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Unit 16 Aim D Distinction Level Marked
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- Astronomy and Space Science
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- PEARSON (PEARSON)
Unit 16 Aim D marked report at distinction level containing points that meet all criteria.
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Unit 16 Aim DLifecycle of the Stars and The Universe
Star Creation
Stars are formed from a nebula, a cloud of interstellar dust and a mixture of gases (mostly hydrogen
gas). Due to its size, the weight of the gases and dust causes the nebula to begin to contract under its
gravitational pull.
As gravity pulls the dust and gas particles together, clusters form inside the nebula. The formation of
these clusters results in them gaining large amounts of kinetic energy, causing them to collide. This
process is called accretion. The kinetic energy of the dust and gas causes a temperature rise of the
matter inside the clusters. This temperature rise goes to over millions of degrees Celsius. This process
forms what is known as a protostar or an infant star.
Nuclear fusion releases energy that produces heat and light. After the protostar reaches a high enough
temperature, nuclear fusion between hydrogen and helium begins to occur in the protostar's core. The
process starts when the protostar's core reaches around 15 million degrees Celsius. This maintains the
core's temperature meaning the fusion reaction is self-sustaining.
("What is the life cycle of a star?" n.d.)
The T-Tauri Phase
The T-Tauri stage occurs when the protostar begins to produce strong winds. These winds push away
any surrounding gas molecules. This process allows the forming star to become visible.
Main Sequence
The main sequence is the longest stage of a star's lifespan. During this process, an equilibrium is reached
within the star. The force pushing outwards created by the expanding pressure from the nuclear fusion
reaction is counteracted by the stars inward gravitational pull trying to collapse the star under its mass.
This process occurs for the main part of a star's lifespan (millions-billions of years).
If a protostar's mass is insufficient, it does not get hot enough for the nuclear fusion reaction. This
causes the star not to emit light or heat and become classified as a brown dwarf. ("What is the life cycle
of a star?" n.d.)
, A star smaller than the Sun does not have enough mass to burn anything but a red glow during this
stage in its life cycle. This classifies them as red dwarfs.
The End of a Star's Lifecycle
As a star expands, the star begins to fuse helium atoms into its core. The energy from this reaction
prevents the core from collapsing. However, once helium fusion ends, the core shrinks, causing the star
to begin fusing carbon. This process occurs until iron appears in the star's core. The iron fusion absorbs
energy which causes the star's core to collapse. The collapse of a star's core can lead to many things,
depending on its size. If a larger star's core collapses, it implodes and creates a supernova, whereas
smaller stars like the Sun begin to contract into white dwarfs. At the same time, its outer shells radiate
away in the form of planetary nebulas.
In some cases, when the fusion process uses up all the hydrogen, stars begin to form larger nuclei. This
causes the star to expand to become a red giant. (Deziel, 2019)
All stars have what is called a Schwarzschild radius. This is the radius below which the gravitational pull
between particles must be for a body to undergo gravitational collapse. This phenomenon is most likely
the cause of the implosions of larger stars to form supernovas. Its Schwarzschild radius is around 3km
for typical stars like the Sun. If the gravitational pull were to go below that radius, its core would
collapse, causing it to become a white dwarf. (The Editors of Encyclopaedia Britannica, 1998b)
A supernova explosion causes most of a star's material to be blown out into space. However, its core
implodes rapidly to form a neutron star or a singularity (a black hole). Smaller stars, however, don't
explode in this way. Due to their cores being too small, their cores contract to form white dwarfs. Stars
smaller than the Sun do not have enough mass to burn with anything apart from a red glow during their
main sequence.
If a singularity is formed, it would contain an accretion disk. The accretion disk is composed of super-
heated gases that swirl around the event horizon of a singularity at extremely high speeds. The event
horizon of a singularity is the boundary that marks the outer edge of the singularity from which light
cannot escape. (Dixit, 2023)
A stellar spectral energy distribution or SED is the energy at each wavelength a star emits. This
measurement allows a star's temperature, luminosity and other properties to be determined. ("What Is
Stellar Spectral Energy Distribution – BosCoin," n.d.)
Degenerate matter is a form of exotic matter that is formed in a massive star's core. The atoms are so
tightly packed that the pressure source is quantum instead of thermal. The Pauli exclusion principle's
limitations state that no two particles can occupy the same quantum state. The most degenerate matter
is metallic hydrogen created under pressures of over a million atmospheres. ("What is Degenerate
Matter?" n.d.)
The Chandrasekhar limit is the maximum theoretical mass possible for a stable white dwarf. The limit
indicates whether a star will become a white dwarf or becoming a neutron star, or a singularity. If a star
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