The lives of celebrities may be more public than yours, but there's a lot that even they don't know. There are unanswered questions about ancestry and missing branches in their family trees. That's where historian and Harvard professor Henry Louis Gates Jr. comes in.

Armed with a team of genealogy researchers and DNA experts, Gates probes the pasts of a variety of notables in the second season of PBS's "Finding Your Roots," premiering on Sept. 23 with author Stephen King and actors Gloria Reuben and Courtney B. Vance.

"This season, we have three guests in each episode, each constructed around a common theme, sometimes hidden, that unites the guests," says Gates. "We have an episode devoted to immigrant food culture with renowned chefs Tom Colicchio, Ming Tsai and Aaron Sanchez, representing what we think of as the holy trinity of the American cuisine; an episode tracing Jewish ancestry with Alan Dershowitz, Carole King and Tony Kushner; An episode exploring what makes a champion by peering into the family histories of Derek Jeter, Billie Jean King and Rebecca Lobo.

"Since the DNA science has improved exponentially, we have found all kinds of new things this season," he adds, using as an example the advancements in DNA testing that have made tracing African-American ancestry much easier. "We reveal to Derek Jeter that his family name originates with the slave master who owned his second great-grandfather, Green Jeter. And we discover that this slave owner passed down not only his name, but also his genes — after DNA analysis proved that he was also Green’s father."

On the other side of the history of slavery, "Anderson Cooper learns that one of his Southern ancestors was killed by one of the 12 slaves that he owned, in what was perhaps an act of vengeance."

"This season's also truly global," Gates continues. “We constructed our largest tree ever, for celebrity chef Ming Tsai, whose family we've traced back more than a hundred generations — all the way back to the year 2598 B.C. We trace Jessica Alba all the way back to ancient Mayan civilization. We traced Tina Fey to an ancestor who miraculously survived one of the worst massacres in Greek history and went on to become a hero in the struggle for Greek independence. We discovered the seafaring English roots of Sting going back five generations."

In this country, notes Gates, "We revealed that Ben Affleck's third great‑grandfather was a man named Almon Bruce French, a popular American spiritualist in the 19th century. We introduced Ken Burns to his Confederate ancestors, which blew his mind, as well to his long‑lost distant cousin who happened to be named Abraham Lincoln. He had ancestors who fought on both sides of the Civil War and the American Revolution."

How does Gates go about selecting his subjects? "We fantasize about guests who we would like to have, and we want a lot of ethnic diversity. We want gender diversity. I'm particularly fascinated by Eastern European Jewish ancestry, which is very, very difficult to do. Then I write to everybody," leaving out how long the process takes, otherwise, "nobody would ever do it. Almost everybody says yes. And then we send them DNA kits and questionnaires," so they can fill in what they know. "It takes months to do the research. We have a mountain of data, but we need to find stories to tell, and that's a real art."

The sit-down interview takes three or four hours. "It's exciting and it's moving," says Gates. "Often people cry, and I feel like Santa Claus, you know, giving people a gift." That's true in the literal sense, because such extensive genealogy studies could cost $50,000, depending on the degree of difficulty. "If you're of English descent, it's cheaper, because you're more deeply documented. If you're a Eastern European Jewish person, it's going to cost more. If you're Greek, forget it — you've got to hire somebody to go back to Greece, which we do for Arianna Huffington and George Stephanopoulos. It all depends on how available the records are and how much original research you have to do."

For the average person,, a sponsor and partner of the series, is a good place to start, says Gates. " has, I believe, 14 billion records scanned. When you join, you just click in the name of your grandfather or grandmother and it ties you to all the documents. But there are things that you can't find out in the paper trail, and that's when you have to hire investigative genealogists."

Gates is already at work on the next season with celebrities Jamie Foxx, Julia Louis-Dreyfus, Jimmy Kimmel and Gloria Steinem on the guest list. "Every family's story is so interesting," he says. "I'm never bored doing this."

Related on MNN:

'Finding Your Roots' fills the gaps of the family tree
The lives of celebrities may be more public than yours, but there's a lot that even they don't know.