- Hi, everybody, Jade here.
I'm at the iTelescope Observatory
in Siding Spring Australia
and today's video is about one of the greatest riddles
in astronomy, which took over 60 years to solve.
It's about a mystery surrounding these lines.
We'll use the amazing equipment around us today
to help us recreate the story
and see if we can solve the pieces of the puzzle ourselves.
It all started in 1864 when an English astronomer named
Sir William Huggins looked through a telescope,
very similar to the one we have here.
He described the night like this,
I directed the telescope
for the first time to a planetary Nebula in Draco
which today is known as the Cat's Eye Nebula.
After a few moments of hesitation
I put my eye to the spectroscope.
Was I not about to look into a secret place of creation?
No spectrum such as I expected,
a single bright line only.
Huggins was extremely excited by what he saw
and here's why.
- See what Huggins saw were spectral lines
and every element on the periodic table
has its own unique signature in the spectral lines.
So how do we get spectral lines?
Well, we need a spectrograph
and this is a very rudimentary spectrograph.
It's just the basics.
So what we need is a light source,
in this case, it's a fluorescent light,
emitting through a collecting lens,
a slit, which is effectively two metal plates
with a very small diameter between them.
Making the light a line of light,
that comes through a collimating lens
and then it hits the diffraction grading.
The diffraction grading fans light out into the spectrum
so that rainbow, that light is then emitted into
in this case a lens, but in a research grade spectrograph
it will be cameras.
- So what was so exciting about this single line
that Huggins saw was that it didn't match
any of the spectral lines of any of the known
elements at the time,
the only conclusion was that he discovered
a new element that didn't exist on earth.
Now, this is where the riddle begins,
as exciting as the discovery of a new element was,
there were some problems,
for one, it was speculated that nebulium
should be a fairly light element.
This is because the list of elements
found in nebulae at the time,
included only substances of low atomic weight,
hydrogen, helium, carbon,
nitrogen, and oxygen.
However, the table of elements was very well
understood at the time
and they didn't seem to be room for another light element.
So the riddle was there was scientific evidence
of a new element, but it didn't fit in anywhere
with the current model of science.
This puzzle remained unsolved for over 60 years,
until an American grad student Ira Bowen came along.
Bowen knew about this riddle
as did most astronomers at the time.
And in 1927, he happened to stumble across
a book at the library.
Little did he know this moment
would change the course of astronomy.
The book was simply a textbook called Astronomy,
written by a permanent astronomer Russell Dugan.
Dugan was talking about the riddle of nebuleum
and he speculated that the nebula lines may be admitted
only in a gas, a very low density.
This would happen for example,
if it took a relatively long time for an atom
to get into the right state, to emit them
and if a collision with another atom in this interval
prevented the completion of the process.
In such a case it might require a great thickness
of the very rarefied gas to emit these lines
strongly enough to be visible.
Even though he didn't fully grasp them,
these words struck a chord with Bowen.
A seed had been planted in his mind,
what it would growing into, he didn't yet know.
Weeks went by, then one night
as Bowen was preparing for bed,
the answer suddenly came to him.
He quickly dressed and rushed back to his office
in tremendous excitement.
Now before I tell you Bowen's revelation,
I thought it would be fun to see
if we could try and figure out the riddle ourselves.
I'm going to give you all the pieces
of information you need as clues
and afterwards, I want you to pause the video
and see if you can solve it yourself.
Just a warning, these clues aren't like regular clues
in that they contain a lot of science
and could be considered quite advanced,
but hey that's why you're here right?
Let's see if we would've made good astronomy detectives,
listen carefully to these four clues.
Clue number one, spectral lines are formed
by photon emission.
We've talked about how spectral lines are used
to identify elements, but we haven't talked about why
they happen in the first place.
The way spectral lines are formed
has a lot to do with the structure of atoms.
You may know that atoms consist of a nucleus in the middle
and electrons whizzing around it.
This isn't exactly the whole story,
because quantum physics likes to complicate things,
but this model is good enough for our purposes
so we're going to use it.
