Stars: Properties and Evolution
Introduction to the topic
Nearly everything you see around you was manufactured in stars. Stars assemble hydrogen atoms into heavier atoms, a process called "stellar nucleosynthesis". In cool environments, atoms connect up to form molecules. Molecular-based life (the only type we know) seems to require complex carbon-based molecules. Life as we know it could not have started before a generation of stars had expired and ejected its heavy elements into space. Due to the importance of atoms in the scheme of our existence, this unit on stars will concentrate on how stars evolve and manufacture elements heavier than hydrogen.
A few notes on jargon
In astrophysics, the terms "heavy elements" and "metals" are not well-defined. Usually they apply to all elements apart from hydrogen. Lithium is a tricky one because it was synthesized in the early universe shortly after the Big Bang, but is destroyed in stars.
We attach an adjective to the word "nucleosynthesis" to differentiate between different environments. Therefore, Big Bang nucleosynthesis is different from stellar nuclesynthesis, is different from explosive nucleosynthesis. The actual physical mechanism, collisional reactions (aka "fusion"), is the same in all cases, but the host environments are different.
The exception to the collision mechanism is hydrogen. Hydrogen was probably formed as a result of an asymmetry in a particular creation-annihilation quantum fluctuation process during the first minute or so of the lifetime of the universe.
During cloud collapse, once the temperature gets high enough for hydrogen to be fused into helium, we say that a star is born.
The fusion reaction is:
At the onset of fusion, this new source of energy inside the star will completely halt the collapse. At this moment, the star balances itself, so that gravity pulling inwards exactly balances pressure pushing outwards. The star is in equilibrium, and its mass will remain constant or perhaps decrease a little.
Fusion reactions only take place in the central portion of the star, a region called the core. The rest of the star is called the atmosphere. At the start of a star's life, the core is quite small, occupying only a few percent of the size of the star. As heavy elements accumulate, fusion moves outwards, so that by the end of the star's life, more than half can be the host of fusion reactions.
Stellar evolution is most easily seen on a diagram that plots the brightnesses of stars against thier temperatures. Such a diagram is called a Hertzsprung-Russell Diagram, or HR diagram. We will return to this diagram later.
Stars are distinguished by their mass - other properties can be determined or derived based on a star's mass. Stellar masses range from 80 MSun to 0.01 MSun. We noted earlier that stars "burn" hydrogen into helium to release energy (here, I use the word "burn" to refer to fusion, although the usage is not strictly correct). This burning occurs in the core of the star. The hydrogen core-burning stage of a star's life is called the main sequence stage. During the main sequence stage, helium will collect in the stellar core over time. It is the changing chemical composition of the star drives stellar evolution. The main sequence stage is by far the longest stage of a star's life, occupying more than 90% of a star's total lifetime.
When about 10% of the hydrogen is burned, then hydrogen burning moves to a shell outside the core. The star's atmosphere expands and cools and the star becomes a red giant - we say that the star is ascending the asymptotic giant branch (AGB) and is an AGB star. We can chart the changing brightness and surface temperature of the star on the HR diagram.
During the AGB phase the core continues to heat up until helium ignites. Helium can be burned in the core to produce carbon. During the core He-burning stage the star settles onto the horizontal giant branch of the HR diagram.
Carbon itself can be burned, and in fact there is a series of fusion products that can be burned to produce a heavier species. The basic fusion ladder looks like this (the other elements lighter than iron (Fe) are produced in secondary reactions):
All stars will produce carbon. The production of heavier elements than carbon will depend upon the mass of the star, with more massive stars able to travel further along the fusion chain. Through other collisional or decay processes, all the elements up to iron are produced (with the exception of lithium which is destroyed in stars). Eventually, all stars will run out of fuel and then experience a mass-loss phase.
Stellar Mass Loss Phase
The mass-loss phase is important. So far we have seen how stars produce the heavy elements up to iron (Fe), but these atoms are still locked up inside the star. We need a way of getting these atoms out of the star and into interstellar gas clouds. These clouds, now enriched with heavy elements, will form the next generation of stars and planets which can use these elements in complex chemical reactions.
The mass-loss mechanisms depend upon the star's initial mass:
Note on stellar lifetimes
See that large stars have very short lifetimes. Intuitively one would think that a larger has more fuel and therefore might live longer, but such is not true. Instead, massive stars are so much hotter that the fuel they have is burned much more rapidly. The result is that stellar lifetimes vary inversely with mass, in a proportion that is shown below. Note that small changes in stellar mass result in large changes in luminosities and lifetimes.
A few easy rules: