Hubble composite 2012 Click to enlarge image
Image composite of galaxies in deep space taken from several years of data from the Hubble Space Telescope. Image: NASA Goddard
© Public Domain

The universe can be defined as the whole of existing things from the scale of sub-micron to outer space.

It consists of approximately 100 000 000 000 (one hundred thousand million) galaxies, each containing approximately 100 000 000 000 stars. The most distant objects that we know of are quasars, some 16 000 000 000 light years away. The age of the universe is approximately 15 000 million years (Ma) or 15 billion years (Ga), though there is some debate on this.

Origin of the universe

Recent astronomical observations have shown that the universe is expanding away from a local group of galaxies. This appears to have begun by a huge explosion some 15 billion years ago. This 'Big Bang' is the most accepted theory which best explains our present observations on the structure of the universe.

In the 'Big Bang' theory, the universe began with a primeval fireball which contained all matter and energy within the universe concentrated within a single body. During the first few minutes, the expansion was dominated by radiation at temperatures of 1011 K (The Kelvin scale is used to measure very high and very low temperatures, 0° C = 273° K). Matter consisted of only protons, neutrons and electrons. Within three to four minutes, the effects of radiation diminished and more complex forms of matter (such as the deuterium nucleus : neutron+proton) evolved. Later, most of the matter became hydrogen (a proton+electron) at a temperature of 109 K.

Hundreds of millions of years after the initial 'Big Bang', immense condensations of matter occurred, eventually forming galaxies. Our own solar system is only some 4600 million years old.

Composition of the universe

The universe is composed mainly of hydrogen with approximately 20% helium. All other chemical elements comprise less than 1%. All elements originated as hydrogen. Within the first two minutes of the 'Big Bang', hydrogen atoms fused together and helium and lithium were produced. Element production ceased at this point in the 'Big Bang' and all other elements heavier than lithium (Li) were produced during later fusion reactions within the interiors of stars.

During supernova events when massive stars exhaust their nuclear fuel in only a few days to months, collapse occurs and the enormous amount of energy caused by intense neutron flux produces the heavier elements such as uranium and gold. The violent explosion also distributes these elements back into space. As the formation of elements proceeds through innumerable star cycles, forming and exploding, the gas and dust produced is slowly enriched in the heavier elements.


The largest objects in the universe are galaxies which can be divided into several types depending upon their shape:

  • Spiral galaxies are the most common and distinctive type (e.g. the Milky Way).
  • Barred spiral galaxies are not as compact as spiral galaxies and have an inner bar which comprises the centre of the galaxy from which arms reach out.
  • Elliptical galaxies are small and difficult to observe because of their low luminosity.
  • Irregular galaxies comprise only a small percentage of all types and lack any distinctive symmetry.


Stars are the most familiar objects in the universe as we can see them almost every night with the naked eye. They vary in size from small white dwarfs to super giants. They are being formed in galaxies all the time (e.g. nearby stars in the Orion Nebula were born less than one million years ago). Stars shine as they burn-up their nuclear fuel and eventually die.

Our sun is an example of a common though smaller than average sized star. It formed approximately 4.6 billion years ago and will most likely last for another 4.6 billion years before it dies out and engulfs all of our solar system's inner planets, including the Earth.

Groupings of stars as seen from the Earth are known as constellations. They are accidental groupings (that is, they are not in any way related to each other).

The luminosity of a star is a measure of its total energy output. The luminosity scale is measured relative to our own sun (a luminosity of 1). The luminosity of stars varies widely from 10-6 to 5 x 105. The temperature of a star is always given as its surface temperature. These range from 3500° K to 80 000° K (The Kelvin scale is used to measure very high and very low temperatures, 0° C = 273° K). The colour of a star is closely related to its surface temperature. The hottest stars are blue, followed by white, and the coolest stars are yellow, to orange and finally red in colour with decreasing temperature.

  • Supergiants and giant stars are those of enormous size and great luminosity despite having low surface temperatures.
  • White dwarfs are relatively hot small stars with a great density.
  • Neutron stars contain matter which is 1014 times denser than water yet have diameters of only 20 km.
  • Pulsars are small rapidly rotating objects (up to 30 times per second) which emit radiation at regular intervals.
  • Quasars are extremely bright distant objects that extend to the limit of the universe as we know it.
  • Black holes are stars that have collapsed under their own gravitational forces. They are of such high density that their gravitational force is strong enough to even prevent any light or matter escaping.

The relative size of our sun

Our sun is not a particularly large star. For example, of the three stars making up the ‘belt’ of Orion, as seen in the Southern Hemisphere, the left hand star, Mintaka, has been estimated as being 20 x the diameter of our Sun. It is actually a double star, both of bluish-white colour. The middle star of the three, Alnitak, has been estimated as 25 x diameter of the Sun. It is a close-spaced double star, with both brilliant white in colour. The right hand star, Alnilan, has been estimated to be 30 x the diameter of our Sun, and bluish-white in colour.

Importance of the sun

The Earth obtains most of its heat and light energy from the sun. This solar energy causes water evaporated on the surface of the Earth to rise into the atmosphere before precipitating elsewhere on the Earth's surface. Wind is a result of air circulation caused by solar heating. The temperature of the surface of the sun is 6000° K (The Kelvin scale is used to measure very high and very low temperatures, 0° C = 273° K).

The temperature at the centre of the sun is 15 000 000° K.

The sun consists of:

  • an interior which contains most of the mass of the sun and where the sun's energy is produced
  • the photosphere is the layer from which the sun's energy escapes and from where light comes from (thickness of 1000 km)
  • the chromosphere has a very low density and is red in colour due to clouds of hydrogen gas (thickness of 500 km)
  • the corona can only be seen during a total eclipse, has a low density and has a temperature of 1 500 000° K

Sunspots are dark markings on the photosphere of the sun between 5° - 35° north and south of the sun's equator. The sun's energy is produced through nuclear fusion reactions in which hydrogen atoms are fused together to form helium.

Measuring astronomical objects

Electromagnetic radiation is the key to our understanding of the universe. It includes all forms of energy that travel through space in the form of waves, visible light, X-rays, ultraviolet, infrared and radiowaves. The electromagnetic spectrum is made up of radiation of varying wavelengths travelling at the speed of light. It is measured and detected using a number of different instruments.

When any element is heated, its gas emits a characteristic spectrum. The chemical composition of many objects can then be determined by examining the spectra of its heated vapours. Conversely, a cool gas absorbs the wavelengths characteristic of the atoms in it.

The fundamental measure of distance in space is the light year which is the distance that light travels through space in one year (9.46 x 1012 km). The speed of light is 300 000 km per second.

The Astronomical Unit (AU) is the mean distance between the Earth and our sun (measured in kilometres).