Now a key characteristic of atoms
is that their electrons exist in each fixed orbitals
around the nucleus.
So an electron can be here or here
but not anywhere in between.
They simply can't exist there,
this has to do with the quantum nature of physics.
It's kind of like when you drink coffee,
there are discreet states of focused,
hyper-focused, anxiety and hyperventilation
and you can't seem to land anywhere in between.
The point is that the electrons need to exist
in these discrete orbitals.
Now, electrons can also jump between orbitals.
When an electron is given some energy
it'll jump to a higher orbital.
One way an electron can get energy,
is if incoming light or a photon hits it
and it absorbs the energy of the photon.
A characteristic of the universe is that it's kind of lazy,
so atoms like to be in the lowest energy state possible
which means after some time, the electron will relax back
to a lower orbital.
Because it was transferred energy in the form of
a photon to jump up, it releases energy as a photon
on the way down.
The released photon has the exact frequency
which corresponds to the energy between the orbital gaps
and this is what is collected by the spectrograph
and produces spectral lines.
As different energy level photons produce
different colors of light.
So why does each element have a unique set
of spectral lines?
Well, the atoms of each element
all have different signature energy spacings
between their orbitals.
Because the admitted photons have a frequency
which corresponds to the energy spacings,
they will have different frequencies
depending on which element they came from.
These frequencies are all collected by the spectrograph
and we get these lovely spectral lines.
So the green line that Huggins saw,
would be coming from photon emissions
from an element in the nebula.
So now you know how spectral lines are formed
and we have our first clue to help us solve the puzzle.
Clue number two.
A photon is not always emitted when an electron relaxes
to a lower energy state.
We just talked about how a photon can excite an electron,
but electrons can also be excited by atoms
that bump into them.
Instead of absorbing the atom
there's a transfer of kinetic energy.
This is called excitation by collision.
Electrons can also relax down to lower orbitals
by collision with other atoms too.
Sometimes an electron in a higher energy state
can transfer some energy to an incoming electron
by bumping into it.
Because it's given some of its energy away,
it relaxes down.
In these cases a photon is not emitted,
instead, the excess energy is transferred to the free atom
as momentum.
The key idea here is that a photon is not emitted
when an electron relaxes to a lower energy state
from a collision.
Clue number three, metastable states.
The most stable state for an electron to be in,
is what's called the ground state.
This is the state with the lowest amount of energy
again because the universe is lazy.
So if an electron is in the ground state,
it's pretty happy just chilling out
and it's not going to move
unless there's some kind of stimulus,
like a photon or collision.
When it is given a stimulus,
it can move into a higher orbital
and because it wants to relax back down
and it doesn't really like being there too much,
we call this an unstable state.
It's not going to be in this state for very long
and by not very long, I mean,
typically less than a microsecond.
Then there are the metastable states
which are kind of stable and kind of not.
The best way to think about them is like this,
imagine a ball at the bottom of a valley
and just up from the valley,
there's another smaller shallower valley.
The ball is in the most stable state, in the lowest valley
as it needs a pretty big push to get out of it.
However, if the ball is somehow pushed to the second valley,
it's pretty stable in that it won't move on its own,
but it also won't take much of a push for it
to get back down to the lower valley.
So it's kind of stable, but not super stable,
it's metastable.
An unstable state would be something like
this tiny cliff sticking out.
The ball sure isn't going to last very long there,
and will probably fall down just from gravity.
So the ball stays longest in the ground state,
shorter in the metastable state
and even shorter in the unstable state.
Likewise, the energy orbitals of atoms
have the stable ground state,
unstable states and metastable states.
And the electrons can stay in the metastable states
much longer than in the unstable states.
They usually spontaneously emit
and release a photon after seconds or even hours
which is a lot longer than the less than
a microsecond the unstable states last.
These metastable states are usually in
lower energy orbitals of the atom
than the unstable states
and electrons are usually excited into them by collisions
rather than photons.
Even though they take a long time to spontaneously emit,
a collision with another atom,
can knock an electron out of a metastable state.
So that's our third clue.
Metastable states take a long time
for an electron to get out of spontaneously,
but can be knocked out by a collision.
