Formation and Structure of Stars

Star formation happens because of a compromise between two of the most awesome forces of nature: Gravity and the Strong Nuclear Force. If gravity wins the star contracts. If, on the other hand, the enormous pressure generated from thermonuclear reactions wins the star expands. The birth, life, and death of stars are controlled by the interplay between  these two awesome forces.

For stars to form a medium is required. This material is the Interstellar Medium. In some cases the interstellar medium is easily visible as a cloud of dust and gas, and we called it a Nebula, such as the Lagoon, Orion, or Horsehead Nebulae.

 
Undersea corral?  Enchanted castles?  Space serpents?  These eerie, dark pillar-like structures are actually columns of cool interstellar hydrogen gas and dust that are also incubators for new stars. These "Gaseous Pillars" protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern.  They are part of the "Eagle Nebula"(also called M16 - the 16th object in Charles Messier's 18th century catalog of "fuzzy" objects that aren't comets), a nearby star-forming region 7,000 light-years away in the constellation Serpens.

Stars form from the cold dark cloud of gas and dust in the  interstellar space. A blast wave from an exploding star, or from some other disturbance going through the gas, causes  clumps of matter to form. Each clump gradually contracts as Gravity pulls it together in a process that can no longer be reversed. At the same time the core rotates. Near the center of the core the mass increases and the collapse accelerates, as the energy of the falling gas heats up the center of the core and a Protostar is formed. A large cloud  of cold gas still surrounds the newly formed star. Throughout the contraction, the protostar converts part of its energy into radiation and part into thermal energy that increases the temperature of the star. The contraction continues and with it the increase of temperature at the core of the star until, finally, it reaches an enormous temperature of 10 million K; this temperature is now large enough for Hydrogen atoms to overcome their Coulomb repulsion and fuse, creating a Thermo-Nuclear Reaction. The pressure in the interior of the star increases greatly and halts the collapse. The wind of gas coming from the star clears away its surrounding cocoon. Finally, the new star settles down into a period without much change, with the star converting Hydrogen into Helium generating an enormous amount of energy. 

The Proton-Proton Chain At a temperature of 10 million K the following three reactions, which form the proton-proton (or pp) chain, are responsible for energy generation in the star: Remember that the star was able to reach this enormous temperature because  of the convertion of gravitational energy into thermal energy.              4 protons (hydrogen nuclei)=6.693x10-27kg                   1 helium nucleus (2p+2n)=6.645x10-27kg                              difference in mass=0.048x10-27kg                    E=Mc2=(0.048x10-27kg)*(3x108 m/sec)2                                =(0.43x10-11 Joules)                  P=(0.43x10-11 Joules)*(1038/sec)  [Sun]                     =(0.43x10+21 MegaWatts)       In one second the Sun generates enough energy to satisfy the        present demands of Tallahassee for more than 10 billion years!!

All Four Fundamental Forces of Nature Operate in a Star!

Gravity enables stars to form from the interstellar medium, but once a cloud contracts to the produce a protostar, something has to stop the gravitational collapse so that a stable star can be produced. That stability is reached as the core of the protostar becomes hot enough to ignite thermonuclear fusion. We then see that nuclear reactions are as important to the life of  stars as gravity is to its formation.

Although bigger stars are more massive than low-mass stars - and one would expect them to live longer - the bigger the star the more quickly it forms, evolves, and dies. This is because nuclear reactions proceed quicker for a bigger star. Indeed, almost all properties of a star, such as its size, color, and ultimate fate, are  fixed by its mass. The most massive stars are blue. The temperature at their surface soars to about 40,000 degrees (almost seven times hotter than the Sun) and they are classified as O stars in the H-R diagram. They are 40 times more massive than the Sun and about 20 times bigger. They shine 500,000 times brighter than the Sun; they are The Blue Supergiants. Moving down through the stellar mass scale we come to the white stars of spectral type B and A and through the F stars until we reach the  yellow G stars, like our own Sun. Stars of lower mass are smaller and dimmer, as their surface temperature decreases and their emission spectrum shifts toward the red. Orange (K) stars have about 3/4 of the Sun's mass and size, while the coolest M stars are the are red color. They are typically 1/5 the Sun's mass and and they have a temperature at the surface of "only" 3,300 degrees. It would take about 100 such stars together to shine as brightly as our Sun.

Estimating the life-expectancy of a star, the time it spends on the main sequence, is not difficult. The more massive the star the more nuclear fuel available; that increases the lifetime of the star. However, the more massive the star the more hotter and bigger the star. These factors increase the luminosity of the star and reduce the lifetime of the star. Hence, lifetime=fuel /rate of consumption, or: 

                       Lifetime = M/L = M/M^3.5 = 1/M^2.5
 
 

Spectral Type	Mass	Luminosity	Years on M.S.
O5 B0        A0         F0 G0 K0 M0	40   15 3.5 1.7 1.1 0.8 0.5	405,000 13,000  80 6.4 1.4 0.46 0.08	1 million 11 million 440 million 3 billion 8 billion 17billion 56 billion

 
Let us follow all the stages in the life of our Sun. About five billion years ago our Sun formed from a cold interstellar cloud. After the gravitational contraction heated the hydrogen core to about 10 million degrees nuclear reactions started and halt the gravitational collapse; we now know that the interior temperature of the Sun is about 15 million K. After this tumultuous process our Sun settled down into a long stable period as a yellow (G2) main-sequence star. In about 5 billion years the Sun is going to run out of hydrogen in its central core. The temperature of the Sun, well below 100 million K, is too low to start Helium burning and a series of very dramatic changes ensue -  marking the beginning of the end.

The Orion Nebula: an Emission Nebula (HII region)

This giant nebula is only 1,500 ly away from us, and 2.5 ly wide. Even though the density is low, it has enough mass to produce thousands of stars! Some of the stars are as young as 8 million years! One star is 40 times the mass of the Sun, which will go into supernova very soon. Hubble Space telescope has taken many pictures of Orion nebula and the details tells us the story of star formation.
 
Located 1,500 light-years away, along our spiral arm of the Milky Way, the Orion nebula is located in the middle of the sword region of the constellation Orion the Hunter, which dominates the early winter evening sky, at northern latitudes. In addition to the cluster of young stars (Trapezium) this stellar cavern contains 700 hundred other young stars at various stages of formation. The stars have formed from collapsing clouds of interstellar gas within the last million years. The most massive clouds have formed the brightest stars near the center and these are so hot that they illuminate the gas left behind after the period of star formation was complete. The more numerous faint stars are in the process of collapsing under their own gravity, but have become hot enough in their centers to be self luminous bodies.
 
 

The Pleiades: an Open Cluster (Reflection Nebula)

The Pleiades - an open cluster of stars - are blue-colored which indicates that they are hot spectral B-type. The color also indicates that the Pleiades are "young" stars - less than 100 million years old; otherwise they would already consumed themselves!

The Globular Cluster M80
 

M80- A globular cluster
 
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.