How we know what the sun is made of

QUESTION: What is the sun made of?

ANSWER: It takes a good amount of physics to describe what the sun is made of and how that stuff changes over time. We really couldn’t answer this question until about 90 years ago, and we weren’t totally sure we had the whole answer until about 15 years ago, as I’ll try to explain.

The sun is made of about three-quarters hydrogen, one-quarter helium, and some other heavier elements like carbon, oxygen and iron, in very small quantities. The hydrogen and helium are in a gas form. But the hydrogen (H) and helium (He) atoms are much closer together in the sun than what you might imagine.

If you filled a balloon with H and He gas at the same average amount (density) as in the sun, the balloon would weigh about 25 pounds here on Earth. But, the density of the gas in the sun changes quite a lot, depending on its location in the sun. If you took the gas from the core of the sun, the balloon would weigh about 2,500 pounds.

The temperature of the sun is also much hotter in the core, where it is about 27 million degrees Fahrenheit, compared with its surface temperature of about 10,000 degrees Fahrenheit.

You might wonder how we know that the sun is made of H and He gas. If you look at the light that comes from the sun after it has traveled through a prism, you will see that the light gets bent into a rainbow of colors. Each color is light of a different energy, with blue light having about one-third more energy than red light. Looking closely, you will find some dark bands, or thin lines, where the light is much fainter. This is because some of the light that left the sun was absorbed by atoms (like H and He) in the sun’s atmosphere. Every atom absorbs specific colors of light, depending on its number of protons and electrons. By examining the pattern of missing colors, or dark bands, you can determine the identity of the atoms that must have absorbed some light.

When looking at light from the sun, this trick gets a little more complicated because of the sun’s high temperature. The pattern of light absorbed by each atom also depends on its temperature. At higher temperatures, the electrons in an atom are more likely to get removed from the atom, which is referred to as ionization, and the atom’s pattern of absorption then changes.

A British astrophysicist named Cecilia Payne-Gaposchkin, working at the Harvard Observatory, put these ideas together in 1925 to describe the composition of stars. She discovered that H and He accounted for most of the atoms of our sun and other stars, contrary to what had been thought.

Meet the student 

Name: Norah Falvey.

School: Johnson City Primary School, Johnson City Central School District.

Teacher: Marie Osick.

Hobbies: Gymnastics, reading.

Career interest(s): Engineer.

Meet the scientist

Answered by: Stephen Levy.

Title: Associate professor of physics, Binghamton University.

Department: Physics.

Hobbies: Parenting.

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