A star is like a huge, luminous ball of plasma that spreads energy and light due to thermonuclear fusion in its core. The closest start to our planet is the Sun, because of what other stars are only visible during the night when they are not out lighted by the Sun.
Astronomers have been fascinated by stars for ages and nowadays can even measure the mass, age, chemical composition besides other descriptive facts by observing the spectrum, luminosity and motion of the star through space. The mass is the main characteristics according to what scientists determine how the star was born and what will happen to it in the future.
Before the real star is born it is only a collapsing cloud that is made out of hydrogen with helium and small parts of heavier elements. When the stellar core of the star is dense enough some of the hydrogen starts to convert into helium through nuclear fusion. More energy is carried away from the core by radiative and convective processes. The internal pressure of those processes help to prevent the star from collapsing under the strength of its own gravity.
Red giants are stars which have exhausted to hydrogen fuel in the core and have at least the 0.4 of the mass of our Sun. Then the star is transformed to a degenerated form, some portions of the matter are altered into the interstellar environment where it forms new generation of stars, but now with a bit higher portion of heavy metals.
