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C - Factary




Coronal Diagnostic Spectrometer, or in other words it is a Spectrometer designed to diagnose the solar corona. You are probably more familiar with the idea of 'diagnosing' from visits to the doctor, but in general it just means finding out what is going on from the symptoms - in the case of the Sun all the symptoms are hidden in the radiation we receive from it.


CDS Spectrum


This ultraviolet spectrometer aboard SOHO is designed to study the solar spectrum from 18 to 80 nanometres. Some of the spectra and images illustrated on these web pages were obtained with the CDS.




Named after the Swedish scientist Anders Celsius.

A scale of temperature. It’s the official name for what is commonly called ‘centigrade’. It is abbreviated or shortened to °C. Water freezes at 0 °C and boils at 100 °C.


Zero degrees Celsius is equal to approximately 273 kelvin, where a kelvin is the official scientific unit of temperature. Zero on the Kelvin scale corresponds to minus 273.15 °C, and is called absolute zero.


Another scale you may have heard of is the Fahrenheit scale. This is a scale which is no longer used by scientists, but is still referred to in some countries (including the UK).


Celsius, Anders (1701-1744)Celsius, Anders (1701-1744)


The Swedish scientist who developed a scale of temperature and various types of thermometer.


If you want to know more about Celsius and what he did have a look at the temperature entry.




This stands for Charge, Element, and Isotope Analysis System. It's an instrument on board SOHO which analyses particles in the solar wind. It tells us what elements are being blown off the Sun.




See Celsius


Charge (electrical)


There are two types of electrical charge, positive and negative. Charges aren't actual particles but 'states' which particles can be in. Just like you are male or female, particles can be positive or negative (yes, some particles can be neutral in terms of electric charge so we will not take the analogy too far!). For instance electrons are negatively charged and protons are positively charged. If two electrons are brought together they will repel (push each other apart). So will two protons. If a proton and an electron are brought together then they will attract (pull together).


That's why a balloon sticks to the wall after you have rubbed it on some woollen clothing. The rubbing of the balloon ‘steals’ electrons from your clothes and the balloon becomes negatively charged. When you put it next to the wall it repels the electrons in the wall and the surface of the wall becomes slightly positively charged. The wall will then attract the electrons on the balloon and so the balloon sticks to the wall – until the charges leak away and the balloon and wall become neutral again.




The layer of the solar atmosphere that is located between the photosphere and the transition region. The chromosphere is hotter than the photosphere but not as hot as the transition region or the corona. The word comes from the Greek word 'chromos' meaning colour because the chromosphere gives out mostly red light and so appears particularly colourful to us.




This is a set of four independent spacecraft launched in 2000 and named Rumba, Tango, Salsa and Samba. The Cluster II satellites are in high orbits around the Earth and are giving us the best information yet on how the Sun affects the space environment near the Earth. For the first time we are able to study the Earth's magnetic field from four viewpoints, like having four cameras at a football match -- one behind the goal and three others at different angles.


Why is it Cluster II as opposed to Cluster I? Because the rocket being used to launch the first version of Cluster blew up shortly after takeoff, destroying all the satellites and so the satellites were rebuilt and launched as the Cluster II mission.





See Coronal Mass Ejection


Colours of stars


Look out on any cloud-free night and you see lots of stars. What colour are they? At first sight they all appear to be white points of light, so why do we talk of the colours of stars? Part of the problem is that the human eye is not very good at seeing colour in faint objects. Take a longer, more careful look at some of those stars and you may see that in fact they are not all 'white points of light'.

One of the easiest coloured stars to spot is in the constellation of Orion. It's best seen on late, winter evenings in the northern hemisphere. Below on the left is a black and white image taken from the 'Students for the Exploration and Development of Space' website. On the right is a trick colour image taken by David Malin. It contains several out-of-focus exposures and is a great way to show the real colour of the stars.

Compare the bright star in the top left with the bright one in the lower right. They are both truly giant stars, but the top left star (called Betelgeuse) is a cool, red giant while the lower right (called Rigel) is a hot, blue giant.

The colour of a star depends on the temperature of its surface and stars show a whole sequence of colours from red to blue depending upon their temperature.




Comets are small objects found in our solar system. They are made of rock and ice and usually have very elliptical orbits around the Sun. They are only visible when they get close to the Sun and are heated up by it. Halley's comet is probably the most famous and that has an orbit which makes it return close to the Sun every 76 years. Over a thousand new comets passing very close to the Sun have been discovered by the LASCO instrument on SOHO. This image shows one of them.


Find out more abouts comets in the section on solar storms and comets




See also, energy.

Conduction is the transfer of energy through matter by collisions of randomly moving atoms and electrons. Conduction occurs most efficiently in solid objects.

Metals are the best conductors of heat and electrical energy. Poor conductors are known as insulators. Plastics are good insulators for both heat and electricity.




Convection is the transfer of energy by the physical movement of matter. This process transports energy from a lower, hotter region to a higher, cooler region. A bubble of gas that is hotter than its surroundings expands and rises. While doing so it gives up its energy to its surroundings and is replaced by cooler material from above. Convection can only occur when there is a big decrease in temperature with height, such as is found in the Sun's convection zone. This motion of a gas or liquid is known as a convection current.

