The Night Sky
The nearest star to Earth is our Sun. It is a large object of burning gas and plasma fuelled by nuclear
There is a large variety of stars in the universe:
• They are classified by size and brightness.
• Our Sun is a medium sized star and is middle aged.
• The colour of stars can vary. They can be blue, orange, yellow and red.
• Stars are born, live and die. They are formed in space by gravity and during their lifetime they
burn up their nuclear fuel.
• The most common form of nuclear fuel for a star is hydrogen.The hydrogen fuses together to
form helium and releases vast amounts of energy.
• Nearly all the objects in the night sky that we can see easily are stars. This is because they emit
light and we can see them. There are man-made objects such as satellites that can also be seen
at night. Other occurrences are, for example, meteor showers.
• The bigger stars usually burn hotter and brighter and therefore have a shorter lifetimes.
Smaller stars are not as hot and usually have longer lifetimes.
Distances in space are vast. One light year is the distance light travels in one year through space at the speed of 30 000 000 metres per second. The distance travelled is 9 500 000 000 000 km.
Evolution of the Universe
At a temperature of 1032 K and at a time of 10-44 seconds gravity and other forces separate. This was followed by an enormous growth of the universe. At a temperature of 1014 K and a time of 10-8 seconds the first particles began to appear. These particles began to form simple atomic nuclei at a temperature of around 1010 K.By 1 million years time, at a temperature of around 3000 to 4000K, matter appears as simple atoms and light appears for the first time. By approximately 1 billion years time matter has started to form in galaxies. Our solar system forms in about 10 billion years and by the time the universe is20 billion years old the temperature has fallen to 2.7K or -273°C.
Understand how light is analysed in terms of spectra.
Theory on Spectra
Atoms can absorb energy as well as releasing energy. This absorption and release of energy occurs with the electrons. An electron can be lifted from a lower orbital to a higher orbital when the electron absorbs heat energy. When the energy is released the electron returns to a lower orbital and at the same time releasing the energy previously gained. This energy is released as electromagnetic radiation and
3 types of spectra
• Continuous spectrum: This is produced by a hot object such as a hot piece of metal.
• Emission line spectrum: Fluorescent tubes emit light at particular frequencies. This gives them a characteristic colour.
• Absorption spectrum. Dark lines can be seen in a continuous spectrum. These lines represent specific elements absorbing certain wavelengths of light.
Yellow light is given off as a result of heating
Sodium ions absorb energy when heated. The electrons are continually absorbing and releasing the energy. Some of this energy can be seen as yellow light. The colour or the frequency of the light is particular to the element that is emitting the light and acts as a fingerprint for that element.
The elements in stars are continually absorbing and releasing energy. The colour or frequency of the energy released or absorbed is a spectral fingerprint of the element(s) in the star. This helps astronomers identify exactly what the elements are that are present in any particular star. Through this method of identifying the elements that are present in stars astronomers are able to understand the structure and nuclear reactions that occur in stars.