Life Cycle of a Star

Noah Waldron

The life cycle of a star can be related to the life cycle of a human, as seen below.
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Prostars: The First Cycle

To understand how prostars are formed, you have to understand what interstellar mediums are. Interstellar mediums are basically the matter that is present in the space between the star systems in a galaxy. Interstellar mediums are composed of gas in ionic, atomic, and molecular form, along with dust and cosmic rays. Interstellar mediums also contain hydrogen (97%) and helium (3%) with trace amounts of trace amounts of carbon, oxygen, and nitrogen. When the dense regions in these interstellar mediums collapse, they form prostars. The gravitational energy of the collapsing star is the source of its energy. Prostars aren't very stable so, they go through a process called accretion to achieve equilibrium (occurs when gas pressure equals gravity inside of a prostar). To achieve equilibrium, prostars go through 6 steps;

1. Gravity pulls gas and dust inward toward the core.

2. Inside the core, temperature increases as gas atom collisions increase.

3.Density of the core increases as more atoms try to share the same space.

4. Gas pressure increases as atomic collisions and density (atoms/space) increase.

5. The protostar’s gas pressure RESISTS the collapse of the nebula.

6. When gas pressure equals gravity, the protostar has reached equilibrium and accretion stops

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The Main Sequence

About 90 percent of stars are currently in the main sequence. The general understanding is that main sequence stars fuse hydrogen atoms together to make helium atoms in their cores. Fusion produces an outward pressure that balances with the inward pressure, caused by gravity, stabilizing the star. The main sequence phase is the longest. When the hydrogen fuel has run out, stars fuse helium into carbon. How long a star stays in the main sequence depends on how massive it is. A higher-mass star might have more material, but it burns through it faster due to higher core temperatures caused by greater gravitational forces.
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Red Giants

The core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon . Once this occurs the star is hot enough to transform into a red giant. The star's outer layers begin to expand, cool, and shine less brightly.
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Death of a Star

Once the helium in the core of the star runs out, depending on the size of the star, it will do one of three things;

Low Mass Stars: The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula. The core left behind is known as a white dwarf.

Medium Mass Stars: Shrinks to neutron star. Supernova happens when a neutron star is created. Neutrons prevent further collapse. The size of a neutron star is about that of a large city. Medium mass stars become explosive supernova. Supernova explosions happen because the core has formed a very stiff neutron star and the in-falling outer layers rebound off of it. After this phase, the star becomes a white dwarf.

More Massive Stars: The highest mass stars have cores that shrink into a point. Before they collapse, they may momentarily create a neutron star and the resulting supernova explosion. Space warps around the object that it leaves a black hole.

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A Star's Light

The Doppler effect (or Doppler shift) is the change in frequency of a wave for an observer moving relative to its source. Light is a wave that can be described by its frequency, the number of wave peaks that pass by each second. Light from distant stars and galaxies can be shifted. Since blue is at the high-frequency end of the visible spectrum, we say the light from an approaching star is shifted toward blue, or blue-shifted. The light from a star moving away from our planet is red-shifted, because red is at the low-frequency end of the visible spectrum.
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The Elements

All liquids, gases, and solids found on earth are made from one, or more, of 92 naturally occurring elements. The nuclei of hydrogen and helium, the lightest and most abundant elements in the visible universe, were created in the moments following the Big Bang. All other naturally occurring elements are generated in the high temperature and pressure conditions present in stars. Expansion and cooling after the big bang allowed the neutrons, and some of the protons, to fuse to helium nuclei. The 73% hydrogen and 25% helium abundances that exists throughout the universe today comes from that condensation period during the first three minutes of the big bang. Other elements are formed in stars such as nitrogen, which is formed inside a red giant star. The red giant smashes lighter atoms together and gets energy to burn, and also has heavier atoms(nitrogen) left over. Iron is formed when a red giant star has changed all of its helium into carbon and oxygen, it then begins to turn the carbon and oxygen atoms into iron atoms.

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