Astronomy Terms: Stars
A star is a luminous plasma sphere, held together by its own gravity. It is formed when gaseous nebula, made mostly of hydrogen (also helium & trace amounts of other elements) collapses. When its stellar core is dense enough, it begins to fuse hydrogen together to make helium. The energy released travels out of the star and into space.
It doesn’t collapse further, because of its own internal pressure. A star 40% the size of our sun (or greater) will become a red giant when the hydrogen is used up. It expands, and as it does so, it throws part of its mass out into space - this will become new stars later. (Some stars fuse heavier elements at their core/shells around the core, as they become a red giant, and these heavy elements will be part of the mass it throws out.) In the end, the core becomes a stellar remnant (a white dwarf, neutron star, or black hole).
Most stars (including all those outside the Milky Way) are invisible to the naked eye. Astronomers can measure properties such as age, composition and mass, by observing its motion through space, luminosity, and frequency of radiation. The most important factor in a star’s life is its mass. Its diameter and temperature change over time; its rotation and movement are affected by its environment.