The departure of Megyn Kelly from Fox News to head over to network news at NBC really shakes things up in the media world. It’s nothing less than a catastrophe for Fox News, where Kelly’s 9 p.m. show is key to the channel’s primetime success — the glue necessary to hold together the increasingly unhinged histrionics of Bill O’Reilly at 8 p.m. and Sean Hannity at 10 p.m. Furthermore, Kelly is that channel’s only prominent female anchor in primetime. At first glance, that may matter less than it would at other channels — most of the Fox News audience doesn’t care about the optics of an all-male primetime lineup. But put aside for a moment the fact that Kelly is/was Fox’s most prominent woman. It’s the tone of The Kelly File — superficially more reasoned than that of O’Reilly’s Factor, Hannity, and, more recently, Tucker Carlson’s 7 p.m. insult-a-thon — that is most valuable for Fox. Without her, Fox News’ nights grow much more dark, grim, and bullying. Expect the channel to take a ratings hit while it gropes for a replacement.

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Meanwhile, Kelly’s shift to network news is a major gamble for NBC. Kelly’s May 2016 broadcast special on Fox — the one where she did her Barbara Walters impersonation, interviewing Laverne Cox, Michael Douglas, and her faux-nemesis Donald Trump — was a ratings disappointment and pointed up a chink in Kelly’s steely armor: namely, she always seems to be wearing an invisible coating of steely armor. It’s being reported that her NBC deal will probably include hosting a daily daytime news show, and as much as Kelly idolizes Dr. Phil, daytime TV requires the kind of warmth and ability to relate to a mostly female, middle-American audience that Kelly has rarely evinced on Fox News. It’s also reported that she’ll anchor a Sunday night news show. Will this go up against CBS’s 60 Minutes? If so, she’ll need a strong team of other correspondents with her to go up against that hoary behemoth. If it’s a primetime gig, good luck to her, trying to compete against all the big-time Sunday-night network and cable offerings in what will have to be a show that’s somehow different from Dateline.

As much as Kelly’s fame and prominence suggest that it’s time for her to leave Fox News, her on-air image and performance suggests she’s going to be a problematic celebrity to place in the constellations of stars that comprise the galaxy of news and showbiz personalities. In the context of Fox, she’s the foxiest in two senses: in on-air shrewdness, and in appeal to that channel’s demographic. (The median age of Fox News viewers is around 68, according to Nielsen.) Emerging from the slew of women that Fox News guru Roger Ailes created to deliver his message, she was a skyrocketing success. In the context of NBC, however, she’ll be forced to compete with everyone from Lester Holt to Savannah-and-Matt to Andrea Mitchell to fellow rising star Hallie Jackson.

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Kelly is excellent — at her best, actually — during breaking-news events, as she proved during two presidential election coverage cycles, when her lawyer’s mind and preternatural calm make her someone to attend to. That’s a gaping hole Fox will have to fill when she departs, and the only part of her NBC arrangement that seems like a sure thing.

Megyn Kelly’s last day at Fox News will be Jan. 6.