Stars and Planets » Stars

Stars are very large and massive bodies in which thermonuclear (burning) reactions are taking place. The Sun is a typical star. It is so huge that its immensity lies beyond words and defies our comprehension. See the diagram on the left above, for which credits goes to Joe Leeson of Jet Propulsion Laboratory NASA. Even our Earth seems enormous by comparison with our ordinary daily lives. Although our Sun is only one of many trillions of similar stars scattered in the stupendous Universe, it is vast beyond our most fantastic dreams. Something of its enormous size can perhaps be dimly grasped by looking at the diagram that compares our star with the size of the Earth, the planet Jupiter and the orbits of their Moons. At a average distance of almost 150 million kilometres from our Earth, the Sun is a few hundred thousand times closer to us than the next nearest star. Because of its ‘close’ proximity we are able to study our star in much, much greater detail than we can the other stars. The Sun is slightly flattened at the poles. It's Sun’s equatorial diameter is about 1,392,000 kilometres. This is equal to 109 Earth diameters and almost 10 times the size of the largest planet, Jupiter. It has about 333,000 times the Earth's mass and is over 1,000 times more massive than Jupiter. It's volume is 1.3 million times that of the Earth.

The Sun was formed from a gas cloud which collapsed under its own gravitational attraction. When a temperature of about fifteen million degrees were reached that the main burning reactions kicked into action. The word burning has a different meaning in astronomy to its usual. In ordinary chemistry burning means a violent reaction in which a substance burns in the oxygen of the air. Examples are carbon in the form of coal burning to give carbon dioxide or hydrogen gas burning to give water.


Although a great deal of heat is given out accompanied by flames (or in the case of hydrogen an explosion if the gases are mixed), the energy given out is extremely small when compared to THERMONUCLEAR BURNING such as occurs in stars. In the core of the Sun four hydrogen nuclei (4 protons) burn to form one helium nucleus. The simplest pathway takes three stages:

Proton + Proton = Deuterium nucleus + Positron + 42.00 million electron volts
Proton + Deuteron = Helium 3 nucleus + 5.49 million electron volts
Proton + Helium 3 = Helium 4 nucleus + 12.86 million electron volts

The overall result is:

4 Protons = Helium 4 nucleus +18.77 million electron volts given out as gamma radiation

These processes are known as thermonuclear reactions and only take place at very high temperatures. The energy evolved in thermonuclear burning is enormous when compared to chemical burning. Hydrogen burning takes place at around 15 million degrees Celsius.

Ray Goodwin

Somewhere there are mountains
Glistening in the snow
Somewhere there are mountains
That we shall never know

Somewhere there are rivers
Flowing fast and free
Somewhere there are rivers
That we can never see

Somewhere there are oceans
And sun drenched island sands
Forests full of creatures
In vastly distant lands

Somewhere there’s a planet
Beneath an alien star
The people watch our tiny sun
And wonder where we are

One day perhaps we’ll find them
Across the void of space
Perhaps through ways as yet unknown
We’ll meet them face to face

The author of this web site Ray Goodwin holds B.Sc. Degrees from London University in Chemistry, Geology and Physiology and an M.Sc. in Biochemistry. He has spent most of his professional life teaching in Colleges of Technology. On his retirement he has entered the fields of astronomy, astrochemistry, astrobiology and space sciences. He has spent a great deal of his retirement in visiting amateur astronomy societies and in attending European Space Agency Symposia in ESTEC in the Netherlands and other scientific conferences in England and Sweden. He regularly attends the yearly European Astrofest in South Kensington London and other meetings in the UK. He has written scientific articles and given a number of lectures on diverse scientific subjects.

Readers of this web site are invited to e-mail the author ( and discuss their opinions of the topics dealt with and suggest any changes which they think may be helpful.

Life in the Cosmos Website
Version 01.00 - April 20, 2015.