Greetings and welcome to the
introduction to astronomy.
In this video, we're going
to talk about star formation.
So how stars actually
form and the processes
that are involved there.
So we're going to start off
looking a little bit at just
some of the basics of stars.
So what we look at
is how do stars work?
Well stars main sequence
star specifically,
which are stars like
our Sun produce energy
through nuclear fusion.
And what that means is that
they are converting hydrogen
into helium at the rate
for our Sun of about 600
million tons of hydrogen into
helium every single second.
However, the Sun has
so much hydrogen in it
that it can do this
for 10 billion years
without running out of
hydrogen. Stars overall come
in a big range of masses
from about a hundred times
the mass of the Sun to a
little under a tenth the mass
of the Sun for the lowest
mass star possible.
And the most massive
stars a little more
in question - Up to about
a hundred solar masses.
Sometimes we see things going
a little bit higher than that.
And often when we start to find
things that are too much higher
than that when we find
stars that we think
might be 200 times
the mass of the Sun.
It usually turns out to be
several stars close together.
Now of these the
small mass stars
the low mass ones
are much more common.
So we see far more of these.
A tenth of a solar mass stars
than we do of the hundred
solar mass stars.
However, these hundred solar
mass stars are the brightest.
They are the hottest
and the brightest.
So they are the
easiest ones to see.
We can see these
over great distances
whereas we can only see
these very small stars.
The cool and faint ones if they
are very, very close to us.
And we look like a galaxy
like our own Milky Way.
It has gas and dust to make
many billions of stars.
So let's take a look at one
of the star forming regions
within our galaxy, which is
the Orion molecular cloud.
And this is one of the
nearest star forming regions
1,400 light years away.
Now remember what
a light year means.
That means we see it as it was.
1,500 years ago.
However, the star
formation process is slow.
So it's not that things
have changed drastically
over 1,500 years.
So it might not look
a whole lot different
than it does right now.
Now you may recognize
the outline of Orion
there is Orion's
belt the sword, which
is the Orion nebula,
which includes
the Orion nebula coming down.
And some of the bright
stars Betelgeuse and Rigel
that make up part of
the body of the hunter.
Now only some parts
of this are visible.
We can see some of the areas
the great loop here coming
around Orion and some
of the other nebula
you watch our
emission now really
caused by the emission
of hydrogen gas
giving off a very bright glow.
But it's hard to see into this
area because it is so dusty.
When we look at this
the material that's here
has formed over
a period of time.
So the stars in the belt are
about five million years old.
These are relatively
older stars.
About five million
years, the stars
in the sword going down
from the belt in here
are about a million years old.
And they're trapezium
stars in the Orion nebula
itself are only a few
hundred thousand years old.
Now we can look
at those trapezium
stars in a little more
detail and here we
see them in an image.
This is visible light here.
And this is in the infrared.
So this is the same image.
And you can see those stars kind
of buried within all the dust
here in the infrared.
They're much more
visible the infrared,
the light is much better
able to penetrate the dust
to be able to let us look
in to this stellar nursery.
So here we are looking
at the trapezium stars
within the Orion nebula.
And these are only a few
hundred thousand years old.
So these are some of the most
massive and hottest stars
that we can see.
But overall, we do see over
2000 stars in this Orion region.
So it's not forming just these
very bright stars that we see.
But lots of others.
And this is, again, one of the
nearest star forming regions
that we have to us
here in the Milky Way
and is an ongoing source
of star formation.
So when you often see images
of nebulae a lot of them
that we see come
from the Orion region
with the Orion nebula
pictured in part
here being one of the main ones.
So how do we go about
once we've as we're
forming stars how
are we going to go
about clearing that nebula?
How are we going
to clear that out?
Well, we need to have this,
we use the stellar winds
of material.
So as these young stars
form the young stars
give us a very
strong stellar wind,
which pushes material away.
And that will clear away
some of the major material.
Many of the mass of
stars do not very
live very long only
a few million years
and they will undergo what
we call a supernova explosion
where they will implode as they
build up iron in their core
and then massively
expand back out.
Both of these serve to
compress nearby clouds
and continue the
star forming process.
So we can see in the image a
star cluster that is forming.
And as its radiation
pressure pushes out
it pushes back against the
nebula and the material
be left behind.
And that material, then gets
compressed and more star
formation continues to go on.
So here's the stars
that have formed here
is the stars that are still
in the process of forming
and things like shock
waves from supernovae.
And the compression
from the stellar winds
will enhance that star formation
and continue the process.
So that will eventually
clear it out,
leaving behind just a
cluster of stars at the end
and eventually the nebulae
itself the nebulosity
will be gone.
So what is the process
of the birth of a star?
Let's take a look at that.
And again, direct
observations are not
possible because of dust.
So we cannot see that visible
light makes it invisible to see
the star formation process.
However infrared or radio
waves can penetrate the dust
and therefore allow us to
get a look or a glimpse
into what is happening
inside some of these clouds.
First, we have the
initial collapse,
which is relatively short.
Taking only thousands of years.
