Astroparticle Physics

174 8 Cosmologytype-Ia supernovae√Lr =4πF . (8.4)Various standard candles can be used, such as Cepheid variablestars, used by Hubble. A plot of speed versus distancedetermined from type-Ia supernovae 1 is shown in Fig. 8.1[7].4000030000Velocity (km/sec)2000010000Fig. 8.1The speed versus distance for asample of type-Ia supernovae(from [7])00 100 200 300 400 500 600 700Distance (Mpc)1 The spectra of SN Ia are hydrogen poor. The absence of planetarynebulae allows to reconstruct the genesis of these events. Itis generally believed that the progenitor of a SN Ia is a binaryconsisting of a white dwarf and a red giant companion. Bothmembers are gravitationally bound. In white dwarves the electrondegeneracy pressure compensates the inward bound gravitationalpressure. The strong gravitational potential of the whitedwarf overcomes the weaker gravity of the red giant. At theperiphery of the red giant the gravitational force of the whitedwarf is stronger than that of the red giant causing mass fromits outer envelope to be accreted onto the white dwarf. Since forwhite dwarves the product of mass times volume is constant,it decreases in size during accretion. When the white dwarfreaches the Chandrasekhar limit (1.44 M ⊙ ), the electron degeneracypressure can no longer withstand the gravitational pressure.It will collapse under its own weight. This goes along withan increase in temperature causing hydrogen to fuse to heliumand heavier elements. This sudden burst of energy leads to athermonuclear explosion which destroys the star.Since the Chandrasekhar limit is a universal quantity, all SNIa explode in the same way. Therefore, they can be consideredas standard candles.