Astro 101
Fall 2013 -- Lecture #3
T. Howard
Using Light to Sample the Universe
Light is a mutual oscillation of electric
and magnetic fields, travelling through
space.
Light is also a stream of fundamental
particles (photons) that can be detected
individually.
Both interpretations are correct.
It is electromagnetic radiation.
Spherical wave
Light from stars starts out like this …
… and reaches us like this.
wavefronts (lines of constant phase)
Plane wave
Why? Because stars are very, very far away.
The curved wave flattens out with distance.
But, the amount of light we can collect …
(with our telescopes)
… falls off as 1/distance2
Why? Three simple reasons:
distant
star
R
A. The total energy = amount of light is conserved
B. As the light flows away from the star, that light
is spread over an imaginary sphere of area
4pR2, where R is the distance from the star
C. Our telescope samples only a very small part
of the area of that imaginary sphere that
surrounds the distant star.
Telescope
(just the front aperture is shown)
We can use this to find either (a) the absolute amount of light that the star
emits, or (b) its distance, if we know the other quantity.
Since light is a wave, it has wave properties …
Direction of wave
travel
“Amplitude” of
the wave = height
of the Electric field
vibration
“Wavelength” = distance between two
successive peaks
This defines the color of the light wave
• Most light we see with our eyes has a spread of many colors mixed
• Generally, some colors stronger than others, so we see objects
in different colors
• White light is a nearly uniform mix of all the colors that we can see
with our eyes
• There are many other colors (= wavelengths) that we can’t see
• Our eyes just not sensitive to vibrations at those wavelengths
• But, electronic detectors and telescopes can see them
So, almost all light reaches us with a spectrum
Spectrum = spread of light wavelengths, mixed in varying strength
that reaches our eye / telescope / camera (plural = spectra)
Spectra can be continuous … (like this) 
or discrete …
(like this) 
Continuous spectra come from warm objects (everything in the
Universe, almost). The color extent and intensity of the
spectrum depends on the temperature of the object. This is
known as thermal
or blackbody radiation.
Even you are emitting thermal radiation, right now.
Example
The astronaut’s space suit is:
a. Reflecting light from its surroundings
incoming
sunlight
… and …
b. Emitting infrared (thermal) radiation
(that we can’t see because our eyes
aren’t sensitive to those colors)
Q: What about the light coming
from the lunar surface? Which is it?
Q: Is the light from the Sun thermal radiation? If so, why
can we see it with our eyes? If not, what is it?
The visible spectrum
400 nm
700 nm
The human eye is sensitive only to the range 400 – 700 nanometers.
(1 nanometer = 10-9 meter = one-billionth of a meter.)
Blue wavelengths are shorter than red wavelengths.
The visible spectrum is only a small part of the overall possible
electromagnetic spectrum. Other regions of the spectrum correspond
to …
gamma rays (very short)
x-rays (pretty short wavelengths)
UV (ultraviolet, shorter than visible blue light)
infrared or IR (long wavelengths, beyond the visible)
radio waves (longer than infrared)
Electromagnetic Radiation
2900K
NIR
< 2 microns
MWIR
3-5 microns

LWIR
8-12 microns
VLWIR
1
10
Wavelength [microns]
Frequency [Hz]
Temperature of Blackbody
with Peak emission at l
29K
290K
100
Infrared
Gamma Rays
e- e+
annihilation
0.511 MeV
X-band
10 GHz
1 eV
1.24 microns
Broadcast AM
0.8 MHz
Radio
frequency [Hz]
3x1025
1024
1023
1022
10-18 10-17 10-16 10-15 10-14
wavelength [m]
1021
1020
1019
1018
1017
10-13 10-12 10-11 10-10 10-9
1016
10-8
1015
10-7
1014
10-6
1013
10-5
10-4
1011 1010
10-3
10-2
109
10-1
108
100
Cellphones
(AMPS, USDC)
850 MHz
X-rays
UV
Medical x-rays
0.1 - 0.5 Angstroms
1012
Peak of
Cosmic Microwave
Background (CMB)
[T =2.736 K]
UV “A” 320-400 nm
UV “B” 290-320 nm
107
101
106
102
105
103
104
104
103
105
Shortwave
10 MHz
Broadcast TV
60 MHz
Wavelength [m]
Photon energy:
E = hn = hc/l
ln = c
Visible
400 nm
[0.4 microns]
4000 Angstroms
Peak
Dark response
510 nm
Peak Day
response
570 nm
700 nm
[0.7 microns]
7000 Angstroms
micron = mm = 10-6 m
nanometer = nm = 10-9 m
Angstrom = 10-10 m
Light of a certain color also has a characteristic frequency
Frequency = number of waves passing an arbitrary location (any
convenient reference point) per second.
