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We have just seen how the elements are synthesized in stars. We are therefore ready
to form solar systems that are not just based on hydrogen and
helium, but include all of the elements in the periodic table.
Many generations of dying stars have ejected
freshly synthesized matter back into interstellar space.
But how can we use this widely scattered matter to form
dense solar systems with planets?
If the ejected material was evenly distributed in the universe,
this would be a hopeless task.
Fortunately, due to the force of gravity,
the matter is concentrated in galaxies, such as our Milky Way.
And within the galaxies,
most of the matter is further concentrated into molecular clouds.
The molecular clouds in the Milky Way are enormous,
even when we observe them from the Earth.
But since they are also very faint, they remain almost invisible to the naked eye.
If you have ever watched the sky you are likely familiar with
the Orion constellation which lies close to the celestial equator.
The three bright stars are also known as Orion's belt.
They are, in fact, massive stars within the Orion constellation.
If we zoom in and increase the brightness, the underlying cloud becomes visible.
Here you may be able to see the Horsehead Nebula, and up here is your Orion Nebula,
which is an active star forming region.
The newly formed stars make this part of Orion very bright.
We cannot observe how our own solar system formed, but
if we observe areas like this, we can see how other systems form in a similar way.
The size of giant molecular clouds may conveniently be measured in
units of light-years.
This cloud measures several hundred light-years across.
Much bigger than the human mind can comprehend.
Molecular clouds look very passive, but in fact, they are highly dynamic.
The stellar winds from massive stars and
supernova explosions push the gas in the cloud around,
at speeds up to several hundred kilometers per second, a truly amazing speed.
But even at a hundred kilometers per second it would take you a couple of
million years to get from here to here.
Therefore, the picture does not change from one knot to the other,
giving us the false impression that we are looking at something rather inactive.
However, if we could fast-forward this picture a few million years,
we would be able to see that this is indeed a very dynamic system.
We can use computer simulations to fast forward this process.
What you see here is a simulation of a giant molecular cloud,
similar in size to the picture you just saw.
The simulation runs for
2 million years, and you can see that on that time scale the cloud is very dynamic.
Note how newborn stars light up within the cloud as the simulation runs.
Despite the fact that the gas is highly concentrated in these giant
molecular clouds, we still need to compress it by many orders of
magnitude to form a solar system.
Although gravity helped us concentrating the gas,
the gas pressure works against gravity and prevents the cloud from collapsing.
However, since these clouds are highly dynamic systems, the gas sometimes becomes
compressed to a point where gravity is stronger than the gas pressure.
The result is a runaway collapse, resulting in star formation.
A famous example of star formation is seen in this Hubble space image of
the Eagle Nebula.
The intense stellar winds from a couple of massive stars have
created a bubble in the cloud.
The stellar winds not only push the gas further and
further away from the star, through a process known as photoevaporation,
it also compresses the gas to the point where gravitational collapse is triggered.
This is exactly what we need to form a new star.
Once a dense core is formed, its gravity field will start to grow as it
attracts more and more gas and thus further increases the gravity field.
This process of growth through gravitational attraction is known
as accretion,
a term I will use several times in the following.
If we zoom in on these pillars, we can see how it works.
As the gas is pushed away, the newly formed stars emerge from the gas.
Each of these small tentacle-like structures contains a newborn
star at the tip.
Eventually, the gas will clear around the young star.
Accretion will cease and the star will begin its life as an independent object.
As the gas collapses toward the growing star it will rotate faster and
faster, much like water running out of a bathtub.
The rotation prevents all of the gas from accreting directly to the star.
Instead, a rotating disk of gas and dust is formed around the young star.
It is from these disks that planets accrete.
The disc structure explains why all of the planets of our solar system orbit in
the same plane around the Sun.
The diameter of the disk is typically up to a few hundred astronomical units,
or about 1 million times less than the diameter of the giant molecular cloud.
Astronomical telescopes cannot resolve the disk around other stars.
They are unfortunately too small.
So what you see here is therefore an artist's illustration of
such a young solar system.
If we look at the disk edge on, we can see how matter from the cloud is
falling in toward the disc.
Within the disc, angular momentum is transferred outward and
most of the mass is drifting inwards toward the star.
The illustration also shows some of the processes that take place in the disc.
Note that the thickness of the gas disc increases away from the star.
Within the disc, and concentrated toward the midplane,
we find solids of different sizes.
Some of the particles which formed during this very early stage have been
preserved in meteorites.
So let me introduce you to the most common types.
In the hot gas, close to the star, millimeter- and
centimeter-sized particles known as CAIs condense out of the gas.
CAI is an acronym for calcium-aluminum rich inclusions.
Calcium and aluminum are elements that condensed at very high temperatures.
So we know that they must have formed in a high temperature environment.
Here's an example of a meteorite with CAIs.
It's the white irregular inclusions that you see here and here.
Further out in the disc we find millimeter to sub-millimeter sized
silicate spheres known as chondrules.
They seem to have crystallized as freely orbiting molten droplets.
The droplets probably formed when pre-existing dust balls were flash melted.
Unfortunately, the nature of the flash heating process remains poorly understood.
Meteorites containing chondrules are also known as chondrites.
They are the most common type of meteorites.
The meteorite we are just looking at is a chondrite.
If you look closely,
you can also see examples of small, round chondrules within the meteorite.
In the next video,
I'll tell you much more about meteorites and the information they provide us with.
So let me return to the previous illustration and
explain what else is going on in the disc.
The disc is composed of dust and gas.
In the inner part of the disc close to the star, temperatures were too high for
water vapor to form ice crystals.
Further out, enormous amounts of ice
crystals were available as building stones for the planet growth.
As a result the outer planets grew faster and bigger.
This had a very visible and significant implication for today's solar system.
The gas in the disc was only present for a few million years.
And unlike the inner planets, the outer planets beyond the snow line grew big
enough to capture gas from the disc while it was still there.
Therefore, these planets captured enormous amounts of gas and became gas giants.
In here in the inner solar system, the smaller rocky or
terrestrial planets, including the Earth, formed.
As far as we know,
these planets are the only ones with the possibility to harbor life.
Before we end this lecture on how the solar system formed,
let's have a last look at the accretion of the planets.
As I explained, the terrestrial planets grew by
attracting solid material from the disc.
In a simpler version, a protoplanetary core, would simply accrete dust,
chondrules and CAIs from the disc, until the disc would be depleted of material.
But in reality the process was probably more complex.
Modelling suggests that many planetary embryos formed and
started sweeping up the material from the disc.
And as the process continued the larger planetary embryos consumed the small ones,
and in the last stages of accretion we were left with a few
large planetary embryos that merged to form the planets we know today.
We believe that one of the last planetary embryos
that accreted to the proto-Earth was probably Mars sized.
And as you can imagine, accreting a Mars sized object to the Earth
resulted in a condition of cataclysmic proportions.
As a result we shall see in a later video,
the presence of our moon was likely a consequence of this event.
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