Two years after becoming the spokeswoman for a new generation of music fans, Billie Eilish faces an age-old challenge: keeping her momentum.

The pop superstar on Thursday night released her second album “Happier Than Ever,” an important test of her artistic and commercial stature in a music business where stars gain and lose luster at an increasingly dizzying pace.

“I feel like crying,” Ms. Eilish said in an Instagram post shortly after midnight. “I grew so much in the process of making this album and experienced so much self-realization.” She called the album “the most fulfilling, most satisfying and profound experience I’ve 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 12 p.m. Eastern on Friday.

‘Happier Than Ever’ by Billie Eilish.

‘Happier Than Ever’ by Billie Eilish.

“Happier Than Ever” is the culmination of an intense period during which Ms. Eilish has become the era’s defining new pop star. The 16-song album follows Ms. Eilish’s four-times-platinum debut, 2019’s “When We All Fall Asleep, Where Do We Go?”; a historic sweep of the 62nd annual Grammy Awards, where she was just the second artist ever 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 music video features stunt drivers weaving around the 19-year-old Ms. Eilish on a dark road at night.

Ms. Eilish’s latest album comes at a pivotal point in her career. New pop stars often face tremendous pressure to follow up 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 Gen Z symbol, Grammy Awards darling and music-streaming powerhouse and “career” artist whose fans flock to all her songs—not just one or two hits.

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Last year, Ms. Eilish was the No. 3 moneymaking artist globally, after Taylor Swift and Queen, thanks in part to her 13.7 billion global streams, which translates into an estimated $13.6 million in royalties, according to Billboard magazine. Her unusually broad appeal—increasingly a rarity in a fragmented music-listening landscape where fans listen to their own favorites on-demand—has inspired a cottage industry of similar-sounding artists signed by major labels.

Yet while “Happier Than Ever” is likely to be a commercial hit, the big question in music-business circles is how it will stack up over time against its 2019 predecessor—and whether it falls short, either in sales, streams or critical praise.

On one hand, “Happier Than Ever” is the most “pre-added” album in Apple Music’s history. The album is also expected to top the Billboard 200 album-sales chart, most likely with very strong vinyl sales (she’s highly popular among vinyl-record buyers). And early album singles, “My Future” and “Therefore I Am,” which were released last July and November, respectively, hit No. 6 and No. 2 on the Billboard Hot 100 singles chart.

But there are a few signs that Ms. Eilish’s more recent singles have elicited a more lackluster response. “Lost Cause” and “NDA,” which were both released in the last few months, have only reached No. 27 and No. 39 on the Hot 100, respectively.

Just one of Ms. Eilish’s newer songs ranks among the 20 most-streamed songs of her career in the U.S.—“Therefore I Am,” which ranks No. 19 with nearly 264 million streams, according to MRC Data. And in the week ending July 22, Ms. Eilish had a single song among the top 100 most-streamed songs in the U.S.: “NDA,” which was No. 87, MRC Data says.

While Ms. Eilish is more of an “album” artist than a deliverer of traditional hits and her most recent songs obviously have had less time to garner streams, their numbers are still considered notably weak on the eve of an album release. Earlier this month, Ms. Eilish clapped back against fans who were suggesting online that she was in a “flop” phase.

Representatives for Ms. Eilish weren’t available for comment.

In the run-up to her new album, Ms. Eilish stoked a cultural debate by dropping her famously casual style, green-and-black hair and oversize clothes for a more glamorous, blonde look. She and her brother Finneas—who cowrites and produces her music—also appear to be operating even more independently of record executives. “No one has a say anymore,” Ms. Eilish recently told Rolling Stone magazine. “It’s literally me and Finneas and no one else.”

At the same time, there has been an extended media blitz, including a documentary film in February (“Billie Eilish: The World’s a Little Blurry” on Apple TV+) and, coming in September, a concert film featuring conductor Gustavo Dudamel and the Los Angeles Philharmonic (“Happier Than Ever: A Love Letter to Los Angeles” on Disney+).

At the 62nd annual Grammy Awards, Billie Eilish was just the second artist ever to win album of the year, song and record of the year and best new artist.

At the 62nd annual Grammy Awards, Billie Eilish was just the second artist ever to win album of the year, song and record of the year and best new artist.

Photo: Emma McIntyre/Getty Images

On the music front, Ms. Eilish’s hushed style—a genre-agnostic blend of pop, rock, folk, electronic music and even hints of jazz and cabaret—remains largely recognizable, though it has expanded to incorporate more vintage sounds and powerful, soulful singing, music critics say. The new songs, which tackle themes of fame, misogyny and abuse, also have a more low-key feel and serious tone, they say.

On the album’s leadoff track, “Getting Older,” for example, Ms. Eilish says she has “more on my shoulders.” “I’ve had some trauma, did things I didn’t wanna, was too afraid to tell ya,” she sings. “But now, I think it’s time.” And on “Not My Responsibility,” she delivers a spoken-word piece about media scrutiny of her and unequal gender norms. “Some people hate what I wear, some people praise it,” she says. “Some people use it to shame others.”

One thing that hasn’t changed at all is Ms. Eilish’s approach to releasing music: As with her 2019 album, Ms. Eilish beforehand dropped a string of one-off songs and album-destined singles, a strategy of pumping out music that keeps the artist—instead of particular songs—foremost in fans’ minds. Before July 30, Ms. Eilish had already released five of her 16 new songs, taking significant pressure off the album.

Like many current pop releases, “Happier Than Ever” also comes in a variety of collectible physical formats aimed at superfans, including a “super deluxe box set” for $100 that includes a CD, satin bandanna and a mirror and brush set.

Write to Neil Shah at neil.shah@wsj.com