Frequency and wavelength are inversely related:
Longer wavelength = lower frequency = slower vibrations
Shorter wavelength = higher frequency = faster vibrations
Most of the time, in this class, we will talk about wavelength rather
than frequency. But if you specify one, you know the other.
The energy of the light is also related to wavelength and frequency.
Some typical wavelengths, temperatures, and frequencies
Object
Temperature
Dental
X-rays
Wavelength or frequency
narrow line near 5 nm
Sun
5700 K
Peak at about 500 nm
Your skin
about 70F
Peak at about 10000 nm
= 10 microns
WiFi signal
GHz
narrow signals at 2.4
or 5 GHz (approx.)
1 GHz = 109 waves/sec.
AM Radio
530 – 1070 kHz
wavelength 280 – 560 m
What about discrete spectra ?
• Narrow, well-defined spectral lines are phenomena caused by
energy transitions in individual atoms or molecules
• These narrow lines correspond to specific light energies
• Both atoms and molecules can exist in many possible energy states
• Only certain energy states are physically possible (“allowed”)
• So, the transition of an atom or molecule from energy state 1
to energy state 2 has a fixed energy difference
• That difference is characteristic of the atom or molecule
• The energy either absorbs light of a certain color, or
emits energy of a certain color
Low  High energy
(absorbs light)
High  Low energy
(emits light)
Spectroscopy and Atoms
How do we know:
- Physical states of stars, e.g. temperature, density.
- Chemical make-up and ages of stars, galaxies
- Masses and orbits of stars, galaxies, extrasolar planets
- expansion of universe, acceleration of universe.
All rely on taking and understanding spectra: spreading out radiation
by wavelength.
Types of Spectra and
Kirchhoff's (1859) Laws
1. "Continuous" spectrum radiation over a broad range of
wavelengths (light: bright at
every color). Produced by a hot
opaque solid, liquid, or dense
gas.
2.
"Emission line" spectrum bright at specific wavelengths
only. Produced by a transparent
hot gas.
3. Continuous spectrum with
"absorption lines": bright over a
broad range of wavelengths with
a few dark lines. Produced by a
transparent cool gas absorbing
light from a continuous spectrum
The pattern of lines is a fingerprint of the element (e.g.
hydrogen, neon) in the gas.
For a given element, emission and absorption lines occur at the
same wavelengths.
Sodium
The Particle Nature of Light
On microscopic scales (scale of atoms), light travels as
individual packets of energy, called photons. (Einstein
1905).
c
photon energy is proportional
to radiation frequency:
1
E n (or E l
)
example: ultraviolet
photons are more
harmful than visible
photons.
The Nature of Atoms
The Bohr model of the Hydrogen atom
(1913):
electron
_
_
+
+
proton
"ground state"
an "excited state"
Ground state is the lowest energy state. Atom must gain
energy to move to an excited state. It must absorb a photon or
collide with another atom.
But, only certain energies (or orbits) are
allowed:
_
_
_
+
a few energy levels of H atom
The atom can only absorb photons with exactly the
right energy to boost the electron to one of its higher
levels.
(photon energy α frequency)
When an atom absorbs a photon, it moves to a higher energy state
briefly
When it jumps back to lower energy state, it emits a
photon - in a random direction
Other elements
Helium
neutron
Carbon
proton
Atoms have equal positive and negative charge. Each element has
its own allowed energy levels and thus its own spectrum. Number
of protons defines element. Isotopes of element have different
number of neutrons.
Ionization
Hydrogen
_
+
Energetic UV
Photon
_
Helium
+
Energetic UV
Photon
+
_
"Ion"
Atom
Two atoms colliding can also lead to ionization. The hotter the
gas, the more ionized it gets.
So why do stars have absorption line spectra?
Simple case: let’s say these
atoms can only absorb green
photons. Get dark absorption
line at green part of spectrum.
“atmosphere” (thousands
of K) has atoms and ions
with bound electrons
hot (millions of K), dense interior
has blackbody spectrum,
gas fully ionized
Stellar Spectra
Spectra of stars differ mainly due to atmospheric temperature
(composition differences also important).
“hot” star
“cool” star
So why absorption lines?
