As you know, the Sun is the largest object in the Solar System. However, compared to other stars in the Milky Way, it is nowhere near the top of the size ranking. The red giant Betelgeuse, for example, is 700 times larger than it and 14,000 times brighter.

Although it is the largest object in our planetary neighborhood, the Sun is not among the largest stars in the Milky Way. Image: Aphelleon – Shutterstock

Since we were children, when we are going to draw the Sun, we start by making a circle. But, in fact, the star is not perfectly round – it is almost. Its equatorial diameter and its polar diameter differ by only 10 km. With an average radius of 696 thousand km, the diameter is calculated to be almost 1.4 million km (specifically, 1,392,000).


According to NASA, these numbers mean that 109 Earths could line up across the face of the Sun, which is a total of 4,370,006 km in circumference. And if it were hollow, more than a million planets the size of ours would fit inside.

How do astronomers calculate these measurements?

According to the website, some characteristics are obtained more directly, others less. For example, the distance to the Earth, called Astronomical Unit (AU), is measured by radar waves directed at a planet in a favorable position of its orbit (for example Venus, when it and the Earth are on the same side of the Sun and aligned with the star).

Contrary to popular belief, the star king is not yellow, but white. Image: Solar Dynamics Observatory (SDO)/NASA

The real size of our star is obtained from its angular size and its distance. Mass can be measured from the orbital motion of the Earth, or any other planet, using Kepler’s third law (in a fixed reference frame on the Sun, the square of the period of revolution of a planet around it is proportional to the cube of the half -major axis of the ellipse representing the orbit of the planet). Knowing, then, the mass and its radius, we have the average density.

Other characteristics are determined from templates. For example, the hydrostatic equilibrium equation, which allows you to determine the pressure and temperature at the center of the star, assuming that they have to be extremely high to support the weight of the outermost layers.

Obviously, all data are estimates. Xavier Jubier, an eclipse researcher, creates detailed models of solar and lunar eclipses to determine precisely where the Moon’s shadow would fall during a solar eclipse. However, when he combined real photos and historical observations with the models, he found that the precise eclipse shapes only made sense if he increased the Sun’s radius by a few hundred kilometers.

Even data from space missions such as NASA’s Solar Dynamics Observatory (SDO) aligned with measurements of the inner planets across the face of the Sun do not refine its radius to the desired accuracy.

“It’s harder than you just putting a ruler on these images and figuring out how big the Sun is. SDO is not accurate enough to nail that,” said NASA researcher Ernie Wright. “Likewise with the transits of Mercury and Venus. A measurement based on them is not as accurate as we would like it to be.” According to Wright, separate papers using a variety of methods have produced results that differ by up to 1,500 km.

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Binary star that broke up or solo career early on?

The Sun is classified as a G-type star – also called a G dwarf star or yellow dwarf (although, like other G-type stars, it is actually white, showing yellow through Earth’s atmosphere).

Generally, stars get bigger as they age. Scientists estimate that in about 5 billion years, the Sun will begin to use up all the hydrogen in its center, swelling into a red giant to expand beyond the orbit of the inner planets, including Earth. The helium will get hot enough to burn to carbon and form oxygen. These elements will then accumulate in the center of the Sun.

Later, the star will eject its outer layers, forming a planetary nebula and leaving behind a dead core of carbon and oxygen – a very dense and hot white dwarf star, the size of Earth.

Although the Sun is a typical star in many ways, it has one quality that stands out from most – it is a solitary being. Most stars have a companion, with some part of a triple or even quadruple system.

However, some lines argue that this solo career may not have always been like this. Research suggests that the Sun’s companion may have been a large binary, located up to 17 times farther from the Sun than Neptune, making it easier to disengage.

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