Chapter 4 Radiation and Spectra The Sun in ultraviolet January 24, 2006 Astronomy 2010

Radiation from Space  Information from the Stars January 24, 2006 Astronomy 2010

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! January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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). January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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 January 24, 2006 Astronomy 2010

Wave Characteristics (1) 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. Amplitude January 24, 2006 Astronomy 2010

Period/Frequency Examples Phenomenon Period Frequency Earth’s orbit around the Sun 365 days 0.00274 days-1 Earth’s rotation 1 day or 86400 sec 1 day-1 or 1.16x10-5 Hz Electrical Power (US) 0.0167 sec 60 Hz Blue light 1.67x10-15 sec 6.0x1014 Hz January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

Distance Units 1 meter = 1 m = 100 cm (centimeter) = 1000 mm (millimeter) = 1000000 mm (micrometer) = 1000000000 nm (nanometer) = 10000000000 Å (Angstrom) January 24, 2006 Astronomy 2010

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 January 24, 2006 Astronomy 2010

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 January 24, 2006 Astronomy 2010

Light Dispersion by refraction Refraction Angle or dispersion function of the wavelength (color) Incidence angle Refraction angle January 24, 2006 Astronomy 2010

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! January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

Visible Spectrum Small wavelength High frequency High energy large wavelength low frequency low energy Remember the Spectrum: ROY G BIV January 24, 2006 Astronomy 2010

The Full Spectrum From the highest to lowest energy Gamma rays X-rays Ultraviolet Visible Infrared Microwave Radio January 24, 2006 Astronomy 2010

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 January 24, 2006 Astronomy 2010

The Full E-M Spectrum

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

Black Body Spectrum January 24, 2006 Astronomy 2010

Star Color vs. Temperature

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. January 24, 2006 Astronomy 2010

Absorption Line Spectrum

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. January 24, 2006 Astronomy 2010

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! January 24, 2006 Astronomy 2010

Spectra (cont’d) January 24, 2006 Astronomy 2010

The Structure of the Atom January 24, 2006 Astronomy 2010

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! January 24, 2006 Astronomy 2010

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. January 24, 2006 Astronomy 2010

Isotopes January 24, 2006 Astronomy 2010

How is light produced? January 24, 2006 Astronomy 2010

Absorption Line Spectra

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. January 24, 2006 Astronomy 2010

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 January 24, 2006 Astronomy 2010

Doppler Shift Moving towards Short wavelength Moving away longer wavelength January 24, 2006 Astronomy 2010

Doppler Shift  Speed of Source January 24, 2006 Astronomy 2010

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!) January 24, 2006 Astronomy 2010

Doppler Shifted Spectra January 24, 2006 Astronomy 2010

Doppler Shift (5) January 24, 2006 Astronomy 2010

Doppler Shift (6) January 24, 2006 Astronomy 2010

Expanding Universe The doppler effect tells us about the relative motion of stars with respect to us. The spectral lines of nearly all of the galaxies in the universe are shifted to the red end of the spectrum. These galaxies are moving away from our Milky Way galaxy. This is evidence for the expansion of the universe. January 24, 2006 Astronomy 2010