Stars over Time 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 proto star 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 proto star 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 of Earth. 4. Neutron Stars.

More massive stars get hot enough to break electron degeneracy and collapse the star even further. The core heats up and begins burning the carbon and other heavier elements. The core squeezes all the protons and electrons together until all that is left is a core packed tight with neutrons. Neutron degeneracy occurs and the core cannot collapse anymore. The collapse creates a huge explosion, called a supernovae explosion, of heat that blows the layers of the star off. The explosion causes the core, now a neutron star, to spin on its axis.

These spinning neutron stars are also called pulsars. Neutron stars are about the mass of our sun packed into the size of Folsom. The solid iron core is one of the densest objects in the universe. 5. Black Holes. The most massive stars in the universe become black holes.

Some massive stars begin to collapse and not even neutron degeneracy can hold the collapsing back. The object gets denser and denser until the escape velocity, the minimum velocity that an object must attain to escape a gravitational field, surpasses the speed of light. Nothing, not even light, can escape from a black hole. Super massive black holes are believed to be at the center of every galaxy. The Milky Way galaxy contains a one-hundred solar mass black hole at its center.

The space surrounding a black hole is called the event horizon. This region is known as the point of no return because once the horizon has been crossed, it is impossible to escape from the black hole. The center of the black hole consists of singularities. These regions are where the laws of physics do not apply. Altec h physicist Kip Thorne describes singularities as regions where gravity 'unglued's pace and time. No one really knows what is in the center of a black hole or what happens to everything it devours.

Some believe that some black holes contain wormholes at the mouth. These space-time tunnels could be shortcuts to different places of the universe or even passageways to different universes. We can never see the evolution of one star in our lifetime. A stars life lasts billions and billions of years.

By studying stars of the same mass but in different stages of life, scientists have categorized the different stars to create a model of all stars evolutionary phases. By knowing evolutionary patterns of stars, scientists can age galaxies and even approximate the age of our universe. They are also used for discovering out how our universe began. The more knowledge we get about stars, the closer we will be to knowing everything about our universe..