Astro unit 3 p1

Chapter 5: Radiation and Spectra

5.1 The Behavior of Light

• Definition of Radiation: A process in which energy is transmitted from one point to another in space without any physical connection between the two points.

• Electromagnetic Radiation: Refers to energy (e.g., light) traveling in the form of oscillating electric and magnetic fields (wave) emitted by electrons inside atoms.

  • Oscillating Electric and Magnetic Fields: These are interconnected; for instance, a changing magnetic field can produce an electric current.

  • Historical Note: James Clerk Maxwell (1831–1879) unified the rules governing electricity and magnetism into a coherent theory, leading to the understanding that all types of electromagnetic radiation could travel through space as waves (mid-1800s).

• Wave Behavior: Light and other electromagnetic waves originate from atoms within stars and other celestial objects. They travel through empty space and take time to reach observers on Earth.

  • Speed of Light in Vacuum: All electromagnetic waves travel at $3 imes 10^8 ext{ m/s}$ in a vacuum.

5.2 The Electromagnetic Spectrum

• Definition of a Wave: A disturbance that transfers energy from one place to another without the physical transfer of material. Waves are patterned and originate from a vibrating source (such as atoms).

  • Wave Characteristics:

  • Amplitude: Height of the wave, which relates to the energy carried by the wave.

  • Frequency (f): The number of complete waves or cycles (wavelengths) that pass a given point per second, measured in Hertz (Hz).

    • Mathematical Definition: f = rac{# ext{ cycles}}{s}

    • Examples include:

    • $f = 3.3 ext{ Hz}$ for $3.3$ wavelengths per second.

  • Period (T): Time duration for one complete wave cycle to pass a point.

    • Mathematical Relation: Period (T) and Frequency (f) are reciprocals:

    • $T = rac{1}{f}$,

    • $f = rac{c}{ ext{total time}}$.

  • Two examples of frequency and period:

    • If 5 waves take 10 seconds:

      • Frequency: $f = rac{5 ext{ waves}}{10s} = 0.5 ext{ Hz}$

      • Period: $T = rac{10s}{5 ext{ waves}} = 2s$

• Velocity of a Wave: Calculated as the speed and direction of wave travel, given by the equation: $V = f imes ext{λ}$,
  where λ is the wavelength.

• Photons: On the microscopic scale, electromagnetic radiation behaves as particles known as photons, with energy proportional to frequency.

  • As light moves farther from its source, it appears less bright due to the inverse square law for light, which states that intensity decreases with the square of the distance.

5.3 Spectroscopy in Astronomy

• Electromagnetic Spectrum Mnemonic:

  • "Roman Men Invented Very Unusual X-Ray Guns" representing the sequence: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, and Gamma rays.

• Temperature and EM Wave Emission:

  • Definition of Temperature: Direct measurement of the average amount of microscopic motion within an object.

  • Higher Temperatures:

    • Particles move faster, radiate more intense energy, and emit more UV radiation.

    • Example: High wattage bulbs produce more infrared, visible light, and UV than low wattage bulbs.

  • Peak Wavelength: As the temperature increases, the peak wavelength of emitted radiation decreases.

• Reflection and Refraction of Light: Light can reflect off a surface or bend when transitioning between materials (e.g., air to glass).

5.4 The Structure of the Atom

• Spectrum Types:

  1. Continuous Spectrum: A full sequence of blended colors from red to violet, produced by luminous solids/liquids or dense gases (e.g., white-hot filament, core of a star).

  2. Emission Line Spectrum: A series of specific wavelengths emitted by particular atoms, produced by low-density hot gases (e.g., energized nebula).

  • Each element has a unique emission spectrum.

  1. Absorption Line Spectrum: Occurs when a continuous spectrum passes through a cool gas, causing the gas atoms to absorb specific frequencies they usually emit, resulting in absorption lines in the spectrum (e.g., seen in the Sun's spectrum).

• Summary of Spectroscopy Laws:

  • Summarized by Gustav Kirchhoff’s laws detailing the relationships between temperature, emission, and absorption:

  • Emission and absorption spectra allow astronomers to determine the composition and conditions of distant objects.

5.5 Formation of Spectral Lines

• Connection to Atomic Structure: The nature of spectral lines leads to insight into electronic structure and transitions in atoms, further informing the study of astrophysics and the characteristics of celestial bodies.

5.6 The Doppler Effect

• Doppler Effect: Observes how sound and light waves change frequency as the source moves closer or farther from the observer—redshift and blueshift applications in astronomy help identify the movement direction of stars and galaxies.

Other Notes

• Andromeda Galaxy: The farthest object visible to the unaided eye, located 2.5 million light-years away, rendering it inaccessible for spacecraft like New Horizons, which travels at approximately 60,000 km/h.

  • Travel Time: Would take about 450 billion years to reach Andromeda and another 2.5 million years to send back information.