A star is a massive, luminous ball of plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star’s interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.
Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.
The motion of the Sun against the background stars and the horizon was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth’s rotational axis relative to the nearest star, the Sun.
The oldest accurately dated star chart appeared in Ancient Egypt. Islamic astronomers gave names to many stars which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars.
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of measurements in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. He is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.
The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. It was discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals.
Important conceptual work on the physical basis of stars occurred during the first decades of the twentieth century. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.
With the exception of supernovae, individual stars have primarily been observed in our local group of galaxies, and especially in the visible part of the Milky Way. In the local supercluster it is possible to see star clusters. The most distant stars resolved have been up to hundred million light years away. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away, ten times the distance of the most distant star cluster previously observed.