Greetings and welcome to the
Introduction to Astronomy.
In this video, we are going
to talk about compact stars.
And what happens to
them in binary systems
and compact stars that
we have looked at so far
are the white dwarf stars.
That is the state of
something like our Sun
and is compacted down down to
about the size of the Earth.
And we had neutron stars,
which occur after a supernova
explosion and are
compacted down to closer
to the size of a city.
So let's go ahead
and get started here.
And what we see is that
in terms of compact stars
we have looked at
the individual ones.
As I've said, we've
looked at the white dwarfs
and the neutron
stars and but what
happens when one of these
stars is in a binary system.
We know that half the
stars in the universe
are in binary systems.
So that implies that there
are a lot of these times when
a compact star is going to be
present in a binary system.
And what we have is we can get
a number of different things
we can get novae, we
can get supernovae
we can get X-ray
and gamma ray burst
and we want to kind of
look at all of these.
Now the supernovae
that we get here
are different than the type two
that we look at the type two
that we looked at previously
was a massive star
at the end of its life.
In this case, we're going to see
a different type of supernova.
So let's look at each
of these in turn.
So what we have.
First of all.
Let's look at what is a nova?
Well, if we have a white
dwarf star in a binary system.
They may be close enough
that material can transfer.
So you have the regular star
on this side, the white dwarf
star at the center
here and material
is being transferred
from one to the other.
So if they are close
enough together material
can be pulled off
of the ordinary star
and into what we
call an accretion
disk around the white dwarf.
And as that material
spirals around and in
towards the white dwarf it
then builds up on the surface.
Now what is building up there?
Well, we know that the
outer layers of a star
are mostly hydrogen.
So hydrogen is now
building up on the surface
of this white dwarf.
And eventually the
temperature can
become high enough that
nuclear fusion will actually
start on the surface
of the white dwarf.
And this star will see
when we look at this,
we will see it become hundreds
or even thousands of times
brighter.
And this is the
kind of thing that
can happen over and over again.
It does not damage
the white dwarf star.
So the material can
build up to the point
where it explodes as a
nova and then 50 or 100
or 200 years later, we
can do the same thing.
We can actually produce
a nova recurring
that it will recur again
in the same system.
So some of these can
occur over and over again,
as long as this star is
still transferring material.
Now generally, this would
happen when this star evolves
and becomes a red giant star.
Remember that when it
becomes a red giant.
It becomes many times larger.
And that allows the outer
layers to actually get closer
to the white dwarf star
and allow the mass transfer
to begin to take place.
So a nova is one
thing that can happen.
Another thing that can happen
is a white dwarf supernova.
It is exactly the same
process as for a nova
that we looked at on
the previous slide.
But the question
here is what happens
if the mass transfer pushes
the white dwarf over the 1.4
solar mass limit.
In that case, the
star can no longer
support itself against gravity.
So it will begin to collapse
and essentially the entire star
will explode.
So our image shows
that a little bit
of that here what
would eventually happen
is you would have the
main sequence star
and you would start
have two stars.
First one would evolve
and become a red giant
and eventually end
up as a white dwarf.
When this main sequence
star becomes a red giant
then we would have
mass transfer occurring
and material is
being transferred
onto the white dwarf.
If this white dwarf is
at 1.4 solar masses then
a little extra material can
push it over that limit.
And it will collapse.
And essentially a carbon fusion
begins throughout the star
all at once throughout
that white dwarf.
And that will rip the
star apart causing
a massive explosion which
we call a type I supernova.
Now that differentiates it from
the type II supernova which
is one of very massive star
reaches the ends of its life
and ends up forming
iron in its core.
This is actually
a white dwarf star
that is right at the
limit of for solar masses.
If it gets pushed over that
limit it cannot survive.
So that means that we do have
these two types of supernovae.
So what we can look at and
just to give some of the ideas
here we have the type II
that is a massive star
at the end of its life and
leaves behind a neutron
star or a black hole.
The different one
way to differentiate
them is that they're light
curves are slightly different.
And that a type II will show
hydrogen lines in the spectrum
because the outer
layers of that star
were a lot of hydrogen
a type I supernova.
On the other hand, occurs
when a white dwarf star
exceeds the 1.4 solar
mass limit and explodes.
In this case, nothing
is left behind.
We do not see hydrogen lines
because the white dwarf was
made primarily of carbon
and a little tiny bit
of hydrogen that was
transferred does not
give very strong hydrogen
lines as it would in a much
more massive star exploding.
These are extremely important
for determining distances
to distant galaxies.
They are what we call standard
candles or standard bulbs.
And that is because each of them
every single time it occurs,
it is the same type
of object exploding
because we know their mass
every single one of these
is a 1.4 solar mass
white dwarf star.
So there should be little
difference between them
when they explode.
We should see the
same brightness occur
and the same patterns
in their light curves.
And that means that we can
use them to then determine
distances because
they should all reach
the same maximum brightness.
So some of what can
happen for other objects.
So those are some
things that can
happen with white dwarfs what
if we have a more massive object
instead of a white dwarf.
