A Star's Life Story
Stars are not eternal. Like living things, they are born, they live for a long time, and then they die. The life story of a star is determined by one key factor: its mass. More massive stars live fast and die young, while less massive stars live long and quiet lives.
The Birth of a Star
All stars are born in a nebula, which is a giant, cold cloud of gas and dust floating in space.
1.Gravitational Collapse: A part of the nebula begins to clump together and contract under the force of its own gravity.
2.Protostar: As the clump gets smaller and denser, it starts to heat up and glow. At this stage, it is called a protostar.
3.Ignition: The protostar continues to pull in more gas and dust, and its core gets hotter and hotter. When the core reaches a temperature of about 15 million degrees Celsius, a process called nuclear fusion begins. This is the moment a star is truly born.
Nuclear fusion is the process where smaller atomic nuclei are fused together to form a larger nucleus, releasing a tremendous amount of energy. For most of a star's life, it is fusing hydrogen atoms into helium atoms. The outward pressure from this fusion energy perfectly balances the inward pull of gravity, making the star stable. This stable, middle-aged phase is called the main sequence. Our Sun is a main-sequence star.
The Life and Death of a Sun-like Star (Low-Mass Star)
Stars like our Sun have enough fuel to stay on the main sequence for about 10 billion years.
1.Red Giant: When the star runs out of hydrogen fuel in its core, it begins to die. The core shrinks, and the outer layers of the star expand, cool, and glow red. The star becomes a red giant.
2.Planetary Nebula: The outer layers of the red giant drift away into space, creating a beautiful, glowing shell of gas called a planetary nebula.
3.White Dwarf: The hot, dense core of the star is left behind. This remnant is called a white dwarf. It is very small—about the size of Earth—but very heavy. A white dwarf no longer produces energy and will simply cool and fade over billions of years.
The Life and Death of a Massive Star
Stars that are more than about 8 times the mass of our Sun have a much more dramatic and violent end.
1.Red Supergiant: Like a low-mass star, a massive star also becomes a giant when it runs out of core hydrogen, but it swells to become a much larger red supergiant.
2.Supernova: The core of a massive star is so hot and dense that it can fuse heavier and heavier elements, all the way up to iron. When the core becomes iron, fusion stops. Gravity instantly wins, and the core collapses in a fraction of a second. The outer layers of the star come crashing down and then rebound off the core in a gigantic explosion called a supernova.
3.The Remnant: What's left after the supernova depends on the star's initial mass.
Neutron Star: The collapsed core of a less-massive star will form an incredibly dense object called a neutron star. They are so dense that a teaspoon of neutron star material would weigh more than Mount Everest.
Black Hole: If the original star was extremely massive, the core will collapse completely, forming a black hole—an object with gravity so strong that nothing, not even light, can escape.