A planet's axis is an imaginary line running through both of its poles. Some planets, including Earth, have an axis that is slightly tilted to one side. The rate at which a planet spins on its axis determines the length of its days. The Earth takes 24 hours to make one full rotation on its axis.
A black dwarf is a star in the final stages of its evolution. It follows millions of years after the white dwarf stage when all energy sources have been expended and the star no longer emits any radiation.
Any object in the Universe possessing matter can be described as a body. However, the term is most often applied to larger objects such as planets and asteroids.
Evolution is the process by which living things develop from their previous form. The evolution of life on Earth begins with the presence of single-cell organisms in the oceans. It then charts their development over millions of years into amphibians, land mammals, apes and ultimately humans - the highest form of evolved life on the planet.
Four of the planets in the outer reaches of the Solar System are described as gas giants - Jupiter, Saturn, Uranus and Neptune. Although they have a solid core, they are composed mainly of various gases. They were formed from the lighter, gassier elements, rather than the heavier, rockier ones that produced the inner planets. Pluto, the other planet in the outer Solar System, is not a gas giant.
The branch of science that deals with the physical structure of the earth and the processes that shape it.
On Earth , a geyser tends to be a hot spring of water bubbling below the surface, which occasionally boils over, sending jets metres into the air. Elsewhere in the Solar System, however, geysers can form from gases and be much more dramatic. The nitrogen geysers on Triton, Neptune's moon, reach several kilometres into the atmosphere.
Gravity is the force that attracts any two masses, bodies or particles towards each other. It is not restricted to the relationship between objects and the Earth - it exists between all things, everywhere in the universe.
Sir Isaac Newton is credited with discovering gravity around the turn of the 18th Century. He realised that for an object to move, change speed or alter its direction, a force had to be applied to it. He called this force 'gravity'. The story of the apple falling from the tree has become a famous example of this effect.
The force of gravity around an object in space is felt in all directions, but decreases rapidly with distance. We never feel the Sun's gravity on Earth because of the massive distance between the two bodies. However, it's the Sun's gravity that keeps our planet in its orbit. Equally, we're not directly aware of the moon's gravity, yet it's powerful enough to influence the tides of the oceans on Earth.
The Greenhouse Effect refers to the increase in temperature of a planet when gases in its atmosphere trap the Sun's radiation A planet with carbon dioxide in its atmosphere will experience a stronger Greenhouse Effect. A good example of this is Venus, which has the most extreme Greenhouse Effect of all the planets. The thick cloud that covers Venus traps the Sun's rays and insulates the planet to create very high temperatures.
A term used to describe one half of a planet, typically divided into northern and southern halves by an equator. The UK can be found in the Earth's northern hemisphere, Australia in its southern hemisphere.
The kelvin (K) is the standard international unit of thermodynamic temperature.
At sea-level, on Earth, water freezes at 0 °C or +273.15 K, and boils at +100 °C or +373.15 K.
A temperature of 0 K corresponds to -273.15 °C. So to convert a kelvin temperature figure to Celsius, subtract 273.15 and vice-versa.
The size of the kelvin "degree" is the same as the size of the Celsius "degree." A change of plus-or-minus 1 °C is the same as a change of plus-or-minus 1 K.
There is no such thing as a minus kelvin figure. A temperature of 0 K represents absolute zero, the absence of all heat.
Nuclear fusion takes place when the nuclei of two atoms collide and fuse together to form one heavier element. Nuclei have the same charge and will initially repel each other, so it takes a huge amount of energy to fuse them together.
This process happens constantly within the Sun, converting hydrogen into helium. These reactions provide sufficient energy to heat and light the Earth. It is also the process that led to the creation of the Sun 4.6 billion years ago. As a cloud of dust in space contracted, it heated to the point where fusion could occur and the early Sun formed.
Any body in orbit around another has an orbital plane. This refers to the level, or "plane" on which it circles another body. Apart from Pluto, all the planets in our Solar System orbit the Sun on the same level, and so all sit on the same orbital plane.
Although it appears to be motionless and static, the Earth's crust is made up of a number of huge 'plates' called tectonic plates that move and shift. It is the movement of these plates that produces mountains and valleys and also causes volcanoes and earthquakes.
The surface of the Earth is broken into large plates that move very gradually over time. Where the edges of two plates come together, there is often intense and sometimes dangerous geological activity, including earthquakes and volcanoes. The massive pressure can also create mountain ranges, which continue to push upwards for many years, as the plates buckle against each other.
Astronomers traditionally looked at the sky by studying the light generated by objects like stars and planets. However, it was later discovered that these objects also emit radio waves, which often contain more detailed information. Since this discovery, scientists have developed sophisticated equipment, including powerful radio telescopes, to detect and measure these waves.
By looking at the sky with both radio and optical telescopes, we have gained a much more complete view of the Universe.
A vortex is a mass of whirling fluid or gas. The Great Red Spot is possibly the most powerful vortex in our Solar System. It has raged on the planet for as long as we have been able to see Jupiter itself - some 300 years. This violent storm maintains its energy mainly due to Jupiter's high rate of spin.
At some point in their existence, stars like our Sun stop burning, lose their brightness and eventually shrink in size. Scientists estimate that it will take another five billion years for the Sun to burn all its hydrogen and reach this 'white dwarf' stage.
The first extinct stars discovered were white in colour, and now all stars that reach this stage are labelled white dwarves, regardless of their actual colour.
When our Sun reaches this stage, it will become smaller, but also incredibly dense. One teaspoon would weigh about 10 tonnes.
