Chemically enriched material is returned to the interstellar medium by stellar mass loss or supernova explosions and then recycled into new stars. At the end of a star's lifetime, its core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.Īlmost all naturally occurring elements heavier than lithium are created by stellar nucleosynthesis in stars or their remnants. For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. The total mass of a star is the main factor that determines its evolution and eventual fate. The observable universe contains an estimated 10 22 to 10 24 stars, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.Ī star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The most prominent stars are grouped into constellations and asterisms, and many of the brightest stars have proper names.
Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky.
Lines have been added.Ī star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
The constellation of Leo as it can be seen by the naked eye.