US actor Leonard Nimoy, who played Mr Spock in the cult sci-fi series Star Trek, has died at the age of 83 in Los Angeles, his family has said.
His son, Adam, said he died of end-stage chronic obstructive pulmonary disease on Friday morning.
Nimoy had a long career as both an actor and director.
However he was best known for his portrayal of the half-human, half-Vulcan character in both the TV franchise and series of films.
Last year, the actor revealed he was suffering from chronic lung disease, despite stopping smoking 30 years ago.
It was reported earlier this week he had been taken to hospital on 19 February after suffering from chest pains.
He later tweeted: "A life is like a garden. Perfect moments can be had, but not preserved, except in memory."
He signed off with "LLAP" - a reference to his character's famous catchphrase, "Live long and prosper". It was to be his final tweet.
George Takei, who played Hikaru Sulu on Star Trek and was a friend of Nimoy's, paid tribute to the actor.
"The word extraordinary is often overused but I think it's really appropriate for Leonard", Mr Takei told US broadcaster MSNBC.
"He was an extraordinarily talented man but he was also a very decent human being."
Among the torrent of tributes on Twitter was a message from Nasa with a picture of the actor standing in front of a replica Space Shuttle.
Thousands took to Twitter to pay tribute after Nimoy's death was announced, including Star Trek actors past and present.
William Shatner, who as Captain Kirk acted alongside Nimoy for years in Star Trek, said he loved the actor "like a brother".
"We will all miss his humor, his talent, and his capacity to love," Mr Shatner said on Twitter.
Wil Wheaton, who played Wesley Crusher in Star Trek: The Next Generation, said: "We stood on your shoulders, and wouldn't have had a galaxy to explore if you hadn't been there, first. Thank you, Leonard, Rest in peace."
More than a Vulcan
It was Nimoy's casting as Spock in 1966 that made him in a star and, in many ways, defined his acting career.
He played the character in all three of the original series of the programme and later in several big-screen spin offs.
Nimoy had an ambivalent relationship with Spock, seeming to both cherish and resent his close association with the role.
His two volumes of autobiography - "I Am Not Spock" in 1975 and "I Am Spock" two decades later - seemed to epitomise his mixed feelings.
Nimoy did have success outside of his Spock costume, in both acting and directing, and he pursued music, painting, and photography.
After the end of Star Trek's initial run, he played master of disguise Paris in the hit adventure series Mission Impossible.
Later he directed two of the Star Trek films, The Search for Spock and The Long Voyage Home, and in 1987 helmed the hit comedy Three Men and a Baby, one of the highest-grossing films of that year.
Nimoy announced his lung condition on Twitter last year, writing: "I quit smoking 30 years ago. Not soon enough. Grandpa says, quit now!!"
The same Twitter account was used by his granddaughter to confirm his death on Friday, in Bel-Air California.