Friday, 26 February 2016

The colours of the heavens

The night's sky is full of colour. It's just that we, poor humans, are having great difficulties to see them because our eyes are... well... almost useless at night. We've got very small pupils, relatively few light-catching cells on the retina and we lack the tapetum lucidum of the true creatures of the night which reflects the light back into the retina, thus doubling the eye's capacity to see in the dark. Many people who're looking in a telescope for the first time feel probably overwhelmed when they see the craters and mountains of the Moon and rejoice when they can make out details on the planets. But during the three decades that I've participated at star parties where the public could come and take a look at the heavens, their enthusiasm often faded when the objects I showed faded as well. Some deep sky objects are incredibly spectacular, such as the Orion Nebula or large globular clusters with their hundreds of thousands of stars. But after a while you have to start showing less impressive objects, such as galaxies, planetary or faint diffuse nebula and the "wows" and "ohs" quickly change into "can you show us Saturn again?". Not that Saturn wouldn't be breathtaking to look at with its elegant rings, but in the end I usually feel a bit disappointed because there are so many other things I'd like to show and which are just as spectacular in their own right, if you know the story behind them and you start to realise what exactly it is that you're looking at (with a certain difficulty). 

Alas, observing through a telescope is a work of patience, relaxing and letting your eye (or eyes in my case) adjust to the dark image. You also have to explain that it helps to look just next to the object because most of the light catching cells on our retinas are next to the point where we focus. But in these times of fast consumerism, patience is becoming a rare quality and I have to admit that I'm no better than anybody else in that regard, perhaps on the contrary. Most of the time, people will glance into the eyepiece for about two seconds and then happily exclaim: "Wow! I've seen it!". Believe me, these people haven't seen a thing.  

Returning to my topic, as much as we're useless at distinguishing light in total darkness, we're even ten times worse at detecting colour in the dark. That's because colour is perceived through a different kind of cells on the retina than those which capture light. As soon as the dark sets in, all colours fade to grey. This is probably another reason why many people feel disappointed after having looked through a telescope for the first time. We've been conditioned too much by all of the wonderful and immensely colourful pictures of our universe that we find in books or on the internet that we tend to forget that those are the fruit of many hours of exposure time and digital rendering. 

And yet... even with our poor little eyes there is a lot of colour to see up there. The object we look at just has to be sufficiently bright for us to be able to perceive the colour. Stars, for instance, are a perfect example. Many stars show a wonderful amount of colour if you take the time to appreciate it, from great red Betelgeuze, orange Aldebaran, yellow Capella (and our sun!), green Procyon, white Vega and bright blue Rigel. The colour of the star is a consequence of its temperature and contrary to what you might expect, red is cold and blue is hot! Think about the flames in a fire... the red flames, which you usually find at the edges, are the coldest, whereas in the heart of the fire the colour changes to yellow and even white. Blue flames on the contrary, like the ones from a gas stove, are extremely hot. The colour game of stars becomes even more spectacular when we're looking at double or multiple stars. Just look at the example on this sketch: Almach or scientifically Gamma Andromedae, the third brightest star in the constellation of Andromeda. It's a beautiful double with the main star being a yellow-orangy giant and its companion a smaller but hot blue star. The companion's actually a lot more complex than you'd guess at first sight because it's a double star on its own! It's sister star can be resolved with large telescopes (with larger magnification than I used here) given perfect sky conditions and it revolves around the other in a period of 64 years. But that's not all. The blue star is itself an extremely close double, the components of which orbit each other in a period of only 2,65 days! So in total, the Gamma Andromedae system is a quadruple star!

But this is only the beginning. In my next posts I'll show you more and very unexpected colours. I'm sure that you'll be amazed!

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