How big is the Sun? An exercise in really big numbers

The Sun, though still very mysterious in a lot of ways, has been studied for a very long time. Rising and setting in our planet’s sky for the last 4.5 billion years, it’s no surprise that someone finally figured out how big it is.

In fact our solar system’s central star is around 1.4 million kilometres in diameter. Simple geometry tells us its volume is 1.4 quintillion cubic kilometres. It weighs 2 with 30 zeroes after it in kilograms, and it makes up 99.8% of the mass of the entire solar system.

It’s at this point that we run into a problem.

While these numbers are impressive, they don’t make any sense to us. They’re so large – so far outside our experience – that we can’t fathom the Sun’s bigness. Let’s try to put things into perspective.

Volume

You can fit 1.3 million Earths inside the volume of the Sun. The Earth itself is 12,742km across and 6 with 24 zeroes kilograms in weight. But this just makes the problem worse. Now we’re trying to measure something incomprehensibly large using another thing which is only slightly less incomprehensibly large as our yardstick.

Let’s try and compare the Sun to something which we can all wrap our heads – or hands – around. An NBA official standard basketball is 0.0071 cubic metres in volume. That means you can fit 200 billion billion billion – 2 with 29 zeroes after it – basketballs inside the Sun.

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Is there any way to make that number comprehensible? There are an estimated 3 trillion trees on Earth. So, the number of basketballs you could fit in the Sun is 67 million billion times greater than the number of trees on Earth. Hmm.

Scientists believe there are 7.5 sextillion grains of sand on our planet. That means there are 27 billion more theoretical basketballs in the Sun than grains of sand on Earth. That includes all the beaches and all the deserts.

Honey, I shrunk our Sun

Not helping? Let’s try shrinking the Earth and starting from there. If the Earth was the size of a tennis ball, the distance from Sydney to Perth would be around 2cm. And the Sun would be a little over 7 metres across. That wasn’t too bad, was it?

Similarly, if the Sun were the size of a 75cm gym ball, the Earth would be about 7mm across – the size of a small blueberry.

How hot is the Sun?

Despite being far away – about 150 million kilometres, equivalent to circling Earth’s equator 3,750 times – the Sun showers Earth in solar warmth every day. In fact, all life on our planet relies on it.

A big, burning ball of gas and plasma, the Sun’s surface is 5000-6000°C. The melting point of stainless steel is around 1,400°C. In fact, tungsten has the highest melting point of any metal at more than 3,000°C – still thousands of degrees lower than the Sun’s surface temperature.

If you think that’s hot, the Sun’s core is estimated to be over a million degrees. In this region, the Sun is hot and dense enough for nuclear fusion to take place – simple elements like hydrogen and helium are forced together to create heavier elements like carbon and oxygen. The nuclear fusion from stars makes all the heavier elements in the universe, from the oxygen and nitrogen in the air we breathe to the aluminium and nickel in the planet’s crust.

The Sun’s fiery crown

Extending millions of kilometres from the Sun’s surface is a mysterious something called the coronasphere, which can be seen from Earth during a solar eclipse. Despite being far from the Sun’s core, the coronasphere is a veil of plasma also reaching one million degrees. Its intense magnetic radiation fires out through the solar system. The hot plasma in this region becomes solar wind, speeding away from the Sun at velocities more than two million kilometres per hour.

We are protected from this harmful radiation on Earth by our own magnetic field. But the Sun’s coronasphere also protects us from more harmful gamma radiation being projected by stars, nebulae and pulsars throughout the galaxy and universe.

The moody monster in the centre of our solar system breaks out in sunspots and solar flares when it is most destructive. The Sun emphasised this in April, when like a blister a sunspot erupted twice in quick succession. The resulting solar flares caused a minor, but long-lasting, radio blackout over Australia and southeast Asia.

Last year, NASA’s Parker Solar Probe became the first spacecraft to “touch” the Sun’s coronasphere. It will not get closer than 6 million kilometres from the Sun’s surface, but the Parker Solar Probe aims to help scientists answer questions like how the coronasphere gets so hot despite extending millions of kilometres into cold, empty space.

While even our home star challenges our perceptions of time and space, as long as we keep asking questions like “how big is the Sun?” we’ll keep looking for answers to the universe’s many mysteries.