The Electromagnetic
Spectrum, Light,
Astronomical Tools
Light and Other Forms
of Radiation
The Electromagnetic Spectrum
In astronomy, we cannot perform experiments
with our objects (stars, galaxies, …).
The only way to investigate them is by analyzing
the light (and other radiation) which we observe
from them.
Light as a Wave

c = 300,000 km/s
= 3*108 m/s
•
•
Light waves are characterized by a
wavelength and a frequency f.
f and  are related through
f = c/
Light as a Wave
• Wavelengths of light are measured in units
of nanometers (nm) or angstrom (Å):
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths
between 4000 Å and 7000 Å
(= 400 – 700 nm).
The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Light as Particles
• Light can also appear as particles, called
photons (explains, e.g., photoelectric effect).
• A photon has a specific energy E,
proportional to the frequency f:
E = h*f
h = 6.626x10-34 J*s
is the Planck constant.
The energy of a photon does not
depend on the intensity of the light!!!
Temperature Scales
• Want temperature scale with energy proportional to T
– Celsius scale is “arbitrary” (Fahrenheit even more so)
• 0o C = freezing point of water
• 100o C = boiling point of water
– By experiment, available energy = 0 at “Absolute Zero” = –
273oC (-459.7oF)
– Define “Kelvin” scale with same step size as Celsius, but 0K = 273oC = Absolute Zero
• Use Kelvin Scale for most astronomy work
– Available energy is proportional to T, making equations simple
(really! OK, simpler)
– 273K = freezing point of water
– 373K = boiling point of water
– 300K approximately room temperature
Planck “Black Body Radiation”
• Hot objects glow (emit light) as seen in PREDATOR, etc.
– Heat (and collisions) in material causes electrons to jump to high energy
orbits, and as electrons drop back down, some of energy is emitted as
light.
• The hotter the material the more energy it emits as light
– As you heat up a filament or branding iron, it glows brighter and brighter
• The hotter the material the more readily it emits high energy (blue)
photons
– As you heat up a filament or branding iron, it first glows dull red, then
bright red, then orange, then if you continue, yellow, and eventually blue
Planck and other Formulae
• Planck formula gives intensity of
light at each wavelength
– It is complicated. We’ll use two
simpler formulae which can be
derived from it.
• Wien’s law tells us what wavelength
has maximum intensity
Max 

3,000,000 nm K 3,000 m K

T
T
• Stefan-Boltzmann law tells us total
radiated energy per unit area
E   T 4 where   5.67  10 -8 J/(m 2 s K 4 )
From our text: Horizons, by Seeds

Example of Wien’s law
• What is wavelength at which you
glow?
– Room T = 300 K so
Max 

3,000 m K 3,000 m K

 10 m
T
300 K
– This wavelength is about 20 times
longer than what your eye can see.
Thermal camera operates at 7-14 μm.
• What is temperature of the sun –
which has maximum intensity at
roughly 0.5 m?
3,000 m K 3,000 m K
T

