T/F: Uranium forms in the core of a star.

Nuclear fusion is the process by which two or more atomic nuclei combine to form a single, more massive nucleus. This process releases a tremendous amount of energy.
In stars, nuclear fusion is the key mechanism that powers them and allows them to shine.
The most common fusion reaction is the fusion of hydrogen nuclei (protons) to form helium. This process occurs in multiple steps, commonly referred to as the proton-proton chain reaction in smaller stars, or the CNO cycle in larger stars.

• First, two protons fuse to create a deuterium nucleus, a positron, and a neutrino.
• Then, the deuterium nucleus fuses with another proton to form helium-3.
• Finally, two helium-3 nuclei fuse to form helium-4, releasing two protons.

As stars evolve and exhaust their core hydrogen supplies, they can fuse heavier elements like helium, carbon, and oxygen in more advanced stages of stellar nucleosynthesis. However, this fusion process only works up to iron.
Beyond iron, fusion reactions are no longer energy-efficient. Hence, elements heavier than iron are not formed in the typical fusion processes that occur in stellar cores.

Heavy element formation refers to the creation of elements heavier than iron. Such elements are not formed through normal nuclear fusion processes in the cores of stars.
Instead, these elements are typically created in events like supernovae or neutron star mergers. During these catastrophic events, there are extreme conditions that allow for the rapid capture of neutrons, leading to the formation of heavier elements.
One important process in the formation of heavy elements is called the 'r-process' or rapid neutron capture process. During supernovae or neutron star collisions, free neutrons are available in large quantities, and heavy nuclei capture these neutrons at a very rapid rate.
As a result, the nuclei become unstable and undergo a series of beta decays, turning neutrons into protons, thus forming new elements.
Another process, known as the 's-process' or slow neutron capture process, occurs in asymptotic giant branch stars (AGB stars). In the s-process, elements slowly capture neutrons, one at a time, and after each capture, beta-decay transformations occur. This process forms stable isotopes of elements heavier than iron but at a slower rate compared to the r-process.

Neutron capture processes are critical to the formation of elements heavier than iron. There are two main types of neutron capture processes: the rapid (r-process) and the slow (s-process).

**R-Process (Rapid Neutron Capture):**
The r-process occurs in environments with a high flux of neutrons, such as supernovae or neutron star mergers. The heavy nuclei rapidly capture neutrons, becoming neutron-rich. These neutron-rich nuclei undergo beta decay, transforming neutrons into protons and thus creating heavier elements.

**S-Process (Slow Neutron Capture):**
The s-process happens in more stable environments like those found in asymptotic giant branch (AGB) stars. Here, neutrons are captured at a slower rate, allowing for the creation of stable isotopes. Beta decay can still occur between captures, but due to the slower pace, the nuclei remain more stable throughout the process.

• In summary:
• R-Process: Fast, unstable environments, rapid neutron capture.
• S-Process: Slow, stable environments, gradual neutron capture.

Both processes are essential to explaining the abundance of heavy elements in the universe. Without them, we wouldn't have many of the heavy elements needed for life and technology.
Understanding these processes helps us learn more about the life cycles of stars and the complexities of nuclear physics.