The Sun, like all stars, is a ball of hot gas. The surface of the Sun sends us the light and heat that provide energy for us on Earth. The Sun's energy is formed deep inside its interior by the process of nuclear fusion and slowly makes its way to the surface. In the Sun's fusion process, every second about 700 million tonnes of hydrogen are converted into 695 million tonnes of helium with the remaining 5 million tonnes being converted to energy according to the equation E = mc2. Because the light from the Sun is so intense, it is important to protect our eyesight when attempting to observe the Sun during an eclipse. It is the dramatic change in lighting levels when the Sun's light is cut off during an eclipse that helps to create such an awe inspiring event.
Figure 6-1 shows the structure of the Sun. The Sun's extremely bright surface is called the photosphere (from the Greek word photos, meaning light). If the Sun is observed using protective filters and precautions, the photosphere appears to be largely uniform in brightness, slightly darker toward its edges and often with sunspots on it.
There is a thin layer of very hot gas just above the photosphere called the chromosphere (from the Greek word for colour, chromos). Glowing loops of hydrogen plasma called prominences can often occur suspended in the Sun's magnetic field above the chromosphere. Prominences are usually associated with sunspots.
The Sun is surrounded by a tenuous layer of extremely hot gas (at about 2 000 000°C) called the corona (from the Latin word for crown). It always surrounds the Sun but is a million times fainter than the photosphere and is normally fainter than the Earth's blue sky. However, during a total solar eclipse when the bright photosphere is hidden and the blue sky is not illuminated, the corona can easily be seen surrounding the Sun and it is one of the most dramatic sights of an eclipse. The chromosphere can usually be seen as a dramatic line of crimson red around the Sun's edge at the start and end of totality and prominences can often be seen rising above.
The corona is irregular in shape with streamers extending millions of kilometres into space. The appearance of the corona is continuously varying and is different at each eclipse. The Sun acts like a giant bar magnet, so thin rays of the corona called polar tufts usually appear at the Sun's north and south poles. Coronal streamers, which appear distributed around the Sun generally appear longer and closer to the Sun's equator at sunspot minimum (low solar activity), while at sunspot maximum (high solar activity) the shape of the corona is usually more uniform with streamers more distributed around the Sun. There are generally more prominences at sunspot maximum. Sunspot numbers are expected to be approaching a maximum around 2013.
Sunspots are relatively cool regions of the solar surface where the Sun's magnetic field is especially strong. The number of sunspots on the Sun is a good indicator of the level of the Sun's activity. Sunspots appear on the photosphere. Each sunspot has a dark centre called its umbra and a less dark region called its penumbra. Sunspots normally form in groups and each spot can last for weeks. Since the Sun rotates about once every 25 days, sunspots can be observed to form and rotate across the photosphere. Figure 6-2 shows sunspots on the Sun's surface.
Sunspots are regions of the Sun where the magnetic field is especially strong, perhaps 3 000 times stronger than the average field of the Sun, of the Earth or of a toy magnet, which are all comparable in strength. Sunspots tend to occur in groups.
The number of sunspots on the Sun waxes and wanes over an 11 year period. Figure 6-3 shows average monthly sunspot numbers over the last few solar cycles. The sunspot number is not actually a direct count; but is a compound value that makes allowance not only for individual spots but also for the presence of sunspot groups.
During sunspot maximum, there are more huge eruptions on the Sun called solar flares. Flares send out particles, X rays and gamma rays that can hit the Earth; these can cause surges in power lines, affect passengers in highflying aircraft or astronauts on space vehicles and contribute to creating the aurora.
The Sun produces electromagnetic radiation over the frequency spectrum including X rays, ultraviolet (UV), visible light, and infrared (IR). Normally we see visible light as white light but it can easily be split into the component colours of the spectrum. Objects appear to us to be their characteristic colour depending on how the light is reflected from their surface. Light can be split into a spectrum by a prism or other devices that refract light.
In the daytime, the sky appears blue because sunlight is scattered by molecules of the atmosphere. This is called Rayleigh scattering. Air molecules scatter about ten times as much blue light as red light, so if you look into the sky it normally looks blue. The light which is left to continue through and is not scattered, is less blue and more red.
When the Sun is close to the horizon and the light that we see has travelled a long way through the Earth's atmosphere, it has lost most of its blue and green due to scattering and so appears to be red and orange. These colours light up the sky to give us the sunset and sunrise colours. The light can be further scattered by clouds and other molecules in the atmosphere or even by volcanic emissions providing us with some truly beautiful sunsets.
During the day, light around the horizon usually looks white, because the blue light scattered from the atmosphere between you and the horizon combines with the light from the distance (which has continued through the atmosphere and is red) to appear white. This can also explain why during a total solar eclipse, the light for 360 degrees around the horizon appears as sunset colours. The area around you at the time of the eclipse is in shadow so there is no blue light scattered from the atmosphere. The light from outside the shadow is the red and orange light, without any blue light from the sky close to you to compensate for it.
- The Sun's diameter is 1 392 000 km (= 109 times the diameter of the Earth or 1 300 000 times the volume of the Earth).
- Perihelion (minimum distance of the Earth from the Sun) is 147 098 000 km; Aphelion (maximum distance of the Earth from the Sun) is 152 098 000 km. The average distance from the Earth to the Sun is 149 597 870 km. This is known as the Astronomical Unit.
- The Sun's mass is 1.99 x 1030 kg (333 000 Earths and about 99.8% of the mass of the solar system.
- Rotation period at the equator is 25.05 days.
- Composition by mass approx 75% hydrogen, 24% Helium, 1% other.
- The time taken for light to travel from the Sun to the Earth is about 8.3 minutes so we see the Sun as it was 8.3 minutes ago.