Newton’s Experiments with Light

Electomagnetic Waves

Properties of Waves: Frequency and

Wavelength

TelescopesYerkesRefractor

AreciboRadioDisk

MaunaKea

HubbleSpaceTelescope

Resolution of Telescopes

Sensitivity of Telescopes

The Earth’s Shroud• The Earth’s atmosphere acts to “screen”

out certain kinds, or bands, of light.• Visible light and radio waves penetrate the

atmosphere easiest; the IR somewhat. Most other bands are effectively blocked out.

• Consequently, telescopes are built at high altitude or placed in space to access these otherwise inaccessible bands.

Transparency of the Atmosphere

Transmission with Altitude

Flux of LightLight carries energy (e.g., perceived warmth from

sunlight)

How does this energy propagate through space? And how does that relate to the apparent brightness of a source?

“Flux” describes how light spreads out in space:with L=luminosity (or power), and d = distance, flux is Watts/square meter = J/s/m2

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F = L4πd2

The Inverse Square Law

Kirchoff’s LawsI. A hot solid, liquid, or dense gas

produces a continuous spectrum of emission.

II. A thin gas seen against a cooler background produces a bright line or emission line spectrum.

III. A thin gas seen against a hotter source of continuous radiation produces a dark line or absorption line spectrum.

Kirchoff’s Laws: Illustrations

Blackbodies1. A common approximation for the

continuous spectrum produced by many astrophysical objects is that a blackbody (or Planckian).

2. A blackbody (BB) is a perfect absorber of all incident light.

3. BBs also emit light!

Temperature Scales

Temperatures

of Note

Sample Blackbody Spectra

Atomic Physics• Atoms composed of

protons, neutrons, and electrons

• p and n in the nucleus

• e resides in a “cloud” around the nucleus

• mp/mn~1• mp/me~2000

Protons p +1 mp

Neutrons n 0 mn

Electrons e -1 me

The Bohr Atom

Atomic Energy Level Diagram

Interaction of Matter and Light

• Absorption: Occurs when a photon of the correct energy moves an electron from a lower orbit to an upper orbit.

• Emission: Occurs when an electron drops from an upper orbit to a lower one, thereby ejecting a photon of corresponding energy

• Ionization: Occurs when a photon knocks an electron free from the atom

• Recombination: Capture of a free electron

Absorption and Emission

The Gross Solar Spectrum

Blackbody-like Blackbody deviations

Thermal Motions of Particles in Gases

Doppler ShiftThe Doppler effect is a change in , , E of light when either or both the source and detector are moving toward or away from one another. So, this is a relative effect.

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Δ0= vradc

Illustration of the Doppler Effect

Composition of the Universe

Brief Overview of Stellar Evolution

• Pre-Main Sequence (really short time):The phase in which a protostar forms out of a cloud of gas that is slowly contracting under gravity

• Main Sequence (long time):The phase in which a star-wannabe becomes hot enough to initiate and maintain nuclear fusion of hydrogen in its core to become a true star.

• Post-Main Sequence (sorta short time):H-burning ceases, and other kinds of burning may occur, but the star is destined to become a White Dwarf, Neutron Star, or Black Hole

Formation of Stars and Planets

Observational Clues from the Solar System:

1. Orbits of planets lie nearly in ecliptic plane

2. The Sun’s equator lies nearly in the ecliptic

3. Inner planets are rocky and outer ones gaseous

4. All planets orbit prograde

5. Sun rotates prograde

6. Planet orbits are nearly circular

7. Big moons orbit planets in a prograde sense, with orbits in equatorial plane of the planet

8. Rings of Jovians in equatorial planes

9. S.S. mass in Sun, but angular momentum in planet orbits

Accretion and Sub-Accretion

Collection of Planetesima

ls into Planets

Solar Nebula TheoryImmanuel Kant (German): 1775, suggested that a

rotating cloud that contracts under gravity could explain planetary orbit characteristics

Basic Modern View –1. Oldest lunar rocks ~4.6 Gyr2. Planets formed over brief period of 10-100 Myr3. Gas collects into “disk”, and cools leading to

formation of condensates4. Growth of planetesimals by collisions

a) Build up minor bodies and small rocky worldsb) Build up Jovian cores that sweep up outer

gases

The Chaotic Early Solar System• Recent computer models

are challenging earlier views that planets formed in an orderly way at their current locations

• These models suggest that the jovian planets changed their orbits substantially, and that Uranus and Neptune could have changed places

• These chaotic motions could also explain a ‘spike’ in the number of impacts in the inner solar system ~3.8 billion years ago

The Moon and terrestrial planets were bombarded by planetesimals early in solar system history.

• The model predicts:

1.After formation, giant planet orbits were affected by gravitational ‘nudges’ from surrounding planetesimals

2. Jupiter and Saturn crossed a 1:2 orbital resonance (the ratio of orbital periods), which made their orbits more elliptical. This suddenly enlarged and tilted the orbits of Uranus and Neptune

3.Uranus / Neptune cleared away the planetesimals, sending some to the inner solar system causing a spike in impact rates

Cosmic Billiards

The early layout of the solar system may have changed dramatically due to gravitational interactions between the giant planets. Note how the orbits of Uranus and Neptune moved outwards, switched places, and scattered the planetesimal population.

20 AU

planetesimals

100 Myr 880 Myr

883 Myr ~1200 Myr

JS

UN

The Big Picture• The current layout of our solar

system may bear little resemblance to its original form

• This view is more in line with the “planetary migration” thought to occur even more dramatically in many extrasolar planet systems

• It may be difficult to prove or disprove these models of our early solar system. The many unexplained properties of the nature and orbits of planets, comets and asteroids may provide clues. Artist’s depiction of Neptune

orbiting close to Jupiter (courtesy Michael Carroll)

Bode’s Law

Planet Bode’s Actual ErrorMercury 0.4 0.4 <1%Venus 0.7 0.7 <1%Earth 1.0 1.0 PerfectMars 1.6 1.5 7%Asteroids 2.8 2.8 <1%Jupiter 5.2 5.2 <1%Saturn 10.0 9.5 5%Uranus 19.6 19.2 2%Neptune --- 30.0 MiserablePluto 38.8 39.4 2%?? 77.2 --- ---

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d(AU) = 4 +{0,3,6,12,24,...}10

Radiative Equilibrium

Global Temperatures of Planets

Planet Predicted Actual Error(K) (K) (%)

Mercury 440 400 10Venus 230 730 68Earth 250 280 11Mars 220 210 5

Jupiter 105 125 16Saturn 80 95 16Uranus 60 60 <1

Neptune 45 60 25Pluto 40 40 <1

Density and Composition<>

(kg/m3)Water 1000Rock 3000Air 1.3

Brass 8600Steel 7830Gold 19300

<> (kg/m3)

Ices 1000Volcanic rock and

stony meteorites

2800 - 3900

Iron rich minerals

5000 - 6000

iron ~7900

Ex: Moon – (surf) ~ 2800 and <> ~ 3300Earth – (surf) ~ 2800 but <> ~ 5500