Astronomy Notes

This is a brief introduction to some of the wonders of the universe. There are many wonderful sights within our universe that most of us never get to learn about. So if you have even a vague interest in deep space, please read on.
The low resolution photographs I have used as illustrations throughout this document are reproduced with the kind permission of and are available in high resolution poster and slide form from The Anglo Australian Observatory. You can use this link, or the one on my Links Page to visit their catalogue and see what else they have to offer.

Stars GalaxiesNebulae
Neutron Stars
Black Holes
The AAO and Me
Where did the Big Bang occur


A star is a large ball of gas that is undergoing a nuclear fusion reaction. The younger stars are fusing hydrogen into helium (see diagram below). This reaction is the most common in the universe and produces a great deal of energy which is manifested as electromagnetic radiation mostly (heat and light are a small part of the electromagnetic spectrum).
As stars age and use up their enormous supplies of hydrogen they start to fuse heavier atoms to form other elements. We are all the product of a fusion reaction! The nearest star to Earth is the sun which supplies all of the energy we have on earth and is about 150,000,000 Km away. The next nearest is the triple consisting of Alpha-a, Alpha-b and Proxima Centauri which are about 4.8 light years away or about 43,000,000,000,000 Km away. One light year (the distance light travels in a year) is about 6,000,000,000,000 miles or 9,600,000,000,000 kilometres.
The largest star so far discovered appears to be about 10,000,000 times the size of our sun. It' diameter is about 310 milliom kilometres or 186 million miles. If it replaced our sun its corona or outer layer would enclose the earth. It should have a lifespan of about 100 million years before it becomes a black hole!

A simplified approximation of the atomic reaction within stars

Neutron Stars, Pulsars and Black Holes

A Neutron Star results when a large star uses up all of its atomic fuel. Without the outward pressure of its atomic reaction there is little to resist the inward pressure of gravity. The star collapses in on itself. In such a star, gravity is so strong that it overcomes the energy that keeps the electrons in orbit about the atoms and so even the atoms collapse. The positively charged protons and the negatively charged electrons within the atoms combine so that the star approximates a ball of neutrons.
As it collapses it produces a large amount of energy and its rate of spin increases, just as a skater increases her spin rate by pulling in her arms. If the majority of this energy is emitted in only one or perhaps two (usually opposite)directions we call it a pulsar.

A Pulsar is like a spinning flashlight. Every time it points its energy beam at us we can detect it and as soon as it points a little away from us we can no longer detect it. It appears to be turning on and off or pulsing. This pulsing can be quite rapid and is usually very regular, more regular even than most of our clocks!

Black Holes
If the star that runs out of energy is very massive, say three or more times the size of our sun, then the gravitational collapse may go to a stage further forming a mass that is so dense that its gravity is strong enough to prevent even the escape of light. Such a massive object is known as a Black Hole. The picture to the left is of Messier 87 which is an elyptical galaxy known to host a black hole.
We can only detect Black Holes because of a phenomenon known as the event horison. We do not know much about the actual nature of a black hole, but the mathematics suggests that even the quantum fields collapse leaving a so called singularity. This is a point in space that has no size, but has the same mass as the massive star it was derived from. This means that it would "warp" the space-time continuum around it - if you can imagine a heavy sinker or lead ball sitting on a rubber membrane you have a simple 3 Dimensional version of this 4 Dimensional phenomenon. It forms a kind of well into which things can fall. The size of the black hole is determined by the diameter of the edge of the well, not the lead ball. The rim of the well is analogous to the Event Horizon of the Black Hole

The word Quasar is the corruption of the name Quasi Stellar Object which was given to a group of super bright starlike objects found over 10 billion light years away from us. Their energy output varies from about 10 to 1000 times the output of our entire galaxy. And remembering that our galaxy, the Milky Way, consists of about 100 million stars, that is a massive output of energy! That energy is output from a region only about the size of our solar system! Quasars are certainly a puzzle! The most common theories state that a Quasar is a supermassive black hole, about 100 million 100,000,000 times the size of our sun. if such an object occupied the place of our sun its Event Horizon would have a diameter as large as the orbit of Saturn.
Of course, what we see is not the black hole itself. Such an object would have such massive gravitational force that even electromagnetic radiation (visible light is a small part of the electromagnetic spectrum) could not escape. What we are most likely seeing is the outpouring of energy from stellar material as it is torn apart just before or just as it crosses the black hole's Event horison
Some astronomers now believe that most galaxies may host a "dormant" Quasar. This theory is supported to some degree by the recent photographs taken by the Hubble Space Telescope of what are believed to be host galaxies of active quasars.