.
. .
. cloud of gas .
.
.
. .
. .
The green photons (say) get absorbed by the atoms. They are emitted
again in random directions. Photons of other wavelengths go through.
Get dark absorption line at green part of spectrum.
Why emission lines?
hot cloud of gas
.
.
.
.
.
.
- Collisions excite atoms: an electron moves to a higher energy
level
- Then electron drops back to lower level
Molecules
Two or more atoms joined together.
They occur in atmospheres of cooler
stars, cold clouds of gas, planets.
Examples
H2 = H + H
CO = C + O
CO2 = C + O + O
NH3 = N + H + H + H (ammonia)
CH4 = C + H + H + H + H
(methane)
They have
- electron energy levels (like atoms)
- rotational energy levels
- vibrational energy levels
Molecule vibration and rotation
Types of Spectra
1. "Continuous" spectrum radiation over a broad range of
wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright
at specific wavelengths only.
3. Continuous spectrum with
"absorption lines": bright over a
broad range of wavelengths with a
few dark lines.
Kirchhoff's Laws
(1859)
1. A hot, opaque solid,
liquid or dense gas
produces a continuous
spectrum.
2. A transparent hot gas
produces an emission line
spectrum.
3. A transparent, cool gas
absorbs wavelengths from
a continuous spectrum,
Doppler shifts
●
Happens for all wave phenomena:
sound => change of pitch
light => change of wavelength (or color)
where V is the velocity of the emitting
source (m/s), c is the speed of light (m/s).
Redshift if receding, blueshift
(negative sign) if approaching.
Spectral lines are used to measure
Doppler shift => gives us
information about the motion of an
object.
Example Doppler shift
●
A spectral line normally seen at 400nm is
shifted to 401nm due to relative motion of the
source. What is the velocity of the source? Is it
approaching or receding?
We've used spectra to find planets around other stars.
Star wobbling due to gravity of planet causes small
Doppler shift of its absorption lines.
Amount of shift depends on velocity of wobble. Also know period
of wobble. This is enough to constrain the mass and orbit of the
planet.
Now 800 + extrasolar planets known. Here are the first few
discovered.
Final note: the Sun’s surface temperature of about
5800 K produces peak emission at 500 nm.
Sun is not yellow-green! What’s going on?
The Earth’s atmosphere scatters short wavelength blue light more
efficiently than red light. Sun appears redder because blue light
scattered away into the sky. Sun even appears red at sunset.
Telescopes
Telescopes
●
●
Basic function of a telescope: extend human vision
–
Collect light from celestial object
–
Focus light into an image of the object
Human eye works from 400 – 700 nm or so and uses a
lens to form an image on the retina. Astronomical
objects emit at much larger range of wavelengths, and
can be very faint!
Optical telescopes
Kinds of optical telescopes
–
Refractor – uses a lens that light passes through, to
concentrate light. Galileo’s telescope was a
refractor.
Large objective lens at the front of the telescope forms the
image, the eyepiece lens at the back of the telescope magnifies
the image for the observer.
Focal length, f, is distance from the lens to the focal point.
Problem with refractors: big diameter
objective lens means huge telescope!
Tube here is 64 ft, biggest lens ever
was 40 inches in diameter.
Solution: use concave mirror, not lens, to focus light.
Reflecting telescope.
Reflector – uses a mirror (shape is conic section–
typically parabolic). Big, modern research telescopes
are reflectors.
Observing with Your Eye vs. Telescope Photography
Eye
Q: Why the difference?
A: Because your eye has
its own “detector”
inside (the retina)
Eyepiece
Objective
Eyepiece provides
magnification
Your eye lens focuses the
light onto the retina
Detector
(or film)
Objective
Aligning the Telescope Axes … It Makes a Difference
“Alt-Az” mounting :
Vertical axis (sweep around horizon)
Horizontal axis (up and down)
“Equatorial” mounting :
Main axis is parallel to Earth N-S axis
Other axis goes up and down in ;atitude
Advantage:
Clock motor
turns the telescope
to match the Earth
rotation, so long
exposures possible
X
Problem:
Stars (& moon, etc.)
drift through
Image as Earth rotates
No good for
Photography,
Spectroscopy
!!
100 inch Telescope, Mt. Wilson Observatory
A small amateur telescope
Reflector advantages
●
●
●
Mirrors can be large, because they can be
supported from behind.
