A star is a luminous ball of gas (mostly hydrogen and helium) held together by its own gravity. Nuclear fusion reactions at its core support the star against gravity, so it does not collapse, and it produces photons and heat, and small amounts of heavier elements. The Sun is the closest star to Earth.
Most stars in the Universe are invisible to the naked eye from Earth and how many can be seen depends on multiple factors; light pollution, atmospheric distortion and whether visual aids are used.
According to current theory, stars are born within gigantic gas clouds that collapse in on themselves. The clouds' material heats up as it falls inward under the force of its own gravity. Stars go through phases and depending on the temperatures at which they burn, they have different colours (hotter stars produce bluer light and cooler stars produce redder light).
When the hydrogen runs out a star cannot burn, so smaller stars become white dwarfs and larger stars explode leading to supernova or black holes. Most massive (and hottest) stars exhaust their energy supply within a few million years, whilst tiny cool red dwarfs can burn for billions of years.
Binary and multi star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits. Their gravitational interaction can have a significant effect on their evolution. Stars can also form part of a larger gravitationally bound structure, such as a star cluster or galaxy.
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