1, All About the Sun

We sometimes take the Sun for granted, and barely notice its movement across the sky during the day. It rises in the east and sets in the west. It is high in the daytime sky during summer yet quite low in the sky during the winter months. Our Sun is important to us as it gives us the light and heat necessary for life to exist on Earth. The Sun powers our climate and weather systems and it has other effects our atmosphere. The Sun is a dwarf yellow star, just like those we see in the night sky. It is so bright because the Earth lies so close to it (at a mean distance of 93,000,000 miles).
Many theories abound about its creation, but one of the most favoured is that the Sun (and the planets) condensed out of a cloud of gas and dust, around 5,000 million years ago. The nebula split and formed into globules, which under the influence of gravity, became smaller and smaller, heating up the interior of the largest globule, which became the Sun. As the gas was compressed by gravity, the temperature climbed higher and higher. The Proto-Sun was started shining, only feebly at first. At a critical point the centre of the Proto-Sun reached just the right temperature to start nuclear fusion, changing vast amounts of hydrogen into the element, helium, in a very complex way. This fusion, or changing of one substance to another, is what keeps the Sun shining. The Sun is nearly all hydrogen, with a small amount of helium and a tiny amount of other trace elements that have come from other stars.

Despite the consumption of a fantastic amount of hydrogen every second (about 6,000,000 tonnes) it is estimated that there is enough hydrogen left for a further 5000 million years. The Sun, although very slightly variable in nature, is highly stable allowing life as we know it here on the Earth.

The Sun is large, with a diameter of 886,000 miles at the equator, and it would take 1,303,600 Earths to equal its volume. It travels around the Milky Way Galaxy taking about 225,000,000 years to make one complete orbit. It would take 109 Earths to fit across the face of the Sun from one edge to the other.


Density (Water = 1) 1.409
Mass (Earth = 1) 333,000
Mass 2 x 1027 tonnes (99% of the Solar System)
Escape Velocity 617 Km/Sec
Surface Gravity (Earth=1) 28
Mean Apparent Magnitude -26
Absolute Magnitude +4 (If Sun was placed at a distance of 32.6 light years or 10 parsecs from Earth)
Spectrum Class G2V
Surface Temperature Around 6,000º Kelvin (K)
Core Temperature About 15,700,000º K
Rotation period (Siderial) 25.38 days
Rotation period (Synodic) 27.27 days
Light Distance 8.3 minutes
Make-up 78% Hydrogen – 20% Helium – 2% other materials


The Sun
Image of the Sun through a “white light” filter taken by Mick Jenkins on 2014 July 4.

If we were able to see inside the Sun, what would we find? At the centre there would be the solar “Core”. It is unimaginably hot, and dense, at around 15,000,000 degrees K (Kelvin). It is here, that the simplest most abundant element in the Universe, hydrogen, is fused into helium under immense gravitational pressure. The core is the powerhouse of the Sun; the outward pressure of energy neatly counterbalanced by the inward gravitational pressure of gas.

The region above the core is called the “Radiative Zone” because energy is transported through it by radiation alone. While the radiative zone is less dense than the core, it is dense enough that energy (such as high energy photon of light) can take a very long time to make it’s way through.  Such a photon is continually absorbed and re-emitted in a “Random Walk” and it can take thousands of  years for the photon to make its way through and escape to the next layer, called the “Convective Zone”.

The convective zone is so called because energy (such as light and heat) are carried to the surface by large convection cells. As the top of the convection cells cools, the now slightly cooler gas sinks back down to be re-heated from below and the process starts again in a continuous cycle of heating and cooling making it a highly turbulent layer.

When we say “surface”, there is no hard surface such as we have on Earth because the Sun is entirely made of hot plasma gas. What we see as a surface is called the “Photosphere”, meaning ‘sphere of light’. Its temperature is around 6,000ºK and it is the region of the Sun where visible light can at last escape the Sun and flow out into space.  The photosphere has a motted appearance often referred to as “granulation”. These are the tops of vast convection cells and are called “Granules”. They have a lifetime of no more than 20 minutes before they disappear and are replaced by new ones. Granules are roughly about the size of the UK. Beneath the granules are the “Supergranules” and these are much larger and typically last about 24 hours.

Image taken by Carl Bowron on 2014 Feb 2 of sunspot group AR1967

The Sun produces its own magnetic field. This strong dipole magnetic field is trapped within the hot plasma gas of the Sun and are the cause of sunspots. As the Sun rotates on its axis, the equatorial regions rotate quicker than the higher latitudes. For example, the solar equator takes nearly 25 days to rotate once but the polar regions take around 35 days to make one revolution. This “Differential Rotation”, as it is called, tangles up the magnetic lines bound-up in the plasma. Where the magnetic lines twist and break through the photosphere, sunspots appear. Where the magnetic lines disappear, sunspots decay and fade from view.

Why is a sunspot dark? As the magnetic lines mentioned above break through the photosphere they inhibit the transfer of heat and reducing the temperature at that location. As the temperature is slightly lower it makes that region of the photosphere appear darker by contrast. If we were able to separate the sunspots from the Sun and see them on their own, they would glow as bright as the full Moon!

