When matter in outer space interacts with light,
we see the results as a spectrum.
In astrophysics, we discussed three primary types of spectra,
summarized by Kirchhoff's three spectral laws.
The First of Kirchhoff's laws describe the conditions for blackbody emission,
while the other two laws deal with atomic emission and atomic absorption,
which we'll cover in the next lesson.
Kirchhoff's first law states that a luminous solid,
liquid or gas emits light at all wavelengths.
This law is a description of the type of light given off by blackbody emitters.
You're probably familiar with at least one type of blackbody emitter, your kitchen stove.
If it has electric coil burners,
like mine does, it will emit a deep red light when you're cooking.
This might be confusing since Kirchoff's law
says it should emit light of all wavelengths.
Well, blackbodies do emit light of all wavelengths just at vastly different intensities.
For example, there are so few x-rays being emitted at
this temperature that we can totally ignore that part of the spectrum.
On the other end,
the burner feels hot because along with a lot of red light,
the burner is also emitting plenty of infrared light.
The spectra of thermal or blackbody emitters are continuous, like rainbows.
You may remember from module two that blackbodies absorbs
all incoming light across all wavelengths and are completely non-reflective.
The kind of light they emit will depend solely on their temperature.
This temperature effect can be seen when
examining a fire poker as it gets hotter and hotter.
First, there is red hot then orange and yellow hot and finally, white hot.
The surfaces of stars,
which are close approximations to black bodies,
exhibit the same properties.
Though they can get even hotter,
getting to blue hot for certain types of stars.
As the thermal emitter gets hotter,
its peak emission is shifted into
more energetic portions of the electromagnetic spectrum.
Black hole accretion disks can be so hot that their peak emission isn't blue hot,
it isn't ultraviolet hot, it's x-ray hot.
When plotted, black body spectra are
smooth continuous curves that look like the hill of roller coaster.
Let's see how changing the temperature of a black body emitter changes its spectrum.
Two laws govern the shape of a blackbody spectrum.
First, Stefan-Boltzmann law states that
a hotter objects emit more light at every wavelength.
What does this mean for our plot?
Well, higher temperature, larger curve.
Additionally, the second law,
Wien's law states that a hotter object emits light with greater average energy.
What does this mean for our plot?
Well, a change in temperature will skew the peak of this graph.
Hotter objects move the peak toward shorter wavelengths,
higher energies and lower temperatures
move the peak towards longer wavelength, lower energies.
Recall that frequency and wavelength are inversely related.
So, the same graph,
plotted for frequency instead of wavelength, will look reversed.
A Higher temperature means that peak emission of
a hotter object is shifted right towards higher frequency which is more energetic.
Kirchhoff's second and third laws are concerned
with how emission and absorption spectra are produced.
Producing these types of spectra relies on a process which affects
individual atoms and molecules called luminescence.
Luminescence is the process that produces light
when electrons drop from higher energy states,
within an atom or molecule,
to lower energy states.
Kirchhoff's second law states,
a low-density hot gas seen against a cooler background,
emits a bright line or emission line spectrum.
As we mentioned before,
luminescence is responsible for this.
When electrons transition from higher energy states to lower energy states,
they emit light based on how far or how many states they drop.
The reverse process can also happen,
which are described by Kirchhoff's third law.
A low density cool gas in front of a hotter source of
a continuous spectrum creates a dark line or absorption line spectrum.
So, if the right energy of light is shined through a low density gas like a nebula,
electrons can steal energy from passing
photons in order to climb to the higher energy states.
By absorbing light, the low density gas,
cool gas takes away portions of the continuous spectrum of
the background emitter leaving behind dark absorption lines.
Energy level transitions are an effect of quantum mechanics.
Electrons surrounding the nucleus of an atom are able only to
accept quanta or in other words specific amounts of energy.
When a passing photon or a collision between
atoms in a gas has the right amount of energy,
the electron will transition to a higher energy state.
Transitioning to a high energy state is much like climbing a ladder.
You can only exist at the top of each rung.
You can attempt to place your foot in between the rungs,
but all that will result in is a banged up shin.
Surely, after an electron transitions to higher energy,
it spontaneously transitions back down to a lower energy state.
The energy the electron had has to go somewhere and what happens is the atom produces
a photon which has the same energy as
the difference between the higher energy state and the lower energy state.
A large number of these downward transitions will produce a bright emission line in
the spectrum and a large number of upward transitions
will produce a dark absorption line.
It's important to note now how we can determine
chemical composition using emission and absorption spectra.
Each type of atom or molecule has a unique set of
energy levels which produce a unique set of emission lines.
Meaning, each type of atom or molecule can be characterized by unique spectrum.
This is a lot like fingerprints on your hand.
Every person has a different set of fingerprints and every type of
atom produces a different emission and absorption spectrum.
A lot of work has been done by scientists to
study the spectra of atoms and molecules in the lab.
So, we know very well what they look like.
When we look at an object in the sky we can match parts of its spectrum
to specific atoms and molecules to determine what it is composed of.
Here's a great diagram that helps us explain Kirchhoff's three laws.
If a light source like a star is shined through empty space,
we see the black body spectrum it produces as continuous rainbow of colors,
like in the leftmost image.
If the star is a cooler outer atmosphere or photosphere,
then some of the cold atoms will will absorb photons with
specific colors causing electrons to move to higher energy states.
This removes light from the continuous spectrum of
a background source creating an absorption spectrum,
as shown in the center image.
If instead, light from a star strikes a nebula from the side,
electrons will be excited and subsequently fall to lower energy states.
In doing so, they will emit light with
specific frequencies in all directions including ours.
This process produces an emission line spectrum like in the rightmost image.
Keep in mind that we are generalizing the emission and absorption processes and that
both can happen simultaneously and are only mediated by the temperature of the object.
Both emission and absorption spectra are important
in determining the chemical abundances of objects in space.
We examine how bright emission lines are or
how dark absorption features are and how each is associated with
a different element or molecule to determine how much and
what is prevalent in all sorts of astrophysical objects.
This is the first step in characterizing
a black hole system but advanced techniques can tell us even more.
