Hubble time

From the point of view of GCE, one is interested primarily in effects averaged over long periods of time of the order of Gyr but in dwarf galaxies which may have experienced only a few star formation bursts over a Hubble time the sporadic character may have appreciable effects, especially when one bears in mind that much of the abundance data for such objects comes from H II regions which are intrinsically the result of a current burst, and there is indeed evidence for a cosmic dispersion in certain element abundance ratios such as N/O in such objects (see Chapter 11). 

Speculations about connection of the cosmological GRB with transition from the hadronic to quark star seems to be interesting, because they explain connection between GRB and supernovae explosion, with arbitrary time delay between these events, including very large, exceeding the Hubble time. 

Low mass stars (0.5 < M/Mq < Mhcf) (MjyeF=l-85-2.2Mo depending on stellar models) ignite He explosively and become C-0 white dwarfs (WD). If M < 0.5M0 they become He WD. The lifetimes range from several 109 years up to several Hubble times. 

Crucial parameters are SN feedback, SN nucleosynthesis, IMF and whether all the stellar ejecta (produced over a Hubble time) can reach the ICM or remain bound to the parent galaxy. 

Second, they can distribute the products of stellar nucleosynthesis over large volumes of the intergalactic medium since the outflow speeds are likely to exceed the escape velocities in many cases. As we have seen, many LBGs are already metal-enriched at 3 and have by then been forming stars for much of the Hubble time. There is therefore at least the potential for widespread pollution of the IGM with metals, thereby explaining at least in part the results on the metallicity of the Lya forest described in 3.1 and 3.2). 

It has become clear that the R CrB and EHe stars probably cannot be formed as a result of a late thermal pulse in particular their surface carbon abundances are too low [164,60]. It has already been seen (Sect. 14.5) how a binary system comprising two white dwarfs could decay and merge within a Hubble time. If the resulting star re-ignites helium to produce a yellow supergiant [182], the immediate result would be as follows. 

From Hubble s law of expansion we can easily establish an upper limit for the age of the universe, this is called the Hubble time Th ... 

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