Some of the most important information in
astronomy comes from a technique of spectroscopy,
which involves dispersing light
into its constituent wavelengths,
as Newton did with his prism hundreds of years ago.
Looking at an image of this star cluster
always sees the stars,
maybe we can measure their colors,
but there's little extra information here.
Look instead of this star cluster where we
can see the spectra of every star within it.
From these spectra and work
first attempt at a 100 years ago,
we can learn not only the temperature of the stars,
but also their density,
their composition, and parts of their life stories.
Spectroscopy gives us a lot more information
than just taking an image.
It is one of the most powerful tools of astronomy.
It gives us remote sensing for
physical conditions far away,
millions or even billions of light years away.
To talk about spectroscopy,
we have to look at the way that
light interacts with matter.
Essentially, everything we know in astronomy comes from
the interactions of light or other forms
of electromagnetic radiation with matter.
We are using remote sensing to learn about
distant objects based on
the radiation they emit or absorb.
In principle, light can be emitted, absorbed, scattered,
or reflected from material
and material objects like stars.
We use terminology like transparent or opaque.
A gas or an object that is invisible to
radiation or where radiation cannot be
transmitted is considered opaque.
An object where radiation is freely transmitted
without interaction is considered transparent.
We're familiar with some of
these interactions in our everyday life.
It's important to be clear about the distinction
between the apparent and
intrinsic properties of something.
When you look at something like
a wall or a sweater that appears to be red,
that's not because that's
thermal radiation from
the object reflecting as temperature,
that's because of a pigment or die in
the paint or the dye used for the cloth,
and it's what that radiation from
the sun or from an overhead light reflects and absorbs.
An object that appears red in that sense,
is just reflecting the red part of
the visible or white light to
you while absorbing the blue light.
That's quite different from the thermal radiation,
where a blue object is
a hotter object and a red object is a cooler object.
To dig deeper into
the interaction between light and matter,
we have to look at
how light interacts with individual atoms.
It was learned about a 100 years ago,
that the atom does not have
a continuous set of energy states.
It has a quantized set of energy states.
This is the base on the so-called quantum theory
of atomic structure,
was first understood by
Bohr Heisenberg and others in the early 20th century.
In other words, atoms cannot have any energy,
and they cannot exchange energy in any amount.
They can only absorb or emit particular amounts
of energy corresponding to
their particular energy levels,
the quantized energy levels of an atom.
Every single unique element
with a unique number of protons and electrons
has a unique set of energy levels that mark
it out is like the fingerprint of an atom,
and it of course means they have
a spectral fingerprint as well or a set of
photons that can be emitted and absorbed
that's completely unique to each chemical element.
Taking the simplest element hydrogen,
it has a set of energy transitions
corresponding to energy that it can lose or gain,
and these energies map by
the electromagnetic spectrum into
particular wavelengths or frequencies of light.
So hydrogen, if it's excited or if it absorbs energy
has a particular fingerprint of
possible transitions, it's spectrum.
This was first seen over a 100 years ago.
We can take a Bunsen burner
and heat up chemical elements,
and then disperse the light that comes off
or is emitted by those elements with a prism,
and we will see this particular emission lines
are sharp spectral features at particular wavelengths,
and they are always unique to the element.
In these examples, we see the uniqueness of this imprint.
Hydrogen, helium, and heavier elements like
neon and mercury have particular sets of transitions.
The appearance of the spectra from
these objects to the naked eye,
is based on the combination of all these ingredients
because the eye does not disperse into a spectrum.
But in the case of neon,
we see that a lot of the energy comes in
two particularly intense lines in
the red or orange part of the spectrum,
which gives neon signs
their purity of their red characteristic.
Other elements like helium have
spectral lines across the blue, red,
and yellow parts of the spectrum,
and so emit a duller glow of yellow or pale yellow light.
Regardless of the particular characteristics,
the fact that each element has
a unique spectral signature,
means that if you have a combination of these gases,
those elements just add.
If you have a mixture of hydrogen and helium
contained within a gassy experiment,
you'll see the lines corresponding to the sum of
both a heat helium lines and
the hydrogen lines superimposed on each other.
Working this logic backwards,
that means if we had an unknown gas
and dispersed it light into a spectrum,
we could diagnose the chemical ingredients.
Spectroscopy actually allows us to go further.
If we have good enough spectroscopy,
we can actually look at the profile or shape of
those sharp spectral features and infer
the temperature or the density of that hot gas,
and that's how we learn about distant stars.
