What is a black hole? Do they really exist? How do they form? How are they related to stars? What would happen if you fell into one? How do you see a black hole if they emit no light? What’s the difference between a black hole and a really dark star? Could a particle accelerator create a black hole? Can a black hole also be a worm hole or a time machine? In Astro 101: Black Holes, you will explore the concepts behind black holes. Using the theme of black holes, you will learn the basic ideas of astronomy, relativity, and quantum physics. After completing this course, you will be able to: • Describe the essential properties of black holes. • Explain recent black hole research using plain language and appropriate analogies. • Compare black holes in popular culture to modern physics to distinguish science fact from science fiction. • Describe the application of fundamental physical concepts including gravity, special and general relativity, and quantum mechanics to reported scientific observations. • Recognize different types of stars and distinguish which stars can potentially become black holes. • Differentiate types of black holes and classify each type as observed or theoretical. • Characterize formation theories associated with each type of black hole. • Identify different ways of detecting black holes, and appropriate technologies associated with each detection method. • Summarize the puzzles facing black hole researchers in modern science.
08.03 – Chopping Up Rainbows
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Astro 101: Black Holes
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Hunting for Black Holes
If black holes absorb all light, how do we see them? In this module, you will explore how astronomers observe real black holes, from studies of accretion discs and jets to the study of material orbiting a black hole.
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Sharon MorsinkAssociate Professor
Department of Physics, University of Alberta
Have you ever wondered how we can determine so
much about objects in the universe, like the Sun.
How do we know that it's made up of
74 percent hydrogen or that its surface temperature is
almost 6,000 kelvin or that the Sun rotates once every 24 and a half days?
The answers to all these questions can in some way
be tied to a field of science called spectroscopy,
which is the study of light and its interactions with matter.
When a scientist observes and records light,
they produce a spectrum or many spectra which tell them,
how much of each color or wavelength of
light is produced by the objects they are studying?
When an astronomer produces a spectrum,
they're spreading incoming light into a band of colors.
Just like you see, when light from the Sun is spread into a rainbow.
This is exactly Newton's famous prism experiment,
where he demonstrated that white light is actually
a combination of every color of the rainbow.
An astronomer collecting spectral data is doing much the same thing.
The act of separating light into its component colors.
Only modern equipment used a special instrument called diffraction gratings.
Just like Newton's prism,
diffraction gratings separates light into its component colors.
However, diffraction gratings use the physics of
waves to spread light out instead of refraction.
An example of this, is the interference on a compact disc.
The rainbows produced are from the interference of
light rays reflected by the lines etched on the CD's surface.
Spectrometers can be combined with telescope technology for use in astrophysics.
Telescopes equipped with spectrometers can do it's called imaging spectroscopy.
By collecting a spectrum for each pixel of an image,
scientists can distinguish what colors are
produced in different locations on the same object.
A spectral image is a type of image that allows us to do very special things.
Even to regions which look the same to our eyes,
if we examine their spectra overlaid on one another,
there can be noticeable and important differences.
From a spectral image, we can extract a wealth of
information about different parts of astrophysical objects.
Since each pixel corresponds to a different location in the sky,
we can examine all the different parts of what we call extended sources.
An extended source is something that doesn't appear point like to us.
They extend over a region of the sky.
Extended sources appear to be large,
either because they're big or because they're very close to us.
Some examples of extended sources are the Sun,
a nebula, or even a galaxy.
Spectroscopy is one of the most frequently used tools in an astronomers toolbox.
Without spectroscopy, we would struggle to define
important characteristics of the objects we view,
speeds, temperatures and compositions, to name a few.
When matter in outer space interacts with light,
we see the results as a spectrum.
In astrophysics, we discuss 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 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.
While 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 hand,
the burner feels hot because along with a lot of red light,
the burner is also emitting plenty of infrared light.
The spectrum of thermal or blackbody emitters are continuous, like rainbows.
You may remember from module two that blackbodies absorb
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 blackbodies exhibit the same properties,
though they can get even hotter,
getting to blue hot for certain types of stars.
As a thermal emitter gets hotter,
it's peak emission is shifted into
more energetic portions of the electromagnetic spectrum.
Black hole accretion disk can be so hot that their peak emission isn't blue hot,
it isn't ultraviolet hot, it's X-ray hot.
When plotted, blackbody spectra's smooth
continuous curves that look like the hill of a roller coaster.
Let's see how changing the temperature of a blackbody emitter changes its spectrum.
Two laws govern the shape of a blackbody spectrum.
First, Stefan-Boltzmann law states that
a hotter object emits more light at every wavelength.
What does this mean for our plot?
Well, higher temperature, larger curve.
Additionally, the second law, Wein'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 wavelengths, 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
high 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 is 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 shining 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,
cooled 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 running 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 and 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.
Shortly after an electron transitions to higher energy,
it spontaneously transitions back down to a lower energy state.
The energy electron have has to go somewhere and what happens is,
the atom produces a photon which is 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 a 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's composed of.
Here's a great diagram that helps us explain Kirchhoff's three laws.
If a light source, like a star is shining through empty space,
we see the blackbody spectrum it produces as a continuous rainbow of colors,
like in the leftmost image.
If the star has a cooler outer atmosphere or photosphere,
then some of the cold atoms will absorb photons with
specific colors causing electrons to move to higher energy states.
This removes light from the continuous spectrum of
the 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-like 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.
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