What if we have a
neutron star present.
Well, we can take a
look at that as well.
The same kind of
process can occur,
you can have a neutron
star in a binary system
and the mass transfer
can occur just
like it did to a white dwarf.
You can have the ordinary star
here and the neutron star here.
And you can have
material transfer
to into an accretion
disk that then
spirals around down to
the neutron star material
would also build
up on the surface.
But the difference is that
you have much stronger gravity
and much higher temperatures
that occur than it
would with a white dwarf star.
That gives us instead a burst
of x-rays from the surface.
So instead of a burst
of visible light
as we get in a nova an
X-ray bursts is essentially
a nova but instead
a neutron star
is the compact object
instead of a white dwarf.
These are many of these have
been detected and interestingly
enough, it depending on the
positioning these can actually
speed up the neutron
star rotation.
So if you are sending material
from a star into a neutron star
if you kick it up and push
it the way it is spinning.
If it is spinning this
direction in the first place,
then if you give it a little
kick this side, kind of
like pushing a child on a swing.
You can give it a little
bit of boost of a boost
and that will allow it
to speed up its rotation
and in fact, we
find what we call
millisecond pulsars pulsing
in thousandths of a second.
And at the absolute
limit of what
a neutron star can actually do
without ripping itself apart.
And these are
believed to be spun up
by mass transfer
from other objects
essentially, each time you
transfer a little bit of mass
to the neutron star
you kick it up give it
a little bit of a push again,
it's pushing a child on a swing
and that would cause it to spin
a little bit faster until it
reaches this limit.
Now we see X-ray bursts.
But we can also find
gamma ray bursts.
So let's take a look at
those and gamma ray bursts
were first detected
back in the 1960s.
In looking for gamma rays
from nuclear detonation.
So military satellites
were detecting these first
because a nuclear blast would
give off a lot of gamma rays
and they were looking
for these detections.
Now And now we have detected
thousands of these from space.
One of the problems is
it is very difficult
to pinpoint the location.
And this is because
gamma ray telescopes
have very poor resolution.
It is very hard to focus
gamma rays as compared
to focusing say radio
waves or visible light.
And that makes it hard to
find the optical or radio
counterpart.
So where is the burst occurring.
We need to say
that if we can only
judge that it's in
this region there
could be multiple
objects that could
be the source of this burst and
we don't know which of them.
It is after some
work we have been
able to detect optical
sources for some of them.
And many of them are found to
be billions of light years away.
So what are these
gamma ray bursts then?
Well, there are two types.
There are the long duration
bursts and the short duration
bursts.
These are defined and given
the limit of two seconds.
So a long duration
burst is something
that lasts longer than two
seconds a short duration burst
lasts less than two seconds.
We believe that the
long duration bursts
are caused by the
collapse of a star,
which has lost its outer
layers of hydrogen.
So as the star collapses
down and gives off
all of this energy.
The energy is then beamed and
gives us a gamma ray burst.
These are the longer ones.
Now we believe that
this short duration
bursts are caused by
colliding neutron stars
- two neutron stars
orbiting each other.
And then spiraling in closer
and closer together over time.
And eventually coalescing
into one single object.
Some of the evidence
for this, we
have things like the kilonova
and gravitational waves
- gravitational waves have now
been detected from black holes.
And our prediction
that would occur
for any massive
objects that are moving
and massive objects moving
fast like two neutron
stars collapsing would
be believed to give off
gravitational waves.
And that is getting to the point
where we can now detect these.
So the difference
is, again, what
happens if some of
the outer layers
have been pushed out of a star.
We may get in that
supernova explosion
- instead of a supernova,
we may actually
get the gamma ray burst
because the outer layers are
no longer present.
We only have the inner layers
of the star in a short duration
burst again, we're looking
at a different process
where two neutron stars are
actually colliding together.
So we can look at
an image - I kind of
drew that on there where
we can actually look
at that as an image as well.
And when we see them.
Here they are actually giving
off those gravitational waves
that will travel out into space,
the closer they get together,
the more rapidly.
They move and the greater
the gravitational waves
therefore making them easier
to detect any object moving
gives off gravitational waves
any the object with mass
at least.
But they are so
weak that they are
difficult to detect unless you
have high mass objects, things
like neutron stars
or black holes that
are moving very, very quickly.
So let's finish up here
as we do with our summary
and what we find is that
compact stars in binary systems
give rise to different
types of phenomena.
The white dwarf stars can give
us novae or supernovae things
that we can see invisible
light neutron stars can give us
X-ray or gamma ray bursts is,
again, more compact object
then can give us
higher energy events.
So we associate the X-ray
bursts or the gamma ray
bursts with neutron
stars and bursts
of visible light for
novae or even supernovae
are believed to be
caused by white dwarfs
in a binary system.
And remember that most
stars in the universe
- half the stars are
part of binary systems.
And that means that
there are a lot
of cases where this can occur.
So that concludes our
lecture on compact stars
in binary systems.
We'll be back again
tomorrow for again next time
for another topic in astronomy.
So until then, have a
great day, everyone.
And I will see you in class.
