The gravity of a white dwarf is opposed by electron pressure and there is an upper mass limit of 1. If material flows rapidly from the binary companion star onto the white dwarf, its mass may exceed 1. The white dwarf is destroyed in a sudden burst of fusion and no remnant is left behind. Type II supernovae involve very massive stars at the ends of their lives.
These stars fuse progressively more massive nuclei in their cores — C, O, Mg, Ne, Si — and finally a core of iron is formed. Since Fe 56 is the most stable nuclei it is not possible to get any more energy from nuclear fusion and the death of the star is imminent.
This type of supernova is less energetic than the Type I and typically a massive object such as a neutron star or black hole is formed. Because supernovae are such energetic events, astronomers can observe them at great distances. In the figure to the left, the supernova is clearly bright enough to be distinguished from its host galaxy.
Mr Toogood Physics - Classification of stars by luminosity
However, they are brief events often lasting only days and astronomers must work diligently to detect them before they reach their peak brightness and begin to fade. They are only useful as distance indicators if it is possible to calibrate them — to relate their observed brightness profile to absolute magnitudes.
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Because there is a direct relationship between that luminosity and the period over which it oscillates, the absolute magnitude of any Cepheid variable close enough to see can be worked out. The identification of this type of star in the Andromeda galaxy, and observations by Edwin Hubble in —3, led astronomers to realise for the first time that Andromeda was much further away than had been thought — outside our own galaxy. Their absolute magnitudes had been underestimated, and so they were much further away than had been believed when they were first identified as extragalactic.
At greater distances or areas in which there is no star formation and hence few Cepheids, such as in elliptical galaxies, other astronomical bodies are used to calculate distances in a similar way. These might be RR Lyrae variables a kind of red giant star or Type 1a supernovae, for example.
For binary systems, the orbital characteristics can be used to calculate mass, and, since there is a direct relationship between mass and luminosity, the absolute magnitude of a star. Other formulae can also be used to determine absolute magnitude, and therefore distance, such as the Tully-Fisher relation, which links the luminosity of a spiral galaxy with the range of its rotational velocities, and the Faber-Jackson relation, from which the luminosity of an elliptical galaxy can be calculated from the dispersion of velocities of the stars in its centre.
The large redshifts of the light from what are now known to be distant galaxies were first noted by the American astronomer Vesto Slipher in , and are a result of the Doppler Effect. Galaxies further from the Earth are moving away from it faster than ones close by.
The cosmic distance ladder
Data from the WMAP satellite in gave a value of For the most distant bodies in the universe, corrections have to be made to take general relativity into account. The quasar discovered in June has a redshift of 7. The Institute is a charity registered in England and Wales no. The cosmic distance ladder Astronomers have identified the most distant quasar yet to be discovered.
Robert Gendler. Chris Martin.