THE EXPLAINER:
THE SUN
HOW FAR IS THE SUN FROM EARTH?
The distance between the Sun and Earth is, on average, about 149.6 million kilometres. That means that the Sun’s light takes about 8.3 minutes to reach Earth. Since the Earth’s orbit is slightly elliptical, at its closest point to the Sun (called ‘perihelion’) it is about 147 million kilometres away, while at its furthest point (called ‘aphelion’) it is about 152 million kilometres away. This 3 per cent or so difference means sunlight is on average 7 per cent stronger at perihelion than at aphelion. But perihelion actually occurs in early January, in the dead of winter for the northern hemisphere. This demonstrates that it is the Earth’s axial tilt which causes the seasons, rather than its proximity to the Sun.
HOW BIG IS THE SUN?
The Sun’s radius is about 696,342km, which is roughly 109 times the Earth’s radius. This means that you could fit more than 1.3 million Earths inside the Sun.
HOW DOES THE SUN BURN WITHOUT OXYGEN?
The Sun is not ‘burning’ in the traditional sense of the word. Instead of chemically combining with oxygen, such as carbon does when coal is burnt, the Sun’s fuel is undergoing thermonuclear reactions. Most of its energy is produced by the fusion of hydrogen into helium deep in the Sun’s core where the temperature and pressure are huge.
Astronomers calculate that the Sun is losing about 4.26 million tons of mass every second due to fusion (or about 140 quadrillion tons per year). This is only 0.000000007 per cent of the Sun’s total mass. This means that even at the end of its life, in about five billion years time, the Sun will still have 99.966 per cent of its current mass!
HOW HOT IS THE SUN?
The Sun is hotter in the middle than at its surface. Right at the core of the Sun, the temperature is about 15,000,000°C, hot enough for thermonuclear reactions to take place. But at the surface of the Sun, which scientists call the ‘photosphere’, the temperature is only 5,500°C. Surprisingly though, the Sun’s outer atmosphere is actually far hotter than the surface; it is about 2,000,000°C, with some regions even reaching 20,000,000°C.
There is, as yet, no complete explanation for the high temperatures in the solar atmosphere, but it is almost certainly a result of the Sun’s magnetic field.
HOW OLD IS THE SUN?
The Sun is about 4.57 billion years old. It is middle-aged, and will live for another five billion years or so. As the Sun reaches old age it will expand and brighten to become a ‘red giant’ star, eventually engulfing Mercury and Venus. Earth may just about survive the expanding Sun, but about three billion years from now the Sun’s energy output will have evaporated Earth’s oceans and atmosphere anyway! After about a billion years as a red giant, the Sun will shake off its outer layers, forming a beautiful ‘planetary nebula’. Left behind will be a small, hot ‘white dwarf’, which will survive, cooling slowly, for maybe another trillion years.
WHAT COLOUR IS THE SUN?
The spectrum of light from the Sun peaks at a wavelength which we would normally describe as green. However, across the narrow range of the visible spectrum the amount of light emitted at each wavelength is almost exactly the same. But more crucially, the human eye does not perceive light by averaging the various colours of the spectrum together. So, a very slight excess of green light does not look green to the human eye – it looks white. The Sun would have to emit only green light for our eyes to perceive it as green. So, why does it generally look yellow? It is because the Earth’s atmosphere scatters blue light more efficiently than red light. This slight deficit in blue light means the eye perceives the colour of the Sun as yellow. In space, though, the true colour of the Sun is obvious; it is white.
HOW STRONG IS THE SUN’S MAGNETIC FIELD?
The most recent measurements have shown that typical magnetic fields just above the solar surface lie between two gauss and six gauss. By way of comparison, the Earth’s magnetic field at the surface ranges between 0.25 gauss and 0.65 gauss, about 10 per cent that of the Sun’s. These are actually quite weak magnetic fields. The strength of a fridge magnet is about 100 gauss. The magnet in a typical audio loudspeaker is about 10,000 gauss, while MRIs use magnets of about 30,000 gauss. The strongest known magnetic fields, around objects called ‘magnetars’ (a type of neutron star), can be as high as a quadrillion gauss. Such magnetic fields would warp all the atoms in your body, killing you instantly!