  Phosphorus ($^{31}$P), which is essential for life, is thought to be
synthesized in massive stars and dispersed into interstellar space when these
stars explode as supernovae (SNe). Here we report on near-infrared
spectroscopic observations of the young SN remnant Cassiopeia A, which show
that the abundance ratio of phosphorus to the major nucleosynthetic product
iron ($^{56}$Fe) in SN material is up to 100 times the average ratio of the
Milky Way, confirming that phosphorus is produced in SNe. The observed range is
compatible with predictions from SN nucleosynthetic models but not with the
scenario in which the chemical elements in the inner SN layers are completely
mixed by hydrodynamic instabilities during the explosion.
