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Colour: The Spectrum of Science

Go beyond the rainbow in this OU/BBC series that delves into colour, how it works and how it has written the story of our planet.

Helen Czerski goes in search of colour. She reveals what it is, what it does, and why colour doesn't exist outside of our perception.

You can see more from Helen exploring colour on the BBC website.

Discover the range of qualifications and modules from the OU related to this programme:

A photo of Helen Czerski and a rainbow-style colour gradient

Copyright: BBC

Ink in water

Separate coloured ink with this experiment

Did you know your black biro isn't black and your green felt tip isn't green? Don't believe this? Try this chromatography experiment.

A row of coloured pens all lined up

Photo by Olia Gozha on Unsplash

Biros, felt tips, marker pens - a pen is an everyday item, found in a myriad of colours and for all uses. But what if I told you that black ink isn’t just black; or brown isn’t brown.

In many pens the ink used is a mixture of chemicals that are brought together to achieve the desired colour and consistency, relying on the use of primary colours (red, blue and yellow) to generate all the others. 

We can study the contents of different inks using a technique called chromatography, literally “drawing a colour graph”.  Chromatography is the science of separation and is used in the chemical industry in many forms from purifying chemicals in a reaction, studying the contents of water, to testing for the presence of illegal compounds in athletes.

Compounds in a mixture will interact with a liquid and an absorbent material in different ways, allowing for separation to occur. In the same way that scientists can separate and study 100’s of chemicals in the human body we can turn the black ink into its components.

Why don't you try for yourself?

A pair of scissors

Photo by Glen Carrie on Unsplash


For this you will need:

  •  a coffee filter paper (ideally, but a sheet of plain kitchen roll or heavy white tissue will work too)
  •  one or more black felt-tip pens with washable (water-soluble) ink (black works best, but you can try other colours too)
  • a tall glass jam jar or tumbler (10–15 cm tall is ideal)
  • a stapler or a paper clip
  • a thin pencil (or stick of similar thickness)
  • water

Remember to do this in the kitchen!

Cut a strip of the coffee filter, or similar paper, a little longer than the height of your jar and about 20 mm wide. Draw a pencil line across the strip about 10 mm from one end of the paper. Fold over the other end, and staple it into a loop (or hold it in a loop with the paper clip), then put the pencil through the loop. Draw a small blob of ink with the felt-tip pen on the pencil line. It’s best to go over this blob several times to get as much ink on it as you can, but don’t let it get more than about 2 mm in diameter. Rest the pencil across the top of your jar, so that the paper hangs down inside it. The paper should not be touching the bottom or the sides of the jar.

Very carefully add water to the jar until it just touches the bottom of the paper (see picture on the left). It must not come up as far as the ink blob. Watch and wait. You should see the water rising up the paper, bringing the ink with it. After a few minutes, you should begin to see different colours appearing in the moving water front, as the different pigments in the ink separate out. Take the paper out when the water front is at least 1–2 cm below the pencil, since the water front keeps on moving for a while after you have removed it and it shouldn’t be allowed to reach the pencil.

Mark the maximum distance travelled by the water and then leave the paper to dry. You might like to repeat this to try the effect of different-sized spots of ink (include a sample with a very large spot) and with different makes of pens and also mixtures of different coloured pens (picture on the right) to see if you can separate out the original colours again.

A visualisation of space, and Orion's nebula

The Colour of Stars

Ever wondered why some stars appear slightly blue and others red? Looking at the life cycle of a star will explain why...

Hertzsprung-Russel Diagram identifying many well known stars in the Milky Way galaxy.

By ESO [CC BY 4.0 ], via Wikimedia Commons under Creative Commons BY-NC-SA 4.0 license

On a clear evening, look up at the night sky. With luck, from a really dark site, you might see a few thousand stars. With dark-adapted vision you should begin to notice that some stars display distinct colours – some will appear slightly redder and others slightly bluer, for instance. With sophisticated detectors attached to telescopes, this colour can be measured more clearly. In fact, the colour of stars is a fundamental clue to their nature, and how they evolve. 

Quite simply, the colour of a star is a measure of its surface temperature. Cooler stars emit more of their light at longer wavelengths and so appear redder; hotter stars emit more of their light at shorter wavelengths and so appear bluer. Beyond the visible are other parts of the spectrum that our eyes cannot see, but which can nonetheless be picked up by appropriate astronomical detectors. Cool, red stars also emit significant amounts of long wavelength infrared radiation, and hot, blue stars also emit a lot of short wavelength ultraviolet radiation.

