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Pin-pricks in the celestial sphere, through
which shines the light of heaven?
Or gods and heroes looking down from their
constellations?
Or lights kindled above middle earth by Varda
Elbereth and brightened with the dew of the
trees of Valinor?
Science has long pondered the mysteries of
the stars.
And this is how we finally figured them out.
The first thing you learn in astronomy is
that the sun and the other stars are giant
balls of fiery hydrogen and helium, powered
by raging nuclear furnaces in their cores.
But this knowledge is surprisingly recent.
A hundred years ago, we were starting to plumb
the deepest mysteries of the universe with
Einstein’s relativity and with quantum theory.
But
we had no idea what that giant bright thing
in the sky was.
We didn’t know what stars were made of nor
where their energy came from.
On the other hand, it’s impressive that
we figured it out at all - afterall, we’ve
never been to a star, never sampled its stuff
to put under a microscope.
And yet in a handful of years during the 1920s
we went from no idea to having a pretty solid
understanding of stellar physics.
And a lot of it was thanks to a brilliant
young astronomer named Cecilia Payne.
You may not have heard of Cecilia Payne - later
Cecilia Payne-Gaposchkin - and that’s a shame.
She not only revolutionized our understanding
of the stars, but she helped blaze a trail
in astronomy and physics for the women who
would come after.
We’ll get to the physics in a minute - but
Payne deserves a quick bio.
She was born in Great Britain in 1900 and
always knew she wanted to be a scientist.
She was tending towards biology at Cambridge
University where she ended up in a lecture
by the great astrophysicist Sir Arthur Eddington,
who recounted his recent solar eclipse expedition
in which he verified Einstein’s new general
theory of relativity.
In Payne’s own words: “The result was
a complete transformation of my world picture.”
She switched from biology to physics, finished
her studies, but couldn’t even graduate
properly - Cambridge simply did not award
such degrees to women.
Cecilia Payne set her eyes on the New World.
Harvard University in Massachusetts was already
proving itself at least a little friendlier
to women.
Annie Jump Cannon and Henrietta Swan Leavitt
- two of the greats of stellar astronomy - had
come through Radcliffe College - the women’s
college adjoining the all-male Harvard.
And Harvard itself was just now opening its
doors to female graduate students.
This was 1923, and just out of her degree
Payne was already extremely broadly knowledgeable.
Enough so that she knew what she wanted to
research - she wanted to unlock the mysteries
of the stars.
We’ll come back to Cecilia - for now let’s
get to some stellar science.
The secret to understanding the stars is not
exactly in the light they send to us.
Rather, it’s in the light that they fail
to send.
This is a modern spectrum of the Sun - it’s
the amount of light we receive at different
colors - or in other words, from photons of
different energies of frequencies.
Most of this light comes the photosphere - a
layer around 100 km deep at the surface of
the Sun.
It’s the glow of the 6000 K hot material
in that layer.
The colour of a star depends on that temperature
- blue for hot stars, red for cooler stars,
and sort of greenish-yellow for stars like
our Sun.
But on its own, that thermal light is a very
smooth curve across the spectrum.
So what about these dark bands?
Those are where photons of very specific energies
have been plucked out of this thermal light.
It works like this.
A photon trying to escape from inside the
Sun encounters a lot of obstacles.
One of the most severe is that the Sun is
full of free electrons - electrons that were
stripped from their atoms due to the intense
heat.
Electrons deflect the path of a photon very
easily.
So any given photon has to bounce its way
between many electrons before finding its
way to the surface.
A photon coming from the core of the Sun will
be or scattered so many times that what should
be a 1-second journey to the surface can take
10s of thousands of years.
Once it gets close to the surface, material
is much less dense, so there are fewer free
electrons to do the scattering.
By the time a photon reaches the photosphere
it has a 50-50 chance of traveling the final
100km to space without bumping into anything.
At least, that’s true for most of the light.
But some photons encounter a new obstacle.
As temperature drops, it becomes possible
for some electrons to be captured by nuclei
to form atoms.
And if free electrons are good at stopping
photons in their tracks, these atoms are even
better.
An atom can absorb a photon if doing so would
cause one of its electrons to jump up to a
higher energy level.
The energy of the photon and the energy of
the electron jump have to be exactly the same.
So any photons trying to escape the Sun that
happen to have one of these particular energies
are going to get sucked up on its way out.
And that’s what these dark lines are - we
call them absorption lines.
Each element on the periodic table produces
a different set of lines corresponding to
its unique energy levels.
Just seeing which absorption lines are present
tells you which elements are present inside
the Sun.
When the spectrum of the Sun was first studied,
it was noticed that the most prominent lines
corresponded to the most common elements on
the surface of the Earth.
