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8. Chemistry
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From the course by University of Arizona
Astronomy: Exploring Time and Space
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Matter and Radiation
Astronomers harvest information across the electromagnetic spectrum, using spectra to measure the composition of distant objects and diagnose extreme physical conditions.
Meet the Instructors
Chris ImpeyDistinguished Professor
Astronomy
Chemistry is the subject of combining ingredients or
elements, by rearranging their electrons to form molecules, liberating and
trapping energy in various ways.
An element is transmutable into another element only by the processes of
fission or fusion.
Chemistry is simply rearranging the electrons in elements and
combining them in different ways.
A lot of the low temperature universe involves chemical reactions,
as do a lot of the phenomenon on the Earth.
But if we're asking about the chemistry of the universe, the most interesting thing
to ask is what is the universe made of in terms of chemical ingredients?
We'll tell this story more fully when we talk about the life and death of stars.
But to see why it's an interesting question to ask,
let's just look at what the Universe is made of.
If you take the sun, which is indeed a typical star,
we'll see that by mass 90% or more of the sun is made of hydrogen, and
most of the rest is helium, the two simplest chemical elements.
All the rest of the elements is less than a percent of the mass of the sun.
And in particular, carbon, nitrogen, and oxygen, which turn out to
be very important for biological material, are trace ingredients.
By contrast, humans or any living creature,
are composed of roughly two parts to one, oxygen and hydrogen,
in the form of water, and about 10% or 11% or 12% carbon, depending on the creature.
Other ingredients, like nitrogen are trace.
Life depends on roughly 12 or 15 different chemical ingredients.
Notice that the proportions of those elements are completely different in
life stuff than they are in star stuff.
Most of the universe is composed of the simplest two elements hydrogen and
helium and roughly the same proportions they are on the sun.
Clearly in biological material, something different is going on.
If instead we look at the rocky objects that surround a star the planets with our
earth as a typical example we see that the chemical constituents are different again.
The Earth contains almost half oxygen and
half of the rest is silicon, with aluminum and other metals, also quite abundant.
That's the Earth's crust.
In the core of the Earth, it's nickel and iron, two metals.
So the rocky material of planets is based on quite different ingredients than
the sun itself or even than humans.
Where did these chemical ingredients come from that lead to
the very different chemistry of a planet, and a star, and a person?
To get a sense of the relative abundance in the universe of
the different chemical ingredients, the el,
elements in the periodic table, let's use the analogy of decks of cards.
If we had a deck of cards of a typical sample of the universe,
which is to say star stuff, or what the sun, or any other star like it is made of,
we'd find that most of the cards are cards that represent hydrogen or helium.
We can image the numbered cards to be the hydrogen atoms, and
the face cards to be the helium atoms.
It's a remarkable thought and something that we'll talk about more when we
discuss stars and cosmology that the universe is one quarter helium by mass.
What about carbon, upon which life on earth and
we imagine life elsewhere in the universe is based.
Carbon is a predicate for
biology because it makes a universal tool kilt of building complex molecules.
Long and perhaps infinite chains of complexity required for
biology to function.
Carbon is actually a trace ingredient in the universe as a whole.
In the deck of card analogy, you'd have to inspect 60 decks of
cards to find the first card representing a carbon atom.
Virtually, all of the rest would be hydrogen and helium.
Heavier and heavier chemical elements are correspondingly rare.
And if we up to a heavy metal like gold, platinum or silver, these atoms
are remarkably rare in the universe as a whole.as indicated by star stuff.
In this analogy, see the building here,
a modern architectural cube somewhere in Germany, you can see the human for scale.
If you fill this entire cube with decks of cards, among average only one of
the cards among the many, many decks of cards would have a gold atom.
Gold is phenomenally rare as are other precious metals in the universe.
The elements we see this way were made by stars.
It's remarkable they exist at all.
Remember, the universe is overwhelmingly the two simplest elements,
hydrogen and helium.
As a precursor to talking about stars and the creation and
dispersal of heavy elements, let's give the story of the gold atom you might find,
perhaps one in a ring on your finger or on an earring.
Where did that come from?
It was not present at the creation of the universe.
The Big Bang consisted of primordial material, subatomic particles,
pure and simple.
There was some helium made in the Big Bang, but almost no heavier elements.
So we have to depend on a history that takes billions of years and
plays out over space and time where heavy stars were able to
generate elements up through the periodic table by a sequential processes of fusion.
At the end of their lives, they disperse these heavy elements out into
space either by ejecting envelopes late in their life or
by violent generation of a supernova at the very end of their life.
To get all the way up to gold,
it might actually take sequential generations of stars.
Perhaps in the first set of fusion processes, only carbon is produced.
And then, that carbon atom must become part of the core of another star
to send the fusion chain out to iron or then gold.
A gold atom, a particular gold atom might have gone through a number of
fusion cycles and dejected into space several times.
Eventually, the gold atom is floating in interstellar space to become part of
a nebula that would one day form a star and planets including the Earth.
By naturally processes, the gold atom finds itself at the cool outskirts of
this nebula and is swept along with other rocky materials, involving silicon and
oxygen, into a rocky object, a terrestrial planet.
The last part of this gold atom's journey takes four and a half billion years,
because that gold atom must of pre-existed the formation of the Earth that long ago.
The gold atom is mixed in with other similar and dissimilar atoms in
the geological context of a living planet churning through magma cycles up and
down in the crust and mantle.
And for it to end up in our possession in the modern age, it must've, at some point,
been distilled enough to shine and reach towards the surface where it might be
eroded by water running, or be dug out by a miner to become part of the human story.
The extraordinary story of gold is a story that involves astrophysics and
space and time extending over lightyears and billions of years.
So the exchange of a ring between two people has a resonance beyond it's
emotional resonance, it involves a deep and profound and
exciting story of astrophysics.
When we talk about chemistry, that is the subject that involves the rearrangement of
electrons and elements to form different combinations.
But in astronomy and astrophysics, chemistry has a different meaning.
It involves the creation and dispersal of elements inside stars.
We can see that this must have taken place by looking at star stuff.
The composition of a typical star, which is almost entirely hydrogen and
helium, the two simplest elements.
With the stuff that humans are made of, or
that the planet we're standing is made of which involves relatively larger amounts,
10 or 20 times more, of carbon, nitrogen, and oxygen and metals.
Even though they are very rare, these metals and
life bearing elements were created inside the cores of stars,
dispersed into space to become part of our story.
This is a story we'll tell more fully, when we consider the evolution of stars.
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