The final clue, clue number four.
The atmosphere of earth is much denser
than the atmosphere of a nebula.
This is the final piece of the puzzle,
on earth our atmosphere has over 10 to the power of 19 atoms
per cubic centimeter.
I know it doesn't look like our atmosphere is filled
with so much stuff, but remember that atoms are small,
crazy small.
Now the density of nebuli is between 1000 to 10,000 atoms
per cubic centimeter,
which is barely anything compared to earth, right?
The density in a nebula is purer
than any vacuum ever produced on earth.
On earth particles collide 10 to the power of
19 times per second.
So that's the final clue.
The density of atoms on earth is crazily large
compared to the density of atoms in the nebuli.
Now let's recap the riddle and clues,
then we'll pause the video
and try and solve it for ourselves.
A spectral line was found in space that does not correspond
to any known element ever seen on earth.
The only conclusion is that it's a new element, nebuleum
which does not exist on earth.
However, there was no space in the current scientific model
for a new element, with the properties of nebuleum
How could this be?
Well, we heard four clues to help us figure it out.
Clue one, spectral lines are formed by photon emission.
Clue two, a photon is not always emitted
when an electron relaxes to a lower energy state.
Clue three, a collision with an atom
can knock an electron out of a metastable state
and clue four, the atmosphere on earth is much, much, much
much denser than the atmosphere in a nebulae.
Pause the video now
and see if you can solve the riddle yourself.
If you to look at any of the clues again
I put the timestamps on the screen,
so you can go and look at them again.
So pause the video
and write your answers in the comments below.
So how did you go?
Let's see Bowen solution to see how yours compares.
Bowen came to the conclusion that the unknown lines
were in fact caused by a known element,
just in a different atmosphere,
as Dugan said "In a gas of very low density, like a nebula"
On earth, if an electron is excited into a metastable state,
because there are 10 to the power of 19
collisions per second,
it won't be long until it's hit again by another atom.
Remember that spontaneous emission
from a metastatic state can take seconds to hours,
but on earth there are 10 to the power of 19
collisions per second.
When electrons are de-excited by collisions with atoms,
the excess energy is carried away with the atom
instead of being emitted as a photon,
therefore in the Earth's atmosphere
we rarely ever see a spectral line produced
by a metastable state.
In a Nebula however, it's a totally different story.
Because of the low density of atoms in a nebula,
if an electron happens to get into a metastable state,
it has a lot of time to spontaneously emit
as there isn't much probability
of it bumping into another atom,
therefore a photon is released when the electron relaxes
and we get these spectral lines
from the metastable transition.
The signature energy gap of these atoms
corresponds to a beautiful green color of light.
So nebulium didn't exist after all,
it was actually the metastable state's photon from oxygen.
These were given the name Forbidden Lines
because they can't be observed anywhere on earth.
Now just quickly, Christian an astronomer
from the Siding Spring Observatory
is going to say a few last words about Forbidden Line.
- Well, we've talked about the oxygen three lines
being forbidden lines and that has been the bone mystery
which you have understood is now solved.
However, these lines are not only spectral lines,
they actually give beautiful colors of our universe
and that's what these telescopes
at Siding Spring are there for.
Let's take a close look here at one of these telescopes.
So you can see a typical telescope consists of
what we call an optical tube assembly,
that's where the reflector is.
A large camera that is cooled
and a huge filter wheel,
which actually contains color filters.
So these color filters are typically red, green, and blue
and in addition to that,
you have the so-called emission line filters
which we also call narrow line photons
and they are oxygen free which is our Forbidden Line
and now we will see all in color.
And the sulfur line as well as the hydrant alpha lines
and that is what makes these beautiful pictures
that you often see with a Hubble telescope
or online in other beautiful sites.
- So that's the story of one of the greatest
astronomy riddles in history.
A huge thank you to the people of Siding Spring Observatory
for helping me make this awesome video.
Also a huge thank you to my patrons on Patreon
who these videos would not be possible without.
Let me know how you did in the comments with the riddle
and I'll see you in the next video.
Bye.