astrophyics and cosmology

• Planets


  An object in orbit around a star with mass large enough for its own gravity to give it a round shape, no fusion reaction, cleared orbit of most other objects
• Planetary Satellites


  A body in orbit around a planet, includes moons and man-made satellites
• Comets


  Small, irregular bodies made of ice, dust and rock, orbit the sun in highly eccentric elliptical orbits
• Solar Systems


  A system containing a star and all the objects surrounding that star
• Galaxies


  A collection of stars and interstellar dust and gas
• Universe


  All of space-time and everything that exists within it
• Formation of a star

  Gravitational attraction between particles of dust and gas forms vast clouds, gravitational collapse accelerates, slightly denser regions get hotter and more dense, protostar forms, fusion starts when extremely high pressures and temperatures are present
• Radiation Pressure


  Outward pressure in a star caused by the photons emitted during fusion, acts with gas pressure against gravity to keep a star in equilibrium
• Gas Pressure


  Outward pressure in a star caused by the nuclei in the core, acts with radiation pressure against gravity to keep a star in equilibrium
• Evolution of a low mass star

  Stars between 0.5M and 10M evolve into red giants, colder than main sequence, inert core, core begins to collapse, fusion starts in a shell around the core, periphery of star expands, outer layers eventually drift away leaving the hot core as a white dwarf
• Characteristics of a white dwarf

  Very dense (e.g mass of Sun with volume of Earth), no fusion, leaks photons created in earlier evolution, surface temp. up to 30000K, core collapse prevented by electron degeneracy pressure up to the Chandrasekhar limit
• Chandrasekhar Limit


  1.44M (1.44 times more massive than our sun)
• Electron degeneracy pressure


  Pauli exclusion principle states two electrons cannot exist in the same energy state, stars under 1.44M can only collapse to a certain point before the electrons exert a pressure that balances its gravity (to prevent them from being in the same energy state as another electron)
• Evolution of a massive star

  Stars of mass > 10M evolve into red supergiants, fusion occurs in shells around the core until the core is entirely iron, instability causes a supernova (implosion which creates all the elements heavier than iron), core left as either a neutron star or black hole
• Characteristics of a neutron star

  Made almost entirely of neutrons, typical mass of 2M (above the Chandrasekhar limit), density similar to atomic nucleus
• Characteristics of a black hole

  Escape velocity of gravitational field is higher than speed of light, mass between 3M and several million M
• Hertzsprung-Russell diagram


  Luminosity-Temperature graph showing main sequence, red giant, red supergiant and white dwarf phases of stars
• Energy levels of electrons in isolated gas atoms

  Discrete set of energies an electron can have in a specific atom, negative, energy level with the most negative value is the ground state
• Emission spectral lines


  Occur when the electrons in excited atoms drop back into lower energy levels, photons emitted with frequencies specific to that element
• Equation for energy which includes the planck constant


  E = h f, E = h c / lambda
• Continuous spectrum


  All visible frequencies or wavelengths are present, atoms of heated solid metal produce this type of spectrum
• Emission line spectrum


  Unique set of emission lines due to the unique, discrete nature of energy levels in atoms of each element
• Absorption line spectrum


  Dark spectral lines against a continuous spectrum, correspond to emission spectral lines for same atom, formed when light from a continuous spectrum source passes through a cooler gas, photons of wavelength equal to the difference in energy levels of the atoms in the gas are absorbed
• Transmission diffraction grating


  Grating consisting of a number of lines ruled on a glass slide, up to 1000 lines in a mm, each diffracts light like a slit, produces clearer diffraction pattern than double slit, useful in spectroscopy
• Condition for maxima


  Dependent on path difference and phase difference, d sintheta = n lambda, where theta is the angle to the normal of the diffraction grating, d is the grating spacing and n is an integer
• Black Body Radiation


  Objects of temp. above absolute zero give off EM radiation of different wavelengths, a black body is an idealised object that absorbs all EM radiation and emits a characteristic distribution of wavelengths at a specific temp. when in thermal equilibrium
• Wien's Displacement Law


  Maximum wavelength is inversely proportional to temperature
• Stefan's Law


  Luminosity of a black body is directly proportional to the fourth power of the absolute temperature of the black body
• Luminosity of a star (Equation)

  L = 4pi r^2 sigma T ^4 where sigma is the Stefan constant
• Estimate the radius of a star

  Use of Wien's law and Stefan's law to estimate radius, once this is determined Newton's law of gravitation can be used to estimate mass and density
• Astronomical Unit (AU)


  Average distance from the Earth to the Sun
• Light-year (ly)


  Distance travelled by light in a vacuum in one year
• Parsec (pc)


  Distance at which a radius of one AU subtends an angle of one arcsecond (1/3600 degrees), d = 1/p
• Stellar parallax


  The apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the Sun, used to calculate distances less than 100 pc
• Cosmological Principle


  Assumption that, when viewed on a large enough scale, the Universe is homogenous (matter is distributed evenly across universe) and isotropic (looks the same in all directions to every observer)
• Doppler Effect


  Apparent increase or decrease in the frequency (and vice versa for wavelength) of a wave moving relative to an observer, blue-shift of wavelength means movement towards observer, red-shift means movement away from observer
• Doppler Equation


  delta lambda / lambda = delta f / f = v / c
• Hubble's Law


  The recessional speed of a galaxy is directly proportional to its distance from Earth, v = H(0) d
• Evidence for the model of an expanding universe

  Hubble's law, almost every galaxy is red-shifted and those that are further away are more red-shifted so are moving faster
• How to calculate the Hubble constant and estimate the age of the universe


  v / d = H(0), age of universe is 1 / H(0)