## Star lightPainting and astronomy

My "Sky" paintings used to be called the "Light" series because of their origins. These small paintings are all inspired by my own astronomy observations in the garden - either with my astronomy binoculars or my small telescope. Because, it may not surprise you, I'm not just a geology buff but also an astronomy buff. (BSc in physics, with focus on astrophysics)

From my out-of-print book Scenes of Art and Science, I'd like to share the chapter on painting stars and light.

Starlight, acrylic on canvas 30x30cm

From my garden one of the simplest and most enjoyable quests is to spot different colour stars with my telescope. Sometimes I use a star map, other times just scan for nice arrangements of stars. In my drawings I try to always note distinct star colours if I can see them.

Though I do paint the star colours, the essence of colour in light is different from colour in pigments.

Paint is reflective colour - colour of physical surfaces, including the tiny particles of pigment in paint. The colour of paint is made by the surface absorbing all the colours of light except the one we see. That colour is reflected back at the viewer. So red paint absorbs all the colours of light except red, and we see just the red bounced back at us.

The colour of stars is actual the light being emitted by the object - the burning star.

The Hertzprung-Russell Diagram is a chart that shows the relationship between star temperatures and luminosity. (actual brightness, ignoring any dimming from our distance from the star) Our Sun lies about half way down the main strip running diagonally through the chart - the “Main Sequence”. Quite simply, our Sun is the star humanity has known the longest so it became our reference to use as the median value for reference. Luminosity value of 1 is defined by our Sun.

#### The Main Sequence is the lifeline of an average star.

It’s born at the top right (hot and bright) and then slowly grows with burning of it’s elements. Some stars evolve off the Main Sequence into giant stars, some continue steadily down the sequence, and some run out of fuel and fall off the sequence down to become a white or brown dwarf.

Looking along the bottom axis, the temperature goes from hottest on the left to coolest on the right. Easy to see the hottest stars from 10,000 to 30,000 Kelvin are the newborn blue stars. The reddest stars at the top right are very bright but actually the coldest temperatures. (but still very very hot compared to on Earth!)

Star 16, acrylic on paper 10x21cm

In the world around us we think of hot things, burning things, as warm colours like red and yellows. The warm sun and sunlight is yellow. Fires are red. Hot metal is red. In contrast, ice and glaciers are blue. This are generalized interpretations. Instead of the psychological temperature we associate with colours in the world, colours in light are directly linked to their wavelength. And wavelength is temperature.

In light, blue is hotter than red. A red star is older and colder than a young hot blue star.

The challenge is to paint this light with non-light. To mix the colours but somehow retain that sense of brightness, the luminosity the starlight has. I need to get that light bouncing off the paint surface as if it were just emitting light itself. This means experiments with yellow sunlight, moonlight, and combining ranges of colours. So to paint a Main Sequence “white” star I actually worked from white to yellow to orange. To paint Betelguese, a red supergiant, I set the red against a dark halo of deep almost-black purple before it then blended into the blue sky. Equally “New Blue” takes the brilliant blue of stars but in paint I’m aiming for the clearly blue - without someone looking at it interpreting it as white light.

New Star, acrylic on canvas 30x30cm

#### Buy why are new stars blue?

It’s simply their temperature. New stars have lots of mass because they’ve only just started burning the hydrogen in their core. Their massive amount of burning means a very high star temperature. In fact some burn so hot they’re ultraviolet, but we can’t see this “colour” so we see it as the next nearest wavelength - blue.

As hydrogen is used up in the star core, the star will start burning helium and creating carbon. The temperature of this reaction is lower, so the light wavelength changes - to white or yellow. When a star becomes a giant it has expanded to be larger and the reactions are also taking place at a lower temperature again, to a cooler red.

Next time you look at the night sky - keep an eye out for hints of blue and red alongside the white points of light.

The eBook (pdf) of Scenes of Art & Science is available at grejgallery.etsy.com:

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