A long, winding road

Life here on Earth depends on our supply of heat and light from the Sun. A question with a suprising answer is how long does the energy produced in the core of the Sun take to reach us?

Let’s see. The Sun is about 150 million kilometres away, and light and heat travel at almost 300,000 kilometres a second. So it would take about 8.3 minutes. That is indeed how long that light and heat takes to reach us from the surface of the Sun, but to get to us from the Sun’s core, we need to add almost a million years to that answer.

The Sun has a diameter of about 1.5 million kilometres. The energy is produced in the core, which has a diameter of around 300,000 kilometres. In that part of the Sun the temperature is around 15 million degrees Celsius and the pressure about 250 billion times the pressure at the Earth’s surface. Under these conditions atoms come apart and rearrange themselves.

The most abundant element in the Sun is hydrogen, and under these extreme pressures and temperatures, four hydrogen atoms combine to form an atom of helium, releasing a lot of energy. This nuclear fusion reaction is by far the source of the Sun’s energy output. Around 4 million tonnes of the hydrogen is annihilated and turned into energy every second. The rest is turned into helium. The energy is released in the form of photons, pulses of electromagnetic waves. Gamma rays, X-rays, ultraviolet, visible light, infrared and radio waves are all forms of electromagnetic waves, and come in the form of streams of photons.

In high densities in the Sun’s core, almost immediately after an energy photon is produced, it collides with a particle and gets absorbed. Then it gets reradiated in some random direction. This process of absorption and reradiation goes on over and over again.

Try this thought experiment. You are in the middle of a field and you want to leave the area. However, after each step or two you pick a new, totally random direction, take a couple more steps, pick a new random direction and so on.

How long do you think it would take for you to get off that field? This sort of moving around is known as a “random walk.”

This is what those photons in the Sun are doing. The part where those photons are random walking around is known as the radiative zone.

As the lucky photons work their way outwards, the density drops, so they get to move further before being bounced into some new direction.

Eventually, after bouncing around for around a million years on average, they find themselves around 70% of the way to the solar surface. This is an important place because at that level the density has fallen to the point where collisions become unimportant, and another transport process takes over, convection.

Here on Earth, we know that hot air rises. This is because it is warmer than its surroundings and less dense, so it floats upwards. As it rises, it cools, however as long as it is warmer than its surroundings it will continue to rise. This condition exists in the outer 30% of the Sun; from this point the energy is carried upwards as flows of hot, rising gas.

This arrives at the surface, the photosphere, a layer a few hundred kilometres thick, which we generally think of as the solar “surface”.

Here that gas radiates its energy into space, cools, and sinks down again, to the base of the convection zone, where it heats, rises, and repeats the process.

The result is a pattern of rising and forming currents, forming convection cells, just like what we see in a pan of heating oil when about to make some french fries.

Once radiated into space, those important energy photons are on the last leg of their trip to us: 150 million kilometres in 8.3 minutes.

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Venus is hard to see, low in the sunset glow. Look for Mars low in the southeast before dawn, and, to its right, Jupiter and Saturn, close together. The Moon will reach First Quarter on the 29th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton