Detailed Desription of Stars. What is a Star? , Classification of stars, Hertzsprung-Russel Diagram, Spectral Classes, Luminosity, Variable Stars, Composite Stars, Neutron Stars, Black Holes, Star Clusters, Supernovae, Binary Star, Chandrashekhar Limit, Limit Value Calculation Formulae, Applications of the limit, Tolman-Openheimer Volkoff Limit, About Subrahmanyam Chandrasekhar

1. Space science
Asif Kareem
Reg No. 814918110005
B.E Aerospace Department

2. Classification of Stars

3. Star Classification
• Stars are classified by their spectra (the
elements that they absorb) and their
temperature. There are seven main types of
stars. In order of decreasing temperature, O,
B, A, F, G, K, and M.
• O and B stars are uncommon but very bright;
M stars are common but dim..
• An easy memoric for remembering these is:
"Oh Be A Fine Girl, Kiss Me."

4. Hertzsprung - Russell Diagram
• Most stars, including the sun, are "main
sequence stars," fueled by nuclear
fusion converting hydrogen into helium. For
these stars, the hotter they are, the brighter.
These stars are in the most stable part of their
existence; this stage generally lasts for about 5
billion years.

5. • As stars begin to die, they become giants
and supergiants (above the main sequence). These
stars have depleted their hydrogen supply and are
very old. The core contracts as the outer layers
expand. These stars will eventually explode (becoming
a planetary nebula or supernova, depending on their
mass) and then become white dwarfs, neutron stars,
or black holes (again depending on their mass).
• Smaller stars (like our Sun) eventually become faint
white dwarfs (hot, white, dim stars) that are below
the main sequence. These hot, shrinking stars have
depleted their nuclear fuels and will eventually
become cold, dark, black dwarfs.

6. Spectral Classes

7. Luminosity
The Yerkes Luminosity Classes
(by William Wilson Morgan and Philip Keenan)
• Luminosity is the total brightness of
a star (or galaxy). Luminosity is the
total amount of energy that a star
radiates each second (including all
wavelengths of electromagnetic
radiation).
• In the Yerkes classification scheme,
stars are assigned to groups
according to the width of their
spectral lines. For a group of stars
with the same temperature, the
luminosity class differentiates
between their sizes (supergiants,
giants, main-sequence stars, and
subdwarfs.

8. Variable Stars

9. Variable Stars
-Stars that Vary in Luminosity-
• A variable star is a star whose brightness
as seen from Earth (its apparent
magnitude) fluctuates.
• This variation may be caused by a change
in emitted light or by something partly
blocking the light, so variable stars are
classified as either:

10.  Intrinsic variables: whose luminosity actually
changes; for example, because the star
periodically swells and shrinks.
 Extrinsic variables: whose apparent changes
in brightness are due to changes in the
amount of their light that can reach Earth; for
example, because the star has an orbiting
companion that sometimes eclipses it.
Many, possibly most, stars have at least some
variation in luminosity: the energy output of
our Sun, for example, varies by about 0.1% over
an 11-year solar cycle.

12.  Intrinsic Variables
These are stars which vary their light output, hence
their brightness, by some change within the star
itself. They are an extremely important and useful
group of stars to astronomers as they provide a
wealth of information about the internal structure
of stars and models of stellar evolution. Perhaps
their greatest value is the role of some types such
as Cepheids and supernovae in distance
determination. Intrinsic variables are further
classified as to whether they exhibit periodic
pulsations are more explosive or eruptive events as
in cataclysmic variables.

