Perhaps the most puzzling result from the studies of the SFR is the relatively short inferred timescale for gas consumption, on the order of a few Gyr. If this timescale is defined as
where 
is the total mass content and SFR the
ongoing star formation rate, most nearby spirals will exaust
the gas in their star forming disk on times
ranging from 0.1 to 0.6 of the Hubble time.
This would suggest that we live in an epoch in which late-type
spirals are being transformed into early-type systems, or
alternatively that large amounts of infall may be needed to
sustain the star formation in late-type disks.
The resolution of this dilemma is the substantial effect that
stellar gas recycling has on the disk lifetimes.
Stellar gas recycling can be quantified
by means of the return fraction R , i.e. the fraction of
mass in a stellar generation that is returned to
the disk over its lifetime. R is sensitive to
several parameters, including the IMF but its mean value
spans from 0.2 to 0.4. As a consequence of this additional, time
delayed gas input the disk star formation extends by factors of 1.5-4.