And again, while that
sounds like a long time
to us astronomically
speaking that is
an extremely short
amount of time,
you start to form a dense core
within a clump of material
and within that the
gravitational force will grow
and eventually become dominant.
So once you get
enough gravity forming
at the center here
it begins to compress
and more material will
fall in the rapid collapse
then begins as the material
begins to collapse into a disk.
Now it's a lot easier
for the material
to fall in along the poles
than it is along the equator
material that is rotating
around in the disc will have
a much harder time losing
that rotational energy
- material that
is just scattered
around not having
a lot of rotation
will be able to
compress a lot easier.
So the material will tend
to collapse into a disk
as the clouds and bits of
clouds collide with each other
and lose energy dropping
down to a lower level.
Now as this happens, we begin
to form first solar mass stars
what we call T Tauri stars.
T Tauri stars are again hidden
in the dust clouds we see them
in the infrared and they
are materials that will then
have outflows of jets.
So jets of material,
but T Tauri stars
have jets of material that
come out and can then impact
into the rest of the material.
So the strong stellar
winds that form
will clear out this material.
There'll be jets of
material coming out
and eventually we'll have
the protostar left behind.
Now what those jets
do is they come out
is to impact into that
remaining material.
So we form what we call
Herbig-Haro objects -
named after the two astronomers
who studied these very early
on.
And here we have the
protostar at the center.
So the protostar is
in here at the center.
And we have the accretion
disk of material around it.
And then perpendicular to that
we have the jets coming out
this way.
And this way that then
impact material around it.
And this is what we call
the Herbig-Haro object.
It's not the protostar star,
it's not the accretion disk.
It is the point where
the jets of material
strike the interstellar
medium and heat that
up and cause it to glow.
Now this is a sketch of it.
What does this
actually look like?
Well, here's an image of it.
And this is an image
of what we would
call one of these
early early forming
stars the star itself would
be shrouded in the center
and hidden by the dust.
But the jet of material
coming out can still be seen.
And then impacts over
here in brighter areas
as it impacts would become
the Herbig-Haro objects
as that material impacts
into the interstellar medium
and causes it to glow.
So this is a very early
stage of star formation
and especially common
for the T Tauri
stars for stars like our own.
So as it goes through this as
it's cleared out the nebula
and it's formed this Herbig-Haro
object or T Tauri star.
Now we begin to settle
into the main sequence.
So the star begins
to settle down
to protostar calms down the
stellar winds the jets diminish
the disk of material
around it will likely
form planets we see that
planets are very, very common.
And the stellar winds and
the radiation pressure
will slowly clear
out the nebula.
But that clears out the dust
and the gas particles leaving
behind the star and most likely
a planetary system behind.
Now, we can also look at the
stellar formation using the HR
diagram.
So we can look at an image
of that here as a star.
It changes it will change its
position on the HR diagram that
has nothing to do with the
star itself moving around.
It has to do with
the temperature
and the luminosity changing.
So when the temperature
and luminosity change
were plotting the temperature
and the luminosity
here when they change its
position on this main sequence
will change.
So if we look here for a
proto star that is collapsing
the tracks are
slightly different
depending on the mass.
They all start up in
the upper right hand
side of the main sequence, which
also contains red giant stars.
However red giant stars are
not buried within nebulae
so these stars are very
hard to actually see
with regular telescopes.
So what happens overall is that
the surface temperature will
increase much more for a
very high mass star going up
to tens of thousands
of degrees, maybe only
a little bit for a low mass
star maybe only increasing
few hundred of degrees.
But in general, the
temperature will increase,
and the luminosity
will decrease.
Why is the luminosity
decreasing as well?
It's in a sense, the
star is becoming smaller.
The large stars are up here.
So a star up in this
corner is very large
as it works its way
down this direction.
The star will become smaller.
Eventually when you reach
the critical temperature
of a little over 10 million
degrees nuclear reactions
- nuclear fusion begins
in the core and the star
will settle on what
we call the zero age
main sequence, which is the
reddish line down below here.
So that is where
the star will settle
and then it will slowly change
over its main sequence lifetime
that main sequence lifetime
may be millions of years
for a very massive star.
It can be billions of years
for a star like our Sun here
and it could be a trillion years
for a very, very low mass star.
If the mass is low enough less
than about 0.075 solar masses
it will never really achieve
high enough temperatures
in its core for nuclear
reactions to begin
and that becomes what we
call a brown dwarf star.
So a brown dwarf
is a failed star.
A star that was never
became hot enough
in its core for nuclear
reactions to begin.
So let's finish up here
as we do with our summary
and what we've looked at
is star formation begins
in a very cool molecular cloud
in space in the Orion region
was a part of one of
these that we looked at
as the material collapses
down you former protostar.
And you get to see a disk an
object that will often form
and we can use the
diagram to study
the early stages of stellar
evolution and also later stages
as well.
So that concludes our
discussion of star formation.
We'll be back again next time
for another topic in astronomy.
So until then, have a
great day, everyone,
and I will see you in class.
