LIFECYCLE
OF STAR
The bright little dots that illuminate our night
sky, the large glowing body we see daily during the day are stars. Stars
are hot spheres of gas that are held together by their own gravity. They emit
immense amounts of heat and light by fusing two nuclei to form a bigger
nucleus, a process known as nuclear fusion. Nuclear fission and fusion deserve
a blog of its own and I will not discuss it here in detail. In layman’s terms,
stars during the majority of its lifecycle fuse two hydrogen nuclei to make a
single helium nucleus, a reaction that takes place in its core. As the star
matures it runs out of hydrogen nuclei to fuse and it starts fusing heavier
nuclei. This simple fact is what gives a lifecycle for stars.
Stars are
born from an interstellar cloud of molecular dust known as stellar nebula.
These are also known as stellar nurseries or star forming regions. This cloud
begins to form local dense regions of sufficient mass which pulls in the rest
of the dust and gas cloud towards it. As more and more mass piles up at the
centre of this collection of gas and dust it heats up and this makes some fusion
reactions possible. At this stage the star is in its infancy and it is called
as a protostar. Protostars continue to gobble up more gas and dust from its
parent cloud to grow, just like an infant would.
Now
let’s meet what I would like to call as “wannabe stars” or “malnourished
stars”. Also known as brown dwarfs, these are the protostars who didn’t feed
enough gas and dust from their parent cloud. They don’t have adequate mass that
would heat up its core just enough to sustain fusion of hydrogen molecules. But
they have the capability of fusing heavier isotope of hydrogen called deuterium
(Hydrogen with a neutron) into helium. This only goes on for a few million
years before it runs out of deuterium. These stars glow very dimly, not because
of nuclear reaction in its core but because of heat leftover from its
formation. The gravitational collapse that forms a brown dwarf releases huge amounts
of energy which gets trapped inside the brown dwarf and radiates into the
vastness of space bit by bit. As it cools the glow gets dimmer and ultimately
becomes infrared radiation.
When
a star cumulates sufficient mass, it becomes a main sequence star. Main
sequence stars are those which can fuse hydrogen nuclei in their core. Red
dwarfs (M star) are smallest and coolest kind of main sequence stars. They are
formed by protostars which accumulate barely minimum amount of mass (7-50
percent mass of the sun) to achieve sustained nuclear fusion of hydrogen.
Seventy percent of stars in the universe are red dwarfs. They are very dim and
cannot be observed with naked eye. Massive stars accumulate their helium in
their cores but red dwarfs have convective currents that mix up its helium throughout.
So, they burn up their hydrogen very slowly and have a prolonged life, so much
so that they will be the last main sequence stars left before the universe
dies.
Further
main sequence stars are classified as average stars (our sun) and massive
stars. These stars unlike red dwarves accumulate helium (fusion product) in
their cores. Average stars have about 0.5- 8 solar masses. Massive stars have
mass above 8 solar masses. Massive stars fuse hydrogen quicker than average
stars so generally have higher surface temperatures. The side effect of this is
that massive stars have much shorter lifespan than average stars. Energy
released by the fusion reaction in the core counters the gravitational force to
keep the size of the star stable.
Entering
the next stage of life cycle, we have giant stars. As the star ages Helium
builds up in the core of the star. But the star's does not have enough
temperature to fuse helium nuclei. So, the star initially begins to shrink in
size. But as it shrinks the insides start to heat up and a shell of unfused
hydrogen nuclei around the core reaches the required temperature to begin
fusing. The energy from this pushes the periphery of the star causing it to
expand. This cycle continues for a few million years. Stars in this phase are
called variable stars. At some point there is enough temperature to fuse
heavier elements which releases tremendous amounts of energy which expands the
star further and forms red giants, red super giants or hypergiants depending on
the initial size of the main sequence star.
Eventually
fusion reactions produce iron in the core of the star. It is not feasible for
fusion to occur beyond Iron because it has the highest binding energy per
nucleon and is the most stable nucleus. So, there will be no nuclei left to
fuse in the stars core. There will be no energy produced by fusion to counteract
the gravitational attraction. The star implodes and rebounds creating an
explosion that can outshine entire galaxies, called supernova. Low mass stars
after supernova forms white dwarfs which are incredibly dense and hot. They
emit most of their radiation as ultraviolet radiation and visible light, making
them some of the brightest objects in the sky. White dwarfs are also incredibly
long-lived, with lifetimes of up to several billion years. This means that they
will continue to shine and emit radiation and will gradually cool over time as
they release their remaining heat energy.
Heavier
stars after supernova form Neutron stars and black holes. A neutron star is a
type of extremely compact star that is composed almost entirely of neutrons.
The density of a neutron star cannot be
comprehended by our minds. Density of a neutron star is equivalent to cramming
the mass of the entire Sun into a sphere the size of a small city. A type of
neutron stars called pulsars emit radiation along their magnetic poles. Black
holes are even more dense than neutron stars. So much mass is crammed in an infinitesimally
small area that it warps space-time to form a singularity. Singularities have
infinite density. I will discuss about neutron stars and blackholes in detail
in my future blogs. The supernova forms a planetary nebula which aids in the
formation of next generation of stars.
x
value of the second term i.e., the term in the bracket will
approach a constant value k which in this case is 0.6931….So, for a random exponential function 
