the term dwarf star refers to any star
of average or low luminosity
mass and size there are important
subclasses of dwarf stars
such as white dwarfs and red dwarfs
main sequence stars such as our sun are
also classified
as dwarf stars whose colors take several
ranges from red to blue
according to their photosphere
temperatures that can be more than 10
000 kelvin or reach a minimum of a few
thousand kelvin
curious to learn more about dwarf stars
and their types
keep watching
[Applause]
[Music]
as we mentioned earlier main sequence
stars are classified as dwarf stars
it's logical to say that most of the
stars in the observable universe
are dwarf stars since they are in the
main sequence
of their life cycle the main sequence
phase is the longest phase in the
lifetime of any star
the characteristic feature of this phase
is the thermonuclear fusion of hydrogen
into helium
in the star's core the star usually gets
cooler
brighter and bigger once it runs out of
the hydrogen
fuel in its core therefore any typical
star
gets bigger before dying although the
main sequence stars are referred to as
dwarf stars
they are quite large even some of them
are several times larger than our sun
our sun is classified as a g2v star
where g2 refers to a photosphere
temperature
of 5500 celsius and
v refers to its dwarf status there are
some special dwarf classifications that
designate small classes of stars
for example red dwarfs brown dwarfs
yellow dwarfs and white dwarfs one
red dwarfs which are the smallest and
coolest stars in the main sequence
additionally they are the most common
type of dwarf stars in the milky way
in fact they constitute three quarters
of the milky way
however they cannot be easily observed
due to their extremely low luminosity
the closest red dwarf to earth is
proxima centauri
located in the southern constellation of
centaurus
red dwarfs are very low mass stars with
masses about 7.5 percent of the solar
mass
and 9 solar radii these conditions
result in low gravitational pressure
which in turn leads to low thermonuclear
fusion rates
that eventually lead to low temperatures
the proton chain mechanism is the way by
which hydrogen is fused to helium
which releases tremendous amounts of
energy
mainly by convection however helium does
not accumulate in the core of low mass
red dwarfs
since they are very convective which
makes them burn
large proportions of hydrogen before
leaving the main sequence phase
the low nuclear fusion rates in the core
along with the convective feature of the
low mass red dwarfs
result in their extremely long life
spans
which are approximated to be longer than
the age of the entire universe
two brown dwarfs whose masses are less
than 0.08 solar masses
the internal temperature and pressure of
the protostar
is insufficient to induce thermonuclear
fusion
that is necessary to fuse hydrogen into
helium
which makes the proto-star a failed star
and it's called a brown dwarf midway
between jupiter
and any regular star to illustrate this
stars radiate light and energy due to
the fusion of hydrogen into helium
in their core however the temperature of
the core must be
at least 3 million kelvin for the
nuclear fusion to occur
and due to the fact that the core
temperature increases
with increasing gravitational pressure
the mass of the protostar
must be at least 75 jovian masses or
0.08
solar masses the dwarf star barely marks
that limit
it's more massive than any observed
supergiant gas planets
and not massive enough to be a star
despite the fact that they are named
brown dwarfs
their colors vary and take a wide range
such as orange red or even mostly
magenta
the existence of brown dwarfs was
theorized by
chives kumar in the 1960s in his theory
kumar named them black dwarfs and
defined them
as dark substellar objects floating
freely in space
that were not massive enough to sustain
hydrogen fusion
3. yellow dwarfs which are stars of
medium size
such as our sun a typical yellow star
belongs to the main sequence
and has a spectral type g weighing
between
0.84 and 1.15 solar masses
yellow dwarfs make up 10 percent of the
total stars in the milky way
the surface temperatures of yellow
dwarfs reach
6000 degrees celsius which makes them
shine bright yellow
almost white yellow dwarfs become red
giants and
white dwarfs at the end of their lives a
typical g-type main sequence star
is burning hydrogen into helium in its
core
by means of thermonuclear fusion our sun
for example converts approximately 600
tons of hydrogen to helium per second
via the proton proton chain mechanism in
which
four hydrogen atoms form one helium atom
alpha centauri a tau ceti and 51 pagassi
are well known examples of yellow dwarfs
besides our sun the ultimate fate of
yellow dwarf stars
is that they will turn into small dense
white dwarfs
after evolving too many faces first
once the yellow dwarf runs out of
hydrogen after approximately 10 billion
years
it expands to larger sizes of its
original volume
and becomes a red giant however at a
certain point
the inner core reaches a typical density
at which the
degeneracy pressure cannot support the
core against its own gravity
sending shock waves to the gaseous outer
layers which eventually are shedded to
form
planetary nebula the core then cools and
becomes a white dwarf
whose chemical composition is carbon and
oxygen
aldebaran or alpha tori is a good
example of red giants
with luminosity 518 times the solar
luminosity
and located around 65 light years away
from the sun
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any of our weekly videos 4. white dwarfs
also called degenerate stars which are
composed of electron degenerate matter
white dwarfs are very distinct
astronomical objects
being stellar remnants they are
extremely dense
