For three decades, Johnny Carson was the greatest single star ever created by American television.

And then came Oprah Winfrey. And they’re now doomed to share that distinction for the rest of eternity.

Both acquired their gigantic stature in the most obvious way: Their shows were the depots through which the nation’s entire celebrity class made arrivals and departures. Careers by the dozens – at least – were born on Carson’s shows. Some (Joan Rivers’) were semi-permanently maimed there, too, when the acknowledged King of Late Night became angry and disenchanted.

Presidential aspirants were required to evince humor and personality next to Carson. Our current president was, to many cynics, virtually appointed to his job by his fellow Chicagoan Oprah Winfrey, who, with her instant imprimatur, gave him a fame and approval rating mere everyday pols couldn’t hope to equal.

We won’t see their likes again. Ever.

Fragmentation is the rule now, in the age of information. So much of Carson’s fame and power came from his running the oldest franchise in late-night TV in an era of three, and then five, channels.

Both Carson and Winfrey were uncommonly fascinating human beings – strange and complex amalgams of immense vulnerability and power. Both offered the utterly amazing spectacle of people who’d successfully turned themselves into arbiters of everyone else’s reputation.

In Henry Jaglom’s “My Lunches with Orson” (see the review in this Sunday’s book pages), Orson Welles complains that he was all but blackballed from Carson’s show when Kenneth Tynan’s New Yorker profile of Carson revealed that of all his guests, the only one who left him awestruck was Welles.

Awe of any other human, it seems, wasn’t Carson’s favorite part of that profile. According to Welles, anyway.

The trouble with Carson in the 21st century is that he’s dangerously close to a lost and obscure subject.

Which is why the current Monday excerpt show “Carson on TCM” is both perfect placement and a hopeless admission that Carson – and most of his guests – are now permanently ensconced in what television considers “the senior demographic” (i.e., out of reach of Kardashians and rappers and, even, out of the much-coveted 18-to-49 demographic altogether, or at least rapidly on the way there).

The Nielsen metric geniuses themselves have been busy pointing out for a long time that being too rigidly exclusionary about people over 50 is hugely self-destructive. America is now full of over-50s with discretionary income, and a well-developed interest in ads telling them how to spend it. No, they’re not, as consumers, nearly as impulsive and promiscuous as young TV consumers are but they are ignored only at TV’s immense peril.

Nevertheless, Carson has semi-officially timed out in the age of late-night’s Jimmys and the imminent disappearance of his career assassination conspirator, Jay Leno. His immensity in American culture is known firsthand only by those older than 50.

That’s one reason why “Carson on TCM” is of enormous value. It’s a place to show that even before Oprah came along, America had one place where fame was established and confirmed and exploited all at the same time. (Please remember that Carson and his producer Fred DeCordova invented the talk show as a conduit for movie, TV, etc., promotion, the place to schedule guests selling something.)

But as its TCM presence confirms, Carson’s encounters with the famous are now rather safely in the ranks of TCM’s stars – huge figures in American culture through history, not currency.

TCM has wisely branched out from movies into the former Olympus and Parnassus of TV talk. First for them was Carson’s ex-writer and brief time slot competitor Dick Cavett, the Nebraska-born Yalie whose show could successfully negotiate highbrow subjects Carson all but abandoned when he moved the show from New York to L.A. (He could always tell himself that a Welles or a Gore Vidal or a Carl Sagan were significant enough to keep highbrows pleased, but he always presented them as if they were spinach for your diet when he did.)

There is no more important cable network in a lot of ways than TCM. It’s where a giant portion of American mass culture and film culture itself now live inside the box. When, somewhat incredibly, Adelphia cable briefly refused to make TCM available to customers inside the city limits, it actually had defenders on the disgustingly philistine grounds that a sports city like Buffalo shouldn’t have to be bothered by minority foolishness like TCM, for all its astonishing and long-lasting supremacy in dealing with the movie past.

Thankfully, Adelphia wasn’t as hopelessly stupid as some of its apologists and in time (not all that long) reversed course and proudly made TCM available everywhere.

No one would dream of doing that anymore. Even in its peculiar senior years – where the “classic” in Turner Classic Movies is actually stretched to include the likes of “Gidget” – TCM has become an irreplaceable cornerstone of the cable dial, right up there with the 24-hour news channels and the ESPN, HBO and Showtime channels.

Monday night was the second show of “Carson on TCM.” I wish, frankly, Cavett had been tapped to introduce the great Carson clips, but I understand their choosing Conan O’Brien. Though never even close to the Carson class of late-night host himself, he’s a well-raised Harvard boy suitable for evincing understanding of his greatest late-night predecessor.

Jerry Seinfeld, in my opinion, would have been infinitely better, but he’s busy with his terrific new Internet TV series “Comedians in Cars Getting Coffee.” (Not to be missed is the one devoted to those near-lifelong friends Carl Reiner and Mel Brooks.)

What no “Carson on TCM” series could ever convey fully was the immense and peculiar power of Carson’s personality – that unique ex-Nebraska boy who was well-raised (his father was the manager of a Light and Power Co.) and was, therefore, someone who understood emotional reserve; an ex-Navy man who learned about hierarchy in the military; and the innocent who had immense power over people whom he so often felt were his cultural superiors.

That’s why he was so great dealing with George Burns, Liz Taylor and Kirk Douglas, among others. He was the stand-in for the audience’s innocence as well as its Middle American yearning for sophistication.

One of my favorite clips of Carson in action had nothing to do with his show. It featured Carson trying in vain onstage to keep up with Frank Sinatra and his Rat Pack – Dean Martin, Sammy Davis Jr. He was trying to fill the Joey Bishop ringmaster role but what was hopelessly apparent was how out of his depth he was with these masters of nightclub anarchy.

By then, Frank, Dean and Sammy could own a nightclub stage in a way Carson couldn’t even dream of.

In their company, you could see the innocent Nebraska kid he always partially remained.

It was ultimately his most enduring strength.

He was both the King of his World and completely alienated from it.

That’s why his audiences never had a truer surrogate.