We’re familiar with the idea that our nearest star, the Sun, appears yellowish. This is a consequence of its surface temperature of around 5500 degrees. It’s a pretty average star, with a fairly average mass, size, luminosity and temperature. The Sun, like the vast majority of stars we can see, is in the mature part of its lifecycle – a phase astronomers refer to as the “main sequence” after the location that stars lie in on a graph of their luminosity versus temperature, known as a Hertzsprung-Russell diagram after the two astronomers who devised it a century ago.

In several billion years’ time, the Sun will begin to run out of nuclear fuel.

The Sun, being fairly average, sits near the middle of the main sequence. More massive, larger stars, are more luminous, and also hotter (and therefore bluer). They sit near the top of the main sequence and are known as blue giants. Less massive, smaller stars, are less luminous, and also cooler (and therefore redder). They sit near the bottom of the main sequence and are known as red dwarfs.

But stars do not stay at the same location on the main sequence for ever. In several billion years’ time, the Sun will begin to run out of nuclear fuel. The hydrogen in its core will all have been converted to helium through the nuclear reactions that power its main sequence lifetime. At this point the Sun will begin to swell up enormously, and its outer layers will cool. It will move off to the upper right of the Hertzsprung-Russell diagram, to a location occupied by luminous, cool stars – the red giants. Eventually, the Sun will settle down to a new phase of its life, undergoing a new set of nuclear reactions in its core, converting helium into carbon and oxygen.

A picture of the night sky, showing the constellation of stars known as Orion

By Mouser (Own work) [GFDL  or CC-BY-SA-3.0 ], via Wikimedia Commons  under Creative-Commons  license

A good example of stars at different stages of their lives may be seen in the constellation of Orion, the hunter. Comprising seven bright stars, Orion is a familiar shape in the night sky – two stars mark his shoulders, and two stars mark his knees, with a belt of three stars across his middle. The star at the top right is Bellatrix, a blue giant star. It’s about 250 light years away and over 6,000 times more luminous than the Sun. Its mass is over 8 times, and its diameter around 6 times, those of the Sun, whilst its surface temperature is around 22,000 degrees. On the opposite shoulder of the constellation, the star at the top left is Betelgeuse, a red giant star. This star is further away but much more luminous than Bellatrix, and is also in a very different state. Betelgeuse is nearing the end of its life, and although it’s only about 8 times the mass of the Sun, it has swelled up to almost 1200 times the Sun’s size. Its surface temperature is a rather cool 3500 degrees.

The Sun too will eventually swell into a cool red giant, like Betelgeuse. About 5 billion years from now, its core will run out of nuclear fuel. At this point, the outer layers of the Sun (comprising about half its mass) will drift off into space, carrying away some of the helium, carbon and oxygen atoms that have been created during its lifetime. This shell of material will be visible for a few thousand years as a so-called planetary nebula, and will seed the interstellar medium with newly created elements, ready to be incorporated into new generations of stars. Meanwhile the remnant core of the Sun will collapse into a white-hot, dense object, known as a white dwarf star. With a mass around half that which the Sun originally possessed, but a size comparable to the Earth, the white dwarf will initially have a surface temperature of around 100,000 degrees. Over the next billion years or so, it will fade in brightness and gradually cool down, moving through the colours until it eventually becomes cold and dead – a black dwarf.

Meet the expert

A photo of Dr Nicholas Turner
Nicholas TurnerVisiting FellowVIEW FULL PROFILE
A photo of Dr Nicholas Turner
Nicholas TurnerVisiting Fellow

My research interests lie in the field of molecular recognition, and in particular the development of artificial (non-biological) recognition elements. Towards this end I use a technique known as Molecular Imprinting.

I am a member of the Royal Society of Chemistry and I sit on the EPSRC Peer Review College.

I am a Senior Fellow of the Higher Education Academy, awarded in recognition for assessment design within our MSc in Medicinal Chemistry.

Professor Andrew NortonProfessor of Astrophysics EducationVIEW FULL PROFILE
Professor Andrew NortonProfessor of Astrophysics Education

I have worked at the Open University since 1992 and am now Professor of Astrophysics Education based in the School of Physical Sciences, where I am currently Deputy Head of School. I’m a former vice president of the Royal Astronomical Society and earned my PhD in X-ray astronomy from Leicester University working on interacting compact binary stars. My current research focusses on time domain astrophysics from large-scale photometric surveys, including variable stars and transiting exoplanets. I have been academic consultant for several OU/BBC TV co-productions and was co-author of the OU’s “60 second adventures in astronomy” videos. I have an Erdos-Bacon-Sabbath number of 13.

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