Star Formation

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Reach for the stars!

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Task 1
Section: The lifecycle and classification of stars
Star formation
Star formation begins with a huge cloud of dust and gases usually stretching light years
across (Schrijver, 2018). The attractive force of gravity then pulls these materials towards each
other. Gradually, a sufficiently large quantity of dust and gas is gathered into a large ball of
material. The intense pressure within the ball of material due to the contraction of the mass under
gravity pull and the collision of the particles raises the temperature of the mass up to the range of
ten thousand Kelvin (Ward-Thompson & Whitworth, 2011). This temperature is high enough to
initiate the beginning of hydrogen ionization. As the contraction progresses, the cloud of material

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breaks into smaller fragments up to the point when the density of the mass becomes high enough
such that it cannot emit any radiation. That is, it develops a photosphere. This radiation which
has been trapped in the interior also aids in heating up the gas cloud. The beginning of the
process of nuclear fusion develops enough energy to balance the contraction process. At this
stage, the cloud can be considered a protostar and it can be represented on the H-R diagram. The
process of nuclear fusion starts when the temperature of the core reaches about 10 million
Kelvin. At this stage the protostar becomes a star and it enters the main sequence in the H-R
diagram and it stays there as long as it has enough fuel to support nuclear fusion.
Death
The lifetime of a star is dependent on its initial mass and it eventually exhausts its main
fuel supply which is hydrogen. This can happen after millions or even billions of years. Once the
fuel supply diminishes, the outward pressure which maintains balance with the inward pull of
gravity disappears. Without this outward force, the star’s outer layers begin to collapse towards
the central core. This renewed contraction generates more heat that counteracts the force of
gravity for some time causing the star to expand. The star can actually expand to a size bigger
than it was initially, up to a hundred times. The star then becomes a red hot giant. From this stage
onwards, what happens to the star is dependent on whether it was a massive star (heavier than
our sun), a low mass star (lighter than our sun) or with a mass equal to that of the sun.
Stars with mass equivalent to that of the sun, black dwarfs
When a star such as the sun reaches the phase of the red giant, its layers on the outer
sphere expand continually. However, the core continues with its inward contraction with carbon
being formed from the fusion of helium atoms (Ward-Thompson & Whitworth, 2011). This

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liberates more energy which slows the contraction only for a short while in the case of stars with
mass equivalent to that of the sun. Due to the strength of the structure of carbon, the surrounding
material mass cannot compress the core any further and it stabilizes. The outer layers of the star
begin to diffuse as a cloud commonly known as the nebula. The end result is that only
approximately twenty percent of the initial mass of the star is left. For the rest of the life of the
star, it shrinks and cools up to several thousand kilometers in diameter and becomes a white
dwarf (Ward-Thompson & Whitworth, 2011). The star then radiates its residual heat until
eventually only a cold and dark mass remains. At this point it is referred to as a black dwarf.
Massive stars
Massive stars have a mass of about 5 times the mass of the sun or more. The end process
of these stars is different and more dramatic than the death of lighter stars. After the star reaches
the red giant phase described before, the core starts to collapse due to imbalance between gravity
and the outward pressure (MacDonald, 2015). The core becomes denser and hotter as it
collapses. This initiates new nuclear reactions which slow down the collapse for a while.
Eventually the core is only made of iron which cannot be fused and the collapse resumes shortly
after. The temperature of the core increases rapidly to the range of hundreds of billions with the
crushing of the iron atoms (MacDonald, 2015). As the iron atoms are crushed together, the
nuclear repulsive forces increase to a point where they become greater than gravity and the star
explodes violently in an event known as supernova sending out a shock wave.
The extreme pressure and temperature involved in the case of supergiant stars sometimes
forces electrons to combine with protons forming neutrons in the process (Ward-Thompson &
Whitworth, 2011). Eventually, the entire core becomes a dense collection of neutrons. If this
dense ball remains after the supernova, it is known as a neutron star. Neutron stars are considered

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to be the objects with the highest density in the universe (Maggiore, 2018). Neutron stars contain
material equivalent to several solar masses compressed into a small diameter of only about 40
km. On the other hand, if the star was about 15 times as heavy as the sun or more, the neutrons
will be incapable of withstanding the collapse of the core and as a result, a black hole will be
formed (Maggiore, 2018). A black hole is a region of space-time where the force of gravity is so
strong that nothing, not even light photons can break free (Wittman, 2018).
The Schwarzschild radius greatly determines the fate of massive stars. This is defined as
the minimum radius of a body necessary to prevent it from gravitational collapse. Below this
radius, the force of attraction between a body’s particles would cause the body to collapse in on
itself ("Schwarzschild radius," n.d.). Mathematically, this radius is expressed as:
Rg = 2GM
c2
The constants in the equation have their usual meaning.
The H-R diagram
The Hertz sprung-Russel (HR) is a very crucial analysis tool for studying the evolution of
stars. The HR diagram was developed in the start of the 20th century by Ejnar Hertzsprung. It was
also independently developed by Henry Norris Russell ("Hertzsprung-Russell Diagram |
COSMOS," n.d.). The HR is a plot of magnitude or the luminosity of stars against their
temperature in Kelvin (for the theoretical plot). The star luminosity is expressed in solar units.
This means that a value of 1 on the diagram is equivalent to the luminosity of the sun. All stars
must undergo a particular process of evolution following certain stages depending on the initial

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mass of the stars. The evolution process is accompanied by changes in temperature, the
composition of the star and the luminous intensity of the star. The stages of evolution can be
represented on the HR diagram hence it is possible to determine the composition of a particular
star as well as its stage of evolution by observing its position on the HR diagram.
Main sequence stars
Using computer models to study the evolution of stars over long periods of time has
shown that stars spend approximately 90 % of their life time undergoing nuclear fusion in their
cores producing helium from the fusion of hydrogen (Saha & Taylor, 2018). This can be said to
be the reason as to why the main sequence band on the HR diagram contains almost 90 % of all
the stars with few in the other regions. The sun is a star on the main sequence band as well as a
collection of other stars which could be of the same mass but different temperature. Other stars
on the main sequence are much heavier than the sun and much hotter and brighter. These stars
are indicated by their blue color. Stars which are lighter compared to the sun are cooler and they
have low luminosity as well as being smaller. As a star gets older, it becomes brighter and moves
up the HR diagram as a result of its increasing luminous intensity (Saha & Taylor, 2018). This
explains why stars on the main sequence are represented by a band.
Giants and supergiants
These are two different groups of stars on the right of the main sequence stars. Giants are
a collection of stars with average luminosity and relatively cooler temperature. Giants stars
derive their energy by fusing heavier elements as a result of having depleted their initial
hydrogen fuel supply (Jones, Jenkins, & Rojo, 2011). Most of the giant stars have been shown to
be unstable with others shining so brightly producing stellar wind. The brightest and largest stars