Modern Astrophysics:

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Modern Astrophysics
Studying Light
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Measuring Light Quantitatively

• Spectroscopy measuring wavelengths (?) and
  frequencies (?) emitted or absorbed by matter
  composition of stars
• Photometry measuring the intensity of light
  luminosity of stars

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Measuring Light Quantitatively

• Polarimetry measurement and interpretation of
  the polarization of light waves.
• Polarization waves that have traveled through or
  have been reflected, refracted, or diffracted by
  some material plane(s) of transmission absorbed

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Radiation

• Radiant Energy
• Electromagnetic (EM) energy
• Energy that spreads out as it travels from its
  source
• Follows an inverse square law
• Can be measured in many different ways

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Properties of Light

• Light is radiant energy
• Can travel through space without a physical
  medium
• Speed 300,000 km/sec
• Speed in a vacuum is constant and is denoted by
  the letter c

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Properties of Light

• c is reduced as it enters transparent materials
• The speed is dependent on color (Blue light slows
  more than red)
• Lenses and prisms work this way
• Light is a mix of electrical and magnetic
  energy

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Nature of Light

• Light shows properties of waves
• Can measure wavelength (?) and frequency (?)

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Nature of Light

• Light also behaves like a stream of particles
  called photons
• Each photon carries a specific amount of energy
• All particles can also behave as waves
• Application Photoelectric effect

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Electromagnetic Energy

• Mathematical relationships
• c ??
• ? wavelength
• ? frequency
• c speed of light
• As wavelength increases, frequency ____________

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Electromagnetic Energy

• Energy and Light
• E h?
• E energy
• h Plancks constant
• As frequency increases, energy ____________

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Light and Color

• The range of colors to which the human eye is
  sensitive is called the visible spectrum
• Color is determined by wavelength (?)
• Frequency (or ?) is the number of wave crests
  that pass a given point in 1 second (measured in
  Hertz, Hz)

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Light and Color

• C Long ? Low ? Low E (Red)
• O
• L Mid ? Mid ? Mid E (Yellow)
• O
• R Short ? High ? High E (Violet)

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Electromagnetic Radiation

• Wavelengths range 10-14 m to 103 m
• Energy range follows the same pattern
• These trends make light a great probe for
  studying the Universe
• E-M spectrum includes radio, microwave, infrared,
  visible, ultraviolet, x-ray, and gamma radiations

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Forms of E-M Radiation
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Forms of E-M Radiation
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Invisible Light in Our Universe
www.warren-wilson.edu/.../sstephens/bragg2.html
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Radio Waves

• Produced in 1888 by Hertz
• First cosmic detection - 1930s
• Long wavelengths (big telescopes needed)
• Temperatures lt 10 K

M87 Galactic Center in radio
Very Large Array New Mexico
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Radio Waves

• For detection/study of
• Cosmic Background
• Cold interstellar medium site of star formation
• Regions near neutron stars white dwarfs
• Dense regions of interstellar space (e.g. near
  the galactic center)

Milky Way in visible (top) and radio wavelengths
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Infrared Radiation

• Discovered by Sir William Herschel (around 1800)
• Long wavelength (?) low frequency (?)
• Temperature range 10 -103 K

Spitzer Space Telescope
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Infrared Radiation

• Useful in detecting
• Cool stars
• Star Forming Regions
• Interstellar dust warmed by starlight
• Planets, Comets, Asteroids

M104 in visible light
M104 in IR
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Ultraviolet Radiation

• Discovered by J. Ritter in 1801
• Photographic plates exposed by light beyond the
  violet
• Shorter ?, higher energy
• Temperatures 104 - 106 K

Hampton UV Telescope
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Ultraviolet Radiation

• Used to detect/study
• Supernova remnants
• Very hot stars
• Quasars

M101 in visible light
M101 in UV light
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X-Rays

• Roentgen discovered X rays in 1895
• First detected beyond the Earth in the Sun in
  late 1940s
• Used to study Neutron stars, Supernova remnants

Chandra x-ray telescope
The sun in x-ray
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spectra
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Kirchhoffs Law
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SPECTRA

• Continuous Spectrum
• produced when dense, hot matter emits a
  continuous array of wavelengths
• we see it as white light

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SPECTRA
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SPECTRA

• Emission Spectrum
• when heated, a low-density gas (low pressure)
  will emit light in specific wavelengths
• the spectrum produced is called a line spectrum
  (also called an emission spectrum

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SPECTRA
Emission spectrum of H
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SPECTRA

• Absorption Spectrum
• Cool, low-density gas between the source and
  observer absorbs light of specific wavelengths
• one gas will absorb and emit in the same
  wavelengths

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SPECTRA
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SPECTRA
Spectra and Stars
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SPECTRA
Hydrogen Atom
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Light The Atom

• Electrons found in discrete energy levels
• Electrons absorb energy, move to higher levels
• Electrons release energy as they move to lower
  energy levels

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Solar Spectrum

• The core of our star produces a continuous
  spectrum
• Atoms in the atmosphere absorb the light
• These atoms emit light in random directions
  that produces dark lines in the spectrum

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Solar Spectrum
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Solar Spectrum

• The dark lines are called Fraunhofer lines

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The Suns Spectrum
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Arcturus Spectrum
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Thermal Radiation Starlight

• Wiens Displacement Law
• Heated bodies generally radiate across the entire
  electromagnetic spectrum
• There is one particular wavelength, ?m, at which
  the radiation is most intense and is given by
  Wiens Law
• ?m k/T
• Where k is some constant and T is the
  temperature of the body

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Thermal Radiation Starlight

• As the temperature of a star increases, the most
  intense wavelengths become shorter
• As an object heats, it appears to change color
  from red to white to blue

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Spectroscopy
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Thermal Radiation Star Light
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Thermal Radiation Star Light

• Stefan-Boltzmann Law
• As the temperature of a star increases, the total
  energy output increases as the 4th power of the
  temperature
• E ? T4

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The Doppler Effect

• Motion-induced change in the observed wavelength
  of any wave light or soundis known as the
  Doppler effect
• If the source is moving toward the observer,
  waves become compressed
• A shorter wavelength will appear blue
• This is called a blue-shift

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The Doppler Effect

• If the source is moving away from the observer,
  waves will be stretched out
• A longer wavelength will appear red
• Known as red-shift

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The Doppler Effect
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Absorption in the Atmosphere

• Gases in the Earths atmosphere absorb
  electromagnetic radiation most wavelengths from
  space do not reach the ground
• Visible light, most radio waves, and some
  infrared penetrate the atmosphere through
  atmospheric windows, wavelength regions of high
  transparency
• Lack of atmospheric windows at other wavelengths
  is the reason for astronomers placing telescopes
  in space

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