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

Cosmos - - Contents -

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?

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