In studying Astronomy it is important to keep an open mind and remember that there may be more than one solution to the observational facts. Tycho Brahe described an Earth-centred universe in which he was able to predict with great accuracy the positions of the planets. His calculations were sound but his theory was wrong! Nevertheless, the majority of his contemporary astronomers believed his theory. It is the aim of this website to present as balanced picture of our understanding of Astronomy as possible.

Structures of the Universe

This page lists the structures of the Universe, starting at the top with the largest and ending at the bottom with the smallest.

The Multiverse

The Universe

The universe in which we live began as a big bang some 12 billion years ago. However, some astronomers do not believe that such a cataclysmic event took place, instead they believe in a steady-state universe that has always looked much the same as it does today.

Galaxies Clusters

• The Great Wall is a string of galaxies that stretches for 100 million light-years
• The Virgo Supercluster also known as the Local Supercluster is centred 50 million light-years away in the constellation of Virgo. The Sun is a member of the Virgo Supercluster.

Galaxies

It is now thought that at the heart of each galaxy is a super-massive black hole. When matter falls onto this black hole, it liberates vast quantities of energy enabling the galaxy to be seen from great distances. Such galaxies are known as quasars and they are some of the most remotest objects in the universe. Some quasars align with seemingly nearby galaxies, these are known as Arp Pairs (or Arp galaxies) after Halton Arp who first pointed them out. According to Arp, this disproves the Big Bang Theory - but most astronomers believe they are just chance alignments. More.

The Sun is a member of a galaxy known as the Milky Way.

Stars, Nebulae and Dark Matter

• Stars - almost spherical bodies that generate light and other energy.
• Nebulae - vast clouds of dust and gas that can only bee seen by reflecting the light of neighboring stars, or they may be 'seen' as a dark cloud silhouetted against a bright background.
• Dark Matter - non-luminous bodies that can not be 'seen' but whose gravity may have an effect of neighboring stars and nebulae.

Planetary Systems

Around many stars can be found planetary systems, like our own Solar System. A planetary system is made up of a number of non-luminous bodies that orbit their parent star (or stars) whose light they reflect making them visible in the sky. There are different types of body in a planetary system.

• Planets - these are the largest objects in the system and orbit the parent star. The Earth is a planet that orbits the Sun (its parent star).
• Natural Satellites (or moons) orbit a planet, just as our Moon orbits the Earth. Some planets have many moons, whilst others have non at all. Although moons are always smaller than the planet they orbit, a few moons are larger than the Planet Mercury.
• Asteroids (or minor planets) - these bodies are fairly small (smaller than most moons) but they orbit the parent star. Some asteroids have moons of their own.
• Comets - these are matter 'left over' from the creation of the system in which they dwell. When they approach their parent star, they give off a tail of dust or gas (or both) by which means the comet can be seen. A comet may have lost all its dust and gas and not present a tail.
• Meteors These are small rocks, often the size of a garden pea or potato that could be either debris from an asteroid, comet or planet. Meteor showers appear regularly in the sky and these are associated with a comet. Most meteors burn up in the Earth's atmosphere but some are large enough to reach the ground - these are known as meteorites.
• Interplanetary Dust - this is the same as a meteor but smaller.

Atoms

These are the smallest parts of normal matter and consist of negatively charged electrons orbiting a positively charged nucleus such that the electrical charge on the atom is neutral. The nucleus itself is composed of positively charged protons and neutrons, which have no charge. When an atom looses electrons it becomes a positively charged ion. Antimatter is equivalent to normal matter but with the electrical charges reversed. Thus a positron has the same mass as an electron but positively charged and orbits a negatively charged nucleus. When matter and antimatter met they anialate each other liberating enormous energy. Star-Trek fans know this as the power source of star-ships.

1. General Astronomy Overview of astronomical phenomena and objects.
2. The Alphastro Guide To The Solar System The latest information about all bodies in the Solar System with associated internet links and books for sale.
3. The Alphastro Guide To Deep Space The latest information on all bodies outside the Solar System with associated internet links and books for sale.
4. The Alphastro Launch Log A manifest of every launch and current status of all artificial satellites and manned spaceflight with associated internet links and books for sale.
5. The Alphastro Bookshop Buy the latest books on astronomy, space research and associated subjects.
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