The Physics of Light, part 2. Astronomy 111

Transcription

1 Lecture 7: The Physics of Light, part 2 Astronomy 111

2 Spectra Twinkle, twinkle, little star, How I wonder what you are.

3 Every type of atom, ion, and molecule has a unique spectrum Ion: an atom with electrons added (negative ion) or taken away (positive ion). Molecule: two or more atoms bonded together. The spectrum of each atom, ion, and molecule is a distinctive fingerprint.

4 The more complicated the atom, ion or the molecule, the more complex the spectrum. electron neutron proton

5 From emission i or absorption lines, we know: 1) which elements are present; 2) whether they are ionized; 3) whether they are in molecules. emission spectrum of the Carina Nebula

6 Kirchoff s Laws of Spectroscopy 1) A hot solid or hot, dense gas produces a continuous spectrum. 2) A hot, low-density gas produces an emission-line i spectrum. 3) A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.

7 Continuum Source Cloud

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9 A hot, low density cloud of gas produces an emission line spectrum Light is emitted only at wavelengths corresponding to energy e differences e between permitted electron orbits. Results: an emission line spectrum. Hydrogen emission spectrum

10 The 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.

11 A cool, transparent gas produces an absorption line spectrum Consider a cold, low density cloud of hydrogen in front of a hot blackbody. Light is absorbed only at wavelengths corresponding to energy differences between permitted electron orbits. Result: an absorption line spectrum.

12 Absorption spectra can tell us about extrasolar planets A planet s atmosphere is a cold, low density cloud of gas illuminated by a background source (its star)

13 The most abundant elements in the Universe are hydrogen and helium It is fairly easy to determine which elements are present in a star. It is much harder to determine how much of each element is present. Strength of emission and absorption lines depends on temperature as well as on the element s abundance.

14 Abundance of elements in the Sun s atmosphere: Hydrogen (H): 75% Helium (He): 23% Everything else: 2% As discovered in 1920 s, other stars are As discovered in 1920 s, other stars are mostly hydrogen and helium, too.

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16 Cecilia Payne- Gaposchkin ( ) was a British- American astronomer. She left England in In 1925, she became the first ever Ph.D. in astronomy from Harvard. Her thesis established that hydrogen was the overwhelming constituent of the stars.

17 Temperature Scale In physics and astronomy, we use the Kelvin scale, which has a zero at absolute zero. Kelvin = Celsius Water boils: 373 Kelvin Water freezes: 273 Kelvin Absolute zero: 0 Kelvin

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19 An object is hot when the atoms of which it is made are in rapid random motion. The temperature is a measure of the average speed of the atoms. Random motions stop at absolute zero temperature.

20 A hot, opaque object produces a continuous blackbody spectrum of light The universe is full of light of all different wavelengths. How is light made? One way to make objects emit light is to heat them up.

21 Blackbody Radiation A Blackbody is an object that absorbs all light. Absorbs at all wavelengths Characterized by its Temperature It is also the perfect radiator: Emits at all wavelengths (continuous spectrum) Total Energy emitted depends on Temperature Peak wavelength also depends on Temperature

22 Wien s Law Wavelength of maximum emission i is inversely related to temperature max T max 2,900,000 nm T wavelength of maximum emission temperature (in Kelvins)

23 Stefan-Boltzmann Law Energy emitted per second per area by a blackbody with Temperature (T): E = T 4 is Boltzmann's constant t (a number). b ) In Words: Hotter objects are Brighter at All Wavelengths ee

24 Blackbody curves:

25 Solar spectrum:

26 Taking the temperature of stars Betelgeuse: a reddish star (cooler). Rigel: a bluish star (hotter).

27 Stellar spectra in order from the hottest (top) to coolest (bottom).

28 Inverse-Square Law of Brightness Luminosity is not the same as Bi Brightness! Luminosity is how much light leaves a source (it does not depend on your location). Brightness is how much light arrives at a particular location (it depends on how far away you are).

29 Doppler shift Light can experience a Doppler shift much like the change in frequency of a train whistle as it passes an observer

30 The reason for Doppler shifts: Wave crests are bunched up ahead of the light source, stretched out behind.

31 The Doppler effect in light Amount of Shift depends upon the emitted wavelength ( em ) and the relative speed v: If the motion is away from observer Wavelength gets longer = REDSHIFT If the motion is towards the observer Wavelength gets shorter = BLUESHIFT

32 If a light source is moving toward you, the wavelength is shorter (called a blueshift ) ). If a light source is moving away from you, the wavelength is longer (called a redshift ).

33 The radial velocity of an object is found from its Doppler shift Radial velocity = how fast an object is moving toward you or away from you. If a wave source moves toward you or away from you, the wavelength is changed.

34 Size of Doppler shift is proportional to radial velocity 0 v r c observed wavelength shift 0 wavelength if source is not moving v r radial velocity of moving source c speed of light

35 Example: Hydrogen absorbs light with λ nanometers But we observe a star with absorption line at λ Δλ v r v r v r nanometers. 01nm. c 0 0.1nm nm 46 km/sec 300,000 km/sec

36 Way to Measure Speeds Observe the wavelength ( obs ) of a source with a known emitted wavelength ( em ) The difference is directly proportional to the speed of the source, v: ( ) obs em em v c