a, &T Telescopes Look, but don t touch. -Astronomers Motto

Transcription

1 Light, Spectra a, &T Telescopes Wednesday, February 4 Look, but don t touch. -Astronomers Motto

2 Visible light is just one form of electromagnetic radiation. The universe contains electrically charged particles: il electrons (-) and protons (+). Charged particles are su urrounded by electric fields and magnetic fields. Fluctuations in these fields produce electromagnetic radiation.

3 Visible light is a form of electromagnetic - but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays. radiation -

4 Light can be thought of as a wave. Wave = a periodic fluctuation travelling through a medium. Ocean wave = fluctuati ion in the height of water. Sound wave = fluctuation in air pressure. Electromagnetic wave = fluctuation in electric and magnetic fields.

5 Wave Char racteristics: (1) Wavelength, (lambda): distance between wave crests (units = meter). (2) Frequency, (nu): ) number of crests passing per second (units = 1/sec = Hertz). (3) Amplitude, a: height of wave crests.

6 Speed of light: Speed of wave, c, equals wavelength times frequency (units = meter/sec): c x The speed of light in a vacuum is always (186,000 miles/sec). c = 300,0 000 km/s

7 Light can be thought of as a particle. Light shows some properties of a wave: diffraction and interference. It shows some properties of a particle: the photoelectric effe ect. (In the photoelectric effect, particles of light, called photons, kick electrons out of atoms.)

8 How sound waves would travel without diffraction: How sound waves actually travel with diffraction: Diffraction happe ens for light, too!

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10 Photons The energy of a photon is related to the frequency of a wave: E = energy e of photon o = frequency of light E= =h h = Planck s constant (A Small Number)

11 Light forms a spectrum from short to long wavelength Visible light has wavelengths from 400 to 700 nanometers. [1 nanometer (nm) = 10-9 meter] Color is determin ned dby wavelength: Blue:4 480 nm Green: 530 nm Red: 660 nm

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13 The complete spectrum of flight Gamma rays ( < 0.01 nanometers) X-rays ( nm) Ultraviolet ( nm m) Visible ( nm) Infrared (700 nm 1 mm) Microwaves (1 100 mm) Radio (> 100 mm) Energy egy

14 Visible light occupies only a tiny sliver of the full spectrum.

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17 Earth s at satmosphere eeis sta transparent to visible light and some microwaves and radio waves. To observe efficiently at other wavelengths, we must go above atmosph here.

18 NASA's Kuiper Airborne Ob bservatory flew a 91-cm telescope to altitudes as highh as 45,000 feet. It operated from 1975 to 1995.

19 Sky: Optical

20 Sky: Radio

21 Sky: Microwaves

22 Sky: Infrared

23 Sky: X-ray

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

25 A hot, transparentt gas produces an Consider a single, isolated atom: A nucleus, containing protons and neutrons, is surrounded by a cloud of orbiting electrons. Electrons can emit or absorb photons. emission spectrum.

26 Consider hydrogen (the simplest atom): one proton, one electron. Behaviour on subatomic scales is governed dby quantum mechanics. cs. One rule of quantum mechanics: electrons can only exist in orbits of particular en nergy (energy is quantizied).

27 When an electron falls from a high energy orbit to a low energy orbit, the energy lost is carried away by a pho oton. In hydrogen, an electron falling from orbit 3 to orbit 2 emits a photon with = nanometers.

28 Consider a hot, low den nsity cloud of hydrogen. Light is emitted only at wavelengths corresponding to energy differences between permitted electron orbits. Results: an emission lin ne spectrum.

29 The Carin na Nebula A cloud of hot, low density gas about 7000 light years away. Its reddish color comes from the nm emission line of hydrogen.

30 A cool, transparent gas produces an absorption 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.

31 Every type of atom, ion, and molecule has a unique spectrum. Ion: an atom with electrons added (negative ion) or taken away (p ositive ii ion). Molecule: twoormoreor atoms bonded together. The spectrum of each atom, ion, and molecule is a dist tinctive fingerprint.

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

33 From emission or absorptio on 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

34 The most abundan nt 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 determ mine how much of each element is present. Strength of emission and absorption lines depends on temperature as well l as on the element s abundance.

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

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37 Cecilia Payne-Gaposchkinn ( ) 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.

38 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.

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

40 The Doppler Effect in Light Amount of fshiftd depends d upon the emitted wavelength ( em ) and the relative speed v: If the motion is aw ay from observer Wavelength gets longer = REDSHIFT If the motion is tow wards the observer Wavelength gets shorter = BLUESHIFT Lecture 2: Light

41 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 ).

42 0 v r Size of Dop ppler shift is proportional to radial velocity. 0 v r c observed wavelength shift wavelength if source is not moving radial velocity of moving source c speed of light

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

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

45 The main purposes of a telescope are to gather light and resolve detail. A telescope is sometimes called a light bucket. Number of photons collect ted per second is proportional to the area of the lens/mirror: Area = x D D 2 where D = diameter of the lens/mirror.

46 A convex lens (thicker in the middle) focuses light to a point: Light from a large area is funneled into a small area.

47 Light with a short wavelen ngth is bent through a larger angle than light with a long wavelength. (This is why prisms spread light into a spectrum.)

48 The world s bigge est telescopes are reflectors (mirrors), not refractors (lenses). The problem with lense es: 1) Lenses absorb light. 2) Lenses sag. 3) Lenses have chromatic aberration: colors do not focus at same point.

49 The world s largestr refracting telescope: Yerkes Observatory, Wisconsin 1 meter diameter Completed 1897

50 A mirror shaped like a par rabola focuses light to a point: Light from a large area is funneled into a small area. Lenses and mirrors (if shaped correctly) produce an accurate image of an obj ject.

51 Soon to be the world s largest reflecting telescope (2x8.4 m):

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53 BIGGER IS BETTER Larger diameter for your lens or mirror means more light, higher resolution. Texas A&M is a founding partner in the Giant Magellan Telescope project

54 Few closing questions: 1) What kind of spectrum will be produced by very hot, but also very densee hydrogen gas? 2) If you have hot gas in front of a star, what kind of spectrum will you see? 3) Which spectrum is more complex: that of hydrogen or that of helium? 4) If you double the diameter of a telescope, how much more light will it collect?