Common sense, Murphy’s law, or pure cultural Darwinism should have prevented the following sentence from ever becoming true: The fifth studio albums by Britney Spears and the Backstreet Boys both reach stores today. That’s nothing against 1990s teen pop; it’s just that few artists make it to a fifth major-label album, even after selling millions of albums and scoring chart-topping hits. “Tossin’ and Turnin'” was a 1961 single that occupied Billboard’s no. 1 spot for six weeks — longer than releases that same year from Chubby Checker, Roy Orbison, and Elvis Presley. Remember the artist? Exactly.

His name was Bobby Lewis, and his brief musical career (1960’s “One Track Mind” was his only other hit) isn’t some obscure cautionary tale. It’s the norm in pop music. Some artists are lucky enough to be flashes in the pan, with only a few hits before petering out — this is why James Blunt looks so nervous all the time. A smaller few develop careers that stretch over their adult lives.

To most followers of popular music, it is nothing short of inexplicable that the Backstreet Boys should have a chance at the latter category. One way or the other, the Boys represent the end of an era. Now that Boys II Men, *NSYNC, O Town, Take That, and their pop brethren have all called it quits, the Backstreet Boys — now the quartet of 27-year-old Nick Carter, 34-year-old Howie Dorough, 34-year-old Brian Littrell, and 29-year-old A. J. McLean — are the last boy band standing. And they have stayed there by not changing anything about their saccharine, user-friendly, clean-cut, non-threatening pop, which they first played for Americans in 1997.

It’s no surprise, then, that the quartet’s new album, “Unbreakable,” sounds like it could have come out at any point in the Boys’ career. It’s full of songs about the ups and downs of love and getting through hard times, or something to that effect. On the downside they opine, “Who’s going to save me now that’s she’s gone,” in “Trouble Is”; and, “You wouldn’t have to lie to me if you would only let me go,” on “Something That I Already Know.” Underneath all the prepackaged passion is a collection of dance-friendly beats, not-too-aggressive guitars, and pristine, machine-aided melodies. Such easy-going, market-tested pop is virtually impervious to criticism: Every single Backstreet Boys album has gone platinum in America — even 2005’s resoundingly panned comeback album “Never Gone” — and it’s easy to hear why. “Unbreakable” is the end result of a team of songwriters (I lost track after the first 20 different names in the credits) and six producers who are dedicated to creating music that conforms to a successful formula. Maybe you didn’t realize that the formula still works. The truth is, it always has and it always will.

“Helpless When She Smiles” is a perfect example of said formula:

Start with a sunny piano line and a warm synth wash to introduce the opening melody. Add a lightly strummed guitar with another voice on the bridge. Then mix in a big drumbeat to underscore the four-part harmony of the chorus. Then, at the climactic moment, introduce the penultimate chorus as everything drops out and the group sings a cappella. If it’s a live album, cue screaming girls.

This is what the Backstreet Boys are: pop as mass-market product. Like the food at burger chains, the product’s desirability lies in its comforting familiarity and unsurprising dependability. And as long as there are America department stores and duty-free shops in Lithuanian airports that need innocuous background music, the Backstreet Boys will occupy a spot in somebody’s heart.

Britney Spears is a different matter entirely.

First, her new album, “Blackout,” is an up-tempo, sassy, and dance-club rocking outing — perhaps her most consistent album to date. Second, Ms. Spears is the most infamously scrutinized woman in pop culture, despite the fact that no matter how many tabloids track her undergarments, schizophrenic hair styles, parenting skills, fashion choices, and rehab stints, nobody has offered any shred of evidence why anybody should care.

Never mind, though, because Ms. Spears comes out swinging: “I’m Miss bad media karma / Another day another drama / Guess I can’t see no harm in working and being a mama,” she sing-speaks in the deliciously bopping “Piece of Me.” The track, produced by Christian Karlsson and Pontus Winnberg, the Swedish duo who put the dance-floor slither into Ms. Spears’s 2003 hit, “Toxic,” is a bouncing, spacious slab of brassy defiance: “I’m Mrs. most likely to get on the TV for slipping on the street when getting the groceries / Now, for real, are you kidding me?” Ms. Spears offers, the entire chorus and verses sounding like a ranting Web log post.

“Piece of Me” is just the tip of the album’s athletic beats. Timbaland protégé Danja inserts rump-shaking bass blasts into the body-moving “Get Naked (I Got a Plan),” which, yes, sounds ready-made for strip clubs, but — as hip-hop singles discovered long ago — if it’s good enough for a woman in five-inch heels, it’ll work in the nightclub. Lead single “Gimme More” knits syncopated clapping with Ms. Spears’s attempts at sultry breathiness for a perfectly excusable dose of PG-13 naughtiness. And “Heaven on Earth” turns an anxious, early-1980s keyboard throb into perfectly disposable dance pop.

“Blackout” may be Ms. Spears’s first album that isn’t riddled with filler surrounding the singles. It’s far from perfect — “Hot as Ice” and “Perfect Lover” are about as sensual as checkout-aisle magazines, and Ms. Spears’s voice still sounds strangely post-human — but it’s a refreshingly listenable album, not something forgotten the very second a song ends. The “Gimme More” remix with Lil Wayne is a genuine dance banger, as are dizzying bonus tracks “Get Back” and “Everybody,” which samples the Eurythmics’ “Sweet Dreams (Are Made of This).”

In other words, “Blackout” is a strong slab of dance-pop, which would be news enough were it not coming from any other artist. Ms. Spears, though, is the oddest of celebrities. Never has the daily life of a white, undereducated, divorced, 25-year-old mother of two from Louisiana ever been of such interest. The culture of Britney-watching has even spawned cottage industries, ranging from the nonsensical — fans started a Web site (www.myspace.com/beproactivetohelp) asking people to boycott her new album until she gets “help” — to the patently absurd: a sock worn by a paparazzi photographer whose foot Ms. Spears ran over sold for $585 on eBay this past weekend.

Why Ms. Spears’s life warrants such minute-by-minute attention is anyone’s guess. She’s not exculpable in this enterprise — she wanted pop stardom and she got it — but she now lives in a peculiar world where her mundane travails have become the reason she’s famous. As such, it’s unclear how the rather confident “Blackout” may perform. Eight years ago, a 17-year-old Ms. Spears released the biggest-selling debut album from a female artist in American chart history. That album has sold more than 83 million copies worldwide. “Blackout” may be strong enough to remind everybody — from fans to culture vulture pundits — just why people started caring about Britney Spears in the first place.