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Astronomy Glossary

Updated Wednesday, 29th September 2004

A glossary of terms used in astronomy

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Stargazing
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Atom:
An atom is a particle which forms a building block of nature. Atoms are made up of protons and neutrons in the nucleus, and surrounding electrons. Each element has its own unique atom, and the atomic properties determine the chemical behaviour of the element.

 

Black Hole:
When a big star (at least 3 times as massive as the Sun) reaches the end of its lifetime, the core collapses and a black hole is the end result. No light can escape from black holes, and space behaves very strangely in their vicinities.

 

 

Brown Dwarf:
A brown dwarf is a failed star. The forces of pressure and gravity reached a stalemate before the temperature in the star’s core became hot enough to ignite nuclear reactions in the core, which is what happens in all other stars to make them hot and bright.

 

Cosmic Background Radiation:
This is the radiation left behind by the Big Bang. Its detectable today as radio-wavelength radiation, for example in the COBE map of the entire night sky.

 

Dark Matter:
This is a term used to describe an abundant substance in the universe whose composition is unknown.

 

Electron:
An electron is another of the fundamental particles, and it has a mass and a negative charge. Electrons often orbit nuclei in atoms.

 

Galaxy:
A galaxy is a massive cluster of stars, bound together by gravity. Galaxies come in different types: spiral, elliptical and irregular. The Milky Way is the spiral galaxy in which we live containing about 10 billion stars.

 

 

Globular Cluster:
Globular clusters are groups of several hundred thousand stars all bound together by gravity. There are globular clusters above the disk of the Milky Way, and in the early twentieth century they were used to determine our position in the Milky Way.

 

 

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Hubble Constant:
The Hubble Constant, or H0 features in Hubble’s Law. Hubble’s Law is a mathematical expression which says that the further away an object is from Earth, the faster it’s moving away from us.

 

Light Year:
A light year is how far light travels in a year. Since light travels at about 300,000 km per second, in a year it travels 9,460,000,000,000 kilometres, or 5,865,696,000,000 miles!

 

Nebula:
This term is used in astronomy to describe fuzzy patches seen in the sky! Nebulae are often vast clouds of dust and gas - some are remnants of a supernova explosions, others are stellar maternity units, where young stars are born.

 

 

Neutrino:
A neutrino is one of several types of fundamental particle, which means it can’t be broken down into anything smaller. Neutrinos are represented in particle physics with the symbol.

 

 

Neutron Star:
Neutron stars are the super dense remains of massive stars, and they are often what is left behind after a supernova explosion.

 

Positron:
A positron is the anti-matter counterpart of the electron. It has the same mass as an electron, but it has a positive charge. If a positron and an electron meet each other, they annihilate to form a photon.

 

 

Pulsar:
A pulsar is a rapidly rotating neutron star that can rotate hundreds of times each second. They give out intense radio waves, that sweep by the Earth like a lighthouse beam with every rotation, registering as a short pulse on radio telescopes. Jocelyn Bell Burnell discovered pulsars in 1967.

 

Quasar:
Quasar is short for ’quasi-stellar radio source.’ Quasars are bright, tiny, distant objects which are 100 to 1000 times as bright as galaxies but are only about as big as a solar system.

 

 

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Red Dwarf:
A red dwarf is a kind of star whose mass is between 0.08 and 0.4 times that of the Sun. The energy released by nuclear fusion in a red dwarf is carried to the star’s surface by the circular motion of the hot gasses inside it.

 

 

Red Giant:
Red giants are stars of similar mass as the Sun but at a later stage in their lifetime - the Sun will eventually become a red giant. Red giants are much bigger than the Sun, and they have a relatively low surface temperature which is responsible for their red colour.

 

Solar Wind:
This is a continuous stream of matter and radiation that comes from the Sun. When a comet approaches the Sun, the effects of the solar wind can be seen as it blows the comet’s tail.

 

 

Star:
Stars are massive hot balls of fire, with nuclear reactions in their cores giving out immense amounts of heat and light energy. There are many different kinds of star and they range in temperature, size and age. Hot stars look blue, and cool stars look red. The lifetime of a star is determined by how massive it is: high mass stars are relatively short lived, whereas low mass stars last for longer. Low mass stars evolve into red giants, and ultimately become white dwarfs and nebulae - our Sun will suffer a similar fate. High mass stars, on the other hand, die spectacularly when their cores collapse under their own gravity, creating a massive supernova explosion. What’s left behind is either a neutron star or a black hole. In fact, most of the elements in the periodic table are produced by supernova explosions, and they are only around today because of supernova explosions in the early universe.

 

 

Stellar Parallax:
This is the apparent shift in the position of a star if it is viewed from the Earth at different times throughout the year. To get an idea of the effect, hold a finger in front of you and using one eye, line it up with a vertical feature in front of you, like the edge of a door. Keep your finger where it is and look at it with your other eye: the change in viewpoint makes it look like your finger has changed position with respect to the vertical background object. The analogy with stellar parallax is that your two eyes are two positions of the Earth separated by six months, your finger is a nearby star and the vertical edge is a distant star, used as a reference point.

 

 

Supernova:
This is the immense explosion which takes place when the core of a massive star collapses under its own gravity.

 

White Dwarf:
A white dwarf is what’s left of the core of a red giant when it has lost its outer layers as nebulae. White dwarfs are extremely dense, with electrons packed very closely together in their core.

 

 

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