September 27, 2016
Astronomy 122
Prof. J. Brau

Outline

• Basics
• Light and Electromagnetic Radiation
• Generation of Electromagnetic Radiation
• Blackbody Radiation
• Astronomical Applications
• Inverse Square Law
• Doppler Effect
• Distance Scales
• Summary

Basics

• Scientific Notation
• Distance Scale
• Temperature Scales
• Light and Electromagnetic Radiation
• Blackbody Radiation
• Inverse Square Law
• Doppler Effect
• Absorption and Emission Lines
• Kirchhoff's Laws
• Bohr Model of the Atom
• Spectral Analysis

Light and Electromagnetic Radiation




We learn about the stars by studying the electromagnetic radiation that they emit. Visible light is one particular type of electromagnetic radiation. The types of electromagnetic radiation are: radio infrared visible light ultraviolet X rays Gamma rays

• Electromagnetic radiation is a wave with a wavelength and an amplitude.



• The frequency of a wave is related to its velocity and wavelength through the formula



wavelength x frequency = velocity



• wavelength is measured in units of length
  
  • meters
  • millimeter (mm) = 0.001 m = 10^-3 m
  • micrometer (μm) = 0.000001 m = 10^-6 m
  • nanometer (nm) = 0.000000001 m = 10^-9 m
  
  
• frequency is expressed in units of inverse time (1/sec)
  • called Hertz (1 Hz = 1/sec)



• Since electromagnetic radiation (or waves) travel at the speed of light
  300,000 kilometers/second
  
  • 1 kilometer(km) = 1000 m = 10³ m
  we have a relationship between wavelength and frequency.



wavelength x frequency = 300,000 km/sec




• The electromagnetic spectrum ranges from radio waves at one extreme to gamma rays at the other extreme.
  
  
• Radiation is composed of "photons" (packets of electromagnetic radiation)
  • the "photons" are "particles"
    • Einstein was awarded the Nobel Prize for understanding that light (and electromagnetic radiation) is composed of these "particles" ("photons")
  • the "photons" have energies related to the "color" or wavelength
    • short wavelengths -> higher energies
      • like gamma rays
    • long wavelengths -> lower energies
      • like radio waves
  
  
  
• So light is both a wave, and particles.
  • This property is known as "wave-particle duality," a basic element of the quantum theory of physics
  
  
  
• Visible light can be split into its components by a prism.
  • the bending of light in going through the prism results from REFRACTION
  • long wavelengths (red) are refracted less than short wavelengths (blue and violet)
  
  
  
• The entire electromagnetic spectrum shows the frequencies and wavelengths of the types of electromagnetic radiation.



• The atmosphere blocks much of the electromagnetic radiation from space. 
  This is referred to as atmospheric "opacity".
  There are "windows" of transparency in the radio and visible parts of the spectrum.

Generation of Electromagnetic Radiation 




Electromagnetic Radiation is generated by the movement of charged particles Nature's primary charged particles are: Proton (positive charge) Electron (negative charge)
Electromagnetic Waves Changing electric fields are accompanied by magnetic fields The Earth has a magnetic field The Earth's magnetic field is relatively static, not a wave
On Earth, we generate radio waves (a form of Electromagnetic Radiation) by moving electrons along an antenna We also detect radio waves when they move electrons along an antenna attached to a receiver

Blackbody Radiation 




A blackbody radiator is a perfect radiator of light that absorbs and re-emits all radiation incident on it. Its light output depends only on its temperature. The sun and stars emit radiation like a blackbody following the Blackbody spectrum. This curve is known as the blackbody curve, or the Planck curve. As an object (a blackbody) is heated, the radiation it emits will always be described by the blackbody spectrum for the temperature of the body, with the curve peaking to higher and higher frequency.  Wien's Law The maximum wavelength of radiation emitted by a blackbody is inversely proportional to the temperature: max wavelength ~ 1/Temperature



Stefan's Law
The total amount of energy emitted by a blackbody is proportional to the 4th power of the temperature:
energy radiated ~ T⁴

Wien's Law and Stefan's Law are evident in the changes in the blackbody spectrum with temperature.

Astronomical Applications


The Sun appears differently in each type of radiation. Examples of the blackbody spectra from cosmic objects: (a.) a cool, invisible galactic gas cloud, Barnard 68 (T=60 K) (b.) a dim, young star (T=600 K) (c.) the Sun, with a surface temperature of 6000 K (d.) a globular cluster of bright stars, Messier 2 (T=60,000 K) The Milky Way Galaxy as it appears at many wavelengths (a.) radio wavelengths (b.) infrared wavelengths (c.) visible wavelengths (d.) X-ray wavelengths (e.) gamma-ray wavelengths

Inverse Square Law




The farther away an object is the fainter it appears. We refer to the amount of radiation for the star at our location as the apparent brightness. The apparent brightness of a star is inversely proportional to the square of its distance: apparent brightness ~ 1/(distance)2 This Law results from the spreading of the energy in the radiation.

The Doppler Effect




Electromagnetic radiation (of any type) always travels through diffuse space at the same speed, the speed of light: 300,000 kilometers/second The observed speed will not depend on relative motion. However, the wavelength of the light does change with relative motion. Stars moving away from us appear red-shifted and stars moving toward us appear blue-shifted. This can be illustrated by considering the wave crests of the wave motion.

Distance Scales 




There are two distance scales in use in Astronomy:  The Light-year (ly):  the distance light will travel in one year 1 ly = 9.46 x 1012 kilometers you can calculate this distance by multiplying the speed of light (300,000 kilometers/second) by the number of seconds in a year (3.15 x 107) The parsec (pc) related to the apparent motion of a close object compared to the relatively more distant background over the course of one year (see illustration - Fig 17.1 of textbook) if the angle of parallax is 1 second of arc, the distance is 1 parsec 1 pc = 3.1 x 10 13 kilometers 1 parsec = 3.26 light-years The "local" neighborhood

Summary

• There are many forms of electromagnetic radiation
  • visible light, radio, ultraviolet, etc.
  • wave characterized by period, wavelength, and amplitude
  • frequency is reciprocal of the period
• Moving electric charges generate radiation
  • Radiation moves through empty space at the speed of light
    • 300,000 kilometers per second
• Blackbody radiation laws based on temperature
• Inverse square law
• Doppler effect