Who will be Dianne Feinstein's replacement? Here are California's rules for replacing U.S. senators.

The death of Sen. Dianne Feinstein leaves behind not just a legacy of someone who shattered glass ceilings, but also an open seat in the United States Senate for California. 

Feinstein, who was 90 years old, had about 15 months of her term left at the time of her death. Under California law, Democratic Gov. Gavin Newsom has sole discretion to fill that seat with an appointee, and that appointee will serve until after voters elect a senator for the seat. 

Newsom will appoint a Democrat, but which Democrat remains to be seen.