 6,000 K
Max
0.5 m
From our text: Horizons, by Seeds
Kirchoff’s laws
• Hot solids emit continuous spectra
• Hot gasses try to do this, but can only
emit discrete wavelengths
• Cold gasses try to absorb these same
discrete wavelengths
Continuous Spectrum
The spectrum of a common (incandescent) light
bulb spans all visible wavelengths, without
interruption.
Emission Line Spectrum
A thin or low-density cloud of gas emits light
only at specific wavelengths that depend on
its composition and temperature, producing a
spectrum with bright emission lines.
Absorption Line Spectrum
A cloud of gas between us and a light bulb can
absorb light of specific wavelengths, leaving
dark absorption lines in the spectrum.
Atomic (Hydrogen) Lines
•
•
•
Energy absorbed/emitted depends on upper and lower levels
Higher energy levels are close together
Above a certain energy, electron can escape (ionization)
•
Series of lines named for bottom level
– To get absorption, lower level must be occupied
• Depends upon temperature of atoms
– To get emission, upper level must be occupied
• Can get down-ward cascade through many levels
n=3
n=2
n=1
From our text: Horizons, by Seeds
Astronomical Telescopes
Often very large to gather
large amounts of light.
In order to observe
forms of radiation other
than visible light, very
different telescope
designs are needed.
The northern Gemini Telescope on Hawaii
The Powers of a Telescope:
Size does matter!
1. Light-gathering
power: Depends on the
surface area A of the
primary lens / mirror,
proportional to
diameter squared:
A =  (D/2)2
D
The Powers of a Telescope (II)
2. Resolving power: Wave nature of light
=> The telescope aperture produces
fringe rings that set a limit to the
resolution of the telescope.
Astronomers can’t eliminate these
diffraction fringes, but the larger a
telescope is in diameter, the smaller the
diffraction fringes are. Thus the larger
the telescope, the better its resolving
power.
min = 1.22 (/D)
For optical wavelengths, this gives
min = 11.6 arcsec / D[cm]
min
Seeing
Weather
conditions and
turbulence in the
atmosphere set
further limits to
the quality of
astronomical
images
Bad seeing
Good seeing
The Powers of a Telescope (III)
3. Magnifying Power = ability of the telescope
to make the image appear bigger.
A larger magnification does not improve the
resolving power of the telescope!
The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope (II)
Paranal Observatory (ESO), Chile
http://en.wikipedia.org/wiki/Paranal_Observatory
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
The Spectrograph
Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a
spectrum tell us about the
chemical composition and
other properties of the
observed object
Radio Astronomy
Recall: Radio waves of  ~ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.
Radio Telescopes
Large dish focuses
the energy of radio
waves onto a small
receiver (antenna)
Amplified signals are
stored in computers
and converted into
images, spectra, etc.
Radio Interferometry
Just as for optical
telescopes, the
resolving power of a
radio telescope
depends on the
diameter of the objective
lens or mirror min =
1.22 /D.
For radio telescopes,
this is a big problem:
Radio waves are much
longer than visible light
 Use interferometry to
improve resolution!
The Very Large Array (VLA): 27
dishes are combined to simulate a
large dish of 36 km in diameter.
The Largest Radio Telescopes
The 100-m Green Bank
Telescope in Green Bank, West
Virginia.
The 300-m telescope in
Arecibo, Puerto Rico
Infrared Telescopes
Spitzer Space Telescope
WIRO 2.3m
NASA’s Great Observatories in Space (I)
The Hubble Space Telescope
• Launched in 1990;
maintained and
upgraded by several
space shuttle service
missions throughout the
1990s and early 2000’s
• Avoids turbulence
in Earth’s
atmosphere
• Extends imaging
and spectroscopy to
(invisible) infrared
and ultraviolet
NASA’s Great Observatories in Space (II)
The Compton Gamma-Ray Observatory
Operated from
1991 to 2000
Observation of
high-energy
gamma-ray
emission, tracing
the most violent
processes in the
universe.
NASA’s Great Observatories in Space (III)
The Chandra X-ray Telescope
Launched in 1999 into a highly
eccentric orbit that takes it 1/3
of the way to the moon!
X-rays trace hot (million
degrees), highly ionized
gas in the universe.
Two colliding
galaxies,
triggering a
burst of star
formation
Very hot gas
in a cluster
of galaxies
Saturn
The Highest Tech Mirrors
Ever!
• Chandra is the first X-ray telescope to
have image as sharp as optical
telescopes.
NASA’s Great Observatories in Space (IV)
The Spitzer Space Telescope
Launched in 2003
Infrared light traces warm
dust in the universe.
The detector needs to be
cooled to -273 oC (-459 oF).
Kepler’s Supernova with all
three of NASA’s Great
Observatories
• Just 400 years ago:
(Oct. 9, 1604)
• Then a bright, naked eye
object (no telescopes)
• It’s still blowing up – now
14 light years wide and
expanding at 4 million
mph.
• There’s material there at
MANY temperatures, so
many wavelengths are
needed to understand it.