Elements heavier than iron originated a. in the Big Bang. b. in the cores of low-mass stars. c. in the cores of high-mass stars. d. in the explosions of high-mass stars.

In the very early universe, right after the Big Bang, the conditions were extremely hot and dense. During the first few minutes, a process known as Big Bang nucleosynthesis occurred. This process formed the lightest elements, including hydrogen, helium, and small amounts of lithium and beryllium. The high temperature and density allowed nuclear reactions to take place, forming these elements from protons and neutrons. Big Bang nucleosynthesis is extremely important because it created the initial supply of these light elements that future stars could use as the building blocks for forming heavier elements. This process did not form elements heavier than beryllium. Heavier elements had to wait for the formation of stars to be created.

Stellar nucleosynthesis refers to the processes by which elements are formed within stars through nuclear fusion reactions. Most stars spend the majority of their lifetimes fusing hydrogen into helium. When a star's hydrogen supply runs low, it starts to fuse heavier elements. The stages and types of fusion depend largely on the mass of the star.

• Low-Mass Stars: These stars, like our Sun, fuse hydrogen into helium in their cores. In later stages, they can produce elements up to carbon and oxygen but cannot create elements heavier than iron.
• High-Mass Stars: These stars have much higher pressures and temperatures in their cores, allowing them to fuse elements up to iron. However, the fusion of elements heavier than iron is not energetically favorable within these stars.

Stellar nucleosynthesis is crucial because it creates a variety of elements that are essential for forming planets, life, and many other structures in the universe. However, as mentioned, it does not produce elements heavier than iron. For these, we need another event.

Supernovae are explosive deaths of massive stars. These explosions provide an environment where elements heavier than iron are formed. Here's how it happens:

• Massive stars form iron in their cores near the end of their lives. Iron fusion does not produce energy, so the core collapses under gravity.
• This collapse results in an explosion that blows off the outer layers of the star, scattering elements into space.
• During this explosion, the high temperature and abundant neutrons in the environment facilitate rapid nuclear reactions (r-process and s-process), creating elements heavier than iron, such as gold and uranium.

Supernovae are significant because they enrich the interstellar medium with heavy elements, which can later be incorporated into new stars, planets, and eventually, life. The elements heavier than iron we find on Earth were all formed in supernova explosions, making supernovae a key process in the chemical evolution of our galaxy.