  Young galactic X-ray point sources (XPs) closely trace the ongoing star
formation in galaxies. From measured XP number counts we extract the collective
2-10 keV luminosity of young XPs, L_yXP, which we use to gauge the current
star-formation rate (SFR) in galaxies. We find that, for a sample of local
star-forming galaxies (i.e., normal spirals and mild starbursts), L_yXP
correlates linearly with the SFR over three decades in luminosity. A separate,
high-SFR sample of starburst ULIRGs can be used to check the calibration of the
relation. Using their (presumably SF-related) total 2-10 keV luminosities we
find that these sources satisfy the SFR-L_yXP relation, as defined by the
weaker sample, and extend it to span about 5 decades in luminosity. The
SFR-L_yXP relation is likely to hold also for distant Hubble Deep Field North
galaxies, especially so if these high-SFR objects are similar to the (more
nearby) ULIRGs. It is argued that the SFR-L_yXP relation provides the most
adequate X-ray estimator of instantaneous SFR by the phenomena characterizing
massive stars from their birth (FIR emission from placental dust clouds)
through their death as compact remnants (emitting X-rays by accreting from a
close donor). For local, low/intermediate-SFR galaxies, the simultaneous
existence of a correlation of the instantaneous SFR with the total 2-10 keV
luminosity, which traces the SFR integrated over (approximately) the last Gyr,
suggests that during such epoch the SF in these galaxies has been proceeding at
a relatively constant rate.
