An important quantity related to SFR history of galaxies is the birthrate parameter b. It is defined as the ratio of the SFR at the current time to the past SFR averaged over the age of the disk
where 
and 
are the stellar mass and age of
the disk, respectively, and
R is the return fraction, i.e. the fraction of mass in a stellar
generation that is returned to the disk over its timelife
(Tinsley, 1980).
The main problem in the determination of b is that the stellar mass of a disk cannot be measured directly, but rather must be estimated using the disk luminosity and an average M/L ratio for galaxies of a given type.
Mean b values for different morphological types are given in
Fig. 7.
The average past SFRs has been determined with:
a) the disk 
/
ratio listed in Table 3
NOTE : The M/L ratios have been converted in
disk M/L ratios using the mean buldge/disk ratio published
for a given Hubble type;
b) the observed V luminosity of each galaxy;
c) a disk age of 10 Gyr;
d) a mean value of 0.4 for the return fraction R.
Figure 7: Distribution of disk birthrate parameter b, the ratio of
the current SFR to the average past SFR, for our sample. Hatched data
include galaxies with individually measured bulge/disk ratios. Open
histograms include all galaxies in the sample.
Table 3: Adopted M/L ratios as a function of Hubble type.
The number of steps listed above emphasizes the considerable uncertainty in b derived with this method.
Nevertheless, Fig. 7 shows a clear trend. A smooth progression in the star formation history with the Hubble type is visible, with the ratio of current to average past SFR increasing from 0.001-0.1 in Sa disks to 0.5-2 in a typical Sc disk. Moreover, from these results we learn that the trend in integrated properties are primarily due to changes in the evolutionary properties of disks, rather than changes in the bulge/disk ratio. Between types Sc and Sa, for example the mean ratio of current to past SFR in the disk decreases by at least a factor of 20, whereas the disk fraction decreases by only 25%.
