What are stars and how they are formed?

 Stars are huge celestial bodies made mostly of hydrogen and helium that produce light and heat from the churning nuclear forces inside their core. They are the building blocks of galaxies, of which there are billions in the universe. It's impossible to know how many stars exists, but astronomers estimate that in our Milky way galaxy alone, there are about 300 Billion stars.

The life cycle of a star spans billions of years. As a general rule, the more massive the star, the shorter it's life span. Birth takes place inside hydrogen based dust clouds called Nebulae. Over the course of thousands of years, gravity causes pockets of dense matter inside the Nebulae to collapse under their own weight.

One of these contracting masses of gas, known as a protostar, represents a star's nascent phase because the dust in the Nebulae obscures them, protostars can be difficult for astronomers to detect.

As a protostar gets smaller, it spins faster because of the conversation of angular momentum - the same principle that causes a spinning ice skater to accelerate when she pulls in her arms. Millions of years later, when the core temperature climbs to about 27 million degrees Fahrenheit (15 Million degree Celsius ), nuclear fusion begins, igniting the core and setting off the next - and longest - stage of a star's life, known as it's main sequence.

Most of the stars in our Galaxy, including the Sun, are categorized as a main sequence stars. They exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating X-RAYS. This process emits an enormous amount of energy, keeping the star hot and shinning brightly. Every star that you see in the sky might be sun to some other solar system .