightness, motion, spectra

³Astrophysics: intrinsic properties of objects (mass, density, temperature, size)

1-5


Our understanding is based on laws of physics:

Electro-magneticGravity

Quantum Mechanics

1-6


1. The Scientific Method: hypothesis, design observations to falsify hypothesis, improve observations. No �proof� theory is correct, just accumulation of supporting evidence

2. No definitive answers3. Sky/universe is ever-changing - a wonderful and

violent place4. Celestial objects evolve: stars are born and die,

universe expands5. Astronomy is a time machine6. An indirect science

1-7


Goals³Explain Scientific Method³Discuss Importance of using physical

laws & lab measurements in Astronomy to investigate remote objects

³Understand scientific notation³Define major units used by Astronomers

to express distance

1-8


³Must assume laws of physics are valid everywhere (space & time)

³Astronomy is a branch of Physics³Modern Astronomers try to determine

physical nature of celestial objects & relationship among the various objects

1-9

Scientific:


³Replacement of geocentric cosmology with heliocentric one ➙ difference between modern philosophy, religion, art, and music and medieval counterparts.

1-10

Philosophical:


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

1-11

Publicity:


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!)

1-12


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 disprovedWe will see later the example of Geo/Heliocentric

views.

1-13

Scientific Method


1-14

Cycle Definition0. Initial Observation1. Create Hypothesis2. Prediction3. Observation - Info

gathering4. Hypothesis testing

(Intent is to disprovehypothesis)

3.Observation

2.Prediction

1. CreateHypothesis

4.Hypothesis

Testing 0. InitialObservation


Scientific Notation1 million billion = 1,000,000,000,000,000,000,000,000

Cumbersome!!So… Scientific Notation: 10 followed by an exponent

or superscript = # of zeroes/digits after �1��Powers of Ten�

1-15


100 = 1101 = 10 (10 x 1)

102 = 100 (10 x 10)103 = 1,000 (10 x 10 x 10)

104 = 10,000 (10 x 10 x 10 x 10) �ten to the fourth�

Distance between Sun and Earth = 150,000,000 km1.5 x 108 km

1-16

Positive exponents


100 = 110-1 = 0.1 (1/10)

10-2 = 0.01 (1/100)10-3 = 0.001 (1/1000)

5.678 x 106 = 5,678,0002.3 x 10-9 = 0.0000000023

Thousand, million, billion, trillion

1-17

Negative exponents


To multiply:

add exponents ➙ (5x105)x(2x1020) = 10x1025 or 1x1026

To divide:

subtract exponents ➙ 6x1023/2x107 = 3x1016

1-18

Math:


Scales


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

8km or 93 million miles)

² Sun to Jupiter is 5.2 AU

But even AUs are awkward…

2-1


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 1012 km = 6 x 1012 miles or about 63,000 AU

2-2


ParsecParsec (pc) = the distance at which 1 AU makes an angle of

1/3600o (= 1 arcsecond) [PARallax SECond]

2-3

Earth

Sun

1 AU

1 parsec

1 arcsecond

1 pc = 3.09 x 1013 km = 3.26 ly Proxima Centauri is at 1.3 pc

1 kpc = 103 pc = kilo pc Sun to center of Milkyway = 8.6kpc

1 Mpc = 106 pc = Mega pc Distance to Virgo Cluster = 20 Mpc


Earth - 103 kmSolar System - 108-10 km

Stars (nearby) - 1013-15 kmGalaxy - 1018 km

Local Group - 1019 kmNearby Clusters - 1020 km

Perceivable Universe - 1023 km

2-4


Time³Remember, 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

2-5


Moon : 1.5 seconds agoSun : 8.5 minutes ago

Pluto : 4-5 hours agoNearest Star : 4 years ago

Center of Galaxy : 25,000 years agoAndromeda Galaxy : 2.6 million years ago

Most distant Galaxies : 8-10 billion years agoQuasars : 11-12 billion years ago

2-6


Astronomical Time Machine

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!!!

2-7


Size/Distance ExampleIf Sun were 1 meter diameter:Earth�s diameter = 1 cmMoon�s diameter = 0.3 cm

²Jupiter�s = 10 cm (~4 inches)

2-8

Object RadiusSun 7 x 1010 cmEarth 6 x 108 cmMoon 2 x 108 cm

At this scale, 1 AU (1.5 x 1013 cm)

Becomes 214 meters

Proxima Centauri, 4.2 ly (4 x 1018 cm)

Becomes 35,700 miles!© Dr. Joseph E. Pesce, Ph.D.