The prevailing wisdom came to be that the
sun was made of exactly the same stuff as
the Earth - just a lot, lot hotter.
But to test this - to figure out the true
composition of the sun from these absorption
lines - was going to take some serious advances
in understanding how both stars and atoms work
Fortunately help was at hand.
The brand new field of quantum mechanics was
emerging in Europe, and a young astrophysicist
named Cecilia Payne had just arrived at Harvard.
Even that early in her career, Payne was widely
read and so she knew about some groundbreaking
work in early quantum theory that she could
use to decode the complex patterns of absorption
lines in stars.
One of the reasons for the complexity of stellar
spectra is that you don’t just get one pattern
of absorption lines per element.
You get a different pattern for every different
ionization state of every different element.
In energetic environments like the Sun, electrons
are regularly kicked free from their atoms.
The atoms are ionized.
That changes the energy levels of the electrons
that remain, resulting in a different set
of possible absorption lines depending on
how many electrons have been kicked free.
So the pattern of absorption lines depends
on the abundance of each element AND on the
abundance of each ionization state of that
element.
Not long before Celilia Payne started her
research, Indian astrophysicist Meghdad Saha
had used early ideas in quantum theory to
crack the ionization problem.
He figured out a formula that told how much
of each ionization state you should get if
you have a cloud of some element at a given
temperature and pressure.
Saha and others began to apply this theory
to stellar absorption lines.
But it was Cecilia Payne to figured it all
out.
Payne realized that it should be possible
to translate a star’s absorption line pattern
into measures of temperature and composition.
But it wasn’t straightforward - there are
multiple competing influences determining
the strength of a given absorption line.
For one thing, each absorption line is formed
as light deeper within the sun traverses a
large distance, over which temperature and
pressure drop dramatically.
And different lines are predominantly formed
at different depths.
In astrophysics, these sort of messy, competing
effects rule the universe.
It’s literally impossible to disentangle
everything.
A big part of making progress in astrophysics
is finding clean relationships amid the chaos.
Cecilia realized a couple of things - first
was that although the strongest absorption
lines were hard to interpret, theoretically
the strength of the weakest lines should be
proportional to the abundance of the particular
ionization state of that element.
That didn’t tell you exactly how much of
that ion type there was - just the relative
amount compared to other ion types.
So the second realization was that although
she couldn’t get the total quantity of each
ion or of the element, with certain assumptions
she could get the relative amounts of the elements
compared to each other.
Cecilia Payne set about analyzing the many
spectra of stars that had been observed at
Harvard Observatory.
She calculated the relative abundances of
the elements and found they varied between
stars, but were generally similar to what
we find on Earth’s surface - but with a
couple of extreme differences.
On Earth, hydrogen is the third most abundant
element after oxygen and silicon, while helium
is extremely rare.
Her results suggested that hydrogen was by
far the most common atom in the sun, followed
closely by helium.
Cecilia Payne had discovered what the sun
and stars were made of.
This was totally against the current scientific
consensus - which was that the Sun was made
of the same stuff as the Earth.
Payne was advised to downplay that result
in her thesis.
She glossed over the result, saying that it
was “almost certainly not real” - that
it was likely the result of not understanding
the atomic theory of hydrogen and helium well
enough.
But the fact of the sun being made mostly
of hydrogen and helium was confirmed only
a few years later, and Cecilia Payne is widely
recognized for discovering this.
By the way, the whole finding out what stars
are made of thing wasn’t even the main point
of Payne’s thesis.
She also developed a way to calculate the
temperatures of stars just based on the absorption
lines.
This was much more precise than the previous
method of just observing the overall color
of the thermal light.
So, yeah, that’s how we know what the stars
are made of.
At around the same time as Ceclia Payne was doing all of this, other scientists were figuring out the rest of the
mysteries of the stars.
Arthur Eddington himself had postulated the
whole nuclear fusion thing just a few years earlier
in 1920, but now knowing what stars are made
of, he and others were able to develop a detailed
theory of stellar physics.
Stars went from being utterly mysterious to
one of the best-understood denizens of the
universe.
And Cecilia?
She stayed in the US and became the first
female professor at Harvard, and the first
female chair of one of its departments.
So, here’s to the stars - both types - Cecilia
Payne-Gaposchkin, star astrophysicist, and
also the type she figured out for us - the
giant balls of burning hydrogen scattered
across space time.
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So we’re going to have to skip comment responses
for a week due to some travels.
We’ll do double comments next week.
But I do want to take this opportunity to
send a big, big thank you to one of our Patreon
supporters, Mark Rosenthal.
Mark supports us at the big bang level - which
basically means we get to film the show, animate
it, AND each lunch.
Mark, we've talked about a lot of stars today, so it’s fitting we end on you, the starriest
star of all.
Thanks for helping us to .... shoot for the
stars?
