Chapter 4 Radiation and Spectra

1. Chapter 4Radiation and Spectra The Sun in ultraviolet Astronomy 2010

2. Radiation from Space  Information from the Stars Astronomy 2010

3. The Nature of Light • At least 95% of the celestial information we receive is in the form of light. • Astronomers have devised many techniques to decode as much of the encoded information as possible from the small amount of light that reaches Earth. • This includes information about the object's temperature, motion, chemical composition, gas density, surface gravity, shape, structure, and more! Astronomy 2010

4. The Nature of Light (cont’d) • Most of the information in light is revealed by using spectroscopy: • Spectroscopy is the separation of light into its different constituent colors (or wavelengths) for analysis. • The resulting components are called the spectrum of the light. Astronomy 2010

5. Electric and Magnetic Fields • Light is composed of electric fields and magnetic fields. • Electric charges and magnets alter the region of space around them so that they can exert forces on distant objects. • This altered space is called a force field (or just a field). Astronomy 2010

6. Electromagnetism • Connection between electric and magnetic fields was discovered in the 19th century. • A moving electric charge or an electric current creates a magnetic field. • Coils of wire are used to make the large electromagnets used in car junk yards or the tiny electromagnets in your telephone receiver. • Electric motors used to start your car or spin a computer's hard disk around are other applications of this phenomenon. Astronomy 2010

7. How it works… • A changing magnetic field creates electrical current---an electric field. • Concept used in power generators---large coils of wire are made to turn in a magnetic field • The coils of wire experience a changing magnetic field and electricity is produced. • Computer disks and audio/video tapes encode information in magnetic patterns... • When the magnetic disk or tape material passes by small coils of wire, electrical currents/fields are produced. Astronomy 2010

8. James Clerk Maxwell • Born/Educated in Scotland • Lived 1831—1879 • Achieved a synthesis of knowledge of electricity and magnetism of his time. • Hypothesis: • If a changing magnetic field can make an electric field, then a changing electric field should make a magnetic field. Astronomy 2010

9. Electric and Magnetic Fields • Consequence: • Changing electric and magnetic fields should trigger each other. • The changing fields move at a speed equal to the speed of light. • Maxwell’s conclusion. • Light is an electromagnetic wave. • Later experiments confirmed Maxwell's theory. Astronomy 2010

10. The electric and magnetic fields oscillate at right angles to each other and the combined wave moves in a direction perpendicular to both of the electric and magnetic field oscillations. Astronomy 2010

11. Electromagnetic Radiation • Light, electricity, and magnetism are manifestations of the same thing called electromagnetic radiation. • Electromagnetic radiation is a form of energy. • This energy exists in many forms not detectable by our eyes such as infrared (IR), radio, X-rays, ultraviolet (UV), and gamma rays. Astronomy 2010

12. EM Waves General Properties • Travels through empty space. • Most other types of waves don’t • The speed of light (EM radiation) is constant in space. • All forms of light have the same speed of 299,800 kilometers/second in space • This number is abbreviated as c. • C = f, f = c/,  = c/f Astronomy 2010

13. Wave Characteristics (1) Amplitude • Amplitude (A) • A measure of the strength/size of the wave. • Period (P) • Duration of a cycle • Units: year, day, hours, seconds, … • Frequency (f) • Rate of repetition of a periodic phenomenon. • f=1/P • Units: Hertz (Hz), or cycle/s. Astronomy 2010

14. Period/Frequency Examples Astronomy 2010

15. Wave Characteristics (2) • Wavelength (l) • Size of one cycle of the wave in space. • Units: meter (m), centimeter (cm), micrometer (mm), nanometer (nm), angstrom (A). • Velocity (v) • Speed at which the wave propagate through space. • v = f x l • Units: m/s, miles/hour, km/hour, etc. Astronomy 2010

16. Distance Units • 1 meter = 1 m • = 100 cm (centimeter) • = 1000 mm (millimeter) • = 1000000 mm (micrometer) • = 1000000000 nm (nanometer) • = 10000000000 Å (Angstrom) Astronomy 2010

