Stars dot the night sky, giving an impressive clue to the vastness of the galaxy we live
in.
I remember being in South Africa on a clear night, far away from any city and its accompanying
light pollution.
I simply can’t describe how spectacular the heavens were.
No photo I could show here could possibly give justice to being in that situation, in
person, and looking into the seemingly infinite.
Most of the dots we see in the sky are stars, all in varying stages of their evolution,
and are many light years away from us.
But apart from being giant balls of light and energy, how much do we actually know about
stars?
Where do they come from?
Are there different types?
And what happens to them eventually?
I’m Alex McColgan, and you’re watching Astrum, and in this small series of videos,
I will go through the lifetime of various types of stars, starting in this episode with
how stars are formed.
When we think of interstellar space, we often think of a cold vacuum of nothing.
And by our Earthly standards, it might as well be nothing, but this is not strictly
true.
Between the stars of our galaxy is what is known as the Interstellar Medium, or in other
words, the matter and radiation that exist between stars.
It is comprised mainly of hydrogen, followed by a small amount of helium and trace amounts
of heavier elements.
It can’t be thought of as something like an atmosphere, as even in the densest parts,
there are only one million molecules per cubic centimetre.
That may seem like a lot, but in the same space at sea level on Earth there are 10 quintillion
air molecules, and even in a laboratory vacuum, there are still 10 billion molecules per cubic
centimetre.
In the more sparse reaches of space, the Interstellar Medium’s density can be less than one molecule
in a cubic centimetre.
But what does this mean in figures we can wrap our heads around?
Well here’s an interesting comparison for you.
If you are sitting on a chair, look directly down at the ground.
If you were to have a cylinder the diameter of your eye, drawn from your eye to the ground,
there are more molecules in that cylinder than if you were to be sitting at the edge
of the solar system and have a cylinder drawn from your eye to the centre of the galaxy,
over 27,000 light years away.
Although very sparse, the interstellar medium is still thought to make up roughly 15% of
the visible mass of the Milky Way.
Much of the molecules in the Interstellar Medium is ionised, and there is a special
scientific instrument called WHAM that can see the densities of ionised gas in space
from our perspective on Earth, and this is what it looks like.
This gives you a perspective that there is a lot more matter in the galaxy than you may
initially think.
This interstellar medium, particularly dust, has a very real effect on us too.
Over vast distances; the dust in the interstellar medium acts like a fog, either blocking the
view of stars thousands of ly away in the visible light spectrum or giving stars a reddish
appearance.
This is one of the reasons the night sky isn’t awash with light.
If there wasn’t an interstellar medium between the stars, you would be able to see the entire
disk of the Milky Way in the night sky.
As I mentioned earlier, the density of the interstellar medium varies greatly around
the Milky Way.
If you’ve been watching my other videos, you may have heard me speak about nebula,
and HII regions.
Nebulae are denser regions of the interstellar medium.
Some of these regions appear as holes in the night sky.
In actuality, they are massive interstellar clouds of gas and dust.
And when I say massive, I’m talking many light years across, some many million times
the mass of our Sun.
You may have seen this famous Hubble image before called the Pillars of Creation.
What you are looking at is an example of these gas and dust clouds.
What you can see of the left pillar in this image is 4 light years long.
Just to give you some perspective, one pixel might just about cover the distance of Neptune’s
orbit if our solar system was placed right next to the pillar.
Incredibly however, this image is in fact just a small segment of this HII region called
the Eagle Nebula.
This nebula is just one of many stellar nurseries where stars are formed in our galaxy.
In the background, you can see lighter colours of greens, blues and reds.
These bright colours represent different ionised molecules, and are known as the HII region
of a nebula.
In this image, greens are hydrogen, red for sulphur and blue for oxygen.
The electrons freed by the ionization process are continually absorbed and reemitted, producing
the different colours for the different atoms.
This is a very similar process to what happens in a neon light, particles are excited to
a higher energy state, and eventually release that energy in a wave of light.
These molecules are extremely hot, heated by nearby stars to temperatures over 8,000
degrees Celsius.
They are also reasonably dense for the interstellar medium from one hundred up to ten thousand
molecules per cubic centimetre.
The pillars themselves are known as molecular clouds, the densest regions of the interstellar
medium , from around 100 to about one million molecules per cubic centimetre.
These clouds are cold and dark at around -260c, and they don’t allow light to travel through
very easily.
In fact, they would be invisible if it wasn’t for the fact some are silhouetted against
brighter HII regions.
Because of the density of the molecular cloud, the inner molecules don’t interact so much
with the UV light of local stars, meaning the molecules stay cool and dark.
Incredibly, it is these clouds of molecules and dust that eventually turn into the same
types of stars that dot our galaxy and universe.
But how, what exactly is the process?
These clouds are generally stable, and would exist for billions of years doing nothing.
Look at this example of how gas behaves in a molecular cloud at -260c.
Even at this temperature, with very little gravity, the molecules spread out in the cloud,
the shape of the cloud supported by a balance of its own gravity and internal pressure.
So in order for these clouds to become stars, there has to be a trigger.
And there are a few different types of triggers that are thought to kick start the star making
process.
Going back to the pillars, you can see one of these triggers in action in this very photo.
Along the outside of the pillars, you can see a sort of aura of light around them.
This is because UV light from hot and bright local stars are blasting the outermost molecules,
exciting them to higher energies.
This process has a weathering effect on these otherwise stable clouds, as the excited molecules
either escape away from the cloud, joining the HII region, or they push against the colder
molecules further in, compressing them.
This pressure can also be created by a supernova shockwave.
A supernova is caused by the death of a massive star, a topic we will come to later on.
It’s interesting to me though that the death of a star can trigger the birth of others.
