1. In 1995 the Hubble Space Telescope was used to observe a 2′ patch of sky for 225 hours [
2. Williams R. E., et al., Astron. J. 112, 1335 (1996);
3. ]. These are the deepest images ever obtained of the high-redshift universe. About 1500 sources were identified in this small field. At the same depth it would be possible to catalog ∼100 billion sources over the full sky.
4. Light from objects receding from Earth produces a spectrum that is shifted toward longer wavelengths. The observed wavelength λ of a spectral line with known wavelength λ o obeys the equation 1 + z = λ/λ o where z is referred to as the “redshift” of the object. Celestial objects have been detected up to redshifts of 5 or more. Because light propagates at a finite speed objects at higher redshifts are seen at earlier times in the universe. The “look back” time τ has a complicated dependence on redshift and the normalized mean density of matter and energy in the universe. A simple form exists for an empty universe that is τ = z /[ H o (1 + z )] where H o is Hubble's constant (∼75 km s −1 Mpc −1 ). The use of redshift will ultimately be supplanted by “distance” and “cosmic time” once the universal parameters have been tied down to sufficient accuracy (5% or better).
5. Elements heavier than helium ( Z > 2) are collectively referred to as “metals.” Some of these can be used to provide a cosmic clock of when the star was born. H and He were mostly formed in the Big Bang and account for 98% by mass of all baryons in the universe. Li Be and B are very depleted as these are fragile elements that are easily destroyed. C and heavier elements up to Fe are mostly fused in stars. Because massive stars evolve rapidly and explode as supernovae there is a general buildup of metals (for example [Fe/H]) with time. Ten million years after the initial starburst through the rapid neutron capture process (r-process) the type-II supernovae from the core collapse of massive stars enhance the even Z elements (so-called alpha particles) with respect to Fe. A billion years later other sources (for example type-Ia supernovae and asymptotic giant branch stars) enhance the odd Z elements through slow neutron capture (the s process). The relative fractions of r - and s -process elements can be used with [Fe/H] to provide a stellar clock.