The Stars Reading Answers

A star is an astronomical object consisting of a glowing spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but due to their gigantic distance from Earth they appear as fixed points of light in the sky. The most prominent stars are grouped into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star collections that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.

A star's life begins with the gravitational breakdown of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. The total mass of a star is the main factor that determines its evolution and eventual fate. For most of its active life, a star shines due to thermonuclear synthesis of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. At the conclusion of a star's lifetime, its centre becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole. Almost all naturally occurring elements heavier than lithium are created by stellar nucleosynthesis in stars or their remnants. Chemically enriched material is returned to the interstellar medium by stellar mass loss or supernova explosions and then recycled into new stars. Astronomers can determine stellar properties including mass, age, metallicity (chemical composition), variability, distance, and motion through space by carrying out observations of a star's apparent brightness, spectrum, and changes in its position on the sky over time.

Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars. When two such stars have a relatively close orbit, their gravitational interface can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. During their helium-burning phase, a star of more than nine solar masses expands to form first a blue and then a red supergiant. Particularly massive stars may evolve to a Wolf -Rayet star, characterised by spectra dominated by emission lines of elements heftier than hydrogen, which have reached the surface due to strong convection and intense mass loss, or from stripping of the outer layers.

When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rise enough to fuse carbon. This process continues, with the successive stages being fuelled by neon, oxygen, and silicon. Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive Star. Each shell fuses a different element, with the farthest shell fusing hydrogen, the next shell fusing helium, 50d so forth. The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly sound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.