Chapter 7:
Atoms and starlight
Not all of these light sources emit the same kind of light
Fluorescent
light
The Sun!
Incandescent Bulb
Halogen bulb
Each part of the EM
spectrum provides
us with different
information.
If you pass white light through a
prism, it separates into different
colors.
R
O
Y
G
B
I
V
spectrum
What can we learn by
analyzing starlight?
• A star’s temperature
• Its composition
An atom consists
of a small, dense
nucleus
(containing
protons and
neutrons)
surrounded by
electrons
- Model
Proposed by
Niels Bohr 1913
Atoms are mostly empty space
So if a nucleus the
size of an orange
(10 cm) was
located at the
center of the
football field,
where would the
electron be?
End Zone?
Grandstands?
On Campus?
In San Jose?
Nucleus
Photons (light-waves) are emitted
from an atom when an electron
moves from a higher energy level
to a lower energy level
Nucleus
Photons (light-waves) can also be absorbed by an
atom when an electron moves from a lower energy
level to a higher energy level
Nucleus
Each chemical element produces its
own unique set of spectral lines when it
is excited
Three types of spectra
prism
Hot/Dense Energy Source
Continuous Spectrum
prism
Hot low density cloud of Gas
Emission Line Spectrum
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
Types of spectra:
Some light sources
emit all colors
(appears as white
light).
Others emit just a
few colors.
Some are missing a
few colors.
Continuous spectrum is also called a “Blackbody
Spectrum”
What color is our 5800K Sun?
The Sun emits all
wavelengths of
electromagnetic
radiation (light);
however, the
wavelengths of light it
emits most intensely
are in the green/yellow
part of the spectrum.
Absorption spectrum: atmosphere of stars
Emission Line Spectrum:
Pictorial
Graphical
Absorption of light: electrons excited to higher energy
levels. Photon is gone!
Absorption Line Spectrum:
Pictorial
Graphical
The Doppler Effect
Astronomy Application
V=0
An Atom’s Structure: The Bohr Atom
Electron orbits in hydrogen
and helium
The pattern of energy
levels is different for
every element.
When electron drops to lower energy level, it releases
(emits) a photon.
Photons
released in
random
directions.
The energy of the photon depends on the size of
the jump between orbits.
Emission spectra of hydrogen and helium
Atomic Fingerprints
H
He
Ne
Kr
Continuous spectrum is also called a “Blackbody
Spectrum”
Hot, low density gases generate
emission line spectra.
 e.g. fluorescent lights
Two stars orbiting one another. The diagram
shows the orbital motion of one of the stars, and
four different positions are marked (A, B, C, D).
At which position is the light from the star
redshifted? blueshifted? not shifted at all? D
a) A, C, B/D
b) C, A, B/D
c) B, D, A/C
A
d) D, B, A/C
B
C
The Earth’s atmosphere is opaque to the
________ part of the electromagnetic
spectrum:
a)
b)
c)
d)
e)
f)
Visible
Infrared
X-Ray
All of the above
A and C are correct
B and C are correct
Which of the following statements is true with
regards to Blackbody radiation?
a) A blackbody emits more energy at longer
wavelengths as it heats up.
b) A blackbody emits energy at all wavelengths
c) The shape of the blackbody spectrum depends
on what the source is made of.
d) All of the above
e) None of the above
Which transitions were responsible for each of these
absorption lines?
a) A: 1-2 B: 2-4 C: 1-4
b) A: 1-4 B: 2-4 C: 1-2
c) A: 4-1 B: 4-2 C: 2-1