SYNAESTHESIA IN ASTRONOMY

Space is so big, astronomers have to combine words for sight, time, and colour just to describe it. GABRIELLA BERNARDI reports.

We need both colour and temperature to describe stars, says GABRIELLA BERNARDI.

IS IT POSSIBLE TO find examples of synaesthesia in science? Were this question addressed to a public audience, it would immediately lead to grey silence into the room.

“Grey silence” is actually an example of synaesthesia: associating a colour with silence is a bit weird, because silence is related to the sense of hearing, not sight.

Synaesthesia, in fact, is a figure of speech which combines words referring to different sensory spheres. We can easily find many examples in art and literature, but what about science? It might look odd, but the field has its own examples, too.

In astronomy, for instance, two interesting cases concern the definition of a unit of measure – the light-year – and the so-called “colour temperature”.

A light-year is the distance covered by light in one year. It is a very well-known expression, but the reason why this peculiar unit of measure is used might be slightly less known. In the universe, distances are so big that using a normal measurement scale, such as kilometres, is totally impractical.

We can better realise the problem by considering the distance between the Earth and Pluto, which is approximately six billion kilometres, or 0.6 thousandths of a light-year. The nearest star is about 40 billion kilometres, or 4.3 light-years, away. The Milky Way is a disk-shaped structure with a diameter of roughly 100,000 light-years. That’s a billion billion kilometres, which in digits would have to be written as 1,000,000,000,000,000,000 km. It’s much easier to jot down 100,000 light years, for sure!

But let’s go back to the subject of synaesthesia. In this

term the first word is linked to the perception of time, and it is combined with the second, which is related to the sense of sight. The result is a unit of length, so that “light-year”, in a certain sense, could be described as a curious example of “multiple synesthesia”.

The term “colour temperature” is probably less known, although it is certainly another example of synaesthesia, but its meaning is not as arcane as it might seem at first.

If we observe a stone while heated, we notice that its colour varies with its temperature, first glowing red when it starts to heat up, then developing increasingly brighter and lighter shades of blue as the temperature gets higher, and finally becoming white when it is incandescent.

It is worth noting that, using another synaesthesia, “white-hot” is actually a synonym of “incandescent”.

Back to astrophysics, the relation between the temperature of a star and the colour of the light it emits is used to estimate its temperature. This information is needed, for example, when we are trying to determine how the star is evolving. (The same principle is also used in other branches of science, allowing temperatures to be deducted in any situation in which a thermometer cannot be used.)

Using this rule, we know that blue stars have a surface temperature of millions of degrees, while the Sun, which has a reddish-yellow color, is at roughly 5000 degrees Celsius. When the surface of a star is cooler than a few thousand degrees, it starts to become less visible because the light is mostly emitted in the infrared band, which cannot be perceived with the naked eye.

Can you think of any other examples?