He suffered a severe stroke last month and never recovered, said Dr. Geoffrey Burbidge, an astrophysicist at the University of California at San Diego who had collaborated with Dr. Hoyle on many research projects.
''Fred was probably the most creative and original person in astrophysics after World War II,'' Dr. Burbidge said.
Dr. Virginia Trimble, an astrophysicist at the University of California at Irvine, said that Dr. Hoyle's opposition to the Big Bang, while considered a mistake, ''was significant in that it went a long way toward making cosmology a true science'' in which competing theories were tested by observations.
A versatile scientist brimming with ideas and a lifelong rebel eager for intellectual combat, Dr. Hoyle was most widely known as an author of the cosmological theory, which now has few adherents, that the universe exists in a steady state. The theory, published in 1948, contends that matter is constantly being created, so the expanding universe remains roughly the same at all times and has no beginning or end.
In a series of popular radio talks in Britain in the 1940's, he coined ''big bang'' to ridicule the rival concept of an explosive origin of the universe, but the term is now widely used and the explosion theory is generally accepted. In recent years Sir Fred joined those arguing for a universe that -- while eternal -- expands and contracts.
The astronomer was instrumental in establishing the Institute of Theoretical Astronomy at the University of Cambridge in England and became its first director. From 1958 to 1972 he was also Plumian professor of astronomy at the university, a post previously held by such leading scientists as Sir Arthur Eddington, whose groundbreaking experiments confirmed the general theory of relativity.
A historic development in astrophysics was explaining how the elements came to be synthesized step by step in the stars, starting from hydrogen and helium. In the 1930's, Dr. Hans Bethe and others showed how stars could derive their energy from the fusion of hydrogen nuclei (protons) to form helium.
The problem Dr. Hoyle and colleagues faced was how slightly larger elements like carbon, nitrogen and oxygen were formed by stars. An element called beryllium-8, which was an intermediate stage in the process of element formation, stood in the way. It did not survive long enough for the fusion process to reach carbon-12, the next stage in the element building process.
Dr. Hoyle solved the problem: he pressed nuclear physicists to look for a special state of carbon-12, that was stable enough for the fusion of heavier elements to occur.
Then, working with three other scientists, Dr. Hoyle figured out how all the heavier elements could have been formed. Their historic paper was published in 1957 in Reviews of Modern Physics. In addition to Dr. Hoyle, the scientists were Dr. William A. Fowler, ofCalifornia Institute of Technology; Dr. Burbidge and Dr. Margaret Burbidge, his wife.
While the formation of the lighter elements, up to iron, could be explained by processes inside stars, extremely high temperatures and violent events were needed. The answer proposed by the four was the supernova, in which a giant star collapses to extreme density, then cataclysmically rebounds.
For this and his subsequent work in astrophysics, Dr. Fowler was awarded the Nobel Prize in Physics. The other three were omitted, probably in part because the prize is rarely, if ever, awarded to more than two people.
Fred Hoyle's noncomformity manifested itself at an early age. Born in Bingley, Yorkshire, he found school boring, preferring to remain at home studying a textbook in elementary chemistry and doing chemistry experiments with equipment he found in his home. As recounted in his 1994 autobiography, ''Home Is Where the Wind Blows,'' in his parents' absence he enjoyed making gunpowder and creating explosions.
As school was compulsory, his absence led to difficulties with the local authorities. The family did not have the funds to send him to a private school, but he finally won scholarships, including one from the West Riding of Yorkshire, and started on the path that eventually led him to Cambridge.
As soon as he reached Cambridge he came under the tutelage of such top physicists as Rudolph Peierls, Eddington, P. A. M. Dirac and R. H. Fowler, whose calculations set the stage for the concept of black holes, stars whose collapse has yielded such density that the gravity prevents even light from escaping.
During World War II he led a radar development group at an Admiralty Signal Establishment center in West Sussex, near the south coast. Working under him were two refugees from Vienna: Thomas Gold and Hermann Bondi. During the day the trio worked on radar. At night they discussed astrophysics, developing the steady-state cosmology. They accepted the evidence for its constant expansion, but proposed that matter is constantly formed to fill the gaps. In 1995 Dr. Bondi and Dr. Gold credited Dr. Hoyle with first proposing such continuous creation of new matter.
Dr. John Faulkner, of the Lick Observatory in California, said that during the ''magical six years'' after establishment of the Institute of Theoretical Astronomy at Cambridge in 1966, it became ''an obligatory mecca'' for young American astronomers, many of whom felt the institute ''fostered their best work.''
But by his own account Sir Fred never shrank from controversy, and in 1972, after a falling out with Cambridge officialdom and rancorous debate on the future of British astronomy, he resigned as director of the institute.
Sir Fred's work on interstellar organic molecules led him to propose that life originated in space. Working with a student, Chandra Wickramasinghe, he championed the unorthodox theory that the seeds of life, including disease viruses, periodically fall from space.
They attributed the onset of various epidemics to such viruses, attempting to document this in the simultaneous appearance of influenza at schools in remote parts of England and Wales.
These theories are reflected in titles of the books he did with Dr. Wickramasinghe: ''Lifecloud'' (1958), ''Diseases from Space'' (1979), ''Space Travelers: The Origins of Life'' (1980) and ''Cosmic Life Force'' (1988). His more conventional writing produced ''Frontiers of Astronomy,'' a widely used text.
Dr. Hoyle, along with Dr. Geoffrey Burbidge and Dr. Jayant V. Narliker, renewed their fight against Big Bang orthodoxy with the book ''A Different Approach to Cosmology'' (Cambridge University Press, 2000).
Dr. Hoyle was also a prolific author of science fiction, producing almost one book a year between 1950 and 1990, some written with his son, Geoffrey. Among the best known were ''The Black Cloud'' (1957) and ''Ossian's Ride'' (1958).
In 1957 he was elected a fellow of the Royal Society and in the early 1970's he was president of the Royal Astronomical Society. He was knighted in 1972.
In recent years he lived in Bournemouth. In addition to his son, also of Bournemouth, he is survived by his wife, the former Barbara Clark, whom he married in 1939, and a daughter Elizabeth Butler, a London stockbroker.
''Home for Hoyle,'' Stephen G. Brush, a science historian, wrote in 1995, ''is not a cozy cottage with an overstuffed chair in front of the fireplace. Not for him the comforts of academic tenure and the polite respect of colleagues. He is at home on the tops of mountains, at the cusps of controversies, where the winds blow fiercely and even God is not omnipotent but, as Hoyle says, just 'doing his best' to make an adequate universe.''
By WALTER SULLIVAN, August 22, 2001 © The New York Times Company