Two years after becoming the spokesperson for a new generation of music fans, Billie Eilish faces a long-standing challenge: maintaining her momentum.

The pop superstar released her second album “Happier Than Ever” on Thursday night, an important test of her artistic and commercial stature in a musical enterprise where stars are gaining and losing their sparkle at an increasingly dizzying rate.

“I feel like crying,” Ms. Eilish said in an Instagram post shortly after midnight. “I grew so much in the process of creating this album and experienced so much self-realization.” She called the album “the most fulfilling, satisfying and profound experience I have ever had with my music.” Earlier Thursday, she announced that the music video for “Happier Than Ever,” the album’s title track, would arrive at noon EST on Friday.

“Happier Than Ever” is the culmination of an intense period in which Ms. Eilish became the defining new pop star of the era. The 16-song album follows Ms. Eilish’s four-time platinum debut, “When We All Fall Asleep, Where Do We Go?” from 2019; a historic sweep of the 62nd annual Grammy Awards, where she was only the second artist to win Album of the Year, Song and Record of the Year, and Best New Artist; and a full year of new singles, including “NDA,” whose clip features stuntmen sneaking around 19-year-old Ms. Eilish on a dark road at night.

Ms. Eilish’s latest album comes at a pivotal moment in her career. New pop stars often face tremendous pressure to continue their initial success, especially if their rise was propelled by a huge hit single. Ms. Eilish is different. She has firmly established herself as a symbol of Gen Z, Grammy darling, music streaming powerhouse, and “career” artist whose fans flock to all of her songs, not just one or two hits.