Astronomy SparkChart

Pr in te d in th e U SA $4 .9 5 $7 .9 5 C A N LIGHT Wave description: The periodic oscillation of electric and magnetic fields in space. Characterized by the following: 1. Frequency of the oscillation, ν , measured in cycles per second (hertz). 2. Wavelength of the oscillation, λ, measured in meters. This is the distance from one peak to the next. • Wave equation: All light travels at a finite speed, c = 3× 10 meters per second. This results in an inverse relationship between the frequency and wavelength. • Mathematically: c = λ× ν . • Thus, light with a high frequency has a short wavelength, and vice versa. Particle description: A stream of photons, individual particles of light that each carry a specific amount of energy, which is directly proportional to the frequency of the light. • Mathematically, Planck’s Law: E = hν E, energy (Joules, J) ν , frequency (Hertz, hz) h = 6.63× 10−34 , Planck’s constant (J × s) • Light with a high frequency (short wavelength) is also very energetic. Electromagnetic spectrum: The collection of all frequencies of light. • Includes (in order of increasing energy) radio, infrared, visible, ultraviolet, x-ray, and gamma ray frequencies. • Different physical processes in the universe emit radiation at different frequencies, so each frequency band probes different phenomena in the universe. Light quantities: • Energy: The capacity to cause change (Joules, J) • Power: Energy emitted per unit of time (J/s= Watts, W=J s−1) • Luminosity: Light energy emitted per unit time (power from a star) (J s−1=Watts, W) • Flux: Energy emitted per unit time per unit area (J s−1m−2 = Wm−2) Mathematically, F = L 4πD2 F , flux from surface of a spherical object (Wm−2) L, luminosity of object (W) D, distance to object (m) Spectroscopy: The technique astronomers use to separate light into its intensity at different wavelengths or spectrum. Components: 1. Continuum: The smooth part of the spectrum (see Figure 1). Most objects emit light at all frequencies, but the shape of the spectrum depends on the physical process that produces the light. • Blackbody: A dense object that reflects no light, and thus emits light only because of the thermal motion of its atoms, measured by its temperature. • Most objects produce their own continuum approximately as a blackbody (e.g., the Sun, an incandescent light bulb, and the human body). • The shape of the curve depends only on temperature. A hot object emits more light at higher frequencies (higher energies) than a cool object (e.g., hot stars appear blue, cool stars appear red). • Wien’s Law for a blackbody: λmax = 3×10 −3 T • λmax, wavelength of maximum intensity (m) • T , temperature of blackbody (Kelvins, K) • Stefan-Boltzmann Law for a blackbody: F = σT 4 F , energy flux from surface of blackbody (W m−2) σ = 5.67× 10−8 (W m−2K−4), StefanBoltzmann constant T , temperature of blackbody (K) • This flux is equal to the area under the curve of intensity versus wavelength for a blackbody. 2. Atomic lines: According to quantum mechanics, electrons bound to an atom can only have particular values of energy; they are unique to that element. Absorption or emission of a photon of light by the atom occurs when the energy of that photon matches the difference between two of these energy levels. • Absorption lines: Narrow, dark regions in a spectrum produced when an electron uses up a photon to jump to a higher energy level in an atom. • Emission lines: Narrow, bright regions in a spectrum produced when an electron spontaneously drops to a lower energy level in an atom. Doppler shift: The difference between the wavelength at which light is observed and the wavelength at which it was originally emitted due to the motion of the emitter relative to the observer. • Mathematically (for objects moving much slower than the speed of light): z = λobs−λem λem = v c z, redshift (dimensionless) λobs, observed wavelength (any length unit, usually nanometers, nm (= 10−9 m) or angstroms, Å (= 10−10 m)) λem, wavelength emitted by the source (same length unit) v, velocity of moving source (m/s) c, speed of light (m/s) • Note: z > 0: Source moving away, shift to longer wavelength (redder) z < 0: Source moving toward, shift to shorter wavelength (bluer)