An Introduction to Stellar Physics

Introduction

No, this is not going to be a college course(!), but this article will present a good overview of the process that causes stars to be born, live and die. This material is generally considered to be out of reach of many amateur astronomers. Many get to the point of understanding that there is a nuclear reaction in stars, but many do not learn much past that. We shall make the attempt here to provide a bit more information than most amateur articles on the subject, and some mathematics may also be presented. Don't worry! It's going to be fun!

The Beginnings

Stars are born from gas, Hydrogen to be specific, floating around in large cool clouds in space as in this image of M16. A lot of Hydrogen is required to create a star. A typical star forming cloud of Hydrogen has a temperature of about 100 degrees Kelvin, a density of 100 atoms per cubic centimeter, and a mass of 2x10⁴ that of our Sun. The star's formation is also dependent upon the relationship between the mass of Hydrogen and the heat energy in the cloud. Gravitational attraction between the gas molecules must win out over the heat energy keeping them apart. At 100 degrees Kelvin, a mass of 2x10⁴ Suns is required to create the instability in the system and allow gravity to bring the atoms closer together. This is the beginning of a star's formation. For those with an interest in the math, there is a formula which describes the mass requirements for a star's formation, called the Jeans Criterion:

M >= 3.7 (kT/Gm)^(3/2)*p^(-1/2)

M = Mass of the gas cloud
k = Boltzmann Constant
T = Temperature
G = Gravitational Constant
m = Mean molecular weight
p = Density

Now, all of this is variable, of course! Just when you think you can nail down the formation of a star with a formula, there are other possibilities. A star can form from a much smaller region of gas if a shock wave from a nearby supernova passes by causing a compression of the atoms. A huge region of gas, such as the Orion Nebula, can be a birthplace to many stars as each portion of the nebula breaks off into compact pockets of gaseous hydrogen.

Ignition!

As the young star shrinks and gets hotter due to converted potential energy, it gets closer to igniting its first nuclear reaction. Before it does this, it is hot enough to be visible and continues to shrink. For those who know the HR Diagram, the star is headed down towards the Main sequence, losing a lot of luminosity, and a gaining little surface temperature. Just as radiative equilibrium is reached (when the star is creating as much thermal energy as it is radiating), the star maintains its luminosity but quickly increases surface temperature as it begins its first nuclear reactions. At this point it becomes a Main Sequence star. The star's interior is now between 1-2x10⁷ degrees Kelvin, and the Hydrogen nuclei are in a chain reaction producing Helium. This enormous amount of energy stops any further gravitational contraction in the star and becomes the star's main lifestyle for much of its future, about 10 billion years in the case of our Sun.

Aging

Even stars age (sad, but true)! It should be noted that since not all stars are born with the same mass, not all stars live their lives in a similar matter. The more massive the star, the more light output it generates, and the more hydrogen burning there must be to counteract the gravitational potential for collapse. Massive stars therefore live their lives by burning their nuclear fuel faster and die out earlier. Very massive stars can actually live and die in a matter of 10 million years, a very short time when compared to our Sun's expected life span. On the other end of the scale, very low mass stars may never evolve far enough towards the Main Sequence to cause the nuclear reactions to begin. Thus they remain brown dwarfs, shining only due to their spent gravitational potential energy.

Now we know that the key to understanding a star's life is its mass. So, let us take it one step at a time and analyze just what happens to stars of different masses as they live out their lives:

Stars of less than 0.4 Solar Masses:

A protostar is formed.
It becomes a Main Sequence M Type star.
Then as it runs out of fuel, it becomes a white dwarf fighting off gravitational collapse by the electron degeneracy of Helium.

Stars of 0.4 to 3.0 Solar Masses:

A protostar is formed.
It becomes a Main Sequence star.
It enter the Hydrogen shell red giant phase.
A bright Helium flash occurs.
The star enters the Helium core burning phase as a red giant.
It then slowly fades into the small star at the center of a planetary nebula having lost its outer shell.
If the central core is less than 1.4 Solar masses, then the star becomes a white dwarf with a Carbon/Oxygen electron degenerate core.
If the central core is greater than 1.4 Solar masses, the star undergoes Carbon detonation, blows apart into a supernova, or blows itself into a neutron star. (More about this later...)

