Early Chemical Evolution

A star’s mass gives a measure of the amount of “fuel”, and its luminosity gives a measure of the rate at which this “fuel” is consumed by nuclear burning, so a star’s lifetime is proportional to its Mass divided by its Luminosity.

Start from BigBang, the Universe generates H and He, H fusion generates He and forms the very first stars with mass ~ 100 mass of the Sun. The more massive they are, the shorter lifetime they have (a few million years). A few million years is not much for a cosmic time scale which means they exploded pretty quickly as gigantic supernovae. They are called the 1st generation of star.

Over time these stars are gone but they left behind all the heavy elements that they had made during fusion processes up to iron and gas. These elements and gas could clump much better to make small next generation of stars.

In 2nd generation, there are more stars with different mass created spanning from low-mass (0.6 - 0.8 mass of the Sun). Due to the low pressure and temperature slowing down the fusion process, these stars have long lifetime (15-20 billion years).

This process happens again and again as there are more stars, more supernovae, and more chemical elements are made. This also leads to the formation of larger structures, such as our Milky Way.

The First Chemical Enrichment Events

A ball of gas that was left over after the Big Bang along with the very first star. Star is not a static object, it actually evolves with time to a red giant and is getting much bigger.

What happens in the evolutionary here, there are two possibilities:

If a star has strong stellar winds, it can lose mass from its surface and the lost gets put back into the reservoir. They don’t explode in supernova and do not contribute to any chemical evolution cycle.

If a star keeps evolving until it explodes as a giant supernova, the outer and inner portions of the star get spilled around and put back into the reservoir again. This introduces new elements from the core of the star that are being put into the reservoir. After the death of these stars, the gas cloud is chemically enriched which enables the next generation of star forms from this enriched material, and they evolve. The massive ones contribute to the creation of new elements. All the massive stars with the new generations contribute to a successive build up of all the elements with time.

Comments:

• Old stars contain little of heavy elements (> H, He).
• Younger stars are more enriched.

Stellar Archaeology

There are old stars called metal-poor. They are usually used as a tool to study the early universe. These stars are long-lived, so they have a low mass (0.6 - 0.8 mass of the Sun, lifetime of 15-20 billion years) and they are still observable in the Milky Way.

Our Milky Way, this is the bulge, the inner part of our galaxy with a supermassive black hole in the center. There are actually two disks in the either sides of the galaxy. The Earth is about 2/3 on the way out of a disk.

The bulge contains a lot of young stars and gas. Over time, more stars and elements are formed, resulting in a bulge that is very metal-rich. The disk is not quite as metal-rich but still pretty enriched.

This is just the most visible part, namely the Milky Way band on the night sky. Another definition to this part is the spiral arms that make up the disk.

For the older stars, because they are located up and below the disk in something that is called the halo of the disk. In the formation of a galaxy, smaller systems actually come together and form a bigger system, known as hierarchical structure formation paradigm. The largest one will be the Milky Way, while the smaller system have a tendency to end up in the outskirts. But they are completely shredded apart and what is left are all the stars that are being spilled into the Milky Way.

Element Production (Fusion)

Why does the sun shine? The Sun’s radiance results from the process of nuclear fusion, which transforms hydrogen into other light elements, and ultimately iron (H..Fe).

What is actually happening is that 4 atoms of Hydrogen come together in a series of steps that we are going to leave out for now, they form a helium atom. That is made from 2 protons and 2 neutrons. Therefore, some conversion of 4 protons into 2 neutrons into 1 Helium. This works because there is quantum mechanical tunneling going on. The 4 positively charged protons would actually repel themselves.

Because this tunneling effect it is hot enough so that these protons can be eventually combined into 1 helium nucleus.

If we weigh 1 He atom and (2n+2p) separately we would find out that 1He < (2n+2p) around ~0.73% (mass defect) and the energy released as the popular equation showed E=mc^2. For atoms that are heavier than iron, the fusion process requires external energy to happen.

In the end, the star ends up with an iron core. These fusion processes have been going on in the center and growing larger and larger as more and more elements are being made. Eventually, there is a big, fat iron core sitting there. Since it can’t get energy out anymore, the star has a big problem because it needs to have an energy source. Without that, it explodes as a supernova.

It is really nice how nuclear physics and astrophysics can come together because nuclear physics governs what is happening inside the core and then the astrophysics provides what we can actually observe. By putting these two together we can understand stellar evolution. That is actually governed by the nuclear physics processes, specifically fusion, in the core.

By measuring the luminosity of a star as well as its temperature we can place it on this diagram and then learn which evolutionary state the star is in, which tells us what is going on in its core.

Spectroscopy

Spectroscopy is the technique we use to observe stars in order to figure out their chemical composition.

Noticing what happens when light comes through a little water droplet and it gets split up into rainbow colors, we do the same thing with a spectrograph mounted at a telescope. What we actually see is less than the rainbow, because there are certain colors of the rainbow missing. And the missing parts contain all the information that we want.

When photons escape from the core, they pass through this outer layer. In the outer layer, we have hydrogen and helium atoms, because that’s what the stars are mostly made of. There will be other atoms of iron, magnesium, carbon, and oxygen. Each element absorbs photons with their very specific energy or wavelength that is equivalent and lets some other photons out.

By measuring the spectra or the line strength, we know about the information of star formation. As the figure shows, the strength corresponds to the abundance of magnesium atoms in the outer atmosphere.

If there are only a little calcium, magnesium, and sodium actually present in the star, which means that the star must have formed at a really early time when the cycle of chemical enrichment had only gone around a few times. These stars must be metal-poor or among the oldest, resulting in spectra displaying very weak lines.

Formation of the Heaviest Elements

The lighter elements are made up in the fusion process that exists in the core of the stars. What about the heavier element?

There are a few little neutrons and an iron nucleus. As the seed nucleus is bombarded with these neutrons, it swells and turn into a much larger nucleus, becoming both radioactive and neutron-rich, ultimately qualifying it is an isotope.

Because it is radioactive, it does not like to stay in this way. In fact, it with all the neutrons are converted into protons, leading to the formation of a stable element that is much larger than the original iron. It could be a carbon atom or uranium.

An example of uranium 238 is technically not a stable element (half-life is 4.7b years). For humans, it is stable but on cosmic timescales, it is not.

The above process so-called neutron capture process. There are 2 ways of neutron capture process: slow n-capture and rapid n-capture. They refer to how fast what timescale this neutron bombardment is occurring.

For the slow n-capture, the timescale is about 10,000 years. Elements produced by this process can contribute to the evolution into a red giant stars. The n-capture process happens for many times until the heaviest element is formed, called Pb.