5.5 - Astrophysics and cosmology

• Doppler shift

  A change in frequency (or wavelength) observed due to the relative motion of the source and observer
• Other than matter, state what else may be present in the universe that may affect its density

  Dark matter

  Black holes
• An astronomer has claimed to find a white dwarf with a mass twice of the sun. Suggest why this claim is incorrect.

  A white dwarf has a mass less than or equal to 1.4x solar masses
• State hubble's law

  Recessional speed is proportional to its distance from us
• Homogeneous

  Uniform distribution of matter
• State how an emission line is produced

  Electrons make a transition to a lower energy level and emit photons
• Suggest why hydrogen spectral lines play an important role in determining the redshift of galaxies

  Hydrogen commonly found in stars
• State and explain how stellar parallax is used to measure distance in space

  Apparent motion or displacement of a star relative to the position of more distant stars Caused by the Earth's orbit around the Sun.

  An angle of parallax of 1 arcsecond when displacement of earth is 1 AU Corresponds to distance 1 parsec
• Describe and explain the next stages of evolution of our sun (exam question 4 marks)

  reduction in energy released by fusion gravitational force is greater than that from radiation and gas pressure core collapses fusion no longer takes place in the core fusion continues in the shell around the core outer layers of star expand and cool outer layers are released reference to planetary nebula reference to white dwarf (left as remnant hot core)
• State characteristics of black holes

  Escape velocity is more than speed of light

  Infinitely dense

  Emits hawking radiation
• Describe the evolution of the universe up to the formation of the first nuclei

  At the big bang the universe is very dense High energy photons but no matter Quarks and leptons form Quarks combine to form neutrons Nucleons combine to make nuclei
• Define luminosity of a star

  Total radiant power of a star
• Explain how a specific absorption line is produced in this type of spectrum in terms of photons and electron

  Continous spectrum

  Electrons excited and jump up to higher energy levels

  Photons absorbed by electrons

  Photons remitted in different directions
• A comet

  A dusty, ice object in a highly elliptical orbit around a star (which lies at one focii)
• A galaxy

  A collection of stars orbiting a Central mass
• A galaxy

  A collection of stars orbiting a Central mass
• A nebula

  Clouds of gas and dust from previous exploded stars
• A neutron star

  A very dense star formed by a collapsed supernova, where all protons have been decayed into neutrons
• A solar system

  A series of planets orbiting a star; formed from the accretion disc of the protostar
• Anti matter

  Particles that have the opposite properties of matter
• Black hole

  An extremely dense neutron star that has collapsed under gravity to a point singularity
• 1 Astronomical unit AU

  Mean distance of earth's orbit radius
• Dark matter

  Mass that can only be observed by the effect of its gravitational fields. Majority of the universe
• Electron degeneracy pressure

  A limiting factor as an atom collapses, when all lower levels are filled, so the electrons prevent further condensing to occur
• Energy levels

  A quantised amount of energy that orbiting electrons of an atom have
• Line spectra

  Discrete frequencies of light emitted/absorbed by a gas, corresponding to energy level differences
• Line spectra

  Discrete frequencies of light emitted/absorbed by a gas, corresponding to energy level differences
• Planet

  A natural satellite of a star (with enough mass to be spherical and clear its orbit of debris)
• Planetary satellite

  A natural or man-made object orbiting a planet
• The big bang theory

  Is our best theory of the start of the universe due to rapid expansion fork a high energy explosion (as evident by CMBR + redshift)
• The chandrasekhar limit

  The upper limit of a stars mass for it to form a white dwarf (rather than go supernova)
• The cosmological principle

  That matter is evenly distributed across the universe and looks the same in all directions
• The cosmological principle

  That matter is evenly distributed across the universe and looks the same in all directions
• Formation of stars

  Stars are formed from nebula - areas of dust and gas - over time the gravitational force brings them together

  As the mass increases gravitational collapse is accelerated. Gravitational energy is converted to thermal. The temperature is increased until it glows which forms a protostar. The pressure and temperature continue to rise until nuclear fusion occurs and produces helium. The star has now entered main sequence
• State some of the properties of the microwave background radiation observed from the Earth. Discuss how the background microwave radiation is linked to the big bang model (3 marks) 


  The intensity of the microwaves is the same in all directions. These microwaves correspond to a temperature of 2.7 K or The temperature of the universe is 2.7 K. The expansion of the universe following the big bang led to cooling and hence we observe microwaves rather than short wavelength e.m. waves / gamma waves.
• White dwarf

  A star left behind after a red giant
• Suggest how microwave background radiation may evolve in the future

  Further expansion leads to cooling. Temperature below 3 kelvin

  Wavelength of Electromagnetic radiation gets longer so microwaves become radiowaves
• Explain what is meant by fusion and explain the conditions necessary for fusion to occur in the core of a star (4 marks)

  Fusion is the joining together of nuclei

  Mass decreases in the reaction and this is transformed into energy

  High temperature needed for fusion

  High pressure and density required in the core
• Describe and explain the evolution of a star much more massive than our Sun.

  When hydrogen and helium run out the outer layers will expand.

  A super red giant is formed

  Core of star collapses. Forms a supernova

  Depending on mass of star a black hole or neutron star is formed
• Explain why the galaxies do not collapse on each other.

  Galaxies are moving away from each other due to big bang

  Acceleration is too small to collapse