spaceSpace and Physics

The Sun May Be Bigger Than We Thought


Robin Andrews

Science & Policy Writer


X-Rays stream off the Sun, as observed by the Solar Dynamics Observatory and the NuSTAR program back in 2014. NASA/JPL-Caltect/GSFC

A common theme in science is that we puny humans aren’t great at measuring things – and we’re not talking about biological features here, ladies and gentlemen. Mount Everest, for example, is an obviously well-known mountain, but it may be smaller, or taller, than people think for a variety of reasons.

The Sun’s another example. Its radius is presumed to be 695,700 kilometers (432,289 miles), but a growing number of researchers suspect this is probably wrong too. Come on science, get it together.


One Xavier Jubier is one such eclipse aficionado who has a problem with the way the Sun is sized up. This rather inventive fellow spends much of his time creating incredibly precise simulations of both solar and lunar eclipses, and he noticed that when they actually occurred, the position of the Moon in front of the Sun was often slightly different to what his models predicted.

Checking the math, he concluded that the only explanation was that the radius of the Sun must be off by several hundred kilometers. If this is true, then how could astronomers get something so wrong about our local star?

At present, the 695,700-kilometer value is defined by the International Astronomic Union (IAU), which itself is based off a 2008 study. As with previous definitions, this one measures the radius up to the edge of the photosphere, the visible envelope of light that we can see with the naked eye.

Due to constant fluctuations and the incredible brightness, this is a surprisingly difficult thing to measure with precision. It’s not a solid boundary, but a fluid and almost ethereal one. Measurements of the photosphere are getting more precise over time, but there’s still a decent margin of error.

The Sun's photosphere (4) is considered to be the "edge" of the star when it comes to its radius. Pbroks13/Wikimedia Commons; CC BY-SA 4.0

In fact, the 2008 study that the IAU get their figure from makes a point of this, noting that their rather precise figure of 695,658 kilometers has a margin of error of 140 kilometers (87 miles). Jubier’s models suggest the value is slightly greater than this, but the point either way remains the same: the Sun is bigger than we think it is.

By “we”, of course, we mean scientists. If you ask most people to state how big the Sun is off the top of their head, they’d probably sigh, extend their arms outwards and say “Like, preeetty big. Huge.”

In reality, of course, our Sun is stupidly small. It’s an average main sequence star that pales in comparison to UY Scuti, the largest star in the universe. This beast has a radius of 2.13 billion kilometers (1.32 billion miles), which makes it more than 3,060 times the size of our own Sun.



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