A star is a big luminous ball of plasma held together by gravity.
Stars are made mostly of hydrogen and helium, along with small amounts of calcium, iron, and titanium oxide.
The amounts of these elements differ from star to star.
Astronomers can tell what elements are in a star, and how much of each, by studying the various wavelengths of a star’s electromagnetic radiation.
A star’s electromagnetic radiation can also tell us how old it is, or when it was formed.