Methods for Measuring & Studying the Sun

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn about how we measure and study the Sun from a distance. This lesson will explain how to figure out the temperature, chemical composition, diameter, and energy source.

Studying the Sun from a Distance

The Sun is a big ball of plasma, one that is vital to our lives. Without it, the Earth would become frozen in no time at all. Without it, our planets would be uninhabitable, and life like us could never have evolved. So the nature and features of the Sun are rather important topics to study. But the Sun is also 93 million miles away from us, which makes studying it rather challenging. Thankfully, humans have found ingenious ways to learn all kinds of things about the universe at a distance.

The Sun is 93 million miles away
The Sun is 93 million miles away

Temperature and Chemical Composition

The temperature and chemical composition of the Sun are determined in the same basic process: measuring the Sun's spectrum. In a nutshell, this involves taking the light from the Sun and separating it into its component colors (or frequencies). At which frequencies is the light brightest? Are there any gaps in the pattern?

When we analyze the light from the Sun and plot a graph of how much light we receive at different frequencies, we get a special curve called a blackbody curve. This is the type of curve that theoretical physicist expect to be created from an extremely hot body like the Sun where all the parts are in contact with one another. To be exact, a blackbody is a perfect absorber and emitter of heat. The Sun isn't a perfect blackbody, but it's pretty close.

Using our knowledge of theoretical physics, we can look at where the peak of the blackbody curve is found - which frequency of light is brightest - and use that to figure out the surface temperature of the Sun. The peak is also related to the main color the Sun has - objects that glow red are hot, ones that glow white are hotter, and ones that glow blue are hotter still. The Sun is white hot, but other stars in the universe are red or blue, and have different temperatures.

Plotting a blackbody curve for the spectrum of the Sun tells us the temperature
Plotting a black body curve for the spectrum of the Sun tells us the temperature

As part of the same analysis, we can look for tiny gaps in the pattern. These are particular frequencies where we receive less light than at other frequencies. These happen because the chemicals in the Sun absorb light of particular frequencies. Every element in the periodic table absorbs different combinations of light frequencies to match the orbits of electrons that the atom allows - every element has different electron orbits. So by looking at these gaps in the Sun spectrum, we can figure out which chemicals are present inside it.

The spectrum of the Sun has gaps in it called absorption lines
The spectrum of the Sun has gaps in it


Figuring out the Sun's diameter is another task that might seem incredibly difficult. We can't just hold a tape measure up to the Sun, at least not without vaporizing first. So how do we do it?

Well, first we have to figure out the distance to the Sun. The way that was first done was pretty complicated, and involved measuring a transit of Venus. But the general idea is that your view of the Sun is different depending on where you are on Earth. Look at some objects around the room, and move your head side to side. You might notice that close objects move around more, and further away objects don't move as much. This effect is called parallax. By measuring the appearance of the Sun from different locations on Earth, we can use parallax to figure out how far away it is.

Animation of parallax
Illustration of parallax

Once we know the distance to the Sun, we can figure out the Sun's diameter using some basic geometry. Half of the Sun's diameter (the radius) can form one side of a right angle triangle. The distance to the Sun is the other side. Last of all we can measure the angle between the middle of the Sun and one edge of the Sun, using various pieces of equipment that involve pointing straight lines at these two locations and then measuring the angle with a protractor.

Once you have an angle and one side of the triangle, you can use the trigonometric funtions of sine, cosine, and tangent (often referred to as SOHCAHTOA) to figure out the other side of the triangle: the Sun's radius, which is half the diameter. To be exact, the tangent of the angle you measured will be equal to the opposite side of the triangle (which is the Sun's radius) divided by the adjacent side of the triangle (which is the distance to the Sun). Solve for the missing variable and there you have it: the Sun's radius, which you just have to double to get the diameter.

Energy Source: Nuclear Fusion

So that's temperature, chemical composition, and diameter, but what about the Sun's energy source? How do we really know what's going on in the center of the Sun?

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