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Lecture 7: The Physics of Light, part 2 Astronomy 111 Spectra “Twinkle, twinkle, little star, How I wonder what you are are.” ASTR111 Lecture 7 Every type of atom, ion, and molecule has a unique spectrum Ion: an atom with electrons added (negative ion) or taken away (positive ion). ion) Molecule: two or more atoms bonded together. The spectrum of each atom, ion, and molecule is a distinctive “fingerprint”. The more complicated li t d the atom, ion or the th molecule, the more complex l the spectrum. electron neutron t proton ASTR111 Lecture 7 From emission F i i or absorption b ti lilines, we kknow: 1) which elements are present; 2) whether they are ionized; 3) whether they are in molecules molecules. emission spectrum of the Carina Nebula ASTR111 Lecture 7 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 i li spectrum. t 3) A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line p p spectrum. p ASTR111 Lecture 7 Continuum Source Cloud ASTR111 Lecture 7 ASTR111 Lecture 7 A hot, low density cloud of gas produces an emission line spectrum Light is emitted only at wavelengths corresponding co espo d g to e energy e gy d differences e e ces between permitted electron orbits. Results: an emission line spectrum spectrum. Hydrogen emission spectrum ASTR111 Lecture 7 The Carina Nebula Neb la A cloud of hot, low density gas about 7000 light years away. y Its reddish color comes from the 656.3 nm emission line of hydrogen. ASTR111 Lecture 7 A cool, transparent gas produces an absorption line spectrum Consider C id a cold, ld llow d density it cloud l d off hydrogen in front of a hot blackbody. Light is absorbed only at wavelengths corresponding to energy differences between permitted electron orbits. Result: an absorption p line spectrum. p ASTR111 Lecture 7 Absorption spectra can tell us about extrasolar planets • A planet’s atmosphere is a cold, low density y cloud of gas g illuminated by ya background source (its star) ASTR111 Lecture 7 The most abundant elements in the Universe are hydrogen and helium It is fairly easy to determine which elements are p present in a star. It is much harder to determine how much of each element is present present. Strength of emission and absorption lines depends on temperature as well as on the element’s abundance. ASTR111 Lecture 7 Abundance of elements i the in th S Sun’s ’ atmosphere: t h Hydrogen (H): 75% Helium (He): 23% Everything else: 2% As discovered in 1920’s, 1920 s, other stars are mostly hydrogen and helium, too. ASTR111 Lecture 7 Cecilia C ili PayneP Gaposchkin (19001979) was a BritishAmerican astronomer. She left England g in 1922. 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. ASTR111 Lecture 7 Temperature Scale I physics In h i and d astronomy, t we use th the Kelvin scale, which has a zero at absolute b l t zero. Kelvin = Celsius + 273 Water boils: 373 Kelvin Water freezes: 273 Kelvin Ab l t zero: 0 K Absolute Kelvin l i ASTR111 Lecture 7 An object is hot when the atoms of which it is made are in rapid random motion motion. The temperature is a measure of the average speed of the atoms atoms. Random motions stop at absolute zero temperature. temperature 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 y to make objects j emit light g is to heat them up. ASTR111 Lecture 7 Blackbody Radiation A Blackbody is an object that absorbs all light light. • Absorbs at all wavelengths • Characterized by its Temperature It is also the perfect radiator: • Emits E it att allll wavelengths l th ((continuous ti spectrum) • Total Energy emitted depends on Temperature • Peak wavelength also depends on Temperature ASTR111 Lecture 7 Wien’s Wien s Law Wavelength W l th off maximum i emission i i is inversely related to temperature max 2,900,000 nm max T wavelength of maximum emission T temperature (in Kelvins) Stefan-Boltzmann Law Energy emitted E itt d per second d per area b by a blackbody with Temperature (T): E = T 4 is i B Boltzmann's lt ' constant t t (a ( number) b ). In Words: “Hotter objects are Brighter at All Wavelengths” a e e gt s ASTR111 Lecture 7 Blackbody curves: ASTR111 Lecture 7 Solar spectrum: ASTR111 Lecture 7 Taking the temperature of stars Betelgeuse: g a reddish star ( (cooler). ) Rigel: a bluish star (hotter). (hotter) ASTR111 Lecture 7 Stellar spectra in order from the hottest (top) to coolest ( (bottom). ) ASTR111 Lecture 7 Inverse-Square Law of Brightness Luminosity is not the same as Brightness B i ht ! Luminosity is how much light leaves a source (it does not depend on your location). location) Brightness is how much light arrives at a particular location (it depends on how far away you are). ASTR111 Lecture 7 Doppler shift • Light can experience a Doppler shift much like the change in frequency of a train whistle as it passes an observer ASTR111 Lecture 7 Th reason ffor D The Doppler l shifts: hift Wave crests are bunched up ahead of the light source, stretched out behind. ASTR111 Lecture 7 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 th motion ti is i towards t d the th observer b Wavelength gets shorter = BLUESHIFT ASTR111 Lecture 7 If a light source is moving toward you, the wavelength is shorter (called a “blueshift”) blueshift ). If a light source is moving away from you, the wavelength is longer (called a “redshift”). ASTR111 Lecture 7 The radial velocity of an object is found from its Doppler shift Radial R di l velocity l it = how h ffastt an object bj t iis moving toward you or away from you. If a wave source moves toward you or away from f you, the th wavelength l th is i changed. ASTR111 Lecture 7 Size of Doppler shift is proportional to radial velocity vr 0 c observed wavelength shift 0 wavelength if source is not moving vr radial di l velocity l i off moving i source p of light g c speed ASTR111 Lecture 7 Example: p Hydrogen absorbs light with λ0 656.3 nanometers t But we observe a star with absorption line at λ 656.2 nanometers. Δλ 0.1 nm c v r 0 0.1 nm vr 300,000 km/sec 656.3 nm v r 46 km/sec ASTR111 Lecture 7 Way to Measure Speeds Observe the wavelength (obs) of a source g ((em) with a known emitted wavelength The difference is directly proportional to the speed of the source, source v: ( obs v em ) em c ASTR111 Lecture 7