Astronomy Semester Exam-Things to know:
Exam will consist of Chapters 3, 4, 5, 6, 7, 8, 9, 10,
11, 12 and 13.
What to know for Chapters 3, 4 and 5 (Radiation,
Spectroscopy & Telescopes):
Light, Electromagnetic Radiation, Spectroscopy



Wavelength
Spectrum (long wavelength -> short)
............(low energy -> high energy)
 Radio (greater than 100 microns- or 100 x
10-6meters)
 Infrared (between 100 microns and 0.7
micron)
 Visible (between about 0.7 microns and 0.4
microns)
. . . (or between about 700 nanometers(nm)
and about 400 nm)
 Ultraviolet (between about 400 nm and
about 10 nm)
 X-ray (between about 10 nm and about 0.01
nm)
 Gamma-ray (below 0.01 nm)
All electromagnetic radiation travels at the speed
of light
. . . . . . . (300,000 kilometers/second)

Radiation is composed of "photons" (packets of
electromagnetic radiation)
 the "photons" behave as particles
sometimes, and as waves at other times
 wave-particle duality
 the "photons" have energies related to the
"color" or wavelength
 short wavelengths -> higher energies
 like gamma rays
 long wavelengths -> lower energies
 like radio waves
E=hf
where E is energy (in Joules)
-h is Planck's constant
(h = 6.63 x 10 -34Joule seconds)
-and f is frequency in 1/sec, or Hz.
Photon energies are very small
For example, for visible light (0.5 μ m), f =
6 x 1014 /sec
wavelength x frequency = velocity = 300,000 km/sec
So, E = 4 x 10-19 Joules

"Windows" in the atmosphere
visible
 radio
 Atmosphere is mostly opaque outside these
two "windows".
Generation of Electromagnetic Radiation

o
Electromagnetic Radiation is generated by the
movement of charged particles
Electrons (negative charge) are most
important
Blackbody Radiation
 Peak wavelength decreases with temperature
(Wien's Law)
 Total energy output increases with temperature
...........(increases as T4 - Stefan's Law)
 Surface temperature of the Sun is 6000 K
...........--> peak of spectrum in the visible light
Inverse Square Law

o
o
The apparent brightness of a star is inversely
proportional to the square of its distance.
o
o
Doppler Effect
 Wavelength of light from moving object is
shifted
 Moving away -> Red shift
 Moving toward -> Blue shift
Absorption and Emission Lines
 Absorption lines (dark lines on continuous
spectrum)
 Emission lines (bright lines)
o
o
o
Kirchhoff's Laws
 Continuous spectrum
 produced by luminous, dense object
 Absorption-line spectrum
 produced by cloud of cool gas on a
continuous spectrum
 Emission-line spectrum
 produced by a low-density, hot gas
Atoms and Molecules
 Bohr Model of the Atom
 Nucleus
 Electrons
 Orbits
 Energy levels
 Ionization
 molecules
Photons
 The particle nature of electromagnetic radiation
 represents wave-particle duality
 photons can be absorbed or emitted by an atom,
boosting the electron to an excited state (on
absorption)
or bringing the electron to a lower energy
state (on emission)
Since only certain energy states of the atom are
allowed, only certain wavelengths of photons are
emitted or absorbed, explaining the spectral lines.
Spectral Analysis
 Spectroscope

o
primitive: prism
 modern: based on diffraction grating
 Each element has a characteristic spectrum
 Comparison of absorption and emission lines of
sodium
Basic Optics
 Refraction
 The bending of light as it crosses the
boundary from one transparent material to
another
 Red (long wavelength) bends less than blue
(short wavelength)

o
Line Broadening

Line broadening is caused by the environment in
which the emission or absorption occurs
 Example of the broadened line
 Thermal broadening
 Rotational broadening
 Collisional broadening
 Broadening by magnetic fields (Zeeman
effect)
Telescopes
o
o
Refracting telescope
 First used by Galileo in early 17th century
 Uses lenses
 Limited by weight
 Chromatic aberration
Reflecting telescope
Invented around 1660
 Optimal mirror is parabolic shape
 limited by turbulence in the atmosphere
 best resolution about 1 arc-second
 Types:
 Newtonian
 Cassegrain
Telescope Size
 Light gathering power
 scales as the area or diameter-squared
 Resolution
 scales as the inverse of the diameter
. . . . large diameter resolves most finely
 also scales as the wavelength
. . . . radio telescope inherently worse than
optical
Hubble Space Telescope
 Launched from the Space Shuttle in 1990
 repaired during a Space Shuttle flight in 1993
 above the atmosphere
CCDs
 Charge-coupled Devices (CCDs)
 finely segmented wafer of silicon
 "electronic" photographic film
Active and adaptive optics
 active
 telescopes environment and focus are
carefully monitored and controlled
 adaptive optics
 blurring effects of the atmospheric
turbulence are corrected in real time

o
o
o
o
achieves 0.1 arc-second resolution
Radio Telescopes

o
First work by Jansky (circa 1930)
. . . . -->Center of the Milky Way Galaxy
Single dish has pretty poor resolution
. . . . Since resolution scales with wavelength
 Interferometry (sharpens image)
. . . . --> VLA (Very Large Array in New
Mexico)
 Value of radio astronomy
 observe objects obscurred by dust in space
(PRIMARY VALUE)
 observe objects that emit strong radio waves
but faint visible waves
 like cool objects
 newly formed stars
 observe at all hours of the day and night
Infrared Astronomy
 Can penetrate dust in space
 Infrared light absorbed by water vapor in
atmosphere
 Aircraft or Space vehicle (or very high, dry
location)
Ultraviolet, X-ray, and gamma-ray Astronomy
 Must be done from outer space due to the
"opacity" of the atmosphere.

o
o