“Star Wars: The Rise of Skywalker” hits theaters today, and over the decades, many of us have become familiar with the iconic “Imperial March,” which debuted in 1980 in “The Empire Strikes Back” (as Darth Vader stood on the bridge of his ship, contemplating his obsessive and unceasing search for Luke Skywalker).

“Imperial March” is the standard by which all movie villain themes are judged. And the bad guys in “Star Wars” get all the best tunes. That’s according to Frank Lehman, music theory professor at Tufts University and author of “The Complete Catalogue of the Musical Themes of Star Wars.”

“Everyone wants to root for the bad guy sometimes. And it's a great technique to make their themes memorable, to leave the audience humming along with the baddies who are so often the most striking and interesting characters in this series,” says Lehman. “The heroes tend to be a little bit maybe milquetoast. And the villains are the ones that are exciting to the audience, and their music is written accordingly to make it catchy and infectious and maybe a little bit seductive.”

Did “Star Wars” creator George Lucas intend for the music to become this iconic? Lehman says everything in “Star Wars” was a bit of an accident, and John Williams (who composed all the music in the series) didn’t think there would ever be a sequel to the first film, let alone a sequel trilogy or prequel trilogy.

Lehman explains that Lucas initially planned to use preexisting classical music. Steven Spielberg, Lucas’ good friend at the time, convinced him that an original score would be more suitable. Spielberg recommended his friend John Williams for the job (the two of them collaborated on “Jaws” two years earlier).

Together, Spielberg and Williams suggested that the music could evolve along with the characters. For example, at the end of the original trilogy, when Darth Vader is dying and his son Luke Skywalker unmasks him, audiences hear a different version of “The Imperial March,” which features a solo harp.

Lehman says the music for Darth Vader’s death is one of the most stunning music transformations in the entire series.

“Up to that point, every instance that we'd heard of Darth Vader's theme had been, well, imperious, overwhelming, loud, militaristic. And finally, when we see him on his deathbed -- this pathetic, ruined, regretful old man -- Williams totally deconstructs the theme. He strips away all of the militaristic garb. There's no more brass, there's no more snare drums,” Lehman describes. “Instead, we get a succession of these sort of solo instruments or very light strings. And finally, this absolutely stripped-down, pathetic version for harp … there is a sense of finality to it, which goes along with the idea of redemption that's so important in this series.”

Jumping ahead to the present, there’s the theme for Darth Vader's grandson, Kylo Ren. Lehman says it’s a statement of music bravado.

“It never quite achieves the heights that Vader's theme does. And I think this is a nice character insight on Williams’ part. It's aggressive and in your face. But in its own way, kind of a concealment or a mask. … Kylo Ren is kind of a character in process, that this dark side facade that we hear in the music is maybe not the whole story. It’s not exactly a carbon copy of Darth Vader's music,” Lehman says.

Finally, there’s the darkest character of the dark side: the Emperor. This theme is maybe Williams’ “musical master stroke in illustrating villainy,” says Lehman.

He observes that this theme is subtle, with a deep and wordless male choir. But what really makes it special are the harmonies.

“There is no real sense of a straightforward natural logic to the way that the chords move from point A to point B. It's very unpredictable. It's perverse in a way. It's a violation of what we expect. So the combination of this deep, ominous, almost liturgical or religious sounding choir with the extremely perverse and deceptive sorts of harmonies makes it, I think, a pretty unmistakably powerful and insinuating sort of theme,” he says.

What will the future of “Star Wars” sound like, especially when 87-year-old John Williams stops working on the series? Lehman says the best thing is for a composer to capture a small element of what makes a ‘Star Wars’ soundtrack unique, but project it through their own voice. “To not feel totally straightjacketed by what has been established by Williams, because no one really can do that, no one can write like Williams,” he adds.

--Written by Amy Ta, produced by Brian Hardzinski