How Do We Determine Where The Sun’s Edges End?

When you look up into the sky at our nearest star and provider of life, it usually appears like a glowing yellow circle. After a second or so, you have to look away due to the intense brightness. Even with the most advanced technology we have available today, it seems hard to believe that we could ever get a clear enough image of the Sun to determine its size, let alone its chemical composition, and yet we know both of those things with great precision.

We might not be able to go there, memeAt the center of the sun, the temperature can reach above 25 million degrees Fahrenheit, and even the furthest edges, the sun’s plasma corona, can reach temperatures of 3.5 million degrees. Given those extreme temperatures, there is no way to set a probe down to measure the star. Those of you who know something about astronomy will also understand that stars have different layers of materials and are more complex than massive balls of burning gas.

With all of these obstacles in place, it’s hard to believe that we know so much about our sun. The question is…. how? Before we get into the details of measuring the size of the sun and determining its edges, let’s take a closer look at the physical characteristics of our Sun.

Structure of the Sun

The Sun can be quite simply broken down into two main areas, each of which have three components. The first three are the actual structure of the sun, while the final three relate to the atmosphere surrounding our star. Each of these areas helps to tell us something about our Sun, including its size, composition and longevity.

The Core

The very center of the sun is an intense place, with extreme pressures and temperatures high enough to cause nuclear fusion, in which hydrogen atoms are compressed into helium atoms, resulting in massive expulsions of energy, which is what generates the light we see from our star. The core comprises approximately 25% of the radius and is made up of hydrogen and helium, as mentioned.

Radiative Zone

This zone acts as a filter for the intense gamma rays generated in the core of the star through nuclear fusion. There are still high pressures in this zone, which makes up roughly 45% of the sun’s radius. In this zone, energy can bounce around, taking a random, zig-zag path up to the convective zone. This journey of energy may take up to 1 million years, in which the energy is perpetually absorbed and re-released through thermal radiation, resulting in lower-level energy particles finally escaping into the convective zone.

The Radiative Zone on a really slow day. meme

Convective Zone

This is the outermost layer of the sun’s physical structure, and composes roughly 30% of the star’s radius. In this region, massive pillars of hot gas rise through to the surface of the star, like water bubbling up to the surface of a heated pot. This is where the energy at the surface of the sun emerges. The temperature in this layer of the sun is approximately 3.5 million degrees Fahrenheit.


This is the first part of the sun that we are physically able to see, stretching from the very surface of the sun to about 250 miles above it. The light energy in this region comes off as the yellow light we discern each day when the sun rises! This innermost part of the Sun’s atmosphere ranges from 6,500-11,000 degrees Fahrenheit.


Between 250 miles and 1,300 miles from the surface is the chromosphere, which has wildly changing temperatures and is—despite being harder to believe—even hotter than the atmospheric layer beneath it.


The corona is perhaps the most mysterious and elusive part of the sun, considering that it cannot be seen except during a total solar eclipse. This region is much hotter than the rest of the atmosphere, extending up to 3.5 million degrees—the same temperature as the convective zone! The corona also doesn’t have an upper limit, extending millions of miles above the surface of the star. Some parts of the corona—including its incredible temperatures—remain a mystery, but research is always ongoing!

When the sun's corona throws a temper tantrum. meme.

How Do We Know Where the Edges Are?

The sections above explain the different parts of the sun that we know about, but that still doesn’t explain how we know what the sun is composed of, and how large it really is. Those two piece of information are acquired through two different means—spectroscopy and angular dimensions.


Until about 100 years ago, we didn’t have a clear way of determining precisely what made up the sun, and it wasn’t until the past few decades that we became absolutely sure. Essentially, when light is released from the sun, it can be passed through a prism, thus spreading out the component rainbow of light beams. Each color represents light of a different energy, but you will also see black bands in the prism, where the light is much fainter. This represents areas where the light was absorbed by a certain element. Each element has a unique band pattern, so we are able to determine what type of materials are found in the sun, and in what concentration.

Thanks to a bit of nuclear astrophysics, we also know how fast the concentrations of those atoms are changing, which can tell us how much fuel the sun has, and thus how long our star will continue burning in the sky!

black band, spectroscopy

Angular Dimensions

Since we can’t get close enough to the sun to take out a tape measure for an accurate reading, astronomers instead have to rely on mathematics to calculate the size of the sun. If you look at the heavens from the perspective of the earth, and think of all you can see as being in a 360-degree sphere, then you can establish angular size and angular distance of those celestial bodies. By utilizing the distance to the object being measured, as well as the angular size and distance, you can use a known quantity, such as the distance to the moon, to thus calculate the size of the sun. If you measure the angular size of the star and multiply it by the physical distance, you should get a decent approximation of its actual diameter. Using this method, astronomers can confidently say that the sun has a radius of roughly 700,000 kilometers.

The atmosphere above the sun extends out for thousands of kilometers beyond the surface, and the corona may well stretch millions of miles away from the edge of the star.

Wanted to measure the sun's edges... meme

A Final Word

The intricacies of astronomical measurement are confusing, to say the least, but there are some rather simple tricks that can be done—even at home!—to determine the size and distance of celestial objects. While the dimensions of the sun are perpetually changing as more and more fuel is burnt up and the relative concentrations of elements change, there is still 4 billion years’ worth of fuel in our star. Thus, for the foreseeable future, there won’t be many changes in the edge dimensions of the sun!


  1. NASA.Gov
  2. Universe Today
  3. Cornell University
  4. American Psychological Association (APA)
  5. Taylor & Francis Group
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About the Author:

John Staughton is a traveling writer, editor and publisher who earned his English and Integrative Biology degrees from the University of Illinois in Champaign, Urbana. He is the co-founder of a literary journal, Sheriff Nottingham, and calls the most beautiful places in the world his office. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.

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