What Is Meant When Our Sun Is Referred to as a Second-Generation Star?

Stars are grouped into generations based on their age. First-generation stars formed shortly after the Big Bang and consist of primarily hydrogen and helium. Second-generation stars, such as the sun, contain some heavier elements. These elements could not exist just based on nuclear fusion inside stars. Instead, some of this material must have come from the supernovae of other stars. Hence, second-generation stars like our sun were formed after first-generation stars.

Stars burn hydrogen in their cores during the first stage of their lives. This phase of a star's life is called the main sequence. Through nuclear fusion, they convert hydrogen atoms into helium atoms. Eventually, as the star becomes a red giant or supergiant, this helium gets converted into carbon. Heavier stars sometimes have enough mass to create even heavier elements. Once a high-mass star's core becomes predominantly iron, it can collapse in on itself and then explode into a supernova, ejecting material into space.

Stars spend a majority of their lives burning hydrogen fuel in the main sequence. The sun has been burning hydrogen in the main sequence for roughly 5 billion years. It will spend roughly another 5 billion years as a main-sequence star before it diverges from the main sequence to become a red giant. During the red giant phase, it will fuse helium into heavier elements such as carbon, nitrogen and oxygen. However, the sun is a relatively low-mass star. This means that it will not be able to fuse carbon, nitrogen or oxygen. Instead, it will gradually burn out all of its fuel as a white dwarf star.

The sun contains roughly 70 percent hydrogen and 28 percent helium. It also contains roughly 1.5 percent carbon, nitrogen and oxygen. This element profile fits that of a main-sequence star of low mass. However, the missing 0.5 percent of the sun's mass is made up of heavier elements. The sun is not massive enough to create these elements through its own nuclear fusion. Therefore, it must have obtained these element from the supernova of another star. Hence, the sun is a second-generation star.

Astronomers can determine the composition of a star by analyzing its absorption spectra. Absorption lines are created when light of certain wavelengths is absorbed by certain atoms. Different atoms absorb different wavelengths of light; therefore, astronomers can study the absorption lines in solar spectra to determine what atoms are present in the sun. The sun's absorption spectrum displays gaps for hydrogen and helium as well as some heavier elements.