Type II-plateau supernovae ( SNe II-P ) are the classic variety of core-collapse events that result from isolated , massive stars with thick hydrogen envelopes intact at the time of explosion .
Their distances are now routinely estimated through two techniques : the expanding photosphere method ( EPM ) , a primary distance-determining method , and the recently developed standard-candle method ( SCM ) , a promising secondary technique .
Using Cycle 10 Hubble Space Telescope ( HST ) observations , we identify 41 Cepheid variable stars in NGC 1637 , the host galaxy of the most thoroughly studied SN II-P to date , SN 1999em .
Remarkably , the Cepheid distance that we derive to NGC 1637 , D = 11.7 \pm 1.0 Mpc , is nearly 50 \% larger than earlier EPM distance estimates to SN 1999em .
This is the first direct comparison between these two primary distance determining methods for a galaxy hosting a well-observed , spectroscopically and photometrically normal , SN II-P. Extensive consistency checks show strong evidence to support the Cepheid distance scale , so we are led to believe that either SN 1999em is in some heretofore unsuspected way an unusual SN II-P , or that the SN II-P distance scale must be revised .
Assuming the latter , this one calibration yields H _ { 0 } { ( EPM ) } = 57 \pm 15 { km s } ^ { -1 } { Mpc } ^ { -1 } 
and H _ { 0 } { ( SCM ) } = 59 \pm 11 { km s } ^ { -1 } { Mpc } ^ { -1 } ; additional calibrating galaxies are clearly desirable in order to test the robustness of both determinations of H _ { 0 } .

The HST observations of NGC 1637 also captured the fading SN 1999em two years after explosion , providing the latest photometry ever obtained for an SN II-P .
The nebular-phase photometric behavior of SN 1999em closely follows that observed for SN 1987A at similar epochs .
The V and I light curves are both declining at rates significantly greater than the decay slope of ^ { 56 } { Co } predicts .
This is likely due to an increasing transparency of the envelope to gamma rays , and perhaps also to the formation of dust in the cooling atmosphere of the supernova .
The absolute V -band brightness of SN 1999em is \sim 0.25 mag brighter than SN 1987A at the same epochs , which suggests that a slightly greater amount of radioactive ^ { 56 } { Ni } , \sim 0.09 M _ { \odot } , was ejected by SN 1999em than was derived for SN 1987A ( 0.075 M _ { \odot } ) .