17. Visible Light color (Å) f (*1014 Hz) Energy (*10-19 J) violet 4000 4600 7.5 6.5 5.0 4.3 indigo 4600 4750 6.5 6.3 4.3 4.2 blue 4750 4900 6.3 6.1 4.2 4.1 green 4900 5650 6.1 5.3 4.1 3.5 yellow 5650 5750 5.3 5.2 3.5 3.45 orange 5750 6000 5.2 5.0 3.45 3.3 red 6000 8000 5.0 3.7 3.3 2.5 Astronomy 2010

18. Color Composition • White light is made of different colors (wavelengths). • White light passing through a prism or diffraction grating, is spread out into its different colors. • First discovered by Newton Astronomy 2010

19. Light Dispersion by refraction Incidence angle Refraction angle • Refraction Angle or dispersion function of the wavelength (color) Astronomy 2010

20. Max Planck’s Photon • Max Planck (lived 1858--1947) • Discovered that if one considers light as packets of energy called photons, one can accurately explain the shape of continuous spectra. • A photon is a particle of electromagnetic radiation. • Bizarre though it may be, light is both a particle and a wave. • Whether light behaves like a wave or like a particle depends on how the light is observed • it depends on the experimental setup! Astronomy 2010

21. Albert Einstein’s Photon Energy Interpretation. • Albert Einstein (lived 1879--1955) • A few years after Planck's discovery Albert Einstein found a very simple relationship between the energy of a light wave (photon) and its frequency: • Energy of light = h  f = (h  c)/ l • where h is a universal constant of nature called ``Planck's constant'' = 6.63  10-34 J·sec. Astronomy 2010

22. Characterizing Light • We now have three ways to characterize electromagnetic radiation: • wavelength • frequency • energy • Astronomers use these interchangeably. • We also divide the spectrum of all possible wavelengths/frequencies/energies into bands that have similar properties. Light is the most familiar of these. Astronomy 2010

23. Visible Spectrum Small wavelength High frequency High energy large wavelength low frequency low energy Remember the Spectrum: ROY G BIV Astronomy 2010

24. The Full Spectrum • From the highest to lowest energy • Gamma rays • X-rays • Ultraviolet • Visible • Infrared • Microwave • Radio Astronomy 2010

25. EM Waves General Properties (cont’d) • A wavelength of light is defined similarly to that of water waves • distance between crests or between troughs. • Visible light (what your eye detects) • has wavelengths 400-800 nanometers. 1 nm = 10-9 m. • Radio wavelengths are often measured in centimeters: 1 centimeter = 10-2 meter = 0.01 meter. • The abbreviation used for wavelength is the Greek letter lambda: l Astronomy 2010

26. The Full E-M Spectrum

27. EM Radiation Reaching Earth • Not all wavelengths of light from space make it to the surface. • Only long-wave UV, visible, parts of the IR and radio bands make it to surface. • More IR reaches elevations above 9,000 feet (2765 meters) elevation. • That is one reason why modern observatories are built on top of very high mountains. Astronomy 2010

28. Earth’s atmosphere is a shield • Blocks gamma rays, X-rays, and most UV. • Good for the preservation of life on the planet… • An obstacle for astronomers who study the sky in these bands. • Blocks most of the IR and parts of the radio. • Astronomers unable to detect these forms of energy from celestial objects from the ground • Must resort to very expensive satellite observatories in orbit. Astronomy 2010

29. Types of Spectra • Continuous spectra consist of all frequencies (colors) - the thermal or blackbody spectrum is the most common example we will see. • Absorption line spectra are continuous spectra with certain missing certain frequencies. • Emission line spectra are a series of discrete frequencies (with or without a continuous spectra). • Often astronomers deal with combinations of the above. Astronomy 2010

