5.5 astrophysics and cosmology

• what is a planet?

  • it has a mass large enough for its own gravity to give it a round shape
  • it has no fusion reactions
  • it has cleared its orbit of most other objects
• what is a planetary satellite?

  A planetary satellite is a body in orbit around a planet
• what is an example of a natural planetary satellite?

  moons
• what is a comet?

  small irregular bodies made up of ice dust and small pieces of rock.

  they have highly elliptical orbits around the sun
• approximately how big are comets?

  few hundred to tens of kilometres across
• what is a solar system?

  a system with a star and objects that orbit around it
• what is a galaxy?

  a collection of stars and interstellar gas and dust
• what is the universe?

  everything
• what is a nebulae?

  a cloud of gas and dust
• how is a protostar formed?

  as the dust and gas get closer together this gravitational collapse accelerated. due to variations in nebulae denser regions begin to form. denser regions begin to attract more dust and gas getting heavier and denser. they also get hotter as gravitational energy begins to transfer to thermal energy.
• when does a protostar become a main sequence star?

  when nuclear fusion occurs
• how does a star remain stable?

  radiation pressure outwards is equal to force due to gravitational acceleration inwards
• what is the range in mass for a red giant to form?

  0.5 - 10 molar masses
• how is a red giant formed?

  when a low mass star has finished fusing all its hydrogen it begins to collapse inwards as the pressure due to radiation is no longer equal to the gravitational pull inwards and a red giant is formed where helium fusion occurs.
• what is chandrasakars limit?

  1.44 Mo
• what are the criteria for a red giant to become a white dwarf?

  its core must have a mass below 1.44 Mo
• what is electron degeneracy pressure?

  when a star begins to collapse electrons are squeezed together and this creates pressure that prevents the core from further gravitational collapse. this pressure us known as electron degeneracy pressure.
• what is the relationship between electron degeneracy pressure and chandrasakars limit?

  electron degeneracy pressure is only sufficient to prevent gravitational collapse if the mass of the core is below 1.44 Mo
• what happens to red giants which have a core above 1.44 Mo?

  they collapse and remain in space as planetary nebulae
• what are characteristics of a white dwarf?

  very dense (often with the mass of our sun but the volume of our earth)
• what happens to a star with a mass above 10 Mo ?

  it becomes a red supergiant
• how does a red super giant form?

  as larger mass stars are hotter helium atoms have more energy and can overcome electrostatic forces to form larger elements
• what are the key features of a red super giant?

  it fuses heavier elements closer to its nucleus
• what happens to a red supergiant after fusion has finished?

  supernova
• what are the products of a supernova?

  neutron star

  black hole
• what are the characteristics of a neutron star?

  extremely dense

  rotate fast
• what are the characteristics of a black hole?

  extremely dense

  have an escape velocity above the speed of light
• what is the typical mass of a neutron star?

  2 Mo
• when does a black hole occur?

  when a core has a mass greater than about 3 Mo
• what happens during a supernova?

  all elements heavier than iron are formed

  they are very luminous
• what does a Hertzsprung-Russel diagram show?

  relationship between luminosity and temperature
• what is luminosity?


  total radiant power output of the star
• label the graph

  A) red giant

  B) red super giants

  C) main sequence

  D) temperature

  E) luminosity

  F) white dwarf

  G) sun

  
• what do these pathways represent?

  A) higher mass stars to red supergiants before supernova

  B) low mass star evolution

  
• what are the characteristics of energy level?

  • an electron cannot have a quantity of energy between two energy levels
  • the energy levels are negative because external energy is required to remove an electron from an atom
  • an electron with zero energy is free from the atom
  • the most negative energy level is called the ground state
• what happens when an electron is excited?

  it moves from a lower energy level to a higher energy level
• what is required to excite an electron?

  energy (photon)
• how do you work out energy required for excitation or energy released during de-excitation?

  E = hf and E = hc / lamda

  where E is the difference between energy levels
• why are all emission line spectra different?

  each element produces a unique emission line spectra because of its unique set of energy levels
• what is a continuous spectra?

  all visible frequencies or wavelength are present. eg - atoms of a heated metal will create this spectra