Atomic Spectra
• A satellite orbiting the Earth contain
gravitational potential energy.
• The satellite can orbit the Earth at any
height. Or, it can contain any amount of
gravitation energy—Its gravitational
potential energy is continuous.
• Similar to the satellite, an electron
orbiting the nucleus of an atom possesses
electric potential energy. However,
unlike satellites in orbits, the electrons can
only stay in a finite number of discrete
energy levels (or orbits).
 The energy levels of the atoms are
‘quantized’.
• There are only certain
allowed Atomic energy
levels
• Different elements have
different energy level
structure…
• Gravitational potential
energy level can be
anywhere along the line…
Element B
i
E
Element A
j
Selection Rules:
There are rules
about if a
transition from
level i to level j
can occur or not!
How does light tell us what things
are made off?
• Every chemical element has
its own unit spectral
fingerprint.
• We can study the chemical
composition of an
astronomical object by
observing its absorption or
emission spectrum.
Emission spectrum of Hydrogen
Absorption spectrum of Hydrogen
With a thermal spectrum background.
Astronomical Spectra
We can learn a lot about a astronomical object by studying the
spectra of the object….
There are three basic types of spectra:
a. Thermal radiation Spectra: All objects with a finite temperature
emit thermal radiation.
b. Absorption Line Spectra:
c. Emission Line Spectra:
Click on image to start animation
Determining the Chemical
Composition of Stars
• By studying the strength of
the absorption spectra of stars,
we can determine the
chemical composition of stars
• The H-R diagram (Chapter
11) on the left shows how
chemical composition is
related to the temperature and
luminosity of stars.
How do we measure the temperature of
Stars (or any object)?
Everything with a temperature emit thermal radiation. We can measure the
temperature of the stars or any object by studying the shape of their overall
spectra.
Black Body
An idealized perfect light absorber that absorbs all the
photons that strikes it (no reflection). It re-emit the absorbed
energy through thermal radiation, with a spectrum
characterized by the blackbody spectrum.
• The shape of the blackbody
spectrum is always the same,
independent of its temperature.
• The peak position (in
wavelength) of the blackbody
spectrum depends only on the
temperature, independent of the
blackbody’s composition, or
size, etc.
Click on image to start animation
Different Temperature Scales
• Celsius temperature
measurement
system is calibrated
against the
properties of water
at sea level…
• Kelvin temperature
scale uses the same
temperature
division as the
Celsius system, but
set the lowest
possible
temperature to 0
degree.
Temperature of Stars
• By analyzing the thermal
spectrum of stars (at what
color is the star light
brightest?), we can
determine their
temperature.
• The H-R diagram (Chapter
11) shows how the
temperature of the main
sequence stars are related to
their luminosity.
How do we study the motion of
astronomical objects?
Doppler Shift
• The frequency (or wavelength) of a wave emitted by a moving
source observed by a stationary observer is different from that
when the source is at rest with respect to the observer.
• If the source is approaching the observer, than the observed
wavelengths will be shorter than the ‘rest’ wavelength—blue
shift…or higher pitch
• If the source is moving away from the observer, than the observed
wavelengths will be longer than the ‘rest’ wavelength—red
shift…or lower pitch
Line-of-Sight Velocity
• Doppler effect only tell
us about the velocity
along the line
connecting the observer
and the source—the
line-of-sight velocity.
• If the object is moving
in a direction
perpendicular to the
line-of-sight, then we
will not see any Doppler
effect.
Animation of Doppler Shift
Doppler Shift of Spectral Lines
• The wavelength of
spectral lines that are close
to each other will be
changed by the (almost)
the same amount, thus the
distance between spectral
lines does not change.
• By studying how much the
pattern of spectral lines is
shifted, we can measure
the velocity of the light
source with respect to us.
Click on image to start animation
Using Doppler Effect to Search for
Extrasolar Planets
• A large planet orbiting a star cause the star to move in a circular
motion also, producing Doppler Effect that can be observed
(Chapter 6).
