All stars are formed from a cloud of gas and dust, as was our Sun. The size of the cloud, which determines the further development of the star, can vary.
When a cloud of gas begins to contract under the influence of its own gravity, it becomes increasingly hotter until it reaches a stage of compression where it begins to emit a dark red glow. At this stage it is called a protostar. The temperature and the luminosity depend on the mass of the star. The T-Tauri stars are thought to belong to this category.
Nuclear reactions, which release enormous amounts of energy, begin when a temperature of about 10 million degrees Celsius is reached. Hydrogen fuses to helium in the center of the star. It has now become a main-sequence_star, with a long life span.
A star the size of our Sun remains in the main-sequence phase for several billions of years. For a star with significantly more mass, this stage of its life lasts "only" a few million years. Then its reserve of hydrogen is used up. Now its interior begins to change. If it is a high-mass star, it becomes a red supergiant and later a supernova, then a tiny neutron_star or a black hole.
In a star of less mass, the helium core inside it continues to contract and further nuclear reactions take place in the stellar envelope surrounding it. The outer layers cool slowly and expand until the star becomes a red giant. The next stages of its development are white dwarf and later black dwarf.
There are two types of brightness associated with a star. The "apparent" brightness observable to us on the Earth, and the "absolute", which is the brightness measured at a standard distance of 32.6 light-years.
