the purpose of this video
is an overview of stellar remnants
we will look at the origins
physical properties
and observational evidence
for white dwarfs neutron stars
and black holes
to skip to the relevant remnant
you want to learn about
use the timestamps in the description
remember that a star
spends most of its lifetime
burning hydrogen into helium
thereby producing the power
and hence temperatures to produce
the pressure to counteract gravity
the store maintains hydrostatic
equilibrium
until the fuel starts to run
low
near the end of a star's main sequence
lifetime
its core is going to
become denser and hotter allowing
for the fusion of heavier elements
for low mass stars like our sun
the final product
of the fusion chain
will still be relatively low
mass elements such as carbon
when even helium fusion starts to
run out
the star becomes unstable
its core shrinks to a dense
remnant known as a white dwarf and its
outer layers
are blown away
in a drawn out process that creates
a planetary nebula
the name planetary nebula is a misnomer
as it has nothing to do with planets
it is merely a descriptor
due to the disk like appearance
through small telescopes
the colors of the nebula
are due to different temperatures and
ionization states
what remains at the very center is known
as a white dwarf
because it is very hot in surface
temperature
and very small in size the mass
of a white dwarf can still
carry
anywhere between half to one and a half
times the mass of the sun
however their size is only that
about the planet earth
this is so dense that a tablespoon's
worth of this stuff
would still outweigh an entire mountain
because of their small size
unless we can identify the planetary
nebula they are associated with
most white dwarfs are detected
as companions to other stars
so what keeps a white dwarf
from collapsing further
previously
the hot plasma fueled by
nuclear fusion is what
counteracted gravity
because we no longer have this fusion
something else must be
pushing back upward
the physical mechanism is not one
based on temperature but on quantum
mechanics
quantum mechanics prohibits two
electrons
from occupying the same
exact orbital state
in a white dwarf
as the atoms get closer and closer
together
due to the pull of gravity
all of the electrons occupy all
possible orbital states
no further
collapse is possible
this is referred to as electron
degeneracy pressure
for high mass stars
the fusion chain continues further
beyond carbon
into heavier elements such as
oxygen magnesium silicon
all the way to iron
iron cannot fuse further
produce net gain of energy
while it too is subject
to resist squeezing
by gravity due to electron degeneracy
pressure
even quantum mechanics has its limits
the loss of power by fusion
and then
a runaway effect
of electrons being squeezed into protons
to create neutrons instead
will cause a cataclysmic core collapse
that releases so much energy
as to completely tear
the star apart
the release of energy
and its heating of the
now ripped apart outer layers
produces a light phenomena
known as a supernova
these outer layers scatter
many of the heavier elements
essential for life back into the
interstellar
medium where they can
mix in with molecular clouds
creating new generations of stars and
their planets
the collapsed core at the very center
carries over one and a half times
solar masses but is squeezed
into a sphere no more than about
10 kilometers in diameter
due to the collapse
it also spins up similar
to an ice skater
what keeps it from collapsing further
is similar to the previous degeneracy
pressure
but now for neutrons
in the atomic nucleus
it is referred to as neutron degeneracy
pressure
because neutron stars are so small they
are even harder to detect
then white dwarfs
when looking at the center
of supernova remnants
we have detected rapid light
and radio pulses
we call these pulsars
what a pulsar actually is
is the remnant rapidly spinning neutron
star
with its magnetic field also highly
concentrated
accelerating particles that then
emit the light and radio pulses
that we detect when looking
towards the supernova remnant
just like white dwarfs had their limits
so the neutron stars
for the most massive stars
not even neutron degeneracy pressure
can stop the collapse due to gravity
the cores of these collapses
become a black hole
what is a black hole
the definition of down
is the direction in which objects
naturally fall
they are being pulled by gravity
near the surface of the earth at
low speeds we observe
that if we shoot a cannonball
in the air it will come down
the higher the initial velocity upward
the higher maximum height it will attain
but eventually it will still fall back
to the ground
however gravity
decreases with distance
this means that if i
have an initial velocity high enough
then the cannonball
can travel
at such a high speed
that it will never slow down enough
despite gravity to fall back to the
ground
this is known as an escape velocity
there is a limit to this as well
the highest speed in the universe is
that
of the speed of light
if you are too close to a massive object
with enough gravity
then the escape speed would be greater
than the speed of light
and hence you will never
escape
the massive object resulting
from the collapse of the core of a
very massive star
is known as a singularity
it has the mass
of over three times the mass of the sun
but it has no volume
it is infinitely dense
at a certain distance away from this
singularity
is the cut off distance
where the gravity is great enough
that the escape velocity
is that of the speed of light
this distance is known as the event
horizon
and it is the point of no return
so if no light escapes how do we know
black holes exist
we need to look at objects that do
emit light near them
if a black hole is part of a binary
system
it can pull material away
from its companion
the material spirals in and heaps up
releasing x-rays
this is the first way black holes
were detected
because black holes have such high
gravity
they warp space-time and hence
the path of light
if a rogue black hole
comes between us
and a far away star it will
act as a gravitational
lens
what we will observe
is a temporary magnification
or brightening of the faraway star
not all black holes
originate from supernova
explosions of massive stars
we have evidence that at the center
of most galaxies
including our own there is
a supermassive black hole
of the order of millions to even
billions of solar masses
for our galaxy
we have observed that stars near
its geometric center are
orbiting an unseen
mass of about
3 million solar masses
the origins of galactic black holes
are not yet entirely understood
but many astronomers hypothesize
that they may have been part of galactic
formation
 
