Nova is one of Marvel Comics' oldest "cosmic" superheroes, a character who began as one of many attempts to recapture the magic of Spider-Man, languished in B-tier superteams for decades, and eventually grew to become a major player in the larger galactic conflicts of the Marvel Universe. He was created by Marv Wolfman and John Buscema in 1976's The Man Called Nova #1, in which Richard Rider, a high school student in New York, is gifted with
a power ring the powers of a Nova Centurion by the dying alien Abin Sur Rhomann Dey, a member of the intergalactic peacekeepers of the distant planet Oa Xandar, the Green Lantern Corps Nova Corps.
Look, bear with me here. It gets better.
Richard acted as an Earth-based superhero for many years, primarily as a member of the New Warriors, a largely street-level team including heroes such as Speedball, Namorita, Darkhawk, and Night Thrasher. (Okay, they weren't exactly the Avengers, but they made it work.) He lost and regained his powers a few times over the years, battled supervillains such as Diamondhead, Condor, and The Sphinx. He made occasional trips to Xandar to help out the Corps with various crises, but never became a major player outside of Earth until 2006's event comic Annihilation.
In Annihilation, Nova was forced to step up when the Annihilation Wave, an invasion force from the Negative Zone led by Annihilus, destroyed Xandar and the Nova Corps, leaving Richard the only survivor. As the last remaining Nova, he became sole host to the Nova Force and the Xandarian supercomputer The Worldmind, and went on to lead a nearly year-long military campaign to fight back the advance of the Annihilation Wave forces. Eventually, Richard won the war by leading a small team into Annihilus's territory and killing Annihilus by reaching into his throat and ripping his fucking insides out.
See, I told you it got better.
Nova went on police the universe as a one-man Nova Corps in an ongoing comic written by Dan Abnett and Andy Lanning, as well as featuring in the cosmic events Annihilation: Conquest and War of Kings, and briefly acting as a member of Captain America's covert ops team, the Secret Avengers. His story came to an end in 2011's The Thanos Imperative, when he and Guardians of the Galaxy leader Star-Lord stayed behind in a dying universe to prevent Thanos from escaping, trapping the Nova Force with him. Richard Rider was presumed killed, and while Star-Lord and Thanos have since returned to the original Marvel Universe (though it hasn't yet been explained how), Richard's current status is unknown.
While Richard Rider is still missing in action, a new Nova, a youth named Sam Alexander, was recently introduced, and will star in his own ongoing series written by Jeph Loeb and drawn by Ed McGuinness starting in 2013. Sam also appears as a character in the animated series Ultimate Spider-Man.
Richard appears as Nova in Ultimate Marvel vs. Capcom 3, and Sam Alexander appears in the MMO Marvel Heroes.