Solar system: 9 light hours diameter

Spiral galaxy: 80,000 light years diameter

Coma cluster of galaxies: 2.5 million light years across

Survey of distant galaxies: 5 to 9 billion light-years away

Cosmic Microwave Background Radiation (after-glow from the Big Bang) - edge of the observable Universe:

14 billion light years away

General Astrophysical Concepts: Astronomical length scales and time scales

First principles of cosmology

• The universe is: (a) HOMOGENOUS (b) ISOTROPIC

• Homogeneity implies isotropy,

• But isotropy does NOT imply homogeneity

Kepler's laws of motion

• (1) Orbits of planets follow ELLIPSES with the Sun at one of the two foci

• (2) Equal areas are swept out in equal intervals of time

• (3) P2 α R3 or P2 = C R3 / M

where P is the period of the orbit, R is the `radius' of

the orbit, and M is the mass of the central object (Sun)

+ ◦Sun

Elliptical path/orbit followed by

planet

Sample Pop-quiz

Which of the following statements is TRUE:

A. When galaxies collide, it is very common for stars to smash into each other

B. Primordial density fluctuations grow under the influence of the electromagnetic force

C. Large galaxies are assembled by the merging of smaller galaxies

D. The furthest galaxies we can see from Earth are 100 billion light years away

Forces of Nature

• Newton's Laws of Motion

• Inverse square law of forces

• Fundamental forces (4)

(1) Gravitational force Example: falling apple

(2) Electromagnetic force Example: horseshoe magnet

(3) Weak nuclear force Example: β-decay or top bottom quark

(4) Strong nuclear force Example: force that binds protons & neutrons in atomic nuclei

The Milky Way and Other Galaxies Like It

• Overview of its constituents

• Dynamics (and mass) of a typical spiral galaxy:

The Milky Way

• Application of Kepler's third law (math application):

- The Solar System and Milky Way compared

- How many stars does the Milky Way contain?

The Detailed Structure of a Spiral Galaxy

Electromagnetic Radiation

• Propagation of Energy in the Form of Oscillating Electric

and Magnetic Fields

• Speed of propagation (in vacuum): c = 300,000 km/s

• Frequency ν: number of oscillations per second

[Unit of frequency: s-1 or Hertz (Hz)]

• Wavelength λ: distance traveled during one oscillation

[Unit of wavelength: meter (m), Angstrom (1 Å= 10-10 m)]

• c = λν or λ= c/ν or ν = c/λ

[ν & λ are inversely proportional to each other]

Electromagnetic Spectrum

• Progression of frequency or wavelength

Radio, millimeter, sub-millimeter, microwave, infrared, optical, ultraviolet, X-ray, gamma rays

• Optical/visible white light spectrum (rainbow colors):

Red Orange Yellow Green Blue Indigo Violet

[Order of decreasing wavelength, increasing frequency]

Atmospheric Windows

• Optical, [sub-millimeter], millimeter, and radio wavelengths

• Impact on astronomy (and on human evolution!)

Sample Pop-quiz

Which of the following statements is FALSE:

A. When galaxies collide, it is very common for stars to smash into each other

B. Primordial density fluctuations “grow” under the influence of the gravitational force

C. Large galaxies are assembled by the merging of smaller galaxies

D. The furthest galaxies we can see from Earth are about 10 billion light years away

Black Body Radiation

• Perfect emitter of radiation; perfect absorber of radiation

• Energy radiated per second depends on temperature: L = AσT4 or L = 4πR2σT4 (sphere of radius R) where: L = luminosity (in erg/s); A = surface area σ = Stefan's constant; T = temperature (in Kelvin = °C + 273)

• `Quality' of radiation (ν or λ or color) depends on the

black body temperature (Wien's law): λpeakT = constant (λpeak is inversely proportional to T)

Black Body Radiation

Wave-Particle Duality• Radiation consists of energy

bundles (quanta) called photons

• Energy of each photon: E = hν where: h = Planck's constant,

ν = frequency of radiation [Energy of each photon depends

on `color' (λ, ν) of radiation]

• The more luminous a source of radiation, the more photons it emits per second

Atomic Energy Levels

• An atom in Quantum Mechanics; discrete energy levels

• Transitions between levels: emission & absorption lines

Continuum, Emission Line, and Absorption Line Radiation