You may also remember me talking about density waves in previous videos, and it is thought
that molecular clouds passing through the Milky Way’s density waves can also be the
trigger for these clouds to collapse.
On a very grand scale, galaxies colliding can cause what is known as starburst within
the galaxies.
Star burst is when a galaxy produces stars at an extremely fast rate, and it can be seen
in these areas of extremely hot, blue stars.
All these triggers destroy the equilibrium of the cloud, and the internal pressure starts
to give in to the gravity of the cloud.
This bumping and shoving of molecules within the dense cloud cause the molecules to clump
together under their own gravity, gradually growing bigger and bigger as the gravity increases
and more molecules are pulled in.
Depending on the size of the cloud, there can be hundreds to millions of these clumps
that get formed within the cloud.
As the density of the molecules increases from the initial trigger, interaction between
the molecules also increases.
The temperature starts in increase.
The higher density causes stronger gravity.
More molecules from the cloud get pulled in by the increasing gravity.
Temperatures rise and gravity continues to increase.
The domino effect continues.
These clumps in the molecular cloud are the beginnings of stars, called protostars.
The protostars keep getting bigger as long as the molecular cloud surrounding them keeps
feeding them material.
And to become a true star, a protostar needs to be at least 0.08 solar masses.
Should the molecular cloud disperse too quickly, the protostar will become a failed star, or
what is known as a brown dwarf.
The difference between a failed star and a main sequence star is if the temperature and
pressure in the core of the star are hot enough for nuclear reactions to begin.
Protostars create energy too, but their energy comes from the impact of material entering
the star.
A star hits the main sequence, or the next stage of its life, if the core begins to fuse
hydrogen to helium.
A brown dwarf doesn’t have the mass to do this.
As a result, it isn’t as hot as a star, and will keep cooling off.
They are not very bright in the visible light spectrum, and would actually appear magneta,
red or orange.
The can have planets, but the chance of life on those planets are slim, as the habitability
zone would be quite small, and would move closer and closer to the star as it cooled.
But what happens if there is enough mass to form a main sequence star?
As material gets sucked into the protostar, the star begins to rotate, and the material
begins to spiral inwards.
There are a few objects NASA have photographed that are thought to be protostars where this
can be seen, these two arms going into the protostar, impacting at tremendous temperatures.
This in turn causes the protostar to rotate faster.
The material surrounding the protostar flattens and turns into what is known as a planetary
disk.
Clumps start to form in the disk too, building up under the same gravity that caused the
star.
Smaller clumps maybe build up to become the star’s planets.
Bigger clumps may begin to form their own planetary disk, and may become another star.
In fact, this is the most common thing to happen in star formation, our Sun being quite
unusual in that respect.
Most stars will be in a binary star system, some stars will have many stars in the same
system, all orbiting each other.
Had Jupiter been fed more mass during its birth, it too might have become a star.
It would have needed a lot more mass though, about 70 times more than it currently has.
We should consider for a moment size scales.
A typical interstellar cloud is 100 trillion km across or roughly 10,000 times larger than
the size of the Solar System.
A cloud fragment which forms one or a few protostars is 1 trillion kms, or roughly 100
times larger than the size of our Solar System.
By the time the core of the fragment has become a protostar, its size will be approximately
10 billion km, and its temperature of order 10,000c.
Interestingly, as material is fed into these protostars, they can unleash shockwaves into
the rest of the molecular cloud they are still surrounded in.
This, in turn, can trigger further gravitational collapse and more protostars will be formed.
Eventually the molecular cloud’s material will be exhausted, and depending on the size
of the cloud, what is left is a star, several stars, a few hundred stars, thousands of stars,
or a whole supercluster of millions of stars.
Some of these clusters will eventually disperse, and the stars will simply join the rotation
of the galaxy, much like our Sun.
Although I’ve talked about this all matter-of-factly, this is still only a theory of how stars are
formed as actually seeing the formation of a star is very difficult due to the timescales
involved, and the fact that these protostars are surrounded by dust that obscures the view.
This is one of the huge advantages of the James Webb space telescope, due to be launched
in 2019.
Its ability to see in the infrared gives it a big advantage over Hubble, in that it will
be able to see through the thickest of dust clouds, and hopefully see protostars in action.
The best way to retain and truly understand knowledge is to apply it, so if you want to
test and expand on what you’ve learned today, try answering this problem - In a gas cluster,
how does density affect gravitational force?
There is more mass, but the distance between molecules is smaller, so what do you think?
(Screenshot of this problem).
This problem is from Brilliant’s recently released course about the life cycle of stars,
which coincides with my upcoming series of videos.
You can review star formation, which is hugely relevant to what you’ve just heard me talk
about today, and get a preview of star death, which you will hear me talk about in future.
As I explained in my previous channel update video, I think what Brilliant offers is quite
relevant to you guys.
Their sequences of problems are really well curated, and can help you to truly understand
astronomy by applying your knowledge to solving similar interesting puzzles.
If you would like to support my channel and learn more about brilliant, you can sign up
for free using the link in the description.
Should you want to truly understand, that link will also get you 20% off an annual premium
membership to the website.
Thank you for watching!
There is still a lot to talk about to do with brown dwarves and the interstellar medium,
but perhaps I will save that for a future video.
That’s the great thing with space, you can really keep going into greater and greater
detail into a subject and it’s still all so fascinating.
If you enjoyed the video, give it a thumbs-up, share and subscribe.
This will be a short three part series of videos, the next two about the main sequence
and then the death of stars.
You may also be interested in going back and checking out the Hubble Top 100 videos I’ve
made, as you may now have more context to what you’re looking at.
Thanks so much again and I’ll see you next time.