The spectral fingerprint is
an extraordinary sensitive technique.
In astrophysicists hands, we can detect
the presence of trace elements in a star at
constitution of one part in a trillion
relative to hydrogen or
helium, extraordinary trace elements.
All this information can be combined to
say what the universe is made of in terms
of constituents in the periodic table
and their abundance relative to hydrogen and helium,
the most abundant elements.
In astronomy, we tend to see
three different types of spectra.
One is the spectrum emitted by a hot gas,
such as the discharge tube.
This is where the gas is excited and emits
the narrow sharp spectral features
corresponding to its element.
This is called an emission spectrum.
We can also see the smooth spectrum that just results
from the random motions of atoms or molecules in a gas,
and this is the smooth thermal spectrum,
which would peak at
optical wavelengths for a hot object like
the sun at an infrared wavelengths
for a cool object like a planet.
The third type of spectrum we can see is when
a hot source has a cooler envelope of gas outside it.
In this case, the high excited transitions
are absorbed by the cooler gas,
but again with a characteristic fingerprint
of the element that gas is made of.
This is actually what the sun's spectrum looks like,
because the hot fusion core
is surrounded by cooler outer layers.
These three types of spectra to astronomers,
I'll call it a continuous spectrum,
the smooth thermal spectrum of a hot object,
an emission spectrum from a hot excited rarefied gas,
and an absorption spectrum where
the hot excited gas is surrounded by cooler material.
The rules by which these three spectra
are produced are called Kirchhoff's Laws.
Another very important attribute
of radiation and astronomy that's
diagnosed by spectroscopy is a Doppler effect.
The Doppler effect is familiar to us from soundwaves,
but it applies to any form of wave.
In a Doppler effect,
a source of sound waves has
those waves compressed in the direction of its motion.
When the ambulance approaches you,
it's catching up with the waves that it emits,
compressing their frequency and raising the pitch.
So you hear the rising pitch of the ambulance.
When it's receding from you,
it's moving away from its waves,
stretching out their wavelength, or pitch,
or lowering it, and you hear the lowering tone.
That's a very characteristic phenomena for sound waves,
but the same thing happens with
light waves or any electromagnetic wave as well.
So in terms of light waves,
the Doppler effect corresponds
to the fact that if a source
of radiation is moving away from us,
the wavelength is shifted towards the
red compared to that source being stationary.
If that source is moving towards us,
the waves are squashed,
or compressed, or shorten
compared to that source being at rest.
So we have red shift for objects moving away from us,
and blue shift for them moving towards us.
The percentage of the shift equals
the velocity of the motion
relative to the speed of light.
Since the speed of light is a very large number,
300,000 kilometers per second,
and the motions of most
astronomical objects are much smaller,
maybe tens or hundreds of kilometers per second,
a Doppler shift is usually a rather small amount,
small fractions of a percent.
Even though it's a small effect,
spectroscopy is such a powerful technique
that it's easy to
detect tiny fractional shifts
corresponding to this effect.
So we can easily map the motions of stars or
even entire galaxies using the Doppler effect.
Notice that the Doppler effect cannot
tell us about the transverse motion of
an object when it's just
tracking on the sky in parallel to us.
It only talks about the component of
its motion towards or away from us.
Therefore, to reconstruct
the full three-dimensional space motion of an object,
we don't have sufficient information,
all we're getting is one component or vector
of the information towards or away from us.
Almost everything we know about the universe comes from
the interactions between radiation or light and matter.
Light can be emitted,
absorbed, scattered, or reflected.
Spectroscopy is the powerful technique that
astronomers use to diagnose the distant universe,
based on dispersing light into a spectrum,
and looking at the details of what's
going on in the emission from an object.
Hot astronomical objects show
characteristic spectral fingerprints which
tell us their chemical constituents,
what ingredients they are made of.
This technique is extraordinarily powerful and sensitive.
We can detect the traces of
heavy elements at a level of one part in a
trillion relative to hydrogen and
helium that are most abundant elements in the universe.
We can even use the shape of
the spectral lines to talk about
the temperature and the density of
a distant star or even a galaxy.
Remote sensing, where the use of spectroscopy to diagnose
the physical conditions of the universe is
perhaps the most powerful technique in astronomy.
We can also measure the Doppler shift or
the relative space motion of an object based on
its waves being compressed or rarefied due
to its relative motion with respect to us.