Galaxies are large collections of stars that are tied together by gravitational forces. They come in several forms, but the prettiest of them are the spiral galaxies.

Our galaxy, the Milky Way, is about 100,000 light years across and 16,000 light years thick at its centre. It is about 3,000 light years thick out here where we are (about two thirds of the way out along one of the spiral arms). It contains about one hundred million (100,000,000) stars and it is just an average sized spiral galaxy.
The overall structure of the universe appears to be as though the galaxies were on the surface of a series of bubbles - somewhat like soapsuds - although this may only be a false impression given due to our relative position in the universe!


A Nebula is a mass of gas and dust in interstellar space. Because different elements and compounds absorb and reflect different parts of the electromagnetic spectrum the colour of a nebula depends on its composition. The true colour of a nebula is not always obvious because of the different photographic techniques used to capture the image on film. What we see is often a "false colour" image. That is, the colour often reflects differences in temperature or photographic techniques rather than true colours. Even so, the images are beautiful.
There are four main types of nebulae. They are:

Emission Nebulae
A Picture of the Orion Emission Nebula

Emission Nebulae are so named because the light by which we see them was emitted rather than reflected by them. This is usually due to the temperature (or excitement) of the particles within the nebula. The normal cause of this "excitement" is the proximity of young hot stars. These Nebulae contain a large quantity of hydrogen gas in them which emits a red light and usually also contain a large quantity of oxygen which emits a green glow. Together these elements provide the yellow to red range of colours we see in emission nebulae. The Orion Nebula pictured here is an example of an emission nebula.

Reflection Nebulae
A Picture of the Orion Reflection Nebula Reflection Nebulae on the other hand are seen in reflected light. They usually have a large proportion of tiny interstellar dust particles in them which tend to reflect or disperse mainly blue light and so overwhelm the rest of the spectrum when the whole spectrum is available to them. If there is little or no blue light to reflect then they can appear quite different. The light they reflect may come from nearby stars or even another nebula. The Reflection Nebula in Orion pictured here is an example of this type of nebula.

Planetary Nebulae
A Picture of the Helix Nebula A Planetary Nebula results when a star sheds its outer shell. This occurs during some traumatic event in the star's life cycle, usually its demise. Hence planetary nebulae tend to be hollow Spheres of stellar gas and debris which expand as they move away from their point of origin until they eventually dissipate. This type of nebula is the most common. The Helix Nebula is an example of this type.

Dark Nebulae
A Picture of the Horse Head Nebula Dark Nebulae are clouds of interstellar dust which are not illuminated by nearby stars or even other nebulae but are only visible due to the light they obstruct. The most famous of these is the Horse Head Nebula pictured below.

To see more exciting deep space photographs you should visit

The Anglo Australian Observatory.

The AAO and Me"

Between 1991 and 1996 (inclusive) I was stationed in Coonabarabran. This is a town of some 3000 people situated in the Warrambungles, a hilly region, about 500 kilometres north west of Sydney. The Anglo-Australian Observatory is situated just 26 kilometres outside this town.
During my time there I had the opportunity to get to know several of the people who worked at the observatory. It was also my privilege to tour the main (3.5 metre) telescope on several occasions. You can also get a tour of the entire site. It is opened up each year for just one day for visitors to get a great view of all of the Telescopes onsite and even to look through the one metre scope if you are there at the right time. Just contact the AAO and ask when their open days are.
You can also go there any morning and see the sky photographs and purchase posters and slides.
If you want to have a good tour of the sky and happen to be in Coonabarabran then you can also go to the SkyWatch Observatory just outside of town. This Observatory was built by a few local residents. It is made from mud bricks and the dome over the 33cm telescope is marine ply. The whole place is beautifully made and well worth a visit. It demonstrates a way that any keen Astronomer could build a personal observatory from inexpensive materials.

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