From this you can appreciate that space is BIG.

Energy from the Sun

Like all radiations from a point source, the energy from the Sun diminishes using an inverse square law.  Although the Sun is a huge object, the distances involved are so big that we can count it as a point source.

If we measure the radiation at a point r metres away from the Sun (provided we are not fried in the process), then we moved out to a point 2r metres from the Sun, we would find that the intensity would reduce to 1/4.  3r metres out, it would be 1/9 and so on.  The picture shows the idea.

Here are some important terms:

• Luminosity (L) - the rate at which the star gives of energy.  Units are watts (W).

• Energy flux (F) or intensity (I)- the energy per unit area.  Units are watts per square metre (W/m²).

From the definition for flux we can write:

Energy flux = total energy ÷ area

The area is the area of the sphere, so:

Luminosity varies between stars.  A big star gives out more radiant energy per second than a small star.  However, there is also the matter of temperature.  A hotter star of the same size will give out more energy per second than a cooler star.

Black body radiation is radiant energy given off by an object.  The black part is there to distinguish it from objects that have been painted.   An object that is black when cold can glow red-hot or even white hot if heated enough.  If we look at the radiation of a star, we see that it gives a spectrum as shown in the picture.

The colour of the 12000 K star is blue-white.  There is a spread of radiation, some of which is in the visible light region.  The most intense part (the peak) is in the ultra-violet region.  If we look at the 6000 K star, the greatest intensity is found in the yellow region of the electromagnetic spectrum.  Hence it is a yellow star (like the Sun).  A yellow star still gives off UV light, so you would quickly tan if you stood by it.

You can work out the luminosity of the Sun if you equate the light of a known luminosity at a known distance.  Robert Bunsen (of Bunsen burner fame) used this method.

Size and Distance

When we view an object, it subtends an angle in our eye.  We call this the angular diameter.  This is shown in the diagram:

From this diagram, we can see that:

Therefore:

Since D is much smaller than d, tan a (in radians) » a

So we can write:

a = D/d

It so happens that the Moon is 1/400 the diameter from the Sun and is 1/400 the distance.  So the two have the same angular diameter.  So in an eclipse the moon completely blacks out the disc of the Sun.  When this happens, we can see that the outer atmosphere (the corona) is about twice the diameter of the Sun's disc.  The Sun does not have a solid surface.  Most of the radiation comes from the photosphere, the bright disc.

How old is the solar system?

Most scientists believe that the Solar System is about 4500 million years old.  It is thought to have evolved from a rotating disc of gas and dust,  brought together as the result of the shock wave of a supernova explosion.

Dating the solar system comes from study of the decay of radioactive isotopes.

Radiation is the process by which an unstable parent nucleus becomes more stable by decay into a daughter nucleus by emitting particles and/or energy.  The basic form can be summed up as:

The decay can consist of several steps.  The unstable nucleus can decay to another nucleus of a different atom by a process called transmutation.  If the new nucleus is unstable it will decay again.  This is known as a decay chain.  There may be several steps, some of which last a very long time indeed, or can be very short.  Some elements have a decay time of thousands of millions of years.  In others the decay time can be microseconds.

Elements have different isotopes.  An element and its isotope have:

• The same number of protons (and electrons)

• Different numbers of neutrons.

If the isotope is unstable, it is radioactive and is called a radioisotope.  We must be aware that radioactive decay is NOT the same as nuclear fission.

There are three kinds of radiation:

• Alpha – a helium nucleus

• Beta – a high speed electron.

• Gamma – an electromagnetic radiation of wavelength about 10^-14 m.

These kinds of radiation can be emitted individually or in any combination, depending on the type of isotope that is emitting the radiation.  Often when an alpha particle is emitted the nucleus is excited and releases the excess energy in the form of a gamma ray or gamma photon.

When specimens of radioactive isotopes decay they do so entirely randomly.  There is no pattern whatsoever, and the rate of decay is not affected by temperature or other physical constraints, or chemical reactions.

The table helps us to compare the properties of radiation

We will look at the mechanisms of production of alpha and beta radiations:

With Alpha radiation, we generally have a big unstable nucleus.  Remember that the nucleus is not a neat collection of billiard balls; it is very dynamic.  Watch the animation:

You can see that the nucleus is very active.  Quite by chance two neutrons and two protons are together in the same part, and are not restrained enough.  The strong nuclear force is not strong enough to hold them in and they are kicked away by electromagnetic repulsion.  The alpha particle is ejected from the nucleus. 

Remember that the alpha is a helium nucleus, not an atom!

The nucleus is often very excited and loses the excess energy very quickly as a gamma photon.

A typical alpha decay is:

And the general equation is:

The term Q stands for energy.