Largest single mirror built: 8.4 m diameter for
the Large Binocular Telescope
There are 10 m telescopes, but in segments
Reasons for using telescopes
●
Light gathering power: LGP  area, or D2
reason for building large telescopes!
Main
Reasons for using telescopes, cont.
●
Magnification: angular diameter as seen through
telescope/angular diameter on sky: m=fobj/feyepiece
–
●
Typical magnifications 10 to 100
Resolution: The ability to distinguish two objects
very close together. Angular resolution:
 = 2.5 x 105 l/D, where  is angular resolution of
telescope in arcsec, l is wavelength of light, D is diameter of
telescope objective, both typically in meters.
Two light sources with angular separation
much larger than
angular resolution
vs.
equal to angular resolution
Detectors
Quantum Efficiency = how much light they respond
to:
–
Eye  2%
–
Photographic emulsions  1-4%
–
CCD (Charge coupled device)  80%
●
Can be used to obtain images or spectra
Photographic film
CCD
Same telescope, same exposure time!
●
Spectrographs: light spread out by wavelength, by
prism or “diffraction grating”
Radio Telescopes
●
Problems – low photon energies, long l
–
●
Remember  = 2.5 x 105 l/D
Single Dish: need big diameter to get decent
resolution.
●
●
●
Can also design clever shapes of reflectors, which minimize
unwanted radio waves bouncing off feed legs into receiver
The Green Bank Telescope
Reflecting surface shouldn’t have irregularities that are
larger than 1/16 of wavelength being focussed – are radio or
optical telescopes easier to construct in terms of surface
accuracy?
●
●
But, wavelength is large – how do we get good
resolution?
Interferometers – e.g., VLA
Use interference of radio waves to mimic the
resolution of a telescope whose diameter is
equal to the separation of the dishes
Aperture Synthesis
-- Combine signals from
multiple apertures.
-- Control the optical (or radio)
path length from each aperture
so that the signals act as if they
were reflected from a single
larger, filled aperture.
-- Computers, high accuracy
time reference, and specialized
signal processing reconstruct
an image.
●
●
Our own
telescope: the
Long Wavelength
Array
Far larger than
the VLA.
“Stations” of 256
antennas, to be
spread across NM
●
Square Kilometer Array, currently being designed, will
be 50 times collecting area of VLA, with baselines to
1000’s km
Optical-mm Telescope sites
●
●
Site requirements
–
Dark skies (avoid light pollution)
–
Clear skies
–
Good “seeing”, stable atmosphere
High, dry mountain peaks are ideal observatory
sites, for optical to cm
Earth at night
●
Adaptive Optics – use a wavefront sensor and a
deformable mirror to compensate for deformations of
incoming wave caused by the Earth’s atmosphere.
Telescopes in space
Pros – above the atmospheric opacity so can work
at l impossible from ground, above turbulence,
weather, lights on Earth
Con – expensive!
The Hubble Space Telescope (HST)
Wide Field Camera 3
Cosmic Origins Spectrograph
• In 1996, researchers
at the Space Telescope
Science Institute used
the Hubble to make a
very long exposure of
a patch of seemingly
empty sky.
Q: What did they find?
A: Galaxies “as far as
the eye can see …”
(nearly everything in
this photo is a galaxy!)
40 hour exposure
JWST (James Webb Space Telescope)
Mirror 6.5 m, segmented
(current) Expected launch 2018
Spectral range:
0.6 – 28 microns
Science instruments:
NIR Camera (0.6 – 5 um) [U. Ariz.]
NIR Spectrograph (0.6 – 5 um) [ESA]
Mid-IR Instrument (5 – 28 um)
camera + spectrograph [Consortium]
Fine Guidance Sensor [Canadian Space Agency]
●
●
Hubble Space Telescope. 2.4 m mirror, 115nm
– 1 micron
Successor: JWST. 6.5 m mirror, 600 nm – 28
microns
Spitzer aperture: D = 0.85m, f/12
Be lightweight mirror, T < 5.5 K
Cooling: Liquid He
Wavelength coverage:
-- Imaging
3 – 180 microns
-- Spectroscopy
5 – 40 microns
-- Spectrophotometry 50 -100 microns
Kepler
Spacecraft
95 M pixels
FOV: 105 deg2
Photometer
accuracy: 0.0001
Kepler Instrument
[Davi10]
21 x 2 Focal Plane
Arrays
73
Chandra X-ray telescope
The sky at different wavelengths
Visible
Infrared
Gamma
ray
Radio
(neutral
hydrogen)
X-ray