In this section, we will learn how the stars, like our Sun, were formed from the death of the first generation of our stars. We also learn about various stages in the life of stars. We discuss the following:
- Main Sequence Stars
- Red Giants
- White Dwarf
- Black Hole
- Neutron Stars
Section Preview: 3D schematic of a Nebula (star-forming region)
When the temperature in the cloud's core reached 15,000,000 degrees, nuclear fusion occurs. The cloud starts to glow brightly, contracted slightly and stabilized. It had now became a main-sequence star and will continue to shine for millions to billions of years. Nuclear fusion converted hydrogen in the core of main sequence star into helium as it glows.
When the star's hydrogen supply runs out and can no longer generate heat through nuclear fusion, the core becomes unstable and contracts. The star's outer shell primarily made up of hydrogen, expands. It cools and glows red as it grows. The star has progressed to the stage of being a red giant. It's red because the outer shell has expanded outward, and it's a giant because it's cooler than it was in the main sequence star stage. Helium fuses into carbon in the red giant's core.
The core of low-mass stars collapse after the helium has fused into carbon and the outer layers of the stars are expelled . These outer layers combined to form a planetary nebula. Newly formed stars are almost entirely made up of hydrogen and helium, but as they progressed, they produce heavier elements through nuclear fusion, which are eventually expelled by powerful stellar winds. These powerful winds recycle elements from planetary nebulae back into the interstellar medium.
When stars 5 times the mass of our Sun or more reach the red giant phase, their core temperature rises as carbon atoms are formed from the fusion of helium atoms. As the temperature rises, fusion processes continues and gravity pulls carbon atoms, forming oxygen, nitrogen, and eventually iron.
Fusion in the core stops when the core is almost entirely made up of iron. Because iron is the most compact and stable of all the elements, breaking up the iron nucleus takes more energy than breaking up the nucleus of any other element. Therefore, creating heavier elements by fusing iron necessitates an energy input rather than a release of energy. Because energy is no longer radiated from the core, the star enters the final phase of gravitational collapse in less than a second. The core temperature rises to over 100 billion degrees as the iron atoms collide. The repulsive force between the nuclei overcomes gravity, and the core recoils in a shock wave, resulting in a supernova explosion.
When a massive star runs out of fuel and collapses, neutron star is formed. The core, the most central part of the star, collapses, crushing every proton and electron into a neutron.
If the core of collapsing star has a mass between 1 to 3 times that of Sun, the newly created neutrons can stop the collapse and leave a neutron star behind. These are the most dense object known. One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth – about as much as a mountain.