Many people's work was needed to prove that the Sun is a star. The
first person we know of to suggest that the Sun is a star up close
(or, conversely, that stars are Suns far away) was Anaxagoras, around
450 BC. It was again suggested by Aristarchus of Samos, but this idea did not catch on. About 1800 years later, around AD
1590, Giordano Bruno suggested the same thing, and was burnt at the
stake for it. Through the work of Galileo, Kepler, and Copernicus
during the 16th and 17th centuries the nature of the solar system and
the Sun's place in it became clear, and finally in the 19th century
the distances to stars and other things about them could be measured
by various people. Only then was it proved that the Sun is a star.
For most of human history, almost all people have thought that the
Earth was in the center of a giant sphere (or ball, called the
"celestial sphere") with the stars stuck to the inside of the sphere.
The planets, Sun, and Moon were thought to move between the sphere of
stars and the Earth, and to be different from both the Earth and the
Anaxagoras, who lived in Athens, Greece, around 450 BC (about 2450
years ago), thought that the Sun and stars were fiery stones, that the
stars were too far away for their heat to be felt, and that the Sun
was perhaps more than a few hundred miles in size. With that
Anaxagoras was, as far as we know, the first one to suggest that the
Sun is a star. His ideas were met with disapproval and he was finally
imprisoned for impiety, because his ideas did not fit the prejudices
of the time.
Aristarchus of Samos (Samos is a Greek island in the Aegean Sea) lived
from about 310 to 230 BC, about 2250 years ago. He measured the size
and distance of the Sun and, though his observations were inaccurate,
found that the Sun is much larger than the Earth. Aristarchus then
suggested that the small Earth orbits around the big Sun rather than
the other way around, and he also suspected that stars were nothing
but distant suns, but his ideas were rejected and later forgotten, and
he, too, was threatened for suggesting such things. Aristarchus and
Anaxagoras had no way of actually measuring the sizes of or distances
to stars (except the Sun), so they had no proof for their ideas.
Claudius Ptolemaeus (commonly called Ptolemy by speakers of English)
of Alexandria (a Greek city in what is now Egypt) around AD 140 (about
1860 years ago) described a geocentric (= earth-centered) model of the
universe, with the Earth in the center of the Universe, the Sun as one
of the wanderers ("planetes" in Greek) that move relative to the
stars, and the stars fixed to the outermost celestial sphere. In this
model, the stars and the Sun were completely different. The universe
described in this book (which book came to be known as the Almagest)
was accepted as the truth by practically everybody for the next 14
centuries, mostly because it was endorsed by the Roman Catholic
Church, which became very powerful during that time. This model
described fairly accurately how planets move, but not why they moved
in just that way, and it lumped the Sun together with the planets
rather than with the stars.
Mikolaj Kopernik (known as Nicholas Copernicus outside of his native
Poland) lived from 1473 to 1543. In 1543, just before he died, he
published a book called "De revolutionibus orbium celestium" in which
he proposed a heliocentric (= sun-centered) solar system with the Sun
in the center and the Earth merely one of the planets orbiting the
Sun, just like the other ones. This model was simpler than Ptolemy's
geocentric model, though either one could be used to predict planetary
motion. The model of Copernicus set the Sun apart from the planets,
but did not say anything about the stars. Copernicus waited as long
as possible before publishing this book because he was afraid the
Church would not approve of it. At first, most opposition to his
ideas actually came from Protestants, not Catholics. Martin Luther,
one of the main early figures in protestantism, declared loudly that
Copernicus was a fool for "setting the Earth in motion".
