nov ae , ▁by ▁the ▁exp ulsion ▁of ▁most ▁of ▁the ▁hydro gen ▁en velope ▁of ▁the ▁pro gen itor ▁star . ▁The ▁plate au ▁phase ▁in ▁Type   II - P ▁super nov ae ▁is ▁due ▁to ▁a ▁change ▁in ▁the ▁opacity ▁of ▁the ▁exterior ▁layer . ▁The ▁shock ▁wave ▁ion izes ▁the ▁hydro gen ▁in ▁the ▁outer ▁en velope ▁– ▁stri pping ▁the ▁electron ▁from ▁the ▁hydro gen ▁atom ▁– ▁resulting ▁in ▁a ▁significant ▁increase ▁in ▁the ▁opacity . ▁This ▁prevents ▁phot ons ▁from ▁the ▁inner ▁parts ▁of ▁the ▁explos ion ▁from ▁esc aping . ▁When ▁the ▁hydro gen ▁co ols ▁sufficiently ▁to ▁re comb ine , ▁the ▁outer ▁layer ▁becomes ▁transparent . ▁ ▁Type   I In ▁super nov ae ▁The ▁" n " ▁denotes ▁narrow , ▁which ▁indicates ▁the ▁presence ▁of ▁narrow ▁or ▁intermediate ▁width ▁hydro gen ▁emission ▁lines ▁in ▁the ▁spect ra . ▁ ▁In ▁the ▁intermediate ▁width ▁case , ▁the ▁e ject a ▁from ▁the ▁explos ion ▁may ▁be ▁interact ing ▁strongly ▁with ▁gas ▁around ▁the ▁star ▁– ▁the ▁circum stell ar ▁medium . ▁▁ ▁The ▁estimated ▁circum stell ar ▁density ▁required ▁to ▁explain ▁the ▁observ ational ▁properties ▁is ▁much ▁higher ▁than ▁that ▁expected ▁from ▁the ▁standard ▁stell ar ▁evolution ▁theory . ▁It ▁is ▁generally ▁assumed ▁that ▁the ▁high ▁circum stell ar ▁density ▁is ▁due ▁to ▁the ▁high ▁mass - loss ▁rates ▁of ▁the ▁Type   I In ▁pro gen itors . ▁The ▁estimated ▁mass - loss ▁rates ▁are ▁typically ▁higher ▁than ▁ ▁per ▁year . ▁There