The Colours of the Night Sky

The Colours of the Night Sky

04/03/2021Written by Alex Thompson

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Betelgeuse Imagined Illustration Credit: ESO, L. Calcada

Look up to the stars and you can see subtle differences straight away.  Due to a combination of their luminosity and distance from Earth, some are brighter than others.

But look closer and you’ll begin to see different colours as well, with some stars having a red, orange or blue glow.

These different colours are based on heat, either due to the age of the star (stars burn brighter in the early part of their lives), the type of star  (some may burn brighter before dying young, whilst others have a longer lifespan at a cooler temperature) or a combination of both.

Understanding what each of these colours represent can be very confusing. Normally we would associate blue with cold and red with hot, however, when it comes to stars the opposite is true. Hotter, younger stars are a bright white or blue in colour, whilst the colder, older stars range towards hues of orange and red.

In addition red stars are usually bigger in size, as many of them swell, becoming red supergiant stars towards the end of their lives.

The easiest example that you can see in the night sky is in Orion.

This constellation is one of the easiest to recognise, as it has three hot blue stars in a row known as Orion’s Belt.

If you take your eye up and left from Orion’s Belt, you will find the  red supergiant named Betelgeuse (pronounced Beetlejuice, just don’t say it three times!) The star forms the shoulder of our Orion constellation.

Betelgeuse is near the end of its life, with experts expecting it to explode in a supernova in the next million years, but for now it gives us a really good example of how and why some stars are a different colour to others.

Stellar Nursery Image credit: NASA/JPL-Caltech/W. Reach (SSC/Caltech)

Using this understanding of the colours of stars can also help understand galaxies.

Regular galaxies, such as the spiral galaxies of the Milky Way or our closest neighbour Andromeda, will often have a very bright orange centre with a bluer outer.

This is because the centre of galaxies is where older stars live, with the expanding outer housing younger stars and the area of “stellar nurseries”.

NASA’s Spitzer Space Telescope captured a glowing stellar nursery within a dark globule.  The image shows us hundreds of bright blue lights, each of which is a new protostars, or embryonic stars, and young stars never before seen.

Like stars and galaxies, the main two colours in nebulae are blue and red. However the reason for this is a little different.

Most nebulae can be described as diffuse nebulae, which can be then broken further into emission nebulae, reflection nebulae and dark nebulae. Whilst the latter does not exhibit light or visible radiation, emission nebulae release spectral line radiation from excited or ionized gas. A spectral line is a very precise colour that is specific to the type of gas being heated, and hydrogen (which is the predominant gas in these nebulae) happens to have a red colour, making the nebulae red. Ironically, only the hottest stars have enough ultra-violet radiation to ionize hydrogen atoms so it’s the blue stars that cause this type of nebulae to be red! If you wondered why you saw little red patches around areas of blue stars on the photo of the Andromeda Galaxy, this is the reason.

Reflection nebulae, as the name suggests, reflects light from nearby stars. As nebulae are good star-forming regions these stars tend to be the hotter stars, but even if they’re not the nebulae would still appear blue as space is better at scattering this colour than others. Some emission and reflection nebulae exist alone, but some nebulae have regions of both which look spectacular.

A little different, however, is planetary nebulae. Despite the name these have nothing to do with planets (the name comes from astronomers in the early days of telescopes confusing them in the night sky for planets); they are in fact glowing, expanding shells of ionized gas emitting from red supergiant stars at the end of their lives. An example of this is the Ring Nebula. Many elements that were created in the star are present, meaning instead of the predominant element being hydrogen there are several and as ionized electrons hit them it creates several different colours.

This photograph of NASA’s Hubble Space Telescope was taken on the fifth servicing mission to the observatory in 2009. Credits: NASA

If you look up to the night sky through a telescope you may be disappointed that, instead of the vibrant colours you see on NASA images, everything looks rather dull and grey.

However, this is not a problem with your telescope, this is due to the ability of the human eye to see these colours.

Our eyes are made up of two types of sensor cells, cones and rods. The rod cells are more sensitive in dark conditions, but don’t provide the brain with information on colour, whilst cones do understand colour but only work in brighter conditions.

Telescopes in space can take in a huge amount of light at once, they can focus on one area of space for long periods of time and they are outside the distortion caused by Earth’s atmosphere.

Unfortunately our eyes cannot do any of these things, and even most ground telescopes will struggle to see distant colour.

Space telescopes will also use different filters to look for specific gases and  anything that falls outside of the visible light spectrum.

Astronomers and scientists can then blend the same image with the different filters together to create stunning images, such as those we see from the Hubble Space Telescope.

So, are the images we see in the media a real representation of a deep-space object, or have they effectively been ‘photoshopped’?  Unfortunately there is not a simple answer to this question.

Sometimes artificial colours can be added for a variety of reasons – often the light will be washed out by other colours in the picture, or the wavelength is outside of the visible part of the electromagnetic spectrum that we can see.

Occasionally colour might be added by researchers to draw attention to a certain element of the image they’re trying to study or highlight to the viewer.

The media may fail to include this information when publishing the images, but the official image released by space agencies will usually tell you how the image was taken and if anything has been enhanced.

So, whilst we’re unlikely to see nebulae or galaxies in glorious technicolour from the safety of our gardens, looking at a static printed image will never be as enjoyable as  looking at these deep-space objects through your own telescope, as you discover the wonders of the Universe for yourself.