• It is very difficult to directly see light reflected or emitted by
the planet, because it is overwhelmed by the star light.
Questions?
Telescopes
There are two different basic designs of telescope:
• Refracting Telescope: Light is focused by transparent glass with curved
surfaces (the lens)
• Reflecting Telescope: Light is focused by reflecting curved mirror.
The 1-meter reflector at the
University of Chicago’s Yerkes
Observatory was the largest
refracting telescope.
The primary mirror of Gemini
is 8 meter in diameter
How Refracting Telescope Works
• Light changes its propagation direction when it enters a
different media (e.g., apparent ‘bending’ of a stick partly
submerged in water).
• The shape of the lens is precisely made such that light
from a point source far away can be focused into another
point.
• Drawback of Refracting Telescope:
– No glass can transmit the entire electromagnetic spectrum.
– Large glass with high quality is difficult to make.
– The size of the lens of the largest refracting telescope is
about 1 meter.
Light from nearby point
Light from very distant point
Focal length of the
lens
How Reflecting Telescope Works
• With the correct shape, light from distant point can be focused into a point by
a curved reflecting mirror.
• A reflecting telescope works for all the wavelength…
• We can build much larger mirrors than lenses…
Keck Telescope on Mauna Kea
Why do we need BIG telescopes?
Two important factors of astronomical observation depends on the size of
the telescope—the larger the better.
• Sensitivity
The farther an object is, the fainter it becomes. A large telescope
collects much more light from a distant object than a small one,
allowing us to see very faint, distant object.
• Resolution:
The farther an object is, the more difficult it is to see the detailed
structure of the object. The angular resolution of a telescope depends
on its size. Thus, the larger the telescope is, the more details it can
resolve.
Car Headlight Simulation
Click on image to start animation
How Earth’s Atmosphere Affect
Astronomical Observations
Some undesirable things of Earth’s atmosphere…
–
–
–
–
–
Day-night cycle.
Light pollution…
Clouds, rains, snows…
The motion of the atmosphere blurs the images formed by the telescopes on the ground.
Some light (wavelengths) cannot reach the ground (gamma ray, X ray, UV, and IR)
Best Place for Observatories
• Dark sky
• Dry air (no clouds, rain, snow, low IR
absorption)
• good seeing ( Calm atmosphere gives sharp
images)
• High (less air to deal with)
 Mauna Kea, Chile
 Space
 Moon?
The primary mirror of Hubble Space
Telescope is only 1.5 meter, but it has
produced much sharper images than the big
telescopes on the ground because it is not
affected by the undesirable effect of the
atmosphere…especially seeing.
Adaptive Optics
With new technology, the seeing effect of the atmosphere can be minimized.
• Adaptive optics (AO) compensate the blurring effects of the atmosphere by
rapidly changing the shape of a mirror to focus the light better.
Image of sunspot with
and without AO
AO corrected image
of a binary system
Telescopes for X-Ray, Radio
Wavelength
• Chandra X-ray Telescope
• Very-Large Array Radio Telescope in
New Mexico…
Observations Across the Spectrum
Crab Nebula observed in four different wavelengths
X-ray
visible
Infrared
Light at different wavelengths are produced by
different physical processes, and/or from different
regions. Thus, we can get a complete picture of
what’s going on only by observing the same object
at all regions of the electromagnetic spectrum…
Radio
• What’s light?
– Basic properties about light…
• What’s spectrum?
– Wavelength, frequency, energy light carries…
• Matter
– Atomic structure
– How do we build different elements?
• How does light interact with matter?
• How does light with different wavelength interact with matter?
– What can we see on the surface of the Earth?
– What can’t we see on the surface of the Earth?
• How do we measure chemical composition?
– Atomic spectra
– Quantization of atomic spectra
• How do we measure speed?
• How do we measure temperature?
• What do we learn about an astronomical object observing with different
telescopes?
• How do telescopes work?
• Why do we need to put telescopes in space?