Stars of 3.0 Solar Masses and Greater:

A protostar forms.
The star enters the Main Sequence.
It becomes a Hydrogen shell burning red giant.
It becomes a Helium core burning red giant.
If the core is between 1.4 and 9 solar masses, Carbon detonation occurs with several possible outcomes:
- The star blows itself apart as a supernova with no surviving core.
- If the core survives the nova phase, then Carbon fuses into heavier and heavier elements eventually giving rise to an Iron core. Once an Iron core is developed there is no hope for the star. The fusion of iron needs more energy to continue than it creates, so the star collapses, losing the gravitational battle in a split second. This endothermic reaction (supernova!) results in a neutron star if the core is between 1.4 and 3 Solar masses. If the core exceeds 3 Solar masses, then even neutron degeneracy can not hold the star from further collapse, and a black hole is formed.
If the core is greater than 9 Solar masses, then there is no Carbon detonation, and fusion continues to heavier and heavier elements until it reaches an Iron core... This leads to a black hole by means of a supernovae (see above).

Pretty exciting? We now have several outcomes for a variety of masses. What's going on in these stars, though? Here is some detail... Once a star begins nuclear fusion, Hydrogen is being converted to Helium. This part of a star's life is as a Main Sequence star and is usually the longest phase of a star's life. Eventually the majority of Hydrogen in the star's core is used up leaving mostly Helium left over. The core no longer will support itself against gravitational contraction, so the star collapses and Hydrogen fusion resumes in the shell surrounding the core. The energy supporting the star against gravitational collapse is now much more intense and cannot be radiated away at the star's surface quickly enough. The result of this is an expansion of the stars outer layers. The star leaves the Main Sequence and becomes a red giant, a more luminous, cooler star. Eventually, all of the Hydrogen will be exhausted in the inner 50% of the star, and Helium will start its fusion process in the degenerate core. This can be a very unstable time for the star. Temperature and density are usually well connected. A change in one usually leads to a corresponding relative change in the other. Now, with Helium burning, the two (temperature and density) are decoupled. The star will no longer expand to meet temperature increases to cool itself resulting in Helium flashes. As Helium burns away, the star can become very unstable and blow away its outer layers leaving behind a planetary nebula like the one pictured here. The star then becomes a white dwarf then eventually a cinder. If the star has lots of mass, it will burn through its Helium then enter further stages of fusion. What is left over by one series of fusions results in the fuel for the next series to begin. The star undergoes a series of fuel depletions, contractions, new fusions, expansions, then ultimately complete depletion of usable fuel. This ends with the star's core being primarily Iron. Iron is nor easily fusable. It requires more energy to fuse than it can give off by doing so. The end result with such a star is gravitational collapse leading to a supernova. This can leave behind several possible objects: a supernova remnant like the Cygnus Veil Nebula, a neutron star, or a black hole. The more massive stars leave behind the denser objects (i.e. black holes).

For those with a hefty set of reference books, you might find, as did I, that the values for the various mass limits may be different depending upon the age and source of your literature. One book states that a mass of 0.8 Suns is required to create a star, others state only 0.4M_Sun is required. The same is true for the more massive end of the spectrum. Some references state that a core of +50 Suns is required for black hole formation. Thus you can see that there is a lot of debate as to the particulars of stellar evolution.

What's Going to Happen to Our Star?

The Hydrogen in our sun was plentiful enough to last for about 10 billion years before being depleted. We are about 5 billion years into that cycle, so we have another 5 billion years to worry before we have to leave! At the point of Hydrogen depletion the Sun will have a mass less than the Chandrasekhar limit (M_{star's core}<= 1.4M_Sun). The sun will eventually start burning Hydrogen in layers, expand into a red giant and shrink into a white dwarf as it depletes it fuels. Along the way, our Sun will likely shed its outer layers into a fine looking planetary nebula. It's a shame that we won't be here to see it!

Clear Skies!

~johnb

Photographs Courtesy NASA.

Contents	What's Up	Blog	Papers	Photos
	Links	Observations	Spectra	Sketches