The Lifecycle and Classification of Stars

Table of Contents
From Birth to Death...................................................................................................................1
Nebula....................................................................................................................................5
Nebula formation...............................................................................................................5
Protostar.................................................................................................................................6
Brown Dwarf..........................................................................................................................6
Spectral class M brown dwarf............................................................................................6
Spectral class L brown dwarf –..........................................................................................6
Spectral class T brown dwarf –..........................................................................................7
Spectral class Y brown dwarf –.........................................................................................7
Main sequence star.................................................................................................................7
Red Giant...............................................................................................................................7
Neutron Star...........................................................................................................................7
Black Holes –.........................................................................................................................8
The Schwarzschild Radius.................................................................................................9
Singularity........................................................................................................................10
Classification of Stars..............................................................................................................11
Hertzsprung-Russell (or H-R) diagram................................................................................11
Group A............................................................................................................................13
Group B............................................................................................................................13
Group C............................................................................................................................13
Group D............................................................................................................................13
The Universe............................................................................................................................14
Big Bang Theory......................................................................................................................14
Timeline of the Big Bang Theory -..........................................................................................15
Singularity Epoch.................................................................................................................15

Inflation Epoch.....................................................................................................................15
Cooling Epoch......................................................................................................................15
Structure Epoch....................................................................................................................16
The fate of the Universe...........................................................................................................17
“Big Crunch” theory............................................................................................................17
The "Big Rip" theory............................................................................................................19
History of the Big Bang Theory...............................................................................................20
Doppler Redshift..................................................................................................................20
Friedmann equations............................................................................................................20
Hubble’s law -......................................................................................................................20
Cosmic Microwave Background Radiations........................................................................21
Olbers’ Paradox........................................................................................................................23
References................................................................................................................................24

Section 1: The lifecycle and classification of stars
From Birth to Death
One would wonder that many light years across the thing that twinkles at night is actually an
enormous cloud of gas and dust! The life cycle of a star starts with what is called a nebula.
Hydrogen is the main constituent of this giant cluster of clouds. With time, hydrogen gas in
the ‘nebula’ is pulled together by the gravitational force of attraction and it begins to spin. As
the gas spins faster, it heats up and becomes a protostar.
If the protostar has not gained enough mass then it turns into a brown dwarf. However, if it
has got enough mass then due to the increased gravity and increased temperature of
upto15,000,000 degrees nuclear fusion occurs in the cloud's core. Hydrogen gas is the major
source to generate energy for the process of nuclear fusion. This cloud of gases begins to
glow brightly, contracts a little and becomes stable. It is now turned to the main sequence
star and will remain in this phase giving out energy for millions to billions of years to come.
Our Sun is also in this state right now.
During the process of energy radiation, hydrogen in the core of the cloud is transformed into
Helium by the process of nuclear fusion. As soon as the hydrogen supply in the core stops,
the process of nuclear fission is also stopped resulting in no further generation of heat. Then,
the core of the cloud becomes unstable and starts contracting. At this stage, the outer shell of
the star is still having mostly hydrogen and starts to expand. Following the expansion, the star
cools as a result of a major drop in temperature and appears red. This stage of the star is
termed as the red giant stage. It is named 'giant' because its outer boundary has an expansion
compared to its previous stage.
If we notice, the mass of the star determines its destiny as to what would be the next phase in
the life cycle.
Further, depending on the mass of the star if it is a low-mass star then the core further
collapses and the outer layer of the star expels. These outer layers form a planetary nebula.
The core, in this case, remains as a white dwarf which further cools to become a black
dwarf. A black dwarf is nothing but a non-radiating ball of gas.
In case, if the mass of the red giant is high which is called a super red giant then it
undergoes into what is called a supernova explosion. In this case, if the remains of the
explosion are comparable about 1.5 to 3 times of the sun's mass, it turns out as a neutron
star.
If the mass of the remains is more than 3 times the mass of the sun then something unusual
happens. Now, the force of gravity overcomes the nuclear forces. The core, in this case, is so
dense that it swallowed by its own gravity giving death to a star and birth to a black hole. A
black hole has very high gravity that even the unbelievably fast 'light' cannot escape from its
grip.
The whole process is explained using a diagram on the next page.

Nebula
'Nebula' is a Latin word for cloud or fog. A nebula is an interstellar cloud of dust made up of
gases like hydrogen, helium, and other ionized gases. A nebula is a place where stars are born
so it is often called "stellar nurseries”. For some centuries, scientists have firmed the view
that outer space is not really a total vacuum but it is made up of what is known as "interstellar
medium". This interstellar medium is nothing but a mixture of gases and dust particles.
Interstellar gases which make 99% of the interstellar medium consist of hydrogen (75%) and
helium (25%). This mixture of gas is made up of atoms, molecules and charged particles
(also known as Plasma).
Nebula formation
When portions of the interstellar medium undergo gravitational collapse, a nebula is formed.
The attractive force of gravity causes matter to clump together, forming regions of very high
density. Here, a center of the collapsing material is formed. The UV ionizing radiation
causes the surrounding gas to become visible at optical wavelengths on the EM spectrum.
Nebula's size has very varied measurements. Nebulae are so vast that their diameter is
measured in hundreds of light years. Surprisingly, a nebula similar in size in sie of the earth
will weigh only a few kgs.
Below diagram shows the formation of Orion nebula –

Protostar
The molecular cloud gains mass due to the gravitational force of attraction. The protostar is
the first step in the stellar evolution. The picture of protostars was first given by Chushiro
Hayashi in 1966. The outer layer of protostar consists of shock waves which came from the
inner edge of the disk. Protostars’ energy is generated from these shock waves. A protostar
approaches its end when it consumes all the available gas due to self-gravity and it then
proceeds towards what is known as pre-main sequence star.
Brown Dwarf
A substellar object whose size is 13 to 75–80 times that of Jupiter which is not able to carry
out nuclear fusion using hydrogen to generate helium. Brown dwarfs are not just brown, they
may be of different colors. Brown dwarfs are classified by their different spectral classes like
type M, L, T, and Y brown dwarfs.
Spectral class M brown dwarf – They have the spectral class of M6.5 or later. Also
called late-M dwarf, some astronomers take them as the red dwarfs.