Professor Dave here, let’s make some stars.
We started our journey with early universe cosmology, and we got from the Big Bang all
the way to a few hundred million years along.
Energy condensed into matter, and as the universe slowly cooled, atomic nuclei formed and eventually
coupled with electrons to make neutral hydrogen and helium atoms, and even molecular hydrogen,
just like we see on earth today.
Now it’s time to dip into some astrophysics, so we can describe what happened to these
particles next.
As we learned in modern physics from Einstein’s general theory of relativity, objects with
mass warp spacetime, inducing a curvature that attracts all massive objects to each other.
Even though the matter in the universe consisted only of tiny atoms at this time, these still
exert gravity, as did all the dark matter lying around, which we will get to later in
the series.
So from around 150 million years to about a billion years after the big bang, all of
this hydrogen and helium slowly began to collect into regions of higher density.
Given the random distribution of matter, some regions became very dense quite early in this
era, while others took a lot longer, but let’s zoom in on one of these dense patches of gas
to get a closer look.
We know from studying gases in chemistry that particles within a sample of gas will exert
a certain pressure, or outward force.
But we just said that all matter exerts gravity, which will manifest as an inward force towards
the center of a large gas cloud.
By large gas cloud, we aren’t talking about the clouds we see in the sky.
We are talking about clouds at least a light year across, and often much larger.
So this is quite a lot of matter involved.
A gas cloud like this, also called a nebula, will remain in equilibrium if the kinetic
energy of the gas pressure, or the force pushing out, is precisely in balance with the gravitational
potential energy, or the force pushing in, a situation called hydrostatic equilibrium.
But if a cloud is massive enough, above a threshold called the Jeans mass, which is
usually at least a few thousand times more massive than our sun, gravity will easily
win the shoving contest, and the minimal kinetic energy from the cold gas pushing out will
not be enough to prevent gravitational collapse.
Any net rotation is amplified as the cloud gets flattened into a disk by the centrifugal
force, like a pizza chef spinning dough in the air, with gravity pulling matter towards
the center of the disk.
The precise physics involved does get rather complicated, but to make a long story short,
things get hotter still as the collapsing continues over millions of years, until the
atoms are reionized back into plasma, with temperatures getting too hot for neutral atoms
to exist.
Eventually, the inner region of gas is so hot that the outward pressure supports the
gas against further collapse, and we call this a protostar, an object that is in a temporary
hydrostatic equilibrium.
More material from surrounding gas continues to collect on the protostar, increasing the
gravitational potential energy.
With sufficient additional mass, collapse then continues, with the inward pressure becoming
so great that it causes temperatures to rise to millions of degrees, until things are hot
enough for nuclear fusion to occur, along with the tremendous energy that fusion generates.
And with that, a star is born.
While this depiction was certainly oversimplified, for our purposes, it will suffice to imagine
a star as a gigantic gas cloud that collapsed on itself with such tremendous gravitational
force that it heated up until it became a spherical furnace of plasma, a hot soup of
nucleons and electrons whizzing around.
The formation of this star results in tremendous radiation which will trigger reionization
in surrounding nebula, stripping these gas particles of their electrons.
This radiation can also push regions of gas around to collide with others, in turn promoting
more star formation.
The precise nature of each star depends largely on its mass, and later we will talk about
all different kinds of stars, how they form, and how they die.
But for now, just imagine that during this period, stars are forming all over the universe,
slowly, over hundreds of millions of years, lighting up the once pitch black cosmos and
bringing the dark ages to an end.
So we’ve gone from tiny atoms scattered around the universe to huge stars, burning
hydrogen fuel and glowing like fireflies.
What happened next?
Well the key factor will continue to be gravity, as will be the case throughout our astronomical
journey.
Massive stars exert a tremendous amount of gravity, so stars began to collect in dense
regions just as the gas particles did.
This same period of time that accounts for star formation is also when the first galaxies
began to form.
Some of these are dwarf galaxies, consisting of around 100 million to a couple billion
stars, and some are much larger, with a few hundred billion stars.
But gravity didn’t stop there.
These huge galaxies, along with the dark matter they are embedded in, exert even greater gravitational
influence, such that they collect to form groups, clusters, and superclusters.
By the end of this roughly one billion year period of star and galaxy formation, the universe
has finally taken on a form that looks quite familiar to us, and although our solar system
hasn’t formed yet, nor any planets anywhere in the universe, we are much closer to the
way things are now than they were in the first few seconds after the Big Bang.
So how did planets, and all of the other things that aren’t stars, come about?
In order to be able to answer that, we have to learn much more about stars first, so let’s
move forward and do just that.