“How did you become a star?” (joke)

A popular joke about an actor/performer is:
 
Q: How did you become a star?
A: I started out in a gaseous state, and then I cooled.
 
The joke has been credited to American television host and comedian Johnny Carson (1925-2005) since at least 1978, in an anecdote about a remark made ten years before (at a taping of The Tonight Show Starring Johnny Carson in New York City). New York-born actor and comedian Milton Berle (1908-2002) included the joke in Milton Berle’s Private Joke File (1989) and often receives credit for it as well.
 
     
Wikipedia: Johnny Carson
John William “Johnny” Carson (October 23, 1925 – January 23, 2005) was an American television host, comedian, writer, producer, actor, and musician, best known for his thirty years as host of The Tonight Show Starring Johnny Carson (1962–1992). Carson received six Emmy Awards, the Governor’s Award, and a 1985 Peabody Award. He was inducted into the Television Academy Hall of Fame in 1987. Johnny Carson was awarded the Presidential Medal of Freedom in 1992 and received a Kennedy Center Honor in 1993.
     
The New Yorker
Google Books
Profiles FEBRUARY 20, 1978 ISSUE
Fifteen Years of the Salto Mortale
BY KENNETH TYNAN
(...)
“Johnny Carson on TV,” one of his colleagues confided in me, “is the visible eighth of an iceberg called Johnny Carson.” The remark took me back to something Carson said of himself ten years ago, when, in the course of a question-and-answer session with viewers, he was asked, “What made you a star?” He replied. “I started out in a gaseous state. and then I cooled.
 
Google Books
The Big Room
By Guy Peellaert and Michael Herr
New York, NY: Summit Books
1986
Pg. 114:
JOHNNY CARSON
“Mr. Carson, how did you become a star?”
“I started out in a gaseous state, and then I cooled.”
 
Google Books
Milton Berle’s Private Joke File:
Over 10,000 of His Best Gags, Anecdotes, and One-liners
By Milton Berle
New York, NY: Crown Publishers
1989
Pg. ?:
A great actor was asked for the ten thousandth time, “How’d you become a star?” He answered, “I started out as a gaseous cloud. Then I cooled.”
 
Google Books
Novel Talking:
The Autotelic Otiose
By Michael H. Riley
Xlibris Corporation (Xlibris.com)
2000
Pg. 173:
Here is our prime example, palely reflected in the words of others (as is our author). Let us call him JC (and thank Kenneth Tynan, again):
 
How did he become a star?
“I started out in a gaseous state, and then I cooled.”
 
Google News Archive
28 March 2002, Milwaukee (WI) Journal Sentinel, pg. 13A, col. 1:
Milton Berle earned his nickname—The Thief of Bad Gags—through jokes such as these, from his book Milton Berle’s Private Joke File.”
(...)
. A great actor was asked for the ten thousandth time, “How’d you become a star?” He answered, “1 started out as a gaseous cloud. Then I cooled.”
   
Google Books
Here’s Johnny!:
My Memories of Johnny Carson, The Tonight Show, and 46 Years of Friendship
By Ed McMahon
Nashville, TN: Rutledge Hill Press
2005
Pg. 34:
The speed of his wit was never shown better than the time I heard a fan ask him, “What made you a star?”
 
Johnny’s answer could have been chiseled in stone at the Comedy Club: “I started out in a gaseous state and then I cooled.”
 
Google Groups: alt.fan.letterman
Transcript of Dave’s tribute to Johnny Carson
katycren
9/19/05
I think Bill requested this. (Please correct any spelling or punctuation errors.)
Tribute to Johnny Carson, given by David Letterman at the 2005 Emmy Awards:
 
Thank you. For those of you who are here for the Governor’s Ball only, it is starting now.
 
In the 1978 New Yorker profile of Johnny Carson, Kenneth Tynan describes an audience question and answer segment that Johnny was doing on the Tonight Show. An audience member asked, “What made you a star?” Johnny replied, “I started out in a gaseous state and then I cooled.”
 
Google Books
Night Train:
New Poems
By Phil Cousineau
San Francisco, CA: Sisyphus Press
2007
Pg. 176:
“Did’cha hear the one about the actor who heard
For the thousandth time how he’d become a star?
He says, ‘I started out as a gaseous cloud. Then I cooled!’”
 
Twitter
ian leslie
‏@mrianleslie
Johnny Carson, on being asked what made him a star: “I started out in a gaseous state, and then I cooled.”
6:35 AM - 28 Jan 2015