30. Black Body Spectrum Astronomy 2010

31. Star Color vs. Temperature

32. Discrete Spectra • Close examination of the spectra from the Sun and other stars reveals that the rainbow of colors has many dark lines in it, called absorption lines. • They are produced by the cooler thin gas in the upper layers of the stars absorbing certain colors of light produced by the hotter dense lower layers. • The spectra of hot, thin (low density) gas clouds are a series of bright lines called emission lines. • In both of these types of spectra you see spectral features at certain, discrete wavelengths (or colors) and no where else. Astronomy 2010

33. Absorption Line Spectrum

34. Spectra • The type of spectrum you see depends on the temperature of the thin gas. • If the thin gas is cooler than the thermal source in the background, you see absorption lines. • Since the spectra of stars show absorption lines, it tells you that the density and temperature of the upper layers of a star is lower than the deeper layers. • In a few cases you can see emission lines on top of the thermal spectrum. This is produced by thin gas that is hotter than the thermal source in the background. Astronomy 2010

35. Spectra (cont’d) • The spectrum of a hydrogen-emission nebula (``nebula'' = gas or dust cloud) is just a series of emission lines without any thermal spectrum because there are no stars visible behind the hot nebula. • Some objects produce spectra that are a combination of a thermal spectrum, emission lines, and absorption lines simultaneously! Astronomy 2010

36. Spectra (cont’d) Astronomy 2010

37. The Structure of the Atom Astronomy 2010

38. Bohr atom • Explanation for the discrete line spectra … • Niels Bohr (lived 1885--1962) provided the explanation in the early 20th century. • Electrons only exist in certain energy levels and as long as an electron stays in a particular energy level, it doesn’t emit any energy (photons). • If an electron changes energy levels, it emits or absorbs energy in the form of a photon. • Set of energies (light frequencies) uniquely identify the type of atom! Astronomy 2010

39. Bohr’s Model • In Bohr's model of the atom, the massive but small positively-charged protons and massive but small neutral neutrons are found in the tiny nucleus. • The small, light negatively-charged electrons move around the nucleus in certain specific orbits (energy levels). • In a neutral atom the number of electrons = the number of protons. • The arrangement of an atom's energy levels (orbits) depends on the number of protons (and neutrons) in the nucleus and the number of electrons orbiting the nucleus. • Because every type of atom has a unique arrangement of energy levels, they produce a unique pattern of absorption or emission lines. Astronomy 2010

41. Isotopes Astronomy 2010

42. How is light produced? Astronomy 2010

43. Absorption Line Spectra

44. Doppler Effect • The wave nature of light means there will be a shift in the spectral lines of an object if it is moving. • Sound Waves: pitch  frequency of wave • Changes the pitch of the sound coming from something moving toward you or away from you • train whistle, police siren • Sounds from objects moving toward you are at a higher pitch because the sound waves are compressed together, shortening the wavelength of the sound waves. • Sounds from objects moving away from you are at a lower pitch because the sound waves are stretched apart, lengthening the wavelength. Astronomy 2010

45. Sound Waves • Spread uniformly from a sound source • Circles -- crests of the sound waves • Think of waves spreading from a pebble dropped into a pool Astronomy 2010

46. Doppler Shift Moving towards Short wavelength Moving away longer wavelength Astronomy 2010

47. Doppler Shift  Speed of Source Astronomy 2010

48. Red shift, Blue shift • Motion of the light source causes the spectral lines to shift positions. • Which way the spectral lines shift tells you if the object is moving toward or away from you. • Blue shift: If the object is moving toward you, the waves are compressed, so their wavelength is shorter. The lines are shifted to shorter (bluer) wavelengths. • Red shift: If the object is moving away from you, the waves are stretched out, so their wavelength is longer. The lines are shifted to longer (redder) wavelengths. • The doppler effect will not affect the overall color of an object unless it is moving at a significant fraction of the speed of light (VERY fast!) Astronomy 2010

49. Doppler Shifted Spectra Astronomy 2010

50. Doppler Shift (5) Astronomy 2010