Nature Description:
A star is massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the cource of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. Historically, the most prominent stars on the celestial sphere were grouped together into constellations, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.
Naturally, stars have been important to civilizations throughout the world. They have been part of religious practices and for celestial navigation and orientation. 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. Stars are fpr,ed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23-28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. The final stage is reached when the star begins producing iron. The rotation rate of stars can be approximated through spectroscopic measurement. The portion of a star that is visible to an observer is called the photosphere. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sunspots, or regions of lower than average temperature, appear. Beyond this is the corona, a volume of superheated plasma that can extend outward to several million kilometres. The corona region of the Sun is normally only visible during a solar eclipse. From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.
