The magnitude of the loss we all suffered when Carrie Fisher and Debbie Reynolds died within a 48-hour period in late December would be evident even without the presence of HBO documentary “Bright Lights,” which depicts the intensely close and complicated relationship between the daughter and her mother.

But the movie, a behind-the-scenes look at Fisher caring for the ailing Reynolds in recent years that also chronicles their respective trajectories in Hollywood, deepens our understanding of who these people were, why they mattered and what their oft-examined and perhaps misunderstood bond might have been really all about.

The documentary by Fisher Stevens and Alexis Bloom is, then, at once an indelible cultural treasure and, now, a poignant cinematic eulogy. Fly-on-the-wall depictions of Fisher and Reynolds together in one of their adjoining California compounds or working to mount one of Reynolds’ last live shows, moments rife with love and concern, effortlessly mesh with home movie footage of Carrie’s youth with her brother Todd, and the parallels between Reynolds’ history as an MGM ingenue with her daughter’s accidental rise to superstardom after she took on what she thought would be a “little B movie” called “Star Wars.”

It is blessed by Carrie Fisher’s fearlessness, the commitment to being open and honest about the successes and struggles that characterized her personal and professional lives. That defining quality — a steadfast authenticity despite being born into Hollywood royalty and spending her entire life inside that suffocating bubble, including playing one of the most iconic characters of all time — shaped her connection with a fanbase that spanned well beyond “Star Wars” fanatics.

She could not have been more different than her mother, an old-fashioned star of the studio system far more consumed by her image, who we encounter living among the ghosts of her past, from Judy Garland to Frank Sinatra, in the form of her famous memorabilia collection and her memories.

The rocky path of such conflicting personalities famously inspired, in at least some capacity, Fisher’s “Postcards from the Edge,” and “Bright Lights” shows us traces of those difficulties. But there’s a deep and cosmic connection there that transcends it all and will for eternity. You know it and believe it after watching this movie. And amid the recent sadness, there’s some small comfort there.