1-4 The Sun

What Makes the Sun Shine?

Hamlet was a play written by William Shakespeare in the early 1600s. In it, Hamlet's late father's friend Polonius reads part of Hamlet's love letter to his sweetheart, Ophelia, out loud:

Doubt thou the stars are fire;

Doubt that the sun doth move;

Doubt truth to be a liar;

But never doubt I love.

Shakespeare may have been a talented playwright, but he obviously wasn't much of a scientist as (a.) the stars are not made of fire, and (b.) the Sun doesn't move. Then again, in the early 1600s, the scientific community was engaged in a huge debate as to what was at the centre of the Universe, the Earth or the Sun, so maybe Shakespeare was trying to chip-in with his opinion.

At any rate, if you want to know what makes the Sun emit light, you have to examine something called nuclear fusion, the combining of smaller elements' nuclei to make a heavier element.

In the very centre of the Sun lies the core, which reaches immense pressures and temperatures of about 15 million degrees Celsius:

Most of the Sun is made from a state of matter we don't often experience: after solid, liquid and gas, there is a fourth state called plasma, which is often called "a soup of charged particles." It is so hot, electrons can't really orbit a nucleus in any sort of stable way, so it's very much like soup in that it has charged particles swimming around all over the place.

About 75% of the Sun, by mass, is hydrogen; as a plasma that basically means it's a bunch of protons and electrons floating around. At the core, with those hot temperatures, nuclear fusion takes place: hydrogen gets squeezed so hard that it makes helium.

If you were to put all the things that go into this change -- the reactants -- on a scale and weigh it, the mass of the reactants would be greater than the mass of the products, the things that come out of it.

In a chemical reaction, this is not allowed: the mass of all the things you put into a reaction must exactly equal the mass of all the things that come out of it. But this is a nuclear reaction, and here we have to introduce the most famous equation in physics:

E = mc²

In this, E stands for energy (measured in joules), m stands for mass (measured in kilograms), and c is the speed of light (a very large number). Albert Einstein published this equation in 1905, which was a very good year for him.

What this means is that the missing mass turns into pure energy. We can use E = mc² to calculate how much energy comes out. When we realize that the Sun loses millions of tons of mass every second, that means the amount of energy the Sun emits every second is unfathomably large.

So, the Sun produces energy through fusion at the core, which makes it hot. If an object is hot, it will glow with visible light:

If an electric stove element is cold, you will not see it give off any visible light. But for the pot of water boiling on a hot stove element, you can see that the element is hot because it gives off an orange-red glow.

The Sun is hot because of nuclear fusion, and it gets this energy from the loss of mass, as described by Einstein, all of which keeps us alive. This video by Veritasium summarizes the process, and you can see that the average person-on-the-street (in Australia, anyway) can't quite explain it.

Structure of the Sun

Here's a more detailed cross-section of the Sun. We won't look at every layer in detail, just a few important parts; for more technical information about all these parts of the Sun, go here.

The core is still at the centre, but the visible surface or photosphere needs to be examined. It's not the "surface" of the Sun, because the Sun has no solid surface. You couldn't stand on it -- not that you would want to, as the temperature is about 5600°C -- you would just sink down into it, like trying to stand on top of a cloud.

The corona is the Sun's atmosphere, which was mentioned in the discussion about Solar Eclipses. For reasons that are still a bit of a mystery, the corona is much hotter than the visible surface; it's about 1 million degrees Celsius.

Sunspots are dark patches on the Sun's visible surface which are much cooler than their surrounding areas.

These may not look very big, but the largest of these spots is about 130 000 km across -- about ten times the diameter of the Earth!

Sunspots are often associated with solar flares, which are eruptions of electrically-charged particles from the Sun's visible surface.

To see a solar flare in action, watch this video. Keep in mind that many Earths could fit inside the looping curve of this flare!

If you want up-to-the-minute conditions on the Sun's visible surface, any active sunspots or solar flares, or a wide variety of other space information, go to SpaceWeather.com.

The Solar Wind

We've all seen flags flying in the wind:

What we think of as "wind" on the Earth's surface is the movement of air from one place to another in the atmosphere.

The solar wind is not like that at all. It is the movement of charged particles -- mostly protons and electrons -- outward from the Sun's corona into space. These particles move very fast: hundreds of kilometres per second.

If there are a lot of solar flares and sunspots, there will likely be an extra burst of charged particles outwards into space, increasing the density of the solar wind.

When those charged particles hit the Earth's magnetic field, they can create auroras, more commonly known as the Northern Lights in the Northern Hemisphere.

While the Northern Lights are beautiful, if there are too many charged particles slamming into our magnetic field (or magnetosphere), that can cause problems, as described in this video:

How the Solar System Formed

The Sun contains over 99% of the mass in the Solar System, but how did it -- and the planets, moons, asteroids and everything else -- get there?

According to our latest theories and observations, a Solar System like ours probably formed in a few steps:

1. Pre-Solar Nebula Forms

A thin gas cloud, or nebula, of mostly hydrogen and helium, along with some other traces of heavier elements, starts to pull itself together with its own gravity. As it does, the density, pressure and temperature at its centre starts to increase.

It's hard to take a picture of such a nebula/cloud, as it's large and dark and cold, and doesn't emit any light.

2. Nebula Collapses, Flattens and Spins

As the cloud pulls itself inward, the forces cause it to do two things: one, it naturally flattens itself into a disc shape. And two, because it didn't start as a perfectly balanced sphere, it will be lop-sided and start to rotate. Below is a picture of several of these protoplanetary disks as other solar systems form; we have no reason to believe ours would have been any different.

The gaps you see in these disks are probably where planets are going to form. If there is a lump of material of higher density in the disk, through its own gravity it's going to start attracting and accumulating, or accreting, material close to itself.

3. The Light Turns On

As the central part of the protoplanetary disk -- where most of the matter is located -- squeezes down even more, its temperature increases. If it is massive enough, the squeezing is enough to make nuclear fusion start to happen, and a star is born.

The above is not a picture, but an artist's conception of what a protoplanetary disk might look like, just after the star is formed.

4. Everything Settles Down, Almost

The accretion of most of the rest of the material in the disk continues, and planets continue forming. However, early in our Solar System, large collisions were much more frequent than they are now. Our current theories suggest a Mars-sized object (retroactively named "Theia") slammed into the Earth, broke off a piece of it, and that became our Moon.

(Again, this is not a picture, but an artist's idea.) Eventually most of the small objects get swept-up, settle into the Astroid or Kuiper Belts, or continue in their orbits around the Sun.

Practice

The Basics

1. What would happen if you tried to stand on the "surface" of the Sun? (Aside from being burnt to a crisp and/or evaporated immediately.)

2. Why do sunspots appear darker than the rest of the Sun's visible surface?

3. Describe what causes the Northern Lights, and how the Solar Wind plays a part.

Extensions

1. Recently, scientists in California announced they managed to make nuclear fusion happen in a laboratory. How could this change our energy generation future?

2. Outer planets, most notably Saturn, also have auroras. Does this mean that Saturn has a magnetic field?