Searle et al. (1973) proposed that abnormally powerful bursts of star formation are responsible for the unusually blue colors in some galaxies. The term starburst is now used to denote an intense star formation event lasting over a short period of time. The poststarburst phase lasts an order of magnitude longer than the starburst phase. Poststarburst galaxies, also called the ``E + A'' galaxies (Dressler and Gunn, 1983), are typically characterized by strong Balmer absorption lines and weak or no emission lines. The E + A refers to the spectroscopic similarity of the appearance of these galaxies to the superimposition of an A-star component over an old elliptical galaxy spectrum (Gunn and Dressler, 1988).
Butcher and Oemler (1984) showed that the proportion of such
blue galaxies is greater in distant clusters. Defining 
to be the fraction of blue galaxies in a cluster i.e., of galaxies
whose rest-frame B-V colors are at least 0.2 mag bluer than the
early type galaxies at that magnitude, they find that 
in nearby clusters (z;SPMlt;0.1) but rises to 
at z=0.5.
Though the study refers not to individual blue galaxies but their proportion in a cluster, the blue galaxies that are the cause of the Butcher-Oemler effect are thought to be E + A galaxies. Thimm and Belloni (1994) have observed galaxies at z;SPMgt;0.8 which are well fit by E + A spectra. E + A galaxies are powerful indicators of galaxy evolution and several models have been advanced to quantify the observable parameters (see e.g. Couch and Sharples, 1987 and Belloni et al., 1995).
Couch and Sharples (1987) classified E + A galaxies into blue,
poststarburst galaxies (PSGs) and red, H 
strong galaxies (HDSs).
Two-parameter models (age and strength of burst) are reasonably
successful in modeling both types of galaxies. However, often
the ambiguity in one of the parameters is large leading to
degenerate solutions. For instance, Liu and Green (1996) conclude
after studying a heterogeneous sample of 8 E + A galaxies that
``the attempts to model such galaxies at high-redshifts using a simple
starburst-plus-galaxy model are inadequate and could lead to serious
systematic errors if interpreted too boldly.''
They find that 5/8 galaxies are well described by E + A model, 2/8
by a burst-plus-spiral model and in one case
multiple bursts have to be invoked to describe
the spectrum. The burst age could vary by upto 30% in each case
without loss of statistical significance. This leads to large
degeneracy in mass as well.
Leonardi and Rose (1996) present a technique to overcome the
age-strength degeneracy. They use two spectral indices viz.
H 
and one formed from the ratio of the
Ca II H + H 
line to the Ca II K. The former
ratio decreases in value as one proceeds from late type stars to
earlier type stars whereas the latter is constant in late type
stars but decreases for stars earlier than F2. Since the two indices
have different behavior for different types of stars, taken together
they are able to resolve the degeneracy between age and strength.
While using two different indices to obtain better estimates
of the burst parameters is an attractive idea, the technique
requires that in addition to
having good spectroscopic data, we also need to study the effect of the
history of star formation on these indices.
What kind of stars give rise to the excess blue color? If the starburst
age is under 10 million years, one interesting possibility is
Wolf-Rayet (WR) stars. Heckman et al. (1997) have observed
Mrk 477, which hosts a powerful type 2 Seyfert nucleus. It is seen
to have a circumnuclear starburst (dimension 
).
Observation from UV to NIR indicate that the likely cause is
Wolf-Rayet stars numbering 
. Such a starburst also
leads to a pronounced excess in the far-IR which is often seen
in type 2 Seyfert nuclei. Thus, in Seyfert galaxies, a WR starburst
could be able to explain various properties.
Most of the models that have been tried so far have been for Solar metallicity, though invoking factors like WR stars may necessiate the use of higher metallicities. One reason why other metallicities have not been extensively tried is the age-strength degeneracy. If one more parameter is to be included, the quality of the data and our understanding of the spectra will have to increase manifold.
In the next section we describe how synthetic spectra are generated and the procedure which is followed to compare colors of our radio galaxies with those from the synthetic spectra.