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.

Introduction

• 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.

Supernovae

• Iron has one of the highest binding energies of all of the elements and is the last element that can be produced by nuclear fusion, exothermically.

• All nuclear fusion reactions from here on are endothermic and so the star loses energy.

• The star's gravity then pulls its outer layers rapidly inward. The star collapses very quickly, and then explodes.

Composite image of Kepler's supernova from pictures by the Spitzer Space Telescope, Hubble Space Telescope, and Chandra X-ray Observatory.

The Solar System

• Our solar system is located away from the galaxy’s center

• Our sun and the planets originated from a solar nebula that had been enriched with heavy elements from nearby supernovae (Stellar Synthesis)

• Solar system is approximately 5 Billion years old• Composition is 75% hydrogen, 23% helium and

2% other materials

Formation of aProtostar

Center contracts

Center continues to heat up

Protostar radiates more heat

Fusion begins in the stars core

Shockwaves radiate outward releasing material

Material coalesces into planets, moons or comets

Other material is ejected to the periphery

Our Solar System

4 inner planets (terrestrial)

4 outer planets (gaseous)

Solar nebula photographed by Hubble