Tom Petty and the Heartbreakers have always recorded under that name, but Petty's 1989 disc Full Moon Fever is credited only to Tom Petty as a solo act. Why? The music features members of the Heartbreakers, the producer is Jeff Lynne (who produced other Petty and Heartbreaker discs around that time), and the most striking musical moment is the solo by Heartbreakers guitarist Mike Campbell on "Runnin' Down a Dream."

What makes music a "solo" effort as opposed to the work of a group? It's almost impossible to imagine rock 'n' roll as music that isn't created collectively. The current taste for big folk-rockestras like Arcade Fire and the National reaffirms rock's drive for community, yet we're still entranced by powerful single stars like Kanye West and Jack White.

Many discs of 2007 tell different stories of the "solo" album. More than the sound, arrangement or who's playing, what counts is the source of the vision, be that an individual's or group's. The conventional solo model is heard on both A Poet's Life by Rancid frontman Tim Armstrong and Jarvis by Pulp frontman Jarvis Cocker. In this mode, the artists spread creative wings away from their former bands, and now sound sorta like, but not quite like, their origins. Armstrong removes the East Bay punk from Rancid and is left with reggae and ska by his new band the Aggrolites. Cocker's current cast of players nudges his wry '90s Brit-pop deeper into piano-ballad ambiance.

A solo disc that doesn't fall far from the tree is Sirens of the Ditch by Jason Isbell, one of the three songwriter-guitarists in the Drive-By Truckers. Isbell may be the best of the three. Here, he makes the same incisive Southern roots-rock that DBT do, commanding his own show with punch and grace. But how solo is he? Sirens is in part unfinished DBT songs from the last few years, featuring the DBT rhythm section, with production and keyboard help from DBT frontman Patterson Hood.

In a related vein, veteran solo acts show that although they've built substantial careers with more longevity than their first bands, they still rely on collaboration. John Doe's A Year in the Wilderness sounds like the folk-blues finale he's chased since the pioneering punkabilly of his band X, and there's a continuous female presence, notably from alt-country fave Kathleen Edwards. Avant-wonder Björk is as whimsical as ever on her latest post-Sugarcubes piece Volta. Her choice of hip-hop producer Timbaland and marginal alt-rock and world-pop guests are acutely clever, picky and personal.

Perhaps a solo act is most practically an artist who rallies others around a shared concept. Hot producer Mark Ronson's slick disc Version is the work of a ringleader who's both inspired and co-dependent. Ronson mashes up current British R&B/rock acts like Amy Winehouse and Lily Allen with classic Brit-pop material from the likes of the Smiths and the Jam. The retro-funky remakes, like Ronson's instrumental originals with his house band the Daptones, speak of a mildly forward-looking scene.

If Ronson is a solo act, then why isn't Josh Homme of SoCal rockers Queens of the Stone Age a solo act? He's the soul behind the band's sideways alt-metal bite and the sensibility to his goofy side project with a childhood buddy in Eagles of Death Metal. Era Vulgaris, QOTSA's latest, focuses Homme's lyrics and riffs in a sharp power-trio format while almost removing his usual use of big-name guests.

The truest recent solo disc comes from the late Joe Strummer, via the soundtrack to his biopic The Future Is Unwritten. In a simulation of shows the former Clash frontman hosted on the BBC late in his career, we hear the charisma of one man linking not only his sources, but his ideal for communal music. "Without people, you're nothing," DJ Strummer reminds us between cuts, which range from the fire of the MC5 to odd Woody Guthrie tunes to cool Latin music to the soul of Nina Simone. From that bigger picture, Strummer and this soundtrack create a single ringing voice.

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