Astronomy 122 Mid-term Exam I Review

Introduction
Radiation and Spectroscopy
Spectroscopy and Telescopes
Some Equations

Fundamentals


• Scientific Notation

  Know it.

• Scientific prefixes
  • kilo (1000) like kilometer = 1000 meters
  • Mega (10⁶) like Megaparsec = 1,000,000 parsec
  • milli (10^-3) like millimeter = 0.001 meters
  • micro (10^-6) like micrometer = 0.000001 meters
  • nano (10^-9) like nanometer = 10^-9 meters
  
  
• Distance Scale
  • Light-year (9.46 x 10¹² kilometers, or about 10¹³ kilometers)
  • Parsec = 3.26 Light-years
  
  
• Temperature Scales
• Fahrenheit(F), Celsius(C), Kelvin(K)
  • Absolute Zero (0 K)
  • Freezing point of Water (273 K)
  • Boiling point of Water (373 K)


• Some Equations
  

The Stars and our Sun


The elements created in the Big Bang were mostly hydrogen and helium

• The Big Bang occurred about 14 billion (13.8 x 10⁹) years ago

Stars are born within dark clouds in interstellar space

The Sun's source of energy is thermonuclear fusion, the fusion of atomic nuclei with the associated release of energy


• This process of fusion creates the heavy elements (such as carbon, nitrogen, and oxygen) from the elements hydrogen and helium

The Sun is approximately 5 billion (5 x 10⁹) years old, and it will continue generating energy much as it is today for another 5 billion years

Nearby Stars
• Closest - Alpha Centauri System
  • Three stars
  • 4 light-years away
  Brightest in Earth's sky - Sirius

Stellar deaths may be violent, such as the supernova explosion observed in the Large Magellanic Cloud in 1987 (SN1987A)

The "star dust" that results from such explosions is the source of heavy elements, making possible the existence of all things, planets, moons, animals, and plants.

Light, Electromagnetic Radiation, and Spectroscopy


• Wavelength
• Spectrum (long wavelength -> short)
  ............(low energy -> high energy)
  • Radio(greater than 100 microns- or 100 x 10^-6meters)
  • Infrared(between 100 microns and 0.7 micron)
  • Visible(between about 0.7 microns and 0.4 microns)
    . . . (or between about 700 nanometers(nm) and about 400 nm)
  • Ultraviolet(between about 400 nm and about 10 nm)
  • X-ray(between about 10 nm and about 0.01 nm)
  • Gamma-ray(below 0.01 nm)
• All electromagnetic radiation travels at the speed of light
  . . . . . . . (300,000 kilometers/second)
• Radiation is composed of "photons" (packets of electromagnetic radiation)
  • the "photons" behave as particles sometimes, and as waves at other times
    • wave-particle duality
  • the "photons" have energies related to the "color" or wavelength
    • short wavelengths -> higher energies
      • like gamma rays
    • long wavelengths -> lower energies
      • like radio waves
  
  

  E = h f

  
  where E is energy (in Joules)
  h is Planck's constant
  
  (h = 6.63 x 10 ^-34Joule seconds)
  
  and f is frequency in 1/sec, or Hz.
  
  Photon energies are very small
  For example, for visible light (0.5 μ m), f = 6 x 10¹⁴ /sec
  
  

  wavelength x frequency = velocity = 300,000 km/sec

  
  So, E = 4 x 10^-19 Joules
  
  
• "Windows" in the atmosphere
  • visible
  • radio
  • Atmosphere is mostly opaque outside these two "windows".


• Generation of Electromagnetic Radiation
Electromagnetic Radiation is generated by the movement of charged particles
• Electrons (negative charge) are most important


• Blackbody Radiation
  • Peak wavelength decreases with temperature (Wien's Law)
  • Total energy output increases with temperature
    ...........(increases as T⁴ - Stefan's Law)
  • Surface temperature of the Sun is 6000 K
    ...........--> peak of spectrum in the visible light
  
  
• Inverse Square Law

  The apparent brightness of a star is inversely proportional to the square of its distance.

  
  
• Doppler Effect
  • Wavelength of light from moving object is shifted
  • Moving away -> Red shift
  • Moving toward -> Blue shift
  
  
• Absorption and Emission Lines
  • Absorption lines (dark lines on continuous spectrum)
  • Emission lines (bright lines)
  
  
  
• Kirchhoff's Laws
  • Continuous spectrum
    • produced by luminous, dense object
  • Absorption-line spectrum
    • produced by cloud of cool gas on a continuous spectrum
  • Emission-line spectrum
    • produced by a low-density, hot gas
  
  
• Atoms and Molecules
  • Bohr Model of the Atom
    • Nucleus
    • Electrons
    • Orbits
    • Energy levels
    • Ionization
  • molecules
  
  
• Photons
  • The particle nature of electromagnetic radiation
    • represents wave-particle duality
  • photons can be absorbed or emitted  by an atom,
    boosting the electron to an excited state (on absorption)
    or bringing the electron to a lower energy state (on emission)
  • Since only certain energy states of the atom are allowed, only certain wavelengths of photons are emitted or absorbed, explaining the spectral lines.
  
  
• Spectral Analysis
  • Spectroscope
    • primitive: prism
    • modern: based on diffraction grating
  • Each element has a characteristic spectrum
  • Comparison of absorption and emission lines of sodium
  
  
• Basic Optics
  • Refraction
    • The bending of light as it crosses the boundary from one transparent material to another
    • Red (long wavelength) bends less than blue (short wavelength)



Line Broadening
• Line broadening is caused by the environment in which the emission or absorption occurs
  • Example of the broadened line
  • Thermal broadening
  • Rotational broadening
  • Collisional broadening
  • Broadening by magnetic fields (Zeeman effect)