Astronomy - The Sun


The Sun

Margret Grove

It is easy to say that it is a seething mass of gas that keeps the Earth warm and gives us light but it is so much more than that.

It is our Star and all the planets revolve around it. It is massive compared to any of the planets in our solar system. Having said that it is known as a Yellow Dwarf Star. This is explained by the comparative sizes of other stars in the Milky Way e.g. Sirius is twice as big as the sun and 25 times more luminous. Pollux is 8 times larger than the sun and Pollux is dwarfed by Arcturus which is 26 times larger. One of the largest star is VY Canus Majoris and could hold about 3 billion suns. Larger in size but not mass, this goes to Peony Nebular star, 170 times the mass of the sun.

Now for a few Stats.

The sun is approx. 93 million miles (150 Km) away from the earth.  Known in the world of Astronomy as AU. 1 astronomical unit.

Diam. 1,392,684 Km

Earth diam.  12,742 Km

Equatorial circum. 4,370,005 Km

Mass 333,060 times that of earth

Surface temp. 5,500 deg. C

Centre temp.   15 million deg. C

The surface area is just under 12,000 times that of earth

Age, at present, is approx. 4.6 billion yrs.

The sun is an almost perfect sphere as there is only a 10 Km difference between the polar and equatorial diameter. No fewer than 1 mil. Earths would fit into the sun. Even Jupiter is 1/000 the volume of the sun.

The Structure and Activity


At the core, energy is generated by Nuclear Fusion as Hydrogen converts into Helium. Because hot objects can expand the sun would explode like a giant bomb if it were not for its enormous gravitational force.

Surrounding the core is the Radiative zone. This zone is so densely packed with matter that energy from the core can take as long as 100,000 yrs. to reach the surface. The radiative zone accounts for 70% of the suns radius and the temperature ranges from 1.5 to 15 million deg C.

Surrounding this radiative zone is the convective zone. Here the pockets of gas rise, by convection, towards the solar surface, carrying energy upwards much faster than the radiative zone.

Covering the convective zone is the Photosphere which is only approx. 60 miles thick.

It appears to have a smooth spherical surface speckled by cooler areas known as sunspots. There are two parts to a sunspot, an inner known as the umbra and an outer known as the penumbra. The darker inside area has a temperature of 2,500 deg C and in contrast the outer is 3,500 deg C. The streaky filaments are called fibrils.   Sun spots are caused by local magnetic activity and they often appear in pairs which have opposite polarity. Like the poles of a magnet.



The Photosphere is also the point at which the suns hot gas becomes transparent letting light flood through.

It then merges with an upper hotter layer that is called the Chromosphere.

Extending far beyond this point is the suns outer atmosphere called the Corona. This is only visible at the time of a total Solar Eclipse.

The Corona is even hotter than the Chromosphere and seethes with activity as eruptions of Plasma burst through it.

These seething balls of Plasma are never the same two days running. They are in constant magnetic turmoil resulting in the most dramatic events in the solar system. Heat and light are not the only things our star gives us as it often hurls vast amounts of electrically charged particles towards us. This can disable power grids and wreck satellite systems.

On the sun’s surface are bright white spots or areas where solar flares have occurred. These are sudden bursts of energy from the surface and are often the warning of a far more dramatic event to take place, that of a Coronal mass ejection. These can send solar particles towards earth at the speed of light. They can overwhelm the earth’s magnetic field channeling energy poleward and producing spectacular aurorae. In massive eruptions the aurorae can be seen near the tropics.



                      Solar Flare                                                               Aurora

The suns magnetic field sometimes tangle to the point that they snap releasing pent up energy. When this happens, loops of plasma known as Prominences erupt from the solar surface following the magnetic lines making beautiful loops. These can extend up into space for anything up to 300,000 miles and last from a few days to months.  If they are seen against a dark space background they are called prominences but if they are seen coming towards the earth, then they are referred to as filaments.


Solar Prominence


The sun is 75% Hydrogen and 25% Helium. There are trace elements of heavier material these include oxygen, carbon, nitrogen, silicon, magnesium, neon, iron and Sulphur.

Every second 4 million tons of matter are converted into pure energy.

As well as the rays of light we can see there are those we cannot see. These are Radio waves and infrared through to ultraviolet waves. Observatories use different wave lengths to capture images that allow us to see things that would otherwise be invisible. 

I will talk a little about H-alpha waves a little later on.

Just a point of interest- scientists traced sunspots activity by studying tree rings. Carbon levels in tree rings are lower during the time of sunspot abundance and greater when there are fewer sunspots.

From a peaceful surface there is a build up to storms and back again. This cycle takes 22 years. The stormy times are called a solar Maximum and the peaceful ones are called a solar minimum. This process is caused by the changes in the magnetic field which becomes twisted before breaking down and renewing itself. This also reverses the suns magnetic poles.  Solar maximum is also associated with not only greater sunspot activity but with solar flares, coronal mass ejection and brighter aurorae on earth.


Solar Eclipse

There are surely not many things that are more spectacular than a total solar eclipse. This happens when the moon is perfectly aligned between the sun and the earth blocking out the light allowing the corona to be seen with clarity. The moon is 400 times smaller than the sun but the sun is 400 times further away enabling us to see this event.