Giordano Bruno, an Italian philosopher, lived from 1548 to 1600. He
decided that if the Earth is a planet just like the others, then it
does not make sense to divide the Universe into a sphere of fixed
stars and a solar system. He said that the Sun is a star, that the
Universe is infinitely large, and that there are many worlds. He was
condemned by both the Roman Catholic and Reformed Churches for this as
well as other things and was burnt alive in Rome in 1600 for heresy
(claiming something that does not fit the ideas accepted by the
Galileo Galilei, an Italian scientist, lived from 1564 to 1642. In
1610, he was the first person we know of to use the newly invented
telescope to look at the stars and planets. He discovered the
satellites of Jupiter, which showed that Ptolemy's and the Church's
idea that there was only one center of orbits in the Universe (namely,
the Earth) was incorrect. Based on his observations, Galilei argued
for the heliocentric model of Copernicus. He noticed that stars look
like little points even when seen through a telescope, and concluded
that stars must be very far away indeed.
In part because Bruno (a convicted heretic) supported them, the ideas
of Copernicus were condemned by the Catholic Church in 1616, and
Galileo was tried and convicted of heresy in 1633. He was forced to
publicly deny the ideas of Copernicus, and was held under house arrest
until he died in 1642. In 1979 a reinvestigation of this conviction
was started by the Church and finally the conviction was overturned,
about 340 years after Galileo's death. A famous story, but perhaps
untrue, has Galileo mutter (of the Earth) "And yet she moves!" on his
death-bed. Yet, Galileo, like Bruno and Aristarchus before him, had
no proof that the Sun and stars are alike.
Johannes Kepler of Germany lived from 1571 to 1630. He studied the
positions of planets very carefully and from that determined three
Laws of planetary motion that firmly put the Sun in the center of the
solar system with the planets orbiting the Sun. It was now clear that
the Sun is not a planet, though why these laws of planetary motion
should be the way the are was still unclear.
Christiaan Huygens of Holland lived from 1629 to 1695. He determined
the distance to the star Sirius, assuming that that star was as bright
as the Sun and appeared faint only because of its great distance. He
found that the distance to Sirius must be very great. At this time,
then, the idea that the Sun is a star was considered seriously by
Isaac Newton, an English scientist, lived from 1642 to 1727. In 1665
he realized that it was gravity that held the solar system together.
Another famous story, probably untrue, has this thought pop into
Newton's head when an apple falls on his head while he sits under an
apple tree, watching the Moon. Newton then determined the formula
that describes how gravity works and showed that this explains the
orbits and motion of the planets around the Sun and of moons around
planets, and therefore also Kepler's three Laws of planetary motion.
The motion of the planets and moons were now explained by a single
formula: Newton's Law of Gravity. People speculated that this same
law might be valid all through the universe.
Finally, in 1838, Friedrich Bessel for the first time measured the
distance to a star without any assumptions about the nature of stars
and found it to be enormous. Distances to other stars followed soon,
and then people could calculate the true brightnesses of stars,
corrected for their distance to us, and discovered them to be about as
bright as the Sun. When other things about the Sun were also found to
be like those of stars, such as its surface temperature and chemical
composition, then the proof was finally here that the Sun is a star.
The Sun is now classified as a G2V star: a main-sequence dwarf star of
You'll notice that during most of the history described above, people
have been persecuted for suggesting things that did not fit the
prejudices of the time, even when (or perhaps because) they presented
proof that those prejudices were incorrect. Things are better
nowadays: you may still be laughed or shouted at for suggesting ideas
that are different from current beliefs, but you will no longer be
burnt at the stake for them.
Encyclopaedia Brittannica: articles on "Cosmos" and on the various
people mentioned above.
Guinness Book of Astronomical Records: sections on "the Sun" and
"history of Astronomy".
The Cambridge Encyclopedia of Astronomy: section on "The history of
Cosmos: by Carl Sagan. This is an execellent video series and book
about astronomy, with lots of background stories about the important
people in the history of astronomy (especially in Chapter 7: "The
Backbone of Night").
In Search of the Big Bang: by John Gribbin. This book is mostly about
the history of ideas about the cosmos outside the solar system, which
have changed considerably during the course of history: Even 100 years
ago people did not know that there were other galaxies outside our