Most people think of astronomy as a night-time pursuit but one of the most impressive objects in the sky is only observable in the daytime sky. It is the Sun, the powerhouse of the Solar System.
The Sun is fairly minor star, like many billions of others in our galaxy, but it dominates our cosmic backyard. Everything in the solar system revolves around the Sun whose diameter of 1,392,000 km is enough to contain a million Earths.
The Sun pumps out the heat we need to survive, not by burning in the sense that a fire burns, but by working like a giant nuclear furnace. Atomic reactions on a huge scale convert 7,000 million tons of hydrogen into helium every second, releasing the vast levels of energy that make the Sun shine.
Incredibly, the Sun has been converting hydrogen like this for more than 4.5 billion years already and will continue to do so for a similar length of time.
As well as being dazzlingly bright, the Sun is the only star with a face that we can study in detail. It is vital to point out at once that you should never look at the Sun directly yourself, except with proper equipment designed for the task, as it is bright enough to blind you!
Images of the Sun taken using correct filtering techniques appear to show a surface that is granulated like sugar crystals, but this is not solid. The Sun is a ball of gas and this visible “surface”, called the photosphere, is really a kind of fog, 500 km deep, where gas is at a temperature of around 5,500 degrees C. The Sun’s core may reach a temperature of 15 million degrees C.
Dark blotches that come and go on the photosphere, like solar acne, are cooler regions due to convection currents driven by disturbances in the Sun’s magnetic field. Termed sunspots, they can be several times the size of the Earth and even visible with the unaided eye, through a suitable filter. Their darkness is an effect of contrast and they would shine brightly if they could be seen in isolation. Sunspots typically last a few days. Their dark centre, called an umbra, is fringed with a lighter penumbra region.
Sunspots helped us to discover how long it takes the Sun to rotate. It turns once every 26 days at the equator but can take up to 36 days closer to the poles.
Other features visible on the Sun’s surface are faculae, which resemble bright rivers of light, and other bright regions called plages. There are also occasional explosions which appear as intense flares.
Special solar telescopes reveal flame-like prominences around the solar disk, stretching thousands of km into space. They also show themselves during a total eclipse of the Sun, along with the chromosphere, or solar atmosphere. At such times, when the Sun is blocked from view, we also see its extensive outer atmosphere, the corona, from which streams the solar wind, a flow of high-speed gas.
Satellites watching the Sun have discovered another explosive event called a coronal mass ejection. These eruptions cause the northern and southern lights, or aurora, when their electrically charged particles collide with the Earth’s magnetic field.
Observing the Sun
Traditionally, amateur astronomers have been advised to view the Sun using a technique called projection – allowing the Sun to shine through the telescope and onto a sheet of paper or card. You can use the telescope’s shadow to make sure everything is lined up – just make sure your eye does not get in the light’s path.
Modern, inexpensive telescopes often use plastic within their tubes, so check that they are suitable for solar projection or you might end up melting the instrument’s insides.
Special filters, made from glass or coated film, can be bought for telescopes, cutting the Sun’s light to a safe level for normal viewing. Only buy such filters from a reputable dealers and you must check them for damage, such as scratches or holes.
Never use the so-called solar filters which used to come with small telescopes. They can crack in the heat and are not safe. Never use coloured film, old photographic slides or CDs either. Even if they seem to filter the sunlight, they will let through invisible radiation that can harm your eyes.