a typical white dwarf has the mass equal
to the sun
trapped in a small planetary volume that
is comparable to the volume
of the earth there's no thermonuclear
fusion that takes place inside the white
dwarf
however its faint luminosity is due to
the stored thermal energy inside it
white dwarfs puzzled many scientists
throughout the last century
in particular eddington who argued in
1924
that white dwarfs have extremely high
density
and hence very strong gravitational
fields that caused the light emitted
from them to be
red-shifted according to the theory of
general relativity
eddington's estimate of the
gravitational redshift of sirius b
was then confirmed one year later by
walter s
adams at the mount wilson observatory in
1928
pokrowski a russian physicist published
a paper about the limiting density
beyond which a star collapses in his
paper
porkrovsky claimed that a star will
collapse
if its degenerate electron pressure can
no longer support its core against the
gravitational pull
nearly a year after that wilhelm
anderson modified prokrowski's idea
he repeated pokrovsky's calculations and
came up with an idea that a star reaches
its limiting density because of
radiation
provided that e equals m c squared there
is an equivalence between radiation and
or luminosity
and the star's mass that is when a star
radiates energy into space
it contracts and decreases its mass
the idea of a star's limiting mass was
then demonstrated by
edmund clifton stoner in his papers he
argued that the limiting density might
exist
due to the squeezing of electrons which
obey fermi dirac
statistics in a small volume not due to
the full ionization of matter
as prokravsky claimed moreover he was
highly dependent on
jean's theory of stellar stability which
states that a star that satisfies the
ideal
gas laws cannot be stable anderson then
made an
analysis of the electron gas in white
dwarfs to check if it
satisfies ideal gas laws or not
also he claimed that as the density of
white dwarfs increases
the electrons are driven to higher
energies
and quickly become relativistic stoner
then came to the picture with a
modification to anderson's mathematical
treatment to the problem
and he finally accepted the role of
special relativity in approaching it
he demonstrated that the density
approaches infinity
as the mass reaches the limiting mass in
1930
at the age of 20 subrahman chandrasekhar
the indian astrophysicist calculated the
correct mass
limit above which the white dwarf cannot
exist
he took into consideration the effects
of special relativity
and applied a better model for density
distribution in white dwarfs
it is so important to indicate that
clifton stoner derived a mass limit
and it was about 2.5 solar masses
however to simplify the calculations he
assumed that the density in the star was
uniform and did not change from the high
density in the center
to the vanishing density on the surface
eventually the underlying assumption of
uniform density
caused him to lose the priority in the
discovery
of the white dwarf's mass limits to sum
this up
white dwarfs are distinct objects with
stellar masses and planetary dimensions
their stability is governed by gravity
and quantum mechanics
and according to the chandrasekhar mass
limit there is a certain mass above
which
white dwarfs cannot exist and it equals
approximately
1.4 solar masses they are also
massive hot conducting spheres sheathed
in a thin
but very effective blanket of insulation
according to some studies regarding
their thermal structure
moreover the chemical composition of
white dwarfs depend on the initial mass
of the star
if the initial mass is between 0.08 and
0.4
solar masses then the star will be a
very low
mass star that's too cool to fuse helium
into carbon and
oxygen it will end up as a white dwarf
made of helium however if the initial
mass is between 0.4
and 4 solar masses then the star will be
a fairly
low star that's too cool to fuse carbon
and oxygen into heavier elements
it will end up as a white dwarf made of
carbon
and oxygen in conclusion stars are very
dynamical objects whose behaviors are
governed by
many macroscopic and microscopic
features
they exist in numerous shapes sizes and
states
and these varieties determine the fate
of any star since we all know the fact
that all stars evolved through the
course of their lives
stars spend most of their life span in a
phase called the main sequence
which is highlighted on the hertzsprung
russell diagram
or hr diagram the most important feature
that highlights the main sequence stars
is the thermonuclear fusion of hydrogen
into helium
in their cores by the proton proton
chain mechanism
the main sequence stars are also
considered to be dwarf stars
despite the fact that they are quite
large in fact several times larger than
our sun
the dwarf stars are classified into many
different types
such as red dwarfs brown dwarfs yellow
dwarfs and white dwarfs
red dwarfs are only a fraction the size
and mass of the sun
and can be as little as 1 10 000 the
sun's brightness
because they are so cool their surfaces
are
orange or red brown dwarfs are failed
stars
since they have extremely low masses
that are insufficient to induce
thermonuclear fusion inside their cores
yellow dwarfs
are medium-sized stars like our sun
which in turn is classified as a g2 v
star
white dwarfs are the dead cores of once
normal stars
they typically are less massive than the
sun and only about the size of earth
they are extremely hot and dense
although they
no longer produce nuclear reactions
thanks for watching everyone
have you understood how dwarf stars are
categorized
do you know what governs the stability
of white dwarfs
or what's their energy source let me
know in the comments below
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