What is called a star ?

 What is called a star ?

A star ( sun )

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. the closest star to Earth is that the Sun. Many other stars are visible to the eye from Earth during the night, appearing as a mess of fixed luminous points within the sky thanks to their immense distance from Earth. Historically, the foremost prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and supply standardized stellar designations. The observable Universe contains an estimated 1×1024 stars,but most are invisible to the eye from Earth, including all stars outside our galaxy, the Milky Way .

For most of its active life, a star shines thanks to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior then radiates into space . most present elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, and for a few stars by supernova nucleosynthesis when it explodes. Near the top of its life, a star also can contain degenerate matter. Astronomers can determine the mass, age, metallicity (chemical composition), and lots of other properties of a star by observing its motion through space, its luminosity, and spectrum respectively. the entire mass of a star is that the main factor that determines its evolution and eventual fate. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of the many stars against their luminosities produces a plot referred to as a Hertzsprung–Russell diagram (H–R diagram). Plotting a specific star thereon diagram allows the age and evolutionary state of that star to be determined.

A star's life begins with the implosion of a diffuse nebula of fabric composed primarily of hydrogen, along side helium and trace amounts of heavier elements. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through fusion , releasing energy within the process. the rest of the star's interior carries energy faraway from the core through a mixture of radiative and convective heat transfer processes. The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant star when the hydrogen fuel in its core is exhausted.In some cases, it'll fuse heavier elements at the core or in shells round the core. because the star expands it throws a neighborhood of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars.Meanwhile, the core becomes a stellar remnant: a white dwarf star , a star , or, if it's sufficiently massive, a region .

Binary and multi-star systems contains two or more stars that are gravitationally bound and usually move around one another in stable orbits. When two such stars have a comparatively close orbit, their interaction can have a big impact on their evolution.Stars can form a part of a way larger gravitationally bound structure, like a star cluster or a galaxy.