13. Pulsating Variables
Pulsating variables periodically expand and contract their
surface layers. In the process they change their size,
effective temperature and spectral properties. As they are
a vital tool in galactic and extragalactic distance
determination and have many types they are discussed in
more detail on separate pages.
Eruptive or Cataclysmic Variables
Eruptive variables can exhibit significant and rapid
changes in their luminosity due to violent outbursts
caused by processes within the star. There is a wide
variety of eruptive or cataclysmic variables. Some event,
as implied by the term cataclysmic result in the
destruction of the star whilst others can reoccur one or
more times. Some are also discussed in more detail on
stellar evolution

14.  Extrinsic Variables
Eclipsing Binaries
The processes behind eclipsing binaries are explained
in more detail in the section on binary stars. They are
regarded as variable too in that as one of the
component stars is eclipsed by the other, the total
brightness of the system decreases. The light curves
produced by eclipsing binaries show distinctive periodic
minima.

15. Rotating Variables
• Our Sun sometimes has sunspots visible on its
surface. These cooler regions appear darker than
the surrounding areas. As the Sun rotates the
sunspots appear to move across its surface. If we
view a side of the Sun with a lot of sunspots it
would have a fractionally lower light output than
an unblemished side. This principle can be
extended to other stars, some of which are
thought to have much more active starspot
activity. Starspots can be either dimmer or
brighter than surrounding regions. As a star with
starspots rotates, its brightness changes slightly.
Stars exhibiting such behaviour are
called rotating variables. One type of rotating
variables are the BY Draconis stars.

16. Composite Stars

17. Red Giants and White Dwarfs
 As a star begins to use up its hydrogen, it fuses helium
atoms together into heavier atoms such as carbon.
 When the light elements are mostly used up the star can no
longer resist gravity and it starts to collapse inward.
 The outer layers of the star grow outward and cool. The
larger, cooler star turns red in color and so is called a Red
giant.
 Eventually, a red giant burns up all of the helium in its core.
What happens next depends on how massive the star is. A
typical star, such as the Sun, stops fusion completely.
 Gravitational collapse shrinks the star’s core to a white,
glowing object about the size of Earth, called a White
dwarf. A white dwarf will ultimately fade out.

18. Neutron Stars
After a supernova explosion, the leftover
material in the core is extremely dense.
If the core is less than about four times the
mass of the Sun, the star becomes a neutron
star.
A neutron star is made almost entirely of
neutrons, relatively large particles that have
no electrical charge.
If the core remaining after a supernova is
more than about five times the mass of the
Sun, the core collapses into a Black Hole.

19. Black Holes
Black holes are so dense that not even light can
escape their gravity. With no light, a black hole
cannot be observed directly. But a black hole can
be identified by the effect that it has on objects
around it, and by radiation that leaks out around
its edges.
Anything that comes within a black hole’s “event
horizon,” its point of no return, will be consumed,
never to re-emerge, because of the black hole’s
unimaginably strong gravity. By its very nature, a
black hole cannot be seen, but the hot disk of
material that encircles it shines bright. Against a
bright backdrop, such as this disk, a black hole
appears to cast a shadow.

21. The stunning new image shows the shadow of the supermassive black hole in
the center of Messier 87 (M87), an elliptical galaxy some 55 million light-years
from Earth. This black hole is 6.5 billion times the mass of the Sun. Catching its
shadow involved eight ground-based radio telescopes around the globe,
operating together as if they were one telescope the size of our entire planet.

22.  There are many remaining questions about black holes
that the coordinated NASA observations may help
answer. Mysteries linger about why particles get such a
huge energy boost around black holes, forming dramatic
jets that surge away from the poles of black holes at
nearly the speed of light. When material falls into the
black hole, where does the energy go?
 A stellar-mass black hole forms when a star with more
than 20 solar masses exhausts the nuclear fuel in its core
and collapses under its own weight. The collapse triggers
a supernova explosion that blows off the star’s outer
layers. But if the crushed core contains more than about
three times the Sun’s mass, no known force can stop its
collapse to a black hole. The origin of supermassive black
holes is poorly understood, but we know they exist from
the very earliest days of a galaxy’s lifetime.

23. Once born, black holes can grow by accreting
matter that falls into them, including gas stripped
from neighboring stars and even other black
holes.

