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Solar rain: the Sun's plasma gets squirted to a height of about 63,000 kilometres, cools and then falls down again (Source: NASA/Solar Dynamics Observatory)

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Surprising Sun facts

Tuesday 11th August 2015 1:13 pm

Sometimes the most familiar things can be the most surprising. And none more surprising than that big ball of nuclear fire in the sky … the Big Fella that keeps us all alive, the Sun.

So how about this for surprising? Our Sun has long lost siblings, and it rains on the Sun.

First, let’s start off with the basics. The Sun is about 150 million kilometres away, and about 1.4 million kilometres in diameter.

Compared to Mother Earth, our Sun is about 109 times bigger, and has a mass about 330,000 times greater.

So far, in its 4.5 billion-year life, the Sun has burnt about 100 times the mass of the Earth and turned it into energy. But the Sun is so big that this loss is just a tiny 0.03 per cent of its total mass.

The Sun is made of 75 per cent hydrogen, 24 per cent helium, with much smaller trace amounts of the elements oxygen, carbon, neon and iron.

The Sun is not made from any of the three states of matter that we are familiar with — solid, liquid or gas. No, the Sun is made of the fourth state of matter — plasma.

You might remember from school, the model of an atom looking like a mini solar system. It had the heavy nucleus in the centre, with a cloud of much lighter electrons surrounding it.

A plasma is just a bunch of atoms that are so hot that the electrons have broken free, and are no longer tied to a nucleus. The electrons float freely through a sea of nuclei. This is ‘plasma’ — what the Sun is made of.

So now you know some background, let’s consider the Sun’s long-lost siblings.

Like most stars, our Sun was born as it condensed from a vast molecular cloud of dust and gas (mostly hydrogen and helium). But it wasn’t a solitary birth. The Sun was born in a cluster of perhaps 1000 other stars — all from the same cloud.

The astronomers went looking, and found some stars with the same trace elements as our Sun. They also did the mathematics of rewinding the motions of some candidate stars over the last four-a-half billion years — back to that primeval cloud.

In that time, our galaxy, the Milky Way, has rotated on its own axis about 18 times — so it wasn’t a trivial problem.

They have found what is almost certainly one of our Sun’s siblings. It’s about 110 light-years away in the constellation Hercules, and has the rather unromantic name of HD 162826. It’s a little bigger than our Sun, and a little hotter — but it’s the same age, because it was born at the same time. (I wonder if, like our Sun, it has planets?)

Okay, so the Sun has at least one sibling we know of, but how can it possibly have ‘rain’? After all, on the surface, it’s about five-and-a-half thousand degrees. Okay, it’s not water, but it works like rain does on Earth.

On Earth, most of us have heard of the so-called water cycle. Water vapour forms (say, over the oceans), and it rises upwards to make clouds. As they rise to a few kilometres of altitude, the clouds cool down to make little water droplets, which fall down as rain. These droplets are a few millimetres across.

On the Sun something similar happens — but it’s with plasma, and the scale is so much bigger. Via various processes, the Sun’s plasma gets squirted to a height of about 63,000 kilometres, cools and then falls down again. But the droplets are huge — about 100,000 square kilometres, or roughly the size of Tasmania. And when they fall, they plummet down to the Sun’s surface at around 50 kilometres per second.

Despite the surprising things that we know about our closest stellar neighbour, there’s still so much we don’t know. For example, we don’t know why the Sun’s atmosphere is over a million degrees hotter than the Sun’s surface.

But in 2017, the European Space Agency will launch the Solar Orbiter spacecraft. Not only will it get within 45 million kilometres of the Sun, but it will also give us our first views of the Sun’s north and south poles.

And the year after, NASA will launch its Solar Probe Plus spacecraft which should get within a cool six million kilometres of the surface of the Sun.

That should be plenty close enough to throw some light on new surprises …

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This blog first appeared on Dr Karl's Great Moments in Science

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