2020 November 12
Our Star – The Sun
The Sun is just one star amongst the several billion that make up our galaxy, the Milky Way. Because every other star is so distant, only the Sun can be seen as a disc, the rest merely as far off pin points of light.
The Earth orbits the Sun at a mean distance of around 93 million miles. Because Earth’s orbit is slightly oval rather than a perfect circle, Earth is nearer to the Sun in the northern hemisphere’s winter and farther away in summer. Because of the angle of Earth’s tilt, the northern hemisphere is angled away from the Sun in winter at our closest approach and towards the Sun in summer when we are more distant; the opposite being true for the southern hemisphere.
About 4.6 billion years ago, the Sun (and the Solar System) was born out of a collapsing cloud of hydrogen containing small amounts of other elements such as helium, carbon, nitrogen and oxygen. Due to the pressures at the centre of this collapsing cloud, temperatures rose to a point where nuclear reactions took place and our young star was born. The Sun was slightly cooler as a young star and has become brighter and hotter with age. Eventually, the Sun’s hydrogen will run out and it will no longer be able to sustain nuclear fusion. In its efforts to secure fuel to maintain itself, the outer layers of the Sun will puff up and expand out into the Solar System, maybe even as far as the orbit of the Earth. This is called the ‘red giant phase’. As the Sun ‘dies’, the outer layers will be ejected into space and leave a very small remnant of our former star called a ‘white dwarf star’, which will eventually fade away. However, this will not happen for many billions of years yet!
The Sun comprises of several layers or zones. In the centre, is the nuclear core where nuclear reactions occur and temperatures exceed 15 million degrees. The force of gravity is causing the star to collapse on itself but the outward pressure of gas from the nuclear reactions in the core create a state of equilibrium and thus the star becomes stable as long as it has enough fuel to burn. The next layer is the ‘Radiative zone’ where energy is absorbed by gas and re-emitted as it tries to make its way to the outer layers of the Sun. Estimates are that it takes over 100,000 years for photons to finally make it to the next layer, the ‘Convective zone’. Once energy reaches this level, it is transported through the Sun’s plasma in convection cells that arrive at the Sun’s Photosphere or surface level. Of course, the Sun is entirely gaseous and does not have a surface as we know it, however the point at which light is released is called the Photosphere and in solar astronomy, we refer to that as the Sun’s ‘surface’. A feature called ‘granulation’ is often referred to on the Photosphere and this is where energy is being released from the convection cells, and cooler gas is flowing back down the convection cells into the solar interior to be re-energised. This cooler gas appears slightly darker and therefore gives the Photosphere a granular appearance.
The Sun’s Photosphere is the layer that solar astronomers observe and where sunspots can be located. First it is very important to understand that you must NEVER stare at the Sun with your naked eye or look through an unfiltered telescope or binoculars at the Sun. Harmful radiation can damage your eyesight permanently and great care must be taken when observing any solar features. Use only professionally manufactured solar filters on your telescope or if you can make your own, use Baader film which is specially constructed to repel harmful solar radiation. Never be tempted to use exposed film or coloured glass or any other substitute. The Photosphere is about 6,000 degrees and appears very bright. Sunspots are slightly cooler magnetic areas that rotate across the Photosphere and because they are cooler, they are darker and we see them as dark ‘spots’ against the brighter, hotter background of the Photosphere. Historically people used to regard sunspots as holes or depressions in the Sun as they look just like that. However, they are not. They represent strong magnetic forces at work at that location and magnetic fields are preventing the full release of energy from the interior, hence the cooler darker appearance of the sunspot regions.
Sunspots appear in a regular cycle. When sunspots are small and scarce, we call this ‘solar minimum’. Over the next few years, sunspots will become more frequent and more complex. They also appear at quite high latitudes in both solar hemispheres and migrate over the years towards the solar equator. After 4 or 5 years after solar minimum, solar maximum will be achieved when sunspots are plentiful, complex and extensive in area. Thereafter, they decline in numbers and strength, until after 11 years or so from sunspot minimum, they will again be scarce and small and so another solar minimum will have arrived. Sunspot groups can be observed as they travel east to west across the solar disc taking about 14 days to travel from one limb to the other. As the Sun is gaseous, it rotates at different speeds, rotating more slowly towards the Poles than at the equator. Solar rotation is given a mean rotational value of 27.38 days taken from the central meridian at zero degrees of longitude, travelling around the entire solar disc back to the same point. Each rotation is given a number called the Carrington rotation number.
Beyond the solar Photosphere is yet another layer, the solar Chromosphere or the Sun’s atmosphere. It is here that the dynamic contortions of the Sun’s magnetic field can be seen at its best but you need a special telescope or filter called a Hydrogen-alpha filter to see this layer. The filter is called a ‘narrow band’ filter as it filters out all the light except the narrow band of hydrogen-alpha which is towards the red end of the spectrum. Thus, when viewing the Sun in hydrogen-alpha the Sun will appear reddish rather than whitish yellow. Through this filter you can see magnetic loops filled with plasma arching out into space from the solar limb or over the brighter disc of the Sun as darker loops. These features are called ‘Prominences’ and ‘Filaments’ and are the same feature but viewed from a different aspect and thus called separate names to distinguish them. You will also see sunspot groups but not as clearly as in white light (the Photosphere) as you are seeing the area of magnetism over the top of the sunspot group. You should be able to see the magnetic disturbance around the active region and observe the magnetic field lines. You should also see bright whitish patches around the sunspot groups called ‘plage’. Sometimes this bright plage becomes very bright indeed when solar flares emanate from these plage regions releasing high energy radiation and sometimes plasma too. When this radiation/plasma reaches the Earth, it interacts with Earth’s magnetic field and the result is magnificent aurora in the northern and southern polar regions.
If you are just starting out in observational astronomy and are thinking of buying your first telescope, then solar observing is a great starting point. Because the Sun is relatively close compared to other stars, you only need a small telescope to observe it. Usually you are trying to obtain as much light as possible to observe but with the Sun we have too much light to deal with and so we need to filter out the harmful radiation even with a small telescope. Usually a small refracting telescope is ideal for projecting the Sun’s light through an unfiltered telescope onto a piece of white card placed behind the eyepiece. Telescopes that use a mirror system are not so good as heat can build up in the telescope and damage the mirror. If you use a mirror system then you must use a full aperture solar filter as in the picture above. The telescope on the left uses a mirror system and so it has a full aperture filter over the objective lens and that makes the telescope safe to look through at the solar Photosphere. The telescope on the right is a narrow band Hydrogen alpha telescope and that comes with filters built in to look at the solar Chromosphere.
We have learned an enormous amount about our nearest star over the last few decades and currently two more missions are en-route to study the solar Corona (the tenuous outer atmosphere of the Sun) and to give us even more understanding and knowledge. Solar astronomy and Space Weather are at the forefront of space exploration in the 21st century. Observing the Sun is a most rewarding and interesting pastime with the Sun’s ever changing dynamic features sure to fascinate. Remember the safety instructions and enjoy solar observing!
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