The Lying Sky

The night sky lies.

It lies from the moment the Sun sets to the moment it rises again. It lies better than any politician or millionaire banker, it lies better than any think-tank apparatchik … it does a remarkable job of looking completely plausible whilst also being utterly, shamefacedly dishonest.

What do I mean by this? Two words: red dwarfs.

Okay, when I accuse the sky of lying, I am obviously being just a little melodramatic! However, there is a serious point, and it goes something like this…

When you look up on a clear, dark night, you can see several thousand stars. They vary in apparent brightness – some look very bright, some faint, a lot vaguely in the middle. Mostly the colours that you see will be whitish, although there are some notable exceptions. (Betelgeuse and Antares are visibly orangey-red, and to my eyes, Sirius and Rigel look somewhat more blue than white.)

But the stars that we see are completely unrepresentative.

The brighter a star is, the greater the distance at which it can still be seen. So obviously we have a selection effect in favour of brighter stars – the distance to which we can see them is greater than the distance to which we can see fainter ones.

Now, what affects a star’s brightness? The ultimate, physical cause is its mass – bigger and heavier objects burn brighter. However, mass isn’t really something you can evaluate by eye. The proximite causes are radius and surface temperature. A bigger radius means a wider object, so more surface area to radiate from. And luminosity scales with the fourth power of temperature, so if you make something twice as hot, you have 2^4 = 2x2x2x2 = 16 times as much luminosity – a rather dramatic increase!

So, considering these factors, what stars would be easiest to see over great distances? The answer is hot, big ones. And, sure enough, the majority of the ones you see from Earth are indeed hot and big. Our night sky is dominated by B- and A-type stars, these being objects with surface temperatures in the 9,000 – 15,000 or so degrees Celsius. (Rigel is type B; Sirius is type A.) There are also a fair numer of cooler F and G-type stars in our skies (our Sun is a type G2, for instance, and Procyon is an F-type).

Then there are the more orangey K-type stars. There are a few of these visible to the naked eye – Epsilon Indi, Epsilon Eridani etc. – but they’re not really that prominent. K-type stars are cooler and fainter than our Sun, perhaps 0.4 solar luminosities down to about 0.1.

We see a few reddish stars, but these are invariably red giants, such as Aldebaran or Betelguese. These stars are vast objects, hundreds of millions of kilometres wide. They’re big enough that their enormous radii compensate for their fairly cool surface temperatures, and they remain relatively bright.

But what of M-type dwarfs?

Not one – not a single one – is visible to naked eye.

And yet, by number, these objects make up around 75% of all stars. Pick a star at random, anywhere in the galaxy, and it will most likely be a red dwarf. The nearest neighbouring star, Proxima Centauri, is a red dwarf. And yet there’s not one – not one! – that we human beings can see without a telescope. Red dwarfs are cool and also small – they’re in the least-favourable spot in the radius-brightness relationship. As such they number amongst the faintest actual stars in the sky. And yet they’re the most numerous.

This is what I mean about the sky lying to us. Until the development of the telescope, we had no clue what was really out there.


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