Early Views


5-2

Goals³Compare/Contrast Ptolemaic & Copernican

Cosmologies³State Kepler�s 3 laws of planetary motion³State Why Galileo�s telescopic observations are

important³State & Give examples of Newton�s 3 laws³State Newton�s law of Universal Gravity


5-3

Early Views³ Early cultures had advanced ideas about astronomy³ Greeks wanted to understand nature

² Aristarchus (300BC) - proposed Sun-centered (heliocentric) modelq Shows Sun is distantq Measures Moon�s diameter relative to Earth�sq Finds distance to Moon/Sun

² Eratosthenes (3rd century BC)q Measures diameter of Earth (shadow)

² Hipparchusq Observed starsq Compiled cataloguesq 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


5-4

Cosmologies³ Greeks observed motions of planets with respect to

background stars

E W

Direct - Eastward motionRetrograde - Eastward, stop,

westward, stop, eastward

Explanation of this was a challenge for Geocentric view


5-5

Cosmologies³ Models became very complex - relying on �epicycles�

E planet

Aristarchus proposed the Heliocentric model to explain


5-6

Cosmologies


5-7

Copernicus³ In the 1500’s, Nicolas Copernicus worked out details of a

heliocentric cosmogony² Determined which planets are closer to Sun, etc² Determined sidereal period of planets (true orbital period) & synodic period (time

between two successive configurations as viewed from Earth² Determined relative distances of planets from Sun² But, incorrectly assumed circular orbits

³ Heliocentric model is not more accurate than Geocentric model, but simpler(Occam’s razor)


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 = 0 0.3 0.5 0.7 0.96 1

Elliptical orbits led to accurate orbit predictions


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

P2 = a3

That is, the closer a planet to the Sun, the more rapidly it orbits & the shorter its year


5-10

Kepler’s Laws (1609)

Focus Focus

SunC

D

A

B


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


5-12

Galileo (1610): Venus Phases


5-13

NewtonFinally, 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)


5-14

Newton2. 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 rotates³ Its mass³ 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


5-15

GravityNewton 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/d2 (used to explain, calculate orbits, etc.)

F = G(m1m2 / d2)


5-16

Proof of Heliocentric Model³ 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 Pendulum³ Coriolis Effects Rotation of Earth


Nature of Light


6-2

Goals³ 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.


6-3

Nature of Light³ 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

reversed³ 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


6-4

HistoryIsaac Newton - 1660s - �light is composed of particles too small to detect.�

Christiaan Huygens - 1678 - light is like a wave

Thomas Young - 1801 - experiments showing wavelike properties

History


6-5

WavesWhat is waving?

Electric and Magnetic fieldsJames Clerk Maxwell - 1860 - 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


6-6

Wavelength³Different �colors� because wavelength of light

is different² l = angstrom (Å, 10-10 m, or nanometers, 10-9 m)²Visible light is 4000-7000Å (400-700nm)


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


6-8

The Spectrum³ The shorter a photon�s wavelength, the higher its energy:

E = (h x c)/lE=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

Red Orange Yellow Green Blue Indigo Violet

Long l Low E

Short l High E

R O Y G. B I V Not all transmitted by atmosphere


6-9

The Spectrum


Electromagnetic Radiation & Spectra


7-2

Goals³Know Stefan-Boltzman law and Wien�s law³State Kirchoff�s 3 laws of spectral analysis³Describe Bohr model of the atom; spectral

lines³Know how spectral analysis provides info

about chemical composition of celestial objects

³Indicate how protons, neutrons, and electrons are used to define elements


7-3

Blackbody - IHeat an Iron Bar

1. As it heats becomes brighter because it emits more EM radiation

2. The color (l of emitted radiation) changes with temperatureCool IR, redHot UV, blue

Blackbody is an object which absorbs all EM radiation which strikes it and is heated. Energy is re-emitted. Amount at each wavelength depends on temperature

Temperature Energy

First noted by Thomas Wedgewood in 1792


7-4

Blackbody - IIBlackbody 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 T4© Dr. Joseph E. Pesce, Ph.D.

7-5

Wien�s LawRelationship between color peak &

temperature found by Wien in 1893

Wien�s Law: T(K)0.29(cm) max =λ

The hotter an object, the shorter lmax

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 blue-green? (scattering)


7-6

Spectra - I Fraunhofer: solar spectrum has dark lines

(spectral lines)Kirchoff-Bunsen: spectra of each element has

characteristic pattern of spectral linesElement: a fundamental substance which can�t

be broken into more basic chemicalsSpectral analysis led to discovery of new elements (e.g., cesium & rubidium)1868, solar eclipse, saw helium on Sun 27 years before detected on Earth


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


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 gasEmission if background is cooler]


7-9

Kirchoff�s Laws


7-10

Spectra


7-11

Solar Spectrum


7-12

Why Do Spectra Occur? Rutherford (1910): Atoms consist of positively charged, massive

nucleus, orbited by tiny, negatively charged electronsNucleus: 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-


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.

protonn=1

23

4

Outer - inner (4-1): e- must lose energy

Inner - outer (1-3): e- must gain energy


7-14Energy-Level

Diagram


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)

crestrest

restobs rv)(=

Δ=

−λλ

λλλ


7-16

Doppler Shift


Thank You!


