The life cycle of stars

In the beginning...

It all starts with a huge cloud of gas and dust floating aimlessly through a galaxy.  These are called molecular clouds, and for good reason.  Molecular clouds may come in a vast range of sizes and shapes, and if their mass reaches approx 1 million times that of the Sun they are known as giant molecular clouds.  Composed of molecular gas (mainly hydrogen, helium, lithium and a few trace metals) they are also strewn with large amounts of dust.  The dust is extremely fine, almost as small as smoke particles, and very important.  The temperature of molecular clouds is very low, just 10-50 degrees above absolute zero (-263 degrees C).  These low temperatures ensure that the clouds contents are moving very slowly i.e. a low kinetic energy.  These low energies are what allow atoms to clump together on dust grains and form molecules.

The structure of these clouds is extremely irregular, with very dense regions and very sparse almost empty regions.  In the areas where the density is high, gravity takes over and very slowly begins to accrete matter together.  Providing that the internal energy of the regions is not too high, gravity will continue to pull in more material toward a central location.  If the energy of the gas is too turbulent i.e. the internal kinetic energy is higher than gravitational potential energy, the cloud will not be able to collapse.  If conditions are right, enormous amounts of gas and dust are pulled inward toward a central point.  Angular momentum then begins to rotate the system and flatten out any material into a disk.  This continues for up to 100 million years until sufficient density is achieved at the centre.  At this point the density is roughly 100,000 kg m^-3, and the temperature is ≥10 million degrees K.

At this point the vast majority of the proto-star is hydrogen, and the conditions are perfect for it to begin fusing into helium.  The first stage of fusion for a star involves converting 4 protons into a helium nucleus.  It does this through a process known as the proton-proton chain.  The PP chain as it is also known is as follows:

 p + p → d + e⁺ + ٧_e

_↓ 

p + d → ³He + γ

_↓

³He + ³He → ⁴He + 2p

The first step is required twice, because two deuterium nuclei are required for step two to produce the two helium nuclei in step 3, hence 4 protons are initially required.  All stars will spend the majority of their life converting hydrogen to helium in a stage called the main sequence.  It is here that the Sun has sat for over 5 billion years, and will continue to do so for another 4.  Once a star has begun its fusion of hydrogen, it will remain in a state called hydrostatic equilibrium, in which the internal pressure is roughly matched by gravitational potential energy.  This prevents the star from further collapse or expansion.

In stars with masses a few times heavier than our sun, the PP chain is not favoured due to the higher temperatures and fusion rates.  As these stars are bigger, they are generally more dense in their cores and therefore hotter.  They burn hydrogen much quicker than smaller stars, and so require hydrogen to actually be recycled in order to maintain constant fusion into helium.  A method of doing this is called the carbon, nitrogen, and oxygen cycle, or CNO cycle.  In order for this to work, sufficient amounts of carbon must be present.  This is not a problem providing the original molecular cloud had been recycled from previous generations of stars.

p + ¹²C → ¹³N + γ

↓

¹³N → ¹³C + e⁺ + ٧_e

↓

p + ¹³C → ¹⁴N + γ

↓

p + ¹⁴N → ¹⁵O + γ

↓

¹⁵O → ¹⁵N + e⁺ + ٧_e

↓

p + ¹⁵N → ¹²C + ⁴He

The keen observer will note that this process also requires 4 protons to ultimately become ¹²C and ⁴He.  Steps 2 & 5 both undergo β decay in which a proton becomes a neutron.  Click here for more information on β decay.  These larger stars will have core temperatures of approx 50 million K and can only convert hydrogen into helium through this highly temperature dependant reaction.

Once most of the hydrogen has been converted into helium, the next step is to fuse helium.  However, there are two problems.  Firstly, the core temperature of a star (even a large star at 50 million K) is still to cool for helium fusion.  And secondly, the most obvious next step is prohibited.  This obvious step would be for ⁴He to combine with a proton to form ⁵?...or...for two ⁴He nuclei to combine to form ⁸Be.  Unfortunately there is not a stable 5 or 8 mass isotope that lasts long enough for any further reactions to take place.  Fortunately, when a star has run out of hydrogen the core temperature begins to fall as fusion ceases.  This in turn reduces the internal pressure of the star, and gravity begins to collapse the core.  The core continues to collapse, increasing in density until the core temperature reaches 100 million K.  At this point helium is fused into carbon through a process known as the triple alpha process.  Due to the higher density of the core,  the unlikely and short lived production of ⁸Be can begin and be sustained providing the fusion rate is high enough.  The rate must be high given the lifetime of ⁸Be is just 2.6 x 10^-16 sec.

⁴He + ⁴He ↔ ⁸Be

↓

⁴He + ⁸Be ↔ ¹²C*

↓

¹²C* → ¹²C + γ

Low mass stars such as the Sun do not burn any more elements past this stage, and are destined to become red giants and planetary nebulae.  High mass stars however do continue burning elements in their cores and are destined to become supernovae.  Click here to find out how.

As helium is turned into carbon, the temperature rises, the core of the star slowly becomes a mix of carbon and oxygen, and helium burning moves outward to a shell surrounding the core.  The increase in temperature is followed by an internal increase in pressure which blows away the the hydrogen shell surrounding the helium.  The outer envelope expands by as much as 200 times its original size, but in doing so cools dramatically.  The surface temperature of the star measures no more than a few thousand degrees K, but its luminosity can increase by almost 10,000 times due to the enormous surface area.  Its radii becomes so large that the outer regions of the star do not have a sufficient gravitational pull to remain part of the star.

The massive internal radiation pressure and temperature blow the outer envelope away to create a planetary nebula.  The intense surface temperature of the core emerges as the envelope disperses.  The intensely hot core (~100,000 K) slowly becomes visible as the envelope is ejected at speeds between 5 - 20 miles per second.  The high levels of UV radiation from the core ionise the surrounding material causing it to glow.  Planetary nebulae can grow to over a light year in size but disappear after a few hundred thousand years.

Not all stars end their lives like this.  If the star happens to have a mass around 8 times larger than the Sun, then it will end its life as a supernova.  Click here to read more.

Example of planetary nebulae