The Amazing Spectral Line
Begin
Table of Contents
A light review
Introduction to spectral lines
What spectral lines can tell us
A Light Review
• Light is both a particle
and a wave.
• Being a wave, it has
both a wavelength
and a frequency.
• Wavelength (l) –
distance between
peaks
• Frequency (f ) –
number of cycles per
second
Some formulas
The speed of light (c) is constant in a vacuum 3.00 * 108 m/s
Wavelength x frequency = the speed of light
l f = c
This means a high frequency wave has a short
wavelength and a low frequency wave has a
long wavelength.
Light carries energy
Energy = frequency * Planck’s constant
E = hf
h = 6.63 * 10-34 J s
Wrap – up
So with light waves, you can convert between wavelength,
frequency, and energy with two equations:
l f = c
E=f h
And two constants:
c = 3.00 * 108 m/s
h = 6.63 * 10-34 J s
In the visible part of the spectrum, different colors
correspond to different frequencies, wavelengths and
energies. Blue light has a short wavelength, high
frequency and high energy. Red light has a long
wavelength, low frequency, and low energy.
An Element’s Fingerprint
• When excited by heat or electricity, gases glow with
characteristic colors.
• A prism can be used to spread out the light from these
hot gases.
• This reveals a series of discrete lines, the element’s
fingerprint.
• Chemists use these fingerprints (called spectral lines) to
identify elements both in the lab and in space.
Here are some spectral lines
Where do they come from?
The Bohr Model
In the Bohr Model of the atom, electrons orbit in
discrete energy levels. When an electron jumps to
a lower energy
level, the extra
energy is given
off as radiation.
This is where
those color
fingerprints
come from.
(learn more)
A Molecule’s Fingerprint
•
•
•
•
•
•
•
•
Hydroxyl radical (OH)
Methyladyne (CH)
Formaldehyde (H2CO)
Methanol (CH2OH)
Helium Isotope (3HeII)
Cyclopropenylidene (C3H2)
Water Vapour (H2O)
Ammonia (NH3)
1612.231 MHz
3263.794 MHz
829.66 MHz
6668.518 MHz
8665.65 MHz
18.343 GHz
22.235 GHz
23.694 GHz
Molecules, like atoms, have characteristic spectral lines.
Usually scientists look for a few specific lines to identify a
molecule. Above is a list of some astronomical chemicals
and their corresponding frequencies. Find radiation at one
of these frequencies, and you’ve found a molecule.
Where do they come from?
Rotation
• Molecules, like atoms, can occupy different
energy states. Diatomic (2 atom) molecules can
rotate in two different ways
• As the molecule changes rotation states, it emits
radiation at a characteristic frequency.
Some Difficulties in
Detecting Molecules
Molecules in space are detected through spectral lines,
often times the line from one rotational state to another.
Symmetric molecules like CH4, N2, H2, and O2 don’t have
rotational states, making them harder to detect. Also, in
order to identify a molecule, a researcher must identify
its spectral line. This is sometimes very difficult because
molecules in space are under very different conditions
(very low pressure, no container walls) than are found in
the laboratory. One example is the “forbidden” line of
O+2. Many spectral lines observed in space have not yet
been identified (learn more).
For us, spectral lines
look like this:
Remember that we could just as easily
use frequency or energy along the x-axis
What can they tell us?
• Emission or
Absorption
• Relative Abundance
• Direction
• Velocity
• Rotation
• Temperature &
Pressure
• Electric &
Magnetic Fields
• Probes
Emission or Absorption
• When the spectral line is emitting by a heated
gas, it appears as a spike.
• When the spectral line is absorbed by a cool
gas, it appears as a valley.
• Knowing whether your spectral line is an
emission or absorption line tells you if the gas it
came from is relatively hot or cold.
Relative Abundance
The following graph shows two spectral lines
for two different atoms or molecules. The line
on the left is much
more intense than
the line on the right.
This indicates that
the atom or
molecule represented by the line on the left is
more abundant
Direction
Spectral lines can tell us the direction in which their source is
moving. If the source is moving towards the receiver, the spectral
line will be shifted to a shorter wavelength (blue shifted). If the
source is moving away from the receiver, the spectral line will be
shifted to a longer wavelength (red shifted).
Not moving
Moving toward receiver
Moving away
from receiver
Velocity
The relative
change in
wavelength is
related to the
velocity of the
source. See:
Doppler Shift.
Not moving
Moving slow
Moving fast
Rotation
When an object rotates, part of it moves towards the observer and is
blue shifted. Part of it moves away and is red shifted. This leads to
Doppler broadening. The degree of broadening reveals the rate of
rotation.
Temperature / Pressure
Broadening
Thermal motion and high pressure can
broaden spectral lines by causing individual
molecules to experience significant Doppler
shifts. The natural width of a spectral line is
very small. It comes from the Heisenberg
Uncertainty Principle.
Electric & Magnetic Fields
Cause line splitting
Stark Effect – Plasma density (learn more)
Zeeman Effect – Magnetic field (learn more)
Paschen-Back Effect – Strong Magnetic Field (learn more)
Probes
Spectral lines can also serve as
Indirect measures of:
Abundance
Temperature
Density
Abundance
The hydrogen molecule (H2) is the most
abundant molecule in the galaxy. It does not,
however, emit a strong spectral line. Several
other molecules (CS, H2CO, HC3N) are used as
probes. When H2 collides with one of the these
molecules, it can produce detectable radiation.
So, in order to look for H2, we look for a probe
molecule.
Temperature
Symmetric top molecules like methane and
ammonia are valuable temperature probes. At
high temperatures, collisions with other particles
excite these molecules to higher energy states.
They return to lower energy states by certain
spectral lines. Detecting these spectral lines
gives information about the temperature of a
region of space.
Density
Some spectral lines are only formed in
dense environments. Find one of these
lines, and you’ve measured the density of
the surrounding region. This is especially
useful when studying star-forming regions.
Doppler Shift
MORE!
The Doppler Shift
is the change in
wavelength of a
wave due to the
relative motion
of the source
and receiver. It
is the reason
why a car seems
to change
sounds as it
passes by.
Another formula
To calculate the change in wavelength due to the
Doppler shift, use the following equation (learn more).
Dl = change in wavelength
Dl
loriginal
=
vsource
vwave