The Night Sky


Goals³Describe Nature and value of

constellations³Define elements of equatorial

coordinate system³Define two solstices & 2 equinoxes³Describe how the orientation of the

ecliptic on the celestial sphere produces seasons

3-2


³Constellations - apparent 2-D groupings of stars (Big Dipper, Leo, Orion)²Formed mostly by Greeks (100 BC - 100 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

3-3

The Night Sky


³ Stars are seen in projection² Stars in constellations are note related - are at

varying distances (3-D)² Stars move, but are so distant it is difficult to

detect

3-4

The Night Sky


³ Project Earth�s geographical features onto sphere to establish directions²Like latitude and longitude on Earth, need 2

coordinates to locate and object:

3-5

The Night Sky

Celestial Equator

Declination

North Celestial Pole

North Pole

Equator

Right Ascension

Declination: N & S of Celestial Equator

Right Ascension: E & W around Celestial Equator


3-6

The Celestial Sphere


360o in a circle90o in a right triangle1o has 60� (arcminutes)1� has 60� (arcseconds)

3-7

Angular Measures

Moon �subtends�1/2o (angular diameter)

Angle, or Angle of arc

Need distance to tell real size


3-8

Earth�s Rotation³Spin on Axis - counterclockwise from N-pole

²Causes apparent motion of stars²Causes day & night (24 hours)

³Earth also Revolves around the sun²1 revolution = 1 year = 365 1/4 days

q Causing different constellations to be seenq Individual stars rise 4 minutes earlier each night

³Noon when Sun is highest in sky (different for different places - longitude)²Timezones (every 15o of longitude)


3-9

Earth�s Rotation


3-10

Earth�s Rotation³ Seasons - caused by tilt of Earth�s axis of rotation

compared to plane of orbit² 23 1/2o

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 daylightq Winter SOLSTICE - 21 December - Sun lowest in N, shortest daylight


3-11

Seasons


3-12

Seasons


Precession & Eclipses


4-2

Goals³Describe precession, its effect on our

observations of the stars, and why it occurs³Explain by diagram how lunar phases are

controlled by positions of the Sun & Moon³Explain why & when solar & lunar eclipses occur

& why they aren�t every month


4-3

Precession³ Gravitational attraction of Earth�s bulge by Moon³ Earth responds by �wobbling� - changing direction in

which rotation axis points on the Celestial Sphere³ Currently, N. Polar axis points to �North star� (Polaris) -

the Celestial North Pole - but wobble traces out circle³ Rate of precession is slow - 26,000 years to complete

circle³ North star varies³ Precession also changes location of equatorial plane, so

Celestial equator precesses, as do equinoxes² Precession of the equinoxes


4-4

Precession


4-5

Phases of the MoonCaused as Moon orbits Earth - phase depends on how much of sunlit side we see:

C

Sunnoon

6 pm

6 am

midnight

B

A

H

D

E

F

G

A B C D E F G H

New

Waxing Crescent

1st Quarter

Waxing Gibbous

Full

Waning Gibbous

3rd Quarter

Waning Crescent

29 1/2 days to go through phases - Can correlate position in sky (time of day) & position

�moonth�


4-6

Phases of the Moon


4-7

Eclipses³ Lunar eclipse - Moon passes through Earth�s shadow

³ Solar eclipse - Moon�s shadow moves across Earth

³ Need proper alignment of Sun-Earth-Moon³ Should happen every month, but…

Earth MoonMoon is fullSun

SunEarthMoon

Moon is new


4-8

Lunar Eclipses

Remember: Moon is full, and seen anywhere Moon is visibleUseful for studying Earth�s

& Sun�s atmosphere.© Dr. Joseph E. Pesce, Ph.D.

4-9

Eclipses…Moon�s orbit is inclined 5o to ecliptic

Sun

Moon

Earth

5o

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)

�line of nodes�


4-10

Solar EclipsesBy coincidence, Sun & Moon have same angular diameter as seen from Earth

Sun

MoonEarth

UmbraPenumbra

Three types of Solar eclipse:1. Total Solar Eclipse - observer is in umbra of Moon�s shadow (moves rapidly over Earth

with a short - 7-10 min - duration2. Partial Solar Eclipse - observer in penumbra only3. 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


4-11

Solar Eclipses


Remember: Moon is new, and seen only on certain areas of Earth because shadow is large

4-12

EclipsesDEMONSTRATION:

Display umbra/penumbra with overhead projector


astro 113 week1 - Physics & Astronomy

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