On Thursday night, a crowd gathered in Union Square in Manhattan for a fond and spontaneous memorial to Michael Jackson. A few hundred onlookers formed a circle, leaving enough space in the middle for the grandstanders and the brave to dance like the King of Pop. Or try to.

Even the lamest moonwalk drew chants of “Mi-chael, Mi-chael!”

Watching this spectacle, you had to wonder: When will this happen again? When will another pop culture figure mean so much to so many that people are moved to assemble, hug and dance?

This is a tribute, of course, to Mr. Jackson’s singular gifts — his voice, songwriting talent, physical grace, and the list goes on and on. But there is the related matter of historical timing. Fame on the level that Mr. Jackson achieved is all but impossible for pop culture heroes today, and quite likely it will never be possible again.

On the most basic level, this is matter of business and math. Michael Jackson has sold an estimated 100 million copies worldwide of the 1982 album “Thriller,” which spent more than 31 weeks at the top of the Billboard charts.

It’s one of those high-water marks that nobody will touch, because record stores are vanishing, and along with them, megahit albums are vanishing, too. A big week on the Billboard charts is a quarter-million units sold, which is about the number of units the Jonas Brothers moved last week with their latest release, which opened at No. 1. And it’s rare for an album to last even three weeks at the top.

People who buy music tend these days to buy — or steal it — online, a song at time.

But even if nobody achieves album sales on a Jacksonian scale, couldn’t he or she be an artist every bit as popular, every bit as loved, every bit as listened to?

Probably not. The pop-idol field — like every field that can lead to super-fame — is more crowded than it has ever been, and the variety of routes to stardom keep growing. When the Beatles were on “The Ed Sullivan Show” in 1964, more