The colour of stars

Why are stars different colours, and which colours can we see in the night sky?

Þ The coolest stars are red with surface temperatures around 3,000°C and the hottest are white at a toasty 40,000°C. Credit: Christian Darkin / Science Photo Library

Despite the fact that the sizes of stars in the night sky are not apparent to to the naked eye, you can in fact discern subtle colours in the brighter stars – if there are any present. In other words, not all stars are the same size or colour.

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Any colour that is present is an indication of how hot a star’s visible ‘surface’ is. The coolest stars are red with surface temperatures of about 3,000ºC.

As a star’s temperature increases, as a result of there being more gas in the star – and hence more fuel to burn – it becomes hotter. Its colour changes from orange, through yellow, to white. The hottest stars are blue, with temperatures up to 40,000ºC.

More about stars:

You might have seen diagrams showing how different stars compare if they are put next to each other. This way you can compare the large, bluish star Vega with the seemingly tiny red star of Proxima Centauri.

The artist’s illustration at the top of this page shows how stars glow different colours according to their temperature.

Antares Region by Rafael Compassi.
The colourful stars of the Antares Region captured by Rafael Compassi.

If you look up on a clear night, you’ll see there are bright gems that sparkle with a tint of their true colours.

There are the red stars of Betelgeuse in Orion, Aldebaran in Taurus and Antares in Scorpius. There’s the golden-looking Arcturus in Boötes, the yellowish Capella in Auriga and the blue-whites of Rigel in Orion and Spica in Virgo.

But this is only a hint of the true nature of the heavens. If only our eyes could see them, the entire night sky would be filled with the most amazing colours.

So what’s up with our eyes? Nothing, of course – they’re just not able to pick up colours in faint light. We evolved as a daytime species, and our eyes evolved to work with the powerful light from the Sun.

The red giant star Betelgeuse (the prominent orange-red star above centre in this image), as it appears in the constellation of Orion. Credit: Rolf Löhr / CCDGuide.com
The orange-red hue of the cooler Betelgeuse above centre is in marked contrast to the hot blue stars that make up the rest of Orion. Credit: Rolf Löhr / CCDGuide.com

How do our eyes see colour?

Inside each eyeball we have cells that enable us to see, called rods and cones. The more numerous rods pick up light intensity, while the cones add in the colour.

As the light intensity falls, the cones begin to switch off and go to sleep. A good example of this can be witnessed in the evening after the Sun has set.

If you are outside, you may have noticed that the general scene begins to lose its colour as dusk sets in. This is simply due to your cones not having enough light to function, so they turn off.

If we translate this to the sky, it’s easy to work out that it is only the brighter stars that are able to set off any of the cones – thus enabling us to see some colour. The other stars are too faint for this and so with the naked eye we only
see them as white.

This mosaic of stars Rigel, Betelgeuse and Sirius was created by capturing images 60 seconds apart to compare the differences in colour, caused by Earth’s atmosphere splitting the starlight and the process being picked up by the DSLR camera. Credit: Amanda Cross
This mosaic of stars Rigel, Betelgeuse and Sirius was created by capturing images 60 seconds apart to compare the differences in colour, caused by Earth’s atmosphere splitting the starlight and the effect being picked up by a DSLR camera. Credit: Amanda Cross

How to see different colours in stars

There are a few things to take into account if you’re interested in seeing a variety of star colours.

1

Time of year

The Pleiades can be found by tracing the three stars of Orion's belt and following the line they create to find what appears as a 'smudge' in the night sky. Credit: Tommy Nawratil / CCDGuide.com
The blue stars of the Pleiades are high in the night sky throughout the winter months. Credit: Tommy Nawratil / CCDGuide.com

For naked-eye stargazing in particular, one of the main factors affecting your chances of seeing any coloured stars is choosing the right time of year. The winter skies in particular are graced with the most bright stars that show a colour tint. However, there is the odd star or two visible at any time in any month.

2

Light pollution

The glare of light pollution can be a menace for amateur astronomers. Credit: Steve Marsh
The glare of light pollution will seriously impede your view of the night sky. Credit: Steve Marsh

It cannot be underestimated how detrimental streetlights or general town sky-glow is. We all know that artificial lighting can wash out the number of stars, and it does nothing for the colours of the stars that remain visible either. Therefore, the darker your observing location, the better.

3

Telescopes & binoculars

Binoculars are a great way for newcomers to get into astronomy. Credit: lucentius / Getty Images
Even a modest pair of binoculars should allow you to pick out different colours in the night sky. Credit: lucentius / Getty Images

By using one of these, you can more or less overcome all the previous hindrances and obstacles to good colour-spotting. A telescope will show the fainter stars and by magnifying their light, it’ll bring out their colours too. You can also try de-focusing, thereby increasing the size of a star’s light from a point source to a coloured disc.

4

Dark adaption

Dunkery Beacon in Exmoor National Park is one of many places in the UK where beautiful vistas of the night sky can be seen. Credit: Keith Trueman
Dunkery Beacon in Exmoor National Park is one of many places in the UK where beautiful vistas of the night sky can be seen. Credit: Keith Trueman

Make sure your eyes are dark-adapted. Staying in a dark place for an hour will enable your rods and cones to get as much out of the feeble starlight as possible. Even better: get yourself to a local dark-sky location. You should also remember to use a red-light torch when finding your way in the dark, the red light won’t affect your dark adaptation as much as other colours.

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This article originally appeared in the July 2011 issue of BBC Sky at Night Magazine.