Although almost all of them seem white to the naked eye, each star has its own colour. What is the physical meaning of these shades?

When we look up at the sky at night, we find ourselves floating, captured by the charm of the firmament; stars seem to us like a multitude of bright whitish spots spread across the sky, more or less bright but basically similar to each other. However, in some cases, a closer look allows us to identify rather interesting details that hide clues about the wonderful physical properties of these objects.

In the summer sky, in our latitudes, since the moments just after sunset it’s possible to see a rather bright star, the first to appear in the twilight. It’s Arcturus, in the constellation of Bootes, the fourth brightest star in the whole sky and the first brightest among those in the boreal hemisphere. The nice name, maybe even ours or that of an acquaintance of ours, actually derives from the Ancient Greek árktos ôuros which literally means “the Guardian of the Bear”; and not by chance, Arcturus is near the famous Big Dipper, a group of stars representing the tail of the largest constellation of the Ursa Major.

As it gets dark, Arcturus stands out well from the other stars and not only for its brightness: in fact it’s not difficult to notice how the colour of this spot is not white, but of a delicate orange shade. Moving now our look towards the zenith (right over our heads) is another very bright star called Vega dominating the sky and it is located in the constellation of Lyra. Vega seems to us just less bright than Arcturus, although the two stars are comparable; what instead stands out no doubt is the difference in the colour of the stars, tending to bluish in case of Vega.

Trying not to fall into trivial football comparisons, we could say that Arcturus is yellow-red while Vega is white-blue. Beyond personal preferences towards one or the other shade, we might wonder why these stars are coloured while the others are not. Sure, there are white stars but there are also red, orange, blue and so on; our Sun, for example, is classified as a yellow star.

The reason why the objects in the night sky seem to us mostly white is a “problem” due to our sight: humans see badly at night. Since we are diurnal animals, we struggle to perceive colours when there is not enough light observing basically in black and white. Moreover, the reaction of our eye to the different shades of colour also changes according to how bright the environment is; we tend to distinguish more red in bright light conditions and blue and vice versa.

This mechanism is known as the Purkinje effect. However, looking through a telescope capable of collecting a greater amount of light than the human eye, the colours of the stars can be highlighted in all their glory; shades are anything but random and connected to the characteristics of these gigantic balls of incandescent gas.

The colour of the light indicates what the energy of the radiation emitted by the star is and in turn connected to the surface temperature of the celestial body; so, summing up, the colour of a star tells us how hot it is.

Let’s try to better figure out : as we have already discussed in this previous article, light waves are characterised by a well-defined frequency, a quantity that tells us how many times the wave repeats itself every second. The higher this frequency, the greater the energy carried by the waves, while a lower frequency also corresponds to a lower energy. Our eye detects these variations precisely through the various colours, with red linked to the lower energy (so to the “lower” temperature) and blue-violet to the higher one (so to the hottest stars).

It sounds like a contradiction, considering that in everyday life we are used to linking red to heat and blue to cold. When we have to open the hot water tap, we usually use the red knob, unless we were really sadistic to scare the life out of those who will use the water after us …

Joking aside, this mental association stems from the fact that on Earth we don’t have spheres of light shining with blue or light blue; and luckily, I would add. If you did encounter them, you would better run like hell as the surface temperature of these objects could reach or even exceed 30,000 degrees. When an object becomes incandescent, such as a piece of heated iron, and begins to emit visible light, the first colour we see is red. By increasing the temperature it would become orange, then yellow, white and finally blue. If it did not evaporate first, of course.

Actually, stars do not emit light of a single colour, but they are characterised by an emission spectrum, that is a set of many frequencies (therefore linked to just as many “colours”) that the body releases in a more or less intense way. For example, although our Sun emits mainly in the yellow colour, its radiation is also partially made up of blue, red, etc. light. Some of it also comes in a form that our sight cannot perceive;  for example as ultraviolet, a more energetic light wave than blue, or as infrared, a radiation characterised by a lower frequency than red light.

But why do the Sun and the stars in general emit light? What allows them to shine for millions or billions of years? The mechanism that powers the stars is nuclear fusion, one of the most efficient processes known in nature. During most of a star’s life, hydrogen is converted into helium in the central regions precisely through fusion, releasing an enormous amount of energy in the form of light radiation. The greater the efficiency of these reactions when they occur, the greater the brightness and the temperature of the star.

One of the reasons why we consider studying the sky so important is the possibility of reproducing what we learn from the celestial bodies here on Earth; if we were able to achieve controlled nuclear fusion, we would have a way to cover the energy needs of the whole planet. It is a renewable mechanism because hydrogen is the most present element in the cosmos, it is clean (Helium is produced, a noble gas that is basically harmless) and extremely performing. Imagining to completely melt only the hydrogen contained in the water of a swimming pool, we would have enough energy to keep the whole world lit for over 24 hours!

Lascia un commento

Il tuo indirizzo email non sarà pubblicato.