The colour of a star tells us how hot it burns. From the dull red of Arcturus to the brilliant blue of Rigel, you can actually see these subtle colour differences with your own eyes when looking up at the night sky.
Just like an iron cast into the blacksmith’s forge, which slowly changes from red to white hot, stars emit light at different frequencies depending on their overall luminosity and energy output.
The Planck-Einstein equation E = hf is a basic way of understanding this. E is energy, f is frequency and h is the famous Planck’s constant. Higher frequency light (blue) is more energetic than lower frequency light (red) and therefore hotter and more luminous stars tend to appear more blue. Meanwhile cooler stars whose external atmospheric envelopes has expanded (red giants like Betelgeuse) appear redder.
A simple way to highlight the colour of star light is to take your smartphone camera or DSLR and manually defocus it on a target star. This will emphasise the colour and you can even produce beautiful star trails like the one below by taking a movie or long exposure star trail.
When we think about the vast array of electromagnetic radiation all around us – from Gamma rays, X-rays , UV, Microwaves and Radio waves – a natural question to ask is why do human eyes see in a very narrow band we call ‘visible light’?
The answer is undoubtably tied to the energy output of our nearest star – the Sun. Its peak radiation just happens to be at this ‘visible’ band of radiation. I’ve illustrated this below with a black body radiation profile of our Sun.
Our eyes have therefore evolved to ‘see’ this particular narrow range of otherwise insignificant wavelengths. There’s nothing inherently important about visible light – in fact it makes up a tiny 0.0035 percent of the entire electromagnetic spectrum.
Understanding this makes me wonder about the potential sensory apparatus of life that might have evolved elsewhere in the universe. Other stars with different stellar classifications to our Sun have different peak radiation profiles.
If we had evolved next to a source of intense gamma rays for instance, we would very likely be completely blind to visible light but adept at observing small granular differences in the intensity of gamma radiation.
This is Bode’s galaxy (M81), an easily accessible island universe in Ursa Major that’s visible all year round from mid to high northern latitudes. It contains approximately 250 billion stars.
It lies over 10 million light years away and has a relatively close galactic companion – the M82 Cigar galaxy. Both of these galaxies can be framed in a low power telescopic eyepiece, and you can even see them very faintly in binoculars if your skies are suitably dark.
I thought I’d use this galaxy to highlight how astronomers use the full electromagnetic spectrum of light to study galaxies and their evolution. Pictured below, therefore, are images of M81 viewed in different wavelengths of light – from X-rays to radio waves (spanning short to long wavelengths). I’ve provided a very brief description of some of the galactic features revealed by each band of light.
X-rays: a central bright patch is revealed, suggestive of a supermassive black hole within the galactic nucleus. The other bright patches correspond to X-ray binary systems.
Ultraviolet: highlights young hot stars and therefore areas of active star formation within the spiral arms of the galaxy.
Optical and Infrared: Shows the bulk of the stellar population and areas of obscuring dust and nebulosity that will seed ongoing star formation.
Radio: Reveals supernova remnants and large H2 regions of ionised gas in the vicinity of very active stellar populations.
As humans our eyes have evolved to see a very narrow band of the full EM spectrum. This evolution is tied to the fact our particular star (the Sun) releases its peak energy in these wavelengths. I always like to imagine how other species, perhaps evolving next to giant sources of x-rays, might have sensory apparatus totally blind to visible light.
Some photo highlights from the Summer Space Camp up in Thurso’s band new Newton Room. I had a great time delivering Mars and astronomy based workshops on day 2. We covered the observational history of Mars, its surface geology, the night sky, the life and death of stars and spectroscopy. Interactive sections included Mars cratering, galaxy frisbees, star cluster balloons and DIY spectrascopes.