24. Star Clusters
Star clusters are very large groups of stars.
Two types of star clusters can be
distinguished:
 Globular Clusters are tight groups of
hundreds to millions of old stars which are
gravitationally bound
 Open Clusters, more loosely clustered groups
of stars, generally contain fewer than a few
hundred members, and are often very young

25.  Open clusters become disrupted over time by
the gravitational influence of giant molecular
clouds as they move through the galaxy
 But cluster members will continue to move in
broadly the same direction through space
even though they are no longer gravitationally
bound; they are then known as a Stellar
Association, sometimes also referred to as a
moving group.
 Star clusters visible to the naked eye include
the Pleiades (M45) , Hyades, and the Beehive
Cluster (M44).

26. The Globular Cluster Messier 15
The Pleiades, an open cluster
dominated by hot blue stars
surrounded by reflection
nebulosity.

28. Supernovae
 A supernova is a cataclysmic stage towards the
end of a star's life that is characterised by a
sudden and dramatic rise in brightness. A
typical supernova may see a star become
brighter by up to 20 magnitudes to an absolute
magnitude of about -15.
 This means that a typical supernova may
outshine the rest its galaxy for several days or
a few weeks.

29. How it is
caused?
1. Supernovae are caused by one of two main
mechanisms. The first takes place when
accreting material falling onto a white dwarf in a
binary system takes it over the mass set by the
Chandrasekhar limit. The resulting instability
triggers a runaway thermonuclear explosion that
destroys the star and releases large amounts of
radioactive and heavy elements into space

30. 2. The second process occurs in very massive stars
once all the material in their core has been
fused into iron.
As fusion cannot occur in elements heavier than
iron the drop in outwards radiation pressure
means that gravitational collapse overwhelms the
core which rapidly implodes. The core material
gets crushed to form degenerate neutron-density
material whilst the extreme temperature and
pressure in the surrounding layers cause rapid (R-
process) nuclear reactions that synthesise the
heaviest elements.

31.  A huge flux of neutrinos is thought to interact with the super-
dense material, ripping the star apart. Such core
collapse supernovae may result in neutron stars and black
holes forming from the remaining core material.

32. Binary star
A Binary Star is a star system consisting of
two stars orbiting around their common Barycenter.
Polaris (the pole star of the Northern Hemisphere
of Earth) is part of a binary star system.
DOUBLE STAR: A Double Star is two stars that
appear close to one another in the sky. Some are
true binaries (two stars that revolve around one
another); others just appear together from the
Earth because they are both in the same line-of-
sight.

33. ECLIPSING BINARY
 An eclipsing binary is two close stars that appear to be a
single star varying in brightness. The variation in brightness
is due to the stars periodically obscuring or enhancing one
another. This binary star system is tilted (with respect to us)
so that its orbital plane is viewed from its edge.
X-RAY BINARY STAR
 X-ray binary stars are a special type of binary star in which
one of the stars is a collapsed object such as a white
dwarf, neutron star, or black hole. As matter is stripped
from the normal star, it falls into the collapsed star,
producing X-rays.

34. Chandrasekhar limit

35. The Chandrasekhar Limit
 The Chandrasekhar limit is the maximum mass of a
stable white dwarf star.
 The limit was first published by Wilhelm Anderson
and E.C Stoner and was named after Subrahmanyan
Chandrasekhar, the Indian American Astrophysicist
who improved upon the accuracy of the calculation
in 1930, at the age of 19.
This limit was initially ignored by the community of
scientists because such a limit would logically
require the existence of black holes, which were
considered a scientific impossibility at the time. The
currently accepted value of the limit is
about 1.4 M☉ (2.765×1030 kg).

36. According to the Limit
 White dwarfs. Unlike main sequence stars, resist
gravitational collapse primarily through electron
degeneracy pressure, rather than thermal
pressure. The Chandrasekhar limit is the mass
above which electron degeneracy pressure in the
star’s core is insufficient to balance the star’s
own gravitational self-attraction. Consequently,
white dwarfs with masses greater than the limit
undergo further gravitational collapse, evolving
into a different type of stellar remnant, such as a
neutron star or black hole. Those with masses
under the limit remain stable as white dwarfs.

