Sun as a Star

Mercury Star Solar System Cosmos Sun Credit:


The Sun is the dominant object in the solar system by mass and total energy content. The irradiance of the Sun drives climate on the planets and is the primary source of energy for the biosphere of the earth. The Sun is a Rosetta Stone for the study of astrophysical processes at resolutions that cannot be easily attained for other stars. The results of these solar studies can be applied toward an understanding of other stars, including the properties of their atmospheres and interior structures. In the realm of physics the Sun plays a unique role. The element helium—the second most abundant element in the universe after hydrogen—was discovered in the solar spectrum. The Sun serves as among the test beds for Einstein’s theory of General Relativity. The nature of subatomic particles called neutrinos—the byproducts of nuclear reactions in the hot and dense core of the Sun and sun-like stars—was elucidated as a result of solar investigations. The Sun serves as a laboratory for the study of plasma physics, i.e., the study of the interactions between ionized gas and magnetic fields.

In addition to its role as the central object of our solar system, the Sun also is a star in a universe of stars. The Sun is sometimes referred to as a “typical” or “average” star. Perhaps if we were to average together in some mathematically appropriate way the properties of the most massive and hottest stars with those of the least massive and coolest stars, we would have the basic properties of the Sun. But stars that are closely similar to the Sun constitute only about 5% of the stars in our Milky Way galaxy! More than 80% of the stars in our galaxy are cool, less massive “red dwarf” stars. The Sun is by no means typical in the sense of common!

The stars offer a range in physical parameter space—rotation rate, mass, temperature, and chemical abundances—that is unavailable with the Sun alone. Thus, stellar studies enable the investigation of the broad astrophysical applicability of models developed purely in a solar context. In other words, since we cannot vary the properties of the Sun, we study other stars with different masses, rotation rates, and temperatures to determine to what extent our results we find for the Sun are applicable to other stars or if our models and theories require modification. A critical and ongoing question in modern astrophysics is whether the Sun is much like other sun-like stars or if it is unique.


Dr. Mark Giampapa
National Solar Observatory

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