Stars over Time

Stars over Time

A star is a self-luminous ball of gas bound by gravity into a single object and powered by nuclear fusion at the core. There are trillions and trillions of stars in our universe and all are different and unique. There are many stages of stars life including main sequence stars, red giants, white dwarfs, neutron stars, and black holes. All stars also have many more variations in each stage of life. The life of a star begins in a nebula, a great collection of gas and dust. Once enough mass has accumulated into a single object, gravity forces the mass to collapse into the center. Due to pressure and friction, the core gets so hot that it begins nuclear fusion and a protostar is made. The age and the mass of stars tell every thing about a stars physical properties and placement into each of the categories. The Hertzsprung - Russell diagram (HR Diagram) graphs stars luminosities over the stars spectral class. Luminosity describes how bright the star is (I, II, III, IV, V); spectral class describes its temperature (O, B, A, F, G, K, M). This graph is the best way to categorize stars.

1. Main Sequence Stars. Once the protostar has stopped the nuclear reactions, it begins to burn up its hydrogen core. This is when it becomes a Main Sequence Star. Main Sequence stars are split into two types: Upper Main Sequence and Lower Main Sequence. They both have luminosity class V. The only difference is how massive each star is. Our sun is a lower main sequence star. The hydrogen in an average star, like the sun, burns for about ten billion years. Upper Main Sequence stars are the hottest and brightest of all Main Sequence stars. They burn hydrogen by using the CNO Cycle, where carbon is fused with hydrogen to get nitrogen, and helium. Lower Main Sequence stars use the Proton-Proton Chain, where hydrogen is fused together to form helium. Both have three layers: a thermonuclear core, a radiative zone, and a convective zone. Upper Main Sequences stars are layered from the center core, to the convective zone, to the radiative zone. Lower Main Sequence stars have the convective and radiative zones flipped.

2. Red Giants. Once the hydrogen supply runs out, the core begins to collapse. During this time the core gets so hot, it begins to burn up the helium filled core into carbon. The helium supply depletes and the core begins to cool. The outer layers heat up and the star expands and a Red Giant is formed. This stage occurs in the last ten percent of a stars life. There are many types of Red Giants including: Supergiants, Giants, and Subgiants. Subgiants are stars that just began to run out of hydrogen and are expanding. The Giants are at the peak of expansion and are the biggest and brightest Lower Main Sequence stars will get. Our sun will become a Red Giant in about five billion years. The most massive of stars become Supergiants; they are the most luminous. While on the Main Sequence, these were the Upper Main stars. Early in the phase, Supergiants are red and enormous. After time, the Red Supergiant loses its expanded atmosphere and becomes smaller, hotter and blue.

3. White Dwarfs. When a Red Giant burns up all the helium, the core begins to collapse again. Electron degeneracy, where the object cannot collapse the atoms more than the electron shell, takes over and the core cools. The outer layers are sloughed off and a planetary nebula is formed. This period last around fifty-thousand years. The ring of gas and dust around the cooling carbon ball is neither a planet nor a nebula, but is about the size of a planet and has the same gas and dust components of a nebula. After the outer layers dissipate, nothing but a cooling ball of carbon is left. This small, hot, dim ball of carbon is called a White Dwarf. These dead stars are about the size of Earth. The sun will eventually become a white dwarf. Just imagine our sun will end up being about the size