5.5 astrophysics and cosmology

• a nebula is a cloud of dust and gas (mainly hydrogen), often many hundreds of times lager than out solar system
• star birth (pt1)

  Tiny gravitational attraction between dust and gas pulls the particles and as they get closer this gravitational collapse accelerates. Denser regions then begin to form and pull in more gas and dust. As it gains mass and denser the gravitational energy is transferred to the thermal energy and in one part of the cloud a protostar forms.
• star birth (pt2)

  for a protostar to become a star, nuclear fusion happens to produce kinetic energy. Extremely high pressures and temperatures are needed to overcome the electrostatic repulsion between hydrogen in order to fuse together to make helium. When the mass of the star reaches a certain point the core becomes so hot that kinetic energy of the hydrogen nuclei are forced together and nuclear fusion begins.
• a protostar is a vert hot and very dense sphere of condensing gas and dust
• star life

  once a star is formed, it remains in a stable equilibrium. , Radiation pressure from the photons emitted during fusion and the gas pressure fro the nuclei in the core push outwards. This balances out with the gravitational attraction that are compressing the star.
• massive star release more power and converting the available hydrogen into helium in shorter time and are only stable for a few million years
• main sequence is the main period on a H-R diagram in a stars life during which its stable
• radiation pressure is the pressure from the photons in the core of a star which acts outwards
• gas pressure in stars is the pressure of the nuclei in the stars core pushing outwards
• a planet is an object in orbit around a star with three characteristics :

  1. it has a mass large enough for gravity to give it a round shape
  2. has no fusion reactions
  3. has cleared its orbit of most other objects
• dwarf planets have not cleared their orbit of other objects
• astroids are objects to small and uneven to be planets
• a planetary satellite is a body in orbit around a planet
• comets are irregular bodies made of ice, dust and small pieces of rock
• solar systems contains a sun and all orbiting planets
• galaxies are collections of stars and interstellar dust and gas. on average they contain 100 billion stars
• red supergiant is a huge star in the last stages of its life before it explodes into a supernova
• supernova is the implosion of a red supergiant at the end of its life, which leads to subsequent ejection of stellar matter into space, laying an inert remnant core
• start with a lower mass are much cooler and will remain in the main sequence for much longer
• solar mass is the mass of the sun
• stars between 0.5M and 10M will evolve into red giants.
• At the end of red giant phase the reduction in energy (released by fusion) means that the gravitational force is greater then the outwards force causing core to collapse and as it shrinks the pressure increases enough to start fusion in a shell around the core
• red giant stars have inert cores so fusion no longer takes place since little hydrogen remains and temperature isn't high enough for helium to overcome the electrostatic repulsion. But fusion of hydrogen does conitiue in the shell. This will cause the periphery of the star to expand as layers slowly move away form the core and as they expand they cool and give star red colour
• planetary nebula are the outer layers of a red giant that have drifted into space leaving.hot core behind and at the centre as a white dwarf.
• a white dwarf is very dense often with mass of our sun and volume of earth. no fusion reactions take place inside white dwarf and emits energy only because it leaks photons created earlier
• the Pauli exclusion principle states two electrons cannot exist in the same energy state
• when the core of a star begins to collapse under the force of gravity the electrons are squeezed together and creates a pressure that will prevent further collapse. This pressure is known as electron degeneracy pressure
• electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than 1.44M this is Chandrasekhar limit
• stars with a mass greater than 10M means their cores are much hotter and consume the hydrogen in their core in much less time
• cores of massive stars are much hotter meaning the helium formed are moving fast enough to overcome electrostatic repulsion so the fusion of helium unto heavier elements occur. These changes cause star to expand former a red supergiant and then high temperatures and pressure are high enough to fuse massive nuclei together forming shells inside the star
• These shells cause heavier elements that are fusing to be deeper within the core. This process will continue until an iron core is developed. Iron nuclei cannot fuse because they can't produce energy. This makes the star unstable and leads to implosion of the layers that bounce of the solid core leading to a shockwave that ejects all the core into space. This is a supernova.
• a supernova can be compressed into one of two objects:

  1. neutron star if mass is greater than the Chandrasekhar limit and typical mass of 2M
  2. black hole if the core has a mass greater than 3M
• black holes are when gravitational field are so strong that in order to escape it the escape velocity must be greater than the speed of light
• supernovae created all the heavy elements and allowed them to distribute these elements throughout the universe
• Schwarzschild radius of an object is the radius of an imaginary sphere sized so that if all the mass of the object is compressed into the sphere and the escape velocity would be greater than the speed of light.
• the radius of a black hole must be smaller than Schwarzschild radius so not even light can escape
• luminosity is the total radiant power output of a star
• hertzsprung-russell diagram is a graph showing the relationship between the luminosity of stars in our galaxy and their average surface temperture
• Hr diagram are often used to show stellar evolution :

  • lower mass stars evolve into red giants and more away from main sequence and gradually lose the cooler outer layers
  • higher mass stars rapidly consume their fuel and swell into red supergiants before they go supernova
• energy levels are a discrete amount of energy that an electron within an atoms is permitted to possess