Stars

We are going to talk about how stars live.

All solar systems began as a huge cloud of gas and dust. Then some force causes this cloud to begin to collapse in on itself. Gravity continues the process after the force that started it could no longer keep it going. The original spin of the cloud accelerates as it shrinks, and it flattens out into a disk. At the center, friction creates a lot of heat. Eventually there is so much heat energy that hydrogen atoms fuse into helium atoms. So much energy is released at this point that the dust in the disk is almost completely blown away, and a star is born. In our solar system, this was the Sun. In solar system with planets, planets may form from dust or debris left behind, or they might already have formed or partially formed. This happened in our solar system.

A Hertzsprung-Russell(HR) Diagram is a diagram used to classify stars based on surface temperature and luminosity. The hottest stars are on the left, and the brightest stars are at the top. Do you notice the big diagonal band of stars? That band is called the main sequence, since almost all stars are found on it. It is where stars who only fuse the element hydrogen are found.

Stars that are about average in size lead long lives, usually billions of years. Our sun is an average-sized star. After an average sized star uses up most of their hydrogen, temperatures in the core are not hot enough to fuse the leftover helium into carbon. The star’s own gravity will cause its layers to collapse in on the core. However, this will facilitate friction and cause the helium in the core to fuse into carbon. At this point, the energy from the fusion will cause the layers to explode outward. The outer layers will cool and become a red color. At this point, the star is a red giant. When the helium runs out, the outer layers will be lost from the core. At this point, all that remains of the star is the core, usually called a white dwarf. After a white dwarf is around for an extremely long time, it will cool down and become a black dwarf. After helium fusion is finished, large mass stars continue fusion, as they have enough mass to produce more heat, and therefore fuse more elements. They continue fusion until they reach iron. When iron is fused, it absorbs more energy than it releases. With nothing to stop it’s immense gravity, the star collapses. However, small particles produced by the core blow the incoming mass outward. The star explodes in a huge supernova, which can outshine the galaxy the star is in. All elements heavier than iron are fused in supernovas. After the supernova, only core remains. However, the core does not turn into a white dwarf. Its mass is too great. It’s collapse continues. Gravity forces protons and electrons together. Their charges cancel, and everything becomes a dense material called neutronium. The core is now a neutron star, only about 20 km wide. If the mass of the core is greater than three solar masses, the neutrons will be forced together. The core will shrink until its gravity is so great, not even light can escape.