The Big Bang • Event that occurred approximately 13.7 BILLION years ago • All the mass and energy concentrated at a point • The universe began expanding and continues to expand • After 1 million years matter began to cool enough to form atoms- Hydrogen- the building block of stars
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The Big Bang Event that occurred approximately 13.7 BILLION years ago All the mass and energy concentrated at a point The universe began expanding and.
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The Big Bang
• Event that occurred approximately 13.7 BILLION years ago
• All the mass and energy concentrated at a point
• The universe began expanding and continues to expand
• After 1 million years matter began to cool enough to form atoms- Hydrogen- the building block of stars
In the Beginning
• Hydrogen and helium with small amounts of lithium, boron, and beryllium were created when the universe was created in the Big Bang.
• The rest of the elements were produced as a result of fusion reactions in the core of stars- stellar necleosynthesis.
In the Beginning
• These reactions created the heavier elements from fusing together lighter elements.
• When the outer layers of a star are thrown back into space (supernovae), the processed material can be incorporated into gas clouds that will later form stars and planets.
• The material that formed our solar system incorporated some of the remains of previous stars.
• All of the atoms on the Earth except hydrogen and most of the helium are recycled material---They were created in the stars.
Galaxies and Stars
• Galaxy- huge rotating aggregation of stars, dust, gas held together by gravity
• Earth, the sun and our solar system is part of the Milky Way
• Stars are massive spheres of incandescent gases (hydrogen and helium)
Stellar Nucleosynthesis
Introduction
• Work on stellar nucleosynthesis in the 1950s has led to our current realization that most of the chemical elements are synthesized in stars.
• Helium is made by hydrogen burning in the core during the main sequence and in a shell above the core in the red giant phase.
• The energy released from nuclear reactions accounted for the longevity of the Sun as a source of heat and light.
• The prime energy producer in the sun is the fusion of hydrogen to helium, which occurs at a minimum temperature of 3 million kelvins.
• The element carbon is created by helium-burning. • For massive (more than ten solar masses, > 10 M-
Sun) stars, direct nuclear burning continues with the production of oxygen, neon, magnesium, silicon and so on, cumulating in the synthesis of iron, the heaviest element possible through direct nuclear burning.
• The other heavy elements, from yttrium and zirconium to uranium and beyond, are produced by neutron capture followed by decay.
Introduction
• For the majority of stars (~95%, corresponding to stars with initial masses of less than 8 M-Sun), direct nuclear burning does not proceed beyond helium, and carbon is never ignited.
• Most of the nucleosynthesis occurs through slow neutron capture during the asymptotic giant branch (AGB), a brief phase (~106yr) of stellar evolution where hydrogen and helium burn alternately in a shell.
• These newly synthesized elements are raised to the surface through periodic "dredge-up" episodes, and the observation of short-lived isotopes in stellar atmospheres provides direct evidence that nucleosynthesis is occurring in AGB stars.
Passive Evolution
• Stellar evolution is relatively well understood both observationally and theoretically
• Massive stars are very hot and blue
• Massive stars are very luminous
• Massive stars have very short lives
Passive Evolution - Single Burst
• Single Burst of Star-formation
• Galaxy starts of very blue as the light is dominated by the massive hot blue stars
• After the burst the massive stars live only a short time and soon the light of the galaxy as a whole is dominated by the red light of the less massive, longer lived stars
• Galaxy gets redder with age
Supernovae• A supernova is a massive explosion of a
star that occurs under two possible scenarios. The first is that a white dwarf star undergoes a nuclear based explosion after it reaches its Chandrasekhar limit from absorbing mass from a neighboring star (usually a red giant).
• The second, and more common, cause is when a massive star, usually a red giant, reaches iron in its nuclear fusion (or burning) processes.