37. Limit according to physics
 Electron degeneracy pressure is a quantum-mechanical
effect arising from the Pauli exclusion principle. Since
electrons are fermions, no two electrons can be in the
same state , so not all electrons can be in the minimum-
energy level.
 Rather, electrons must occupy a band of energy levels.
Compression of the electron gas increases the number of
electrons in a given volume and raises the maximum
energy level in the occupied band.
 Therefore, the energy of the electrons will increase upon
compression. So pressure must be exerted on the
electron gas to compress it, producing electron
degeneracy pressure. With sufficient compression,
electrons are forced into nuclei in the process of electron
capture, relieving the pressure.

38. Calculated values for the limit
Calculated values for the limit will
vary depending on the nuclear
composition of the mass
Chandrasekhar gives the limit’s
expression, based on the equation of
state for an ideal Fermi gas

39. Limit Value Calculation
Formula

40. Limit Value Calculation Formula
Where,
W is the reduced Planck’s Constant
 c is the speed of light
 G is the Gravitational constant
 μe is the average molecular weight per
electron, which depends upon the chemical
composition of the star.
mH is the mass of the hydrogen atom.
ω0
3 Y2.018236 is a constant connected with
the solution to the Lane-Emden equation.

41. Applications of the limit
 The core of a star is kept from collapsing by the heat
generated by the fusion of nuclei of lighter elements
into heavier ones.
 At various stages of stellar evolution , the nuclei
required for this process will be exhausted, and the
core will collapse, causing it to become denser and
hotter.
 A critical situation arises when iron accumulates in
the core, since iron nuclei are incapable of getting
furtger energy through fusion
 If the core becomes sufficiently dense, Electron
Degeneracy Pressure will play a significant part in
stabilizing it against gravitational collapse.

42.  If a main sequence star is not too massive less than
approximately 8 Solar Masses, it will eventually shed
enough mass to form a white dwarf having mass
below the Chandrasekhar Limit, which will consist of
the former core of the star.
 For more massive stars, EDP will not keep the iron
core from collapsing to very great density, leading to
formation of a Neutron Star, Black Hole or
speculatively a Quark Star. During the collapse,
neutrons are formed by the capture of electrons by
protons in the process of electron capture, leading to
the emission of Neutrinos.
 The decrease in gravitational potential energy of the
collapsing core releases a large amount of energy
which is on the order of 1046 joules.
 Most of this energy is carried away by the emitted
neutrinos. This process is believed to be responsible
for some types of Supernovae.

43. Tolman-Oppenheimer Volkoff limit
 After a supernova explosion, a neutron star may
be left behind.
 Like white dwarfs, these objects are extremely
compact and are supported by degeneracy
pressure
 But a neutron Star is so massive and compressed
that electrons and protons have combined to form
neutrons and the star is thus supported by
Neutron Degeneracy Pressure instead of EDP
 The limit of Neutron Degeneracy pressure,
analogous to the Chandrasekhar Limit is known as
the Tolman-Oppenheimer-Volkoff Limit.

44. Subrahmanyam Chandrasekhar
Subrahmanyam Chandrasekhar was an Indian –American
Astrophysicist , best known for his work on the theoretical
structure and evolution of stars and particularly on the later
evolutionary stages of massive stars and the calculation of The
Chandrasekhar Limit. He won the Nobel Prize in Physics
shared with William Fowler in 1983 largely for his earlier
work, although his research also covered many other areas
within theoretical Physics and Astrophysics. He calculated the
maximum non-rotating mass which can be supported against
gravitational collapse by EDP. This limit describes the max.
mass of a white dwarf star or the minimum mass above which
a star will ultimately collapse into a Neutron Star or a black
hole, following a Supernova event rather than remaining as a
White Dwarf.

45. THE END
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