Convection currents are common in the Earth’s atmosphere. Birds and gliders use them to stay up in the atmosphere; the convection currents are known as thermals. Convection currents can also occur in a cup of tea. They help to cool the tea because hot tea rises to the top of the cup and gives up its energy to the surrounding (colder) air.

BrainiacNote that convection needs gravity to be able to work - you can't get convection without it. If you think that's pretty obvious or irrelevant, consider the problem of sleeping astronauts. When we breathe out, our breath contains a higher concentration of carbon dioxide (CO2) than the air we breathed in. When we are asleep and lying still, the CO2 is usually carried away into the surroundings by convection currents driven by the heat from our bodies. In a spacecraft, with a so-called 'zero-gravity' environment, no convection will occur in the spacecraft 'atmosphere' and a sleeping astronaut will soon become surrounded by air with an ever-rising concentration of CO2. That could very quickly become serious - possibly fatal! As a result, astronauts always have to sleep with a fan nearby to create the air circulation that convection would normally take care of.



Convection Zone


A layer in a star in which convection is the main way in which energy is transported outward. The Sun’s convection zone is a region inside the Sun which stretches from a distance of 70% of the radius from the centre outwards to just below the photosphere.




In solar astronomy, this is the innermost part of the Sun, where energy is released by the conversion of hydrogen to helium in nuclear reactions (see nuclear fusion).




The outermost layer of the solar atmosphere. The corona consists of a very thin plasma with a temperature greater than one million degrees Celsius. It is visible to the naked eye only during a total solar eclipse.


The solar corona is the Sun’s atmosphere. The bright glow surrounding the main body of the Sun in this image, taken by a camera on board the YOHKOH satellite, shows the radiation emitted by plasma in the solar corona.






A telescope for observing the corona by producing an artificial eclipse. It contains an occulting (nothing to do with black magic but another word for 'blocking out') disk, which covers the disk of the Sun so that the faint corona may be more easily observed. Before coronagraphs were built astronomers had to wait for a natural total solar eclipse to blot out the Sun's disk in order to let them study the corona.


Coronagraphs were invented by the French astronomer Lyot. You might not think that building a telescope with a black disk in front to block out the Sun's image would be difficult. The problem is that the very bright light from the Sun tends to get reflected a lot inside a telescope and bounces around all over the place. This so-called stray light can end up being much brighter than the Sun's corona - so a special, clever design of telescope, which avoids most of the stray-light, really is necessary.


Coronal HoleImage of a coronal hole


A region of the corona which appears dark in X-ray and ultraviolet images of the Sun.
It's dark because there is no X-ray or ultraviolet radiation emitting plasma there; there's a 'hole' in the corona - a coronal hole!

Coronal holes are usually located at the poles of the Sun (as in this image), but can occur in other places as well. They occur where the Sun’s magnetic field extends out into the solar wind rather than coming back down to the Sun's surface as it does in other parts of the Sun. In these regions the magnetic field allows the plasma of the solar corona to escape into space.

Find out more about coronal holes



Coronal Mass Ejection (CME)


A huge cloud of plasma that erupts from the Sun's corona and travels through space at high speed. The plasma typically travels at speeds of 300-400 km/s (about 750,000 mph!) but the world record (or should that be the solar system record) is a whopping 2000 km/s (4.5 million mph).


This set of images, taken with the LASCO instrument on SOHO, shows a CME erupting from the lower left limb of the Sun.


Find out more about coronal mass ejections (CME) in the section on solar storms


Coronal Streamer


Large scale magnetic structures observed in the Sun's corona. They appear as bright ‘spikes’ radiating from the Sun in this SOHO/LASCO image.



Cosmic Ray


A cosmic ray is not a ray at all, it's a particle. Cosmic rays are usually ions, ranging from a single proton, up to an iron nucleus and beyond. They come from space and are produced by a number of different sources, such as stars (including the Sun), supernovae, neutron stars, black holes and the nuclei of some galaxies.


Cosmic rays can travel at very nearly the speed of light. The most energetic particle ever observed had about 50 joules of kinetic energy (equivalent to the energy carried by the ball from a fast bowler in cricket!). No one knows what process was able to give that particle so much energy.


Cosmic rays were discovered by Victor Hess in 1911. He was an Austrian-American physicist who sent up balloons carrying electroscopes (devices capable of detecting charges) in order to try and locate the source of radiation which was creating ions in the air. Hess believed the radiation would be weaker higher up in the atmosphere, but found instead that it increased in intensity by up to eight times. As a result, he suggested that the radiation came from outer space.




COmprehensive SupraThermal and Energetic Particle analyzer.


Instrument on board the SOHO satellite which analyses high energy particles in the solar wind.


Coulomb (C)


A quantity of electricity. It's the amount of electricity used when one ampere flows for one second, and is named after Charles Augustin de Coulomb.


Coulomb, Charles Augustin de (1736-1806)Coulomb, Charles Augustin de (1736-1806)


Coulomb was trained as an engineer and in his early career was recognised as the founder of the study of friction. However, it is for his later studies of electricity that he is recognised by the naming of the coulomb unit of electrical charge.


Current (electrical)


The name given to a flow of electrical charges, usually electrons. If a potential difference (voltage) is applied across a conductor, a current will flow.


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