IN THE CONSTELLATION of pop music history, some stars shine brightly for eons; others are fated to memory. Washington's own Starland Vocal Band, Bill and Taffy Danoff, Jon Carroll and Margo Chapman, burned as bright as any with the success of their first single "Afternoon Delight."

Besides topping the national charts for six weeks, winning two Grammy awards and selling nearly 2 million copies, the band's big hit became a national catchword of sorts, literally on the tip of everyone's tongues during the summer of 1976. Clyde's of Georgetown even dubbed their afternoon snack plate the "Afternoon Delight."

It wasn't their first taste of fame. The Danoffs, fixtures in the local music firmament since their Georgetown University days, co-wrote and sang backing vocals on "Country Roads," a No. 1 hit for John Denver in 1971. The royalties from that record, when they finally arrived, rescued the Danoffs from their leaky Q Street basement apartment.

After the hit, the group tried again with a second album, "Rear View Mirror"; headlined a star-crossed summer variety series for CBS in 1977, with then-unknown comic David Letterman ("It stunk," Carroll comments candidly); and offered several more singles, none of which hit the same heights on the charts.

The Danoffs'--Bill and Taffy, now separated--are trying new directions. Taffy is studying acting with the Theater School of Georgetown. Bill moved back to Georgetown and has started his own music publishing company with former VISTA director Margery Tabankin. Danoff and Carroll wrote several songs for Norman Lear's "I Love Liberty" television special, and Danoff manages Carroll's new band, Jon Carroll's Metro.

"We play piano-based pop--more R&B than what Starland did," Carroll says of his new group, which is doing a showcase in June at Trax nightclub in New York. Margo is doing a solo demo under her maiden name, Margo Ann Kunkel.

In addition to a different sound, Carroll has a new strategy for Metro, which has been well-received critically for their opening dates with the Four Tops and LeRoux at local clubs like the Bayou and the Wax Museum. "What happened with Starland was, we had 10 people including sound people, and we'd fly out to do a $10,000 gig in Arkansas, and we'd lose money.."

Carroll and Kunkel were married on Ebbitt's Farm in McLean in 1978 and live in Falls Church with 3-year-old son Ben, "a real good drummer." "When we're not doing the group, we're doing TV and radio jingles. Remember the one that went 'Milk's a soft-soft drink'? That was us."

Carroll recently achieved a new dream--he sold a song to Linda Ronstadt. "In December, I had finished my first three-song demo and Bill took it with him to L.A. where he was visiting friends--Emmylou Harris is an old friend, and Linda Ronstadt was there, just hanging out. And Bill said, 'This is what Jon's up to,' and played the tape, and Linda liked "Love, Get Closer!" In 36 hours, they had called and put the song on hold. Bill called to tell me about 5 in the morning. The last time I heard from Bill at 5 a.m., he had this great idea--'Do you want to start a group?'

"Bill and I sometimes still end up at 5 in the morning, drunk, wondering 'what if?' I've been thinking of writing a book about the Starland experience and all we learned. It was like Murphy's Law. Everything went real well from the word go--we walked right into a record contract, good producer--a lot of the stuff was real storybook. And all of those things turned out wrong. The record company and our manager were misguided--we were misguided."

"Are we making it financially? Yes and no," Carroll says. "There's nothing like being a hit. I can't wait to be a hit again."