A galaxy is a huge mass of stars nebulae and interstellar dust and gases. Nebulae are clouds of dust and gas(mainly hydrogen and ionized hydrgen) that float in space. Often,the force of gravity pulls the particles of gas and dust cloesr into a swirling mass. Ifthe mass is heavy enough, the spinning cloud will to glow. this is a PROTOSTAR, or the first stage before a star is born.
Stare are self-luminous bodies of hot and glowing gas(hydrogen) born inside the nebulae itself. our Sun is a small star. Massive stars may have a mass of at least three times that od the Sun and some stares as massive as 50 times the Sun exist. stars produce energy by nuclear fusion at the core. after millions to billions of years, when the hydrogen starts running out, the core contracts while the outer portion continues expanding. The star is now called a red giand. the amount of mass a star has determines the rest of its life cycle. the cores of low-mass stars collapse expelling the outer layers which form a nebula. The core remains as a WHITE DWARF and eventually cools to become a black dwarf.
A massive star will undergo a SUPERNOVA explosion. If the remnant of the explosion is 1. 4-3 times as massive as the Sun, if will become a NEUTRON STAR. After the explosion, a star with a core more than three times the Sun, will be swallowed by its own gravity. If now becomes a BLOCK HOLE which can readily attract any matter or energy that comes near it.
