Stars (full version)

Stars

The stars are one the most curious objects in space, being all over the universe (there are around 300 billion stars only in the Milky Way); they are formed by clouds of matter called nebulas, which are mostly made of hydrogen and helium. Nebulas are known to be the star’s birthplace, because inside them the matter is condensed, starting the star’s cycle. The Solar nebula was the responsible for the formation of the solar system, 4.6 billion years ago. With the evolution of the star formation process, an object known as protostar is formed. The protostar is a young, cold and small star. As more matter is added to the protostar via accretion disk (wrap of matter orbiting a central body), its mass and temperature are increased, until the process of nuclear fusion starts, setting the end of the matter addition (because of the production of strong star winds) to the so formed star.

The process of nuclear fusion is responsible for the Sun’s radiation. On it, two atom’s nuclei bound together resulting in the releasement of radiation (in form of photons) and the formation of a heavier element. That process occurs in the star's core. It is curious the fact that some stars (like our Sun), do not have the temperature required to fuse hydrogen into helium; so how do they still accomplish it? Without the heat needed, quantum tunneling (phenomena described by quantum mechanics in which particles can go through potential barriers even without the energy required) is the responsible for nuclear fusion.

The hydrostatic balance is one of the keys in order to understand the behavior of the stars. When a star starts nuclear fusion, a balance between gravity (what induces de star to contract itself) and the pression generated by nuclear fusion (which prevents the collapse) is created. After billions of years (in general) the star won't have more elements to fuse, which means the balance will be interrupted, resulting in a collapse.

The fate of each star depends near exclusively on one factor: its mass. According to its mass, a star can become a white dwarf, a neutron star or even a black hole. When a star mass goes beyond the limit of Chandrasekhar (maximum mass a white dwarf can assume, which is about 1.44 times the mass of the Sun) the degeneracy pression of the electrons is just not enough to contain the collapse, and so that star will become one of the most extreme objects known, a neutron star/magnetar/pulsar, quarks star or a black hole. Stars more massive than the Sun go through a common process when they are close to "death", spreading a huge amount of matter into space in an explosion called supernova.

When the Sun reaches its final steps (approximately 5 billion years to come), it won't turn into a Supernova (because it doesn’t have enough mass); it will fuse hydrogen on outer layers, causing a big increase on size which will “eat” Mercury, Venus and possibly Earth. After the “swelling”, the star is named red giant.

Once all the hydrogen available in the outer layers is fused, the star suffers a collapse (due gravity) which induces the formation of a planetary nebula (one of the most beautiful objects to be seen in space).

The star’s core, now called white dwarf, is what is left of the formal star, being extremely dense.

Reference link: https://www.if.ufrgs.br/oei/stars/formation/form_st.htm