At the same time our atmosphere becomes very eerie as the temperature drops, it goes dark and the birds stop singing and everything appears very still.

Because of the suns tilt of 5deg an eclipse occurs approx. every 18 months.  This can be seen from various parts of the world according to the position of the moon.

Sometimes the moon disk fails to completely cover the sun allowing the edge to be seen. This is called an annular eclipse. In fact, an eclipse can look both total and annular depending on where you are on earth. This is caused by the elliptical orbit of the moon around the earth. The further away the moon is from the earth the more sun we will be seen hence an annular eclipse.



Annular Eclipse

Astronomers will capture wonderful pictures of this event.

The next total eclipse is 2026 when it is expected that up to 95% of the sun will be covered. The next complete one is September 2090 with maximum duration in Cornwall.

Life of the Sun

The suns life began approx. 4.5 billion years ago and will continue for another 4-5 billion years when it will have burned off all its hydrogen and helium and will eventually collapse into a white dwarf. That is it in a nutshell. Now let’s take a closer look.

Before the sun was born space looked empty but it was in fact filled with gas and dust. Most of the material was hydrogen and helium and some of it was left over remnants of the violent death of a star. Waves of energy travelling through space pressed these particles closer together and gravity caused them to collapse in on themselves.

It began to spin into a disk and in the centre the material formed what is called a protostar. Over the course of the next 100,000 years. the temperature and pressure increased causing the start of the fusion of hydrogen into helium which drives the sun we know today.

Not all the material was used to form the sun and the remainder eventually formed the planets. Although the sun is considered an average star it is perfect for us to orbit. It is neither too large and fast burning or too small and dim.

The sun is approx. half way through its life and possibly its most stable. It has not changed much in the first half and will remain stable for another 4 billion years.

This is now the time for change. It will have burned off all its hydrogen by then but the helium will continue to burn. During this last period of time and while the hydrogen and helium burn the core will continue to shrink allowing the outer layers to move closer to the centre increasing the spin and gravitational pull. This in turn increases the rate at which the burning takes place.

In 3.5 billion years. the sun will be 40% brighter. An increase in luminosity also means an increase in heat. This will cause the oceans to boil and the ice caps will have long disappeared. In these conditions life as we know it will not exist.

The earth’s atmosphere will absorb this heat producing a moist greenhouse effect. This is what gave Venus the environment it has today.

In short our earth will be another hot dry Venus.

The last stage of the sun the core will burn and with the ash it will become unstable and collapse under its own weight. This will cause the core to heat up and become denser, the sun will then grow into a Red Giant. At this stage it will take in Mercury Venus and possibly our earth. Even if earth survives the intense heat would have scorched everything out of existence.


Red Giant

Finally The Death of the Sun

The red giant stage will last for approx. 120 million years. Eventually the core will ignite violently in a helium flash where most of the mass will be converted into carbon in a matter of minutes. It will shrink to around 10 times its current size and 50 times its luminosity with a temperature a little lower than today.

When all the activity has ceased then it will become a white dwarf star slowly cooling down over trillions of years.


To learn more about the sun any observations has to be made above the earth’s atmosphere. A number of countries have launched missions to do just that. Information received has improved our knowledge and understanding of the suns magnetic field and the way its solar wind interacts with the planets.

Hopefully we will be able to predict any massive and dangerous solar storm that may be unleashed in our direction.

Here are just a few     

1960 – 1968 Pioneer 5-9. Nasa

1974 Nasa and Germany sent Helios A

1976 Nasa and Germany sent Helios B

1991 and 2006 Japan

Others have been Soho, Genesis, Sterio A and B

There are others planned.

Pioneer 5 the earliest mission took a path between Venus and the earth and confirmed the existence of an interplanetary magnetic field for the first time.

Also how this field was affected by solar flares.

The Helios spacecraft studied the solar wind and magnetism.

These were also the fastest spacecraft achieving 44 miles per second. They are no longer functional but remain in orbit.

Soho is still working today returning spectacular images of the suns activities and violent weather

Do we need the Sun?

The simple answer is Yes.

Sunlight is absorbed through our skin to help our bodies to produce and use vitamins.

Plants need the sunlight to produce energy through photosynthesis and since plants form the basis of the food chain it would seem to be essential.

All animals depend on plants either directly or indirectly.

It gives us warmth, warming the atmosphere we enjoy today.

And of course it gives us light.

With these benefits there are always downsides.

Too much sun and just like anything else that is hot it will burn. Many people will have experienced this and degrees can vary from red areas through to blistering and deep skin burns needing skin grafts.

The more tragic result of long and frequent exposure to the sun is the abnormal changes made to the skin cells causing cancer. We have all been warned and given the appropriate advice but alas not always heeded.

We are continually being warned not to look at the sun with the naked eye.

Permanent damage to the retina would result rendering the person irreversibly blind.

Anyone having a telescope is aware never to point it at the sun let alone look through an eye piece. Not only would you be blind it would ruin your telescope.

So, it is imperative that we have the sun and I think we will all agree that it gives us a wonderful feeling of wellbeing.

And my personal last word is to treat the sun with respect.