Stellar Formation

• Stellar

  nucleosynthesis is the process by which elements are formed within stars.
• According to the star formation theory, stars are formed when gravity starts acting on

  matter and particles expanding with the universe. These dense regions of molecular clouds,

  known as stellar nurseries, collapse to form young stellar objects known as protostars

  which eventually become mature stars.
• The abundance of the elements in a star change as it evolves.
• Stellar evolution is the

  process by which a star changes during its lifetime.
• The primary factor that determines how

  stars evolve is mass.
• All stars are born from clouds of gas and dust called nebulae or molecular clouds that

  collapsed due to gravity. As a cloud collapses, it breaks into smaller fragments which

  contract to form a superhot stellar core called a protostar.
• The protostar continues to accumulate gas and dust from the molecular cloud and

  continues to contract while the temperature increases, forming a main sequence

  star.
• A main sequence star transforms into red giants if hydrogen atoms successfully fuse

  to form the helium core.
• When the core can no longer produce energy to resist gravity, the star undergoes an

  explosion, called a supernova.
• The sun is said to be in the middle of its main sequence phase of stellar evolution and will

  continue to be in this phase for about five more billion years. Red, small stars called red

  dwarfs stay on the main sequence phase for hundreds of billions of years or longer. In

  these stars, hydrogen fuses slowly, and core energy is stabilized.
• Sooner, the proton-proton chain reactions will exhaust all hydrogen in the core of a main

  sequence star. Helium, which is the product of these nuclear fusion reactions, will become

  the major component of the core. Hydrogen fusion becomes significant on the outer shell,

  while some of it is also fused to the core’s surface.
• When most of the helium in the core has been converted to carbon, the rate of alpha fusion

  processes decreases. Gravity again squeezes the star. The star’s fuel is depleted, and over

  time, the star's outer material is blown off into space as a planetary nebula. The only thing

  that remains is the hot and inert carbon core. The star becomes a white dwarf.
• The composition of a white dwarf depends on how much mass is in it before it becomes

  such. The white dwarf discussed previously is assumed to have come from the main

  sequence low mass stars.
• Unlike low mass stars, the fate of a massive star (or high mass star) is different. A massive

  star has enough mass such that temperature and pressure increase to a point where

  carbon fusion can occur.
• The star goes through a series of stages where heavier elements are fused in the core and in

  the shells around the core. Carbon fusion formed oxygen; oxygen fusion formed neon; neon

  fusion formed silicon; silicon fusion formed iron. The star then becomes a multiple-shell

  red giant.
• Elements lighter than iron can be fused because when two of these elements combine, they

  produce a nucleus with a mass lower than the sum of their masses. The missing mass is

  released as energy. The fusion of two elements lighter than iron, therefore, releases energy.
• Extinction in astronomy means the

  absorption and scattering of electromagnetic radiation by gases and dust particles between

  an emitting astronomical object and an observer.
• The IR measurements are used to

  approximate the energy, temperature, and pressure in the protostar.
• Energy in the form of infrared radiation (IR) is detected from different stages of star

  formation.
• A protostar is a stellar core formed when the fragments of a collapsed molecular cloud contract.
• proton-proton chain reaction is the mechanism that explains how hydrogen is fused into helium in the core of a main sequence star.
• Carbon is the new element is formed from He in a red giant star.
• Gravity is the force that squeezes stars when mass, temperature or pressure is altered.
• white dwarf is formed when a star becomes an inert carbon core.
• Helium is converted to carbon
• hydrogen is converted to helium
• Nucleosynthesis inside the star, which is responsible for the formation of Heavy Elements
• Hydrogen Burning is the process that results in the production of helium-4
• Helium Burning - Reaction that uses Helium to produce heavier elements.
• The triple-Alpha Process is the Fusion reaction that start with three helium-4 that are converted to carbon-12.
• Alpha Process or Alpha Ladder is the process that converts helium into heavier elements. Always involve the CAPTURE of an alpha particle.
• The fusion reactions cannot produce nuclei higher

  than iron-56 because the fusion reaction becomes

  unfavorable. This is because the nuclear binding

  energy per nucleon, the energy that holds the

  nucleus intact decreases after iron-56.

  Therefore, different pathways are needed for the

  synthesis of heavier nuclei.
• The temperature after a supernova is tremendously high

  that the neutrons are very fast. Because of their speed,

  they can immediately combine with the already heavy

  isotopes.
• A NEUTRON IS ADDED to a seed nucleus. The addition of

  neutron produces a heavier isotope of the element.
• Rapid Neutron Capture - happens when there is a large number of

  neutrons. It is termed rapid because the rate of

  neutron capture is faster than an unstable nucleus

  may still combined with another neutron just

  before it undergoes beta decay.
• Slow neutron capture - Happens when there is a small number of

  neutrons. It is termed slow because the rate of neutron capture is slow compared to the rate of Beta-decay.
• Proton capture is the addition of a proton in the nucleus. It happens after a supernova when there is a tremendous amount of energy available.
• Alpha decay - the radioactive substance will lose 2 neutrons and 2

  protons to reach stability.
• Beta decay - the atom will increase its atomic number while

  maintaining its mass number.
• Supernova nucleosynthesis - The temperature after a supernova is tremendously high that the neutrons are very fast. Because of their speed, they can immediately combine with the already heavy isotopes.