The answer is Halo stars formed before those elements were made. The correct answer is option C.
Stars in the halo of a galaxy have almost no heavy elements such as carbon, nitrogen, and oxygen because these elements were not present when the halo stars formed.
Halo stars are the oldest stars in a galaxy, and they formed early in the history of the galaxy. At that time, the universe had a very low abundance of heavy elements. The elements carbon, nitrogen, and oxygen are considered heavy because they have more protons and neutrons in their nuclei compared to lighter elements like hydrogen and helium.
The heavy elements in the universe are primarily created through nuclear fusion in the cores of stars. During the life cycle of a star, nuclear reactions occur that convert lighter elements into heavier ones. However, the process of creating these heavy elements takes time, and it occurs over successive generations of stars.
In the early universe, the first stars formed from mostly hydrogen and helium, with only trace amounts of heavier elements. These first-generation stars, including the halo stars, did not have enough time to produce significant amounts of carbon, nitrogen, and oxygen through nuclear fusion.
It was only in later generations of stars, particularly more massive stars, that these heavy elements began to be synthesized. As these more massive stars eventually exploded as supernovae, they released their enriched material into space, including carbon, nitrogen, and oxygen. This material then mixed with the interstellar medium, enriching it with heavier elements.
Therefore, the halo stars formed before the heavier elements were created, which is why they have almost no carbon, nitrogen, and oxygen. This is an important characteristic of halo stars and provides valuable information about the early universe and the evolution of galaxies.
Therefore the correct answer is option C.