“Supernova” does not have a happy ending. But that’s how stories about Alzheimer’s go, and the film, which recently began streaming, is worth watching. It’s one of those occasional movies that come along and portray the emotional aspects of this disease with nuance. The films, by using big-name stars like Stanley Tucci and Colin Firth in “Supernova,” lift some of the stigma around dementia that can isolate its victims and their caregivers. Dementia “is still very much a taboo topic,” said Bobbi Matchar, who, as director of the Duke Dementia Family Support Program, facilitates group discussions for people with dementia and their families. “Having movies that more accurately portray the face of dementia is really helpful.” The newest of thes…