Sunspots are made up of two main areas. The very dark parts are called the “Umbra” (plural: “Umbrae”), and has a temperature of about 4,000ºK,. The lighter outer area is called the “Penumbra” (plural: “Penumbrae”), and it is about a thousand degrees hotter.

A sunspot can start as a “Pore” (this is a very small sunspot without a penumbra). It might stay as that and fade away or it might develop into a large complex group or develop more slowly into smaller, singular spots. After time sunspots usually become smaller, decaying into a pore once again before disappearing altogether. Only by watching a particular sunspot group over time, as it crosses the face of the Sun, can you see this development. Sunspots never appear at the Sun’s poles but are concentrated within 40 north or 40 south degrees either side of the solar equator.

Sunspots are carried by the Sun’s rotation from east to west, taking nearly 14 days to cross the face of the Sun. From the Earth the Sun appears to make a complete rotation in about 27 days (slightly longer than the true rotation of nearly 25 days and is due to the Earth having moved slightly further along its orbit around the Sun). A complete solar rotation (of around 27 days) is often called a “Carrington Rotation”. This numbering system was started by the English solar amateur astronomer, Richard Carrington, in 1853 starting at “1”.This system has survived to this day and it is still widely used particularly by amateurs. We are now at Carrington Rotation Number 2155 and this number will go on increasing by one every 27.2753 days into the future.

The number of sunspots rises and falls with the 11-year sunspot cycle. It is not exactly 11 years long but can vary by several years either way. The strength of the sunspot cycle also varies and some are stronger and more active than others. The current sunspot cycle (No. 24) does not seem to be as strong as previous cycles 22 and 23 but the period of sunspot maximum when the solar disk is more spotted than other time especially near sunspot minimum, seems to be lasting longer. See our sunspot activity graphs under Reference/Graphs and Forms for starters. Both graphs (1978-1995 and 1995-2018) are based on the solar observations made by the members of the SPA Solar Section.

There are many shapes and forms to sunspots and there is an established sunspot classification system. If you want to know more please contact me for further information. If a sunspot group contains one or more spots it is designated an “Active Region” (or “AR”) number by NOAA (the National Oceanic and Atmosphereic Administration). The current numbering system was restarted at zero again by NOAA in 1972 (there was a similar previous numering system in place before that) and they are widely used. We also use these numbers to identify the sunspot groups as they cross the face of the Sun. You will see the AR numbers in the monthly reports under Solar Highlights and on the SPA Solar news pages. If a group disappears over the western limb and then re-appears 14 days later on the eastern limb, it is designated a new AR number by NOAA. These AR number continue to increase with time currently (2014) at around 12055 (in 2002, 10,000 was reached so the sequence was started again with the “1” being ignored so we have “AR2055”).

The very large spots can be see with the (fully protected) eye alone, and are called “naked eye” sunspots (see: Top 32 largest sunspots).

The Sun is not bright all over. Through a telescope it appears as a brighter centre and a slightly darker edge (or “limb” as solar observers call it). This gentle fading of light is called “limb darkening”, and is caused by looking at the outer regions of a sphere, where the light is attenuated somewhat.

Limb darkening helps us see another item: “faculae”. Faculae are the brighter hotter (by some 300 degrees K) patches of light that are seen on the photosphere mainly near to, or around, sunspots. Limb darkening makes them easier to see. In Latin faculae means ‘little torches “. Faculae often preceed sunspots and last well after sunspots have disappeared.

In specially made narrowband filters we can often see solar prominences, filaments, plages and solar flares. The image here is slightly more orange-looking due to digital processing and it would look more red through a H-alpha telescope. The aeroplane is optional!

The chromosphere is lower layer of the solar atmosphere. Surprisingly, at around 10,000 degrees K, the chromosphere is hotter than the photosphere. It here here that we find the prominences. These are large clouds of slighly cooler gas held aloft and shaped by the strong lines of magnetic force that rise up from below. Some are quiet (or “quiescent”) but others are highly active (eruptive), changing shape as you watch them reaching great heights above the solar limb. When a prominence is seen on the solar disk it is then called a “filament” and appears dark by contrast against the brighter solar disk.

Plages are the bright areas usually seen around sunspots in H-alpha and are associated with the solar chromosphere.  The word “plage” is taken from the French language for “beach”.

Solar flares are highly eruptive events producing short bright bursts of energy, including light. They can lead to Coronal Mass Ejections (CMEs) that can affect the Earth producing aurorae.

H-alpha Sun
Image of H-alpha Sun being crossed by a jet on 2014 Sept 7 taken by Julia Wilkinson

We cannot see the solar Corona, the outer atmosphere of the Sun, as the daylight sky is far too bright and obscures it. Only during a total solar eclipse can we view the beautiful Corona. On these special occasions, when the Moon covers exactly the disk of the Sun, are the long and short delicate white streamers of the Corona visible extending away from the fully eclipsed Sun.

The outer regions of the solar “atmosphere” is the Heliosphere. Much of the Solar System is in the Heliosphere iincluding the Earth.  It is only when we travel far enough away from the Sun that we finally reach the Heliopause. At this boundary the Sun’s influence comes to an end and intersteller space begins.

Next: Looking at the Sun safely