LIGHT and Telescopes

Transcription

1 Astro 201: Sept. 14, 2010 Read: Hester, Chapter 4 Chaos and Fractal informa@on on class web page On- Line quiz #3: available amer class, due next Tuesday before class HW #3 on line Today: Light LIGHT and Telescopes 1

2 LIGHT Astronomers study the Universe by analyzing the light from cosmic objects LIGHT: Maxwell s equa@ons predict that moving charged par@cles will produce periodic waves of electric and magne@c field Periodic func@on, Travels through space 2

3 Ocean wave = fluctua@on in height of water. Sound wave = fluctua@on in air pressure. Electromagne@c wave = fluctua@on in electric and magne@c fields. 3

4 Wavelength, Frequency, c The speed of a wave equals frequency. c for celeritas, the La@n word for speed The speed of light in a vacuum is ALWAYS c = 300,000 km/sec (186,000 miles/sec). 4

5 Figure 4.6: Wavelength, Speed, Frequency, and Amplitude Light is made of PARTICLES. Light shows some of such as the photoelectric effect. of light, called PHOTONS, kick electrons out of atoms. 5

6 The ENERGY of a photon is related to the FREQUENCY of the wave. E = energy of photon f = frequency of light wave h = Planck s constant (a number) Light forms a spectrum from short to long wavelength. 6

7 Roy G. Biv Op/cal, or Visible Light ROY G BIV 7

8 Bright at all wavelengths Bright at specific wavelengths Bright at all except some specific wavelengths An example of a Con@nuum Spectrum: Blackbody A blackbody is an object that absorbs all light that falls on it It radiates light which has a dis@nc@ve spectrum, specified by its temperature The peak of the spectrum for blackbodies with T=98.6 F is in the infrared part of the spectrum T=6000 degree black bodies have peaks in the visible part of the spectrum (e.g. the SUN) Incandescent light bulb filament: deg. 8

9 Stars have spectra that are more or less like those of blackbodies Absorp'on and Emission Lines in Spectra: Produced when light interacts with atoms A nucleus, consis@ng of protons and (usually) neutrons, is surrounded by a cloud of electrons. 9

10 HYDROGEN: One Proton, one electron. proton electron Behavior on subatomic scales is governed by QUANTUM MECHANICS. Rule: electrons can only exist in orbits of energy. (Small orbit = low energy, big orbit = high energy). Electron falls from high energy to low energy orbit: energy is carried away by a photon. λ=656.3 nm nm nm Photon has a fixed ENERGY, corresponding to a specific WAVELENGTH. 10

11 Consider a hot, low density glob of hydrogen gas. λ = nm (3 2) λ = nm (4 2) λ = nm (5 2) Light is emimed with wavelengths or frequencies corresponding to energy jumps between electron orbits. 11

12 1) Hot, low density gas produces an emission line spectrum Spectrum of hydrogen at visible wavelengths. Carina Nebula: a cloud of hot, low density gas about 7000 light- years away. Its reddish color comes from the nm emission line of hydrogen. 12

13 A cool, low density glob of hydrogen gas in front of a light source. Light is absorbed at specific wavelengths corresponding to energy jumps between electron orbits. 2) Cool, low density gas produces an absorp/on line spectrum Spectrum of hydrogen at visible wavelengths. 13

14 Every type of atom has a unique spectrum. The spectrum of the Carina Nebula: Hydrogen line at nm 14

15 The RADIAL VELOCITY of an object is found from the DOPPLER SHIFT of its spectrum. Radial velocity = how fast an object is moving toward you or away from you. Chris@an Doppler ( ) DOPPLER SHIFT: If the wave source moves toward you or away from you, the wavelength changes. 15

16 A modern of Doppler s famous experiment: Doppler effect of sound hmp:// The reason for Doppler shims: Wave crests are bunched up ahead of wave source, stretched out behind wave source. 16

17 Figure 4.16: The Doppler shim of light If a light source is moving TOWARDS you, the wavelength appears shorter (called blueshim ). If the light source is moving AWAY from you, wavelength is longer (called redshim ). 17

18 Doppler shims are easily detected in emission or line spectra. Figure 4.17: Doppler shims of astronomical objects 18

19 Size of Doppler shim is to radial velocity: Δλ = observed wavelength shim = λ- λ 0 λ 0 = wavelength if source isn t moving V = radial velocity of moving source c = speed of light = 300,000 km/sec One of the nimy applica@ons of the Doppler effect has been the detec@on of Planets orbi@ng other stars: Extrasolar Planets The star and planet orbit around their center of mass periodic red and blue shims of the lines in the star s spectrum To date: about 358 planets have been discovered, mostly larger than Jupiter 19

20 Another nimy of the Doppler effect : Doppler radar to predict weather Not so nimy: Police radar guns to catch you speeding 20