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Why do the stars shine in the sky?

Stars in the sky: how often have we taken their presence for granted? Let's stop and think about what lies behind these night lights: where their energy comes from, how we can see them from Earth, what they can be used for and what threatens our ability to admire them.

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The journey of light through the Universe

Anyone who looks up can’t help but notice the spectacle that the night sky offers us. But how many people have ever wondered what is behind a starry canopy, why the stars shine in the sky and what can be learned by looking up at night? The time has come to shed some light on these questions!

What is the origin of the ‘brilliance’ of stars?

The first step is to understand the source that allows light to travel the immense cosmic distances and hit our retinas; so why do stars shine in the sky, i.e. emit light radiation? The reason is nuclear fusion, which takes place in the nucleus of such bodies, where hydrogen is transformed into helium. The effect is twofold: on the one hand, the pressure prevents a collapse due to gravity and, on the other hand, it ‘illuminates’ the star.

Perchè le stelle brillano nel cielo: origine della luce Credits: tech.everyeye.it
Why the stars shine in the sky: origin of light Credits: tech.everyeye.it

We can now make an initial distinction and classify stars according to the colour of their emission: possible wavelengths include the frequencies of blue, yellow and red. The higher the surface temperature of the star, the more the light it emits will tend to take on shades of blue; conversely, the more the star tends towards red, the lower its temperature.

This classification also involves a variation in size. Hotter stars will generally be larger than those with a lower temperature. This distinction is summarised in a very explanatory way on the H-R (Hertzsprung-Russell) diagram, the main sequence cuts diagonally across the diagram and describes exactly the pattern illustrated above. The exceptions are mainly red giants like Betelgeuse.

Diagramma HR Credits: onthestar
Diagramma HR Credits: onthestar

Our Sun occupies the central position in the diagram, in fact, it represents the intermediate class of yellow stars with a surface temperature around 6000K. At first glance we might say that the biggest stars will be the brightest, but this statement is incorrect because it does not take into account a very important factor.

Why do stars shining in the sky have different intensities?

So far we have reasoned in general terms, in fact the luminosity we have referred to is the absolute magnitude; that is, the magnitude that measures the luminosity of stars if they were all at a distance of 10 parsecs (32.6 light years). The instruments that allow such measurements are photometers that associate a brightness with a magnitude that decreases as the brightness increases (a star with magnitude 1 is 2.5 times brighter than one with magnitude 2).

Why do stars shine in the sky? Credits: frontierarieti
Why do the stars shine in the sky? Credits: frontierarieti

The other major variable is therefore the distance of the observer, in this case us on Earth: we introduce the apparent magnitude, i.e. the brightness we can perceive on our planet. If we take our Sun as an example, its apparent magnitude is -26.8 (extremely bright, and we know it!); its absolute magnitude is 4.7 (it would have such a low brightness that it would blend into the firmament).

The last obstacle for our eyes: the Earth’s atmosphere

We have learned that the brightness of a star depends on two ‘external’ factors: the size of the celestial body and its distance. The last variable is precisely the conformation of our planet, which is enveloped by the shield that has allowed life to exist: the atmosphere. Skywatchers will know what problems the presence of this layer of air brings to astronomical observation.

Refraction effect on the sun's rays Credits: scienceabc
Refraction effect on the sun’s rays Credits: scienceabc

We have already dealt with this problem in a previous article: the atmosphere acts as a filter for observation due to the physical phenomenon of refraction. The column of air above us is constantly subject to motion and temperature variations that make it turbulent and unstable; the light, which must necessarily pass through it, is affected and generates the shimmering that appears to our eyes.

This phenomenon is more acute during the summer season, when the air is warmer; the altitude at which we find ourselves can also influence the observation: the higher the altitude, the smaller the layer of atmosphere that the starlight has to travel through. The last difference can come from an observation at the zenith compared to one close to the horizon; in the first case the path crossed will be less than in the second.

Why do we need the stars shining in the sky?

Since ancient times, starlight has enabled man to orientate himself while exploring the four corners of the planet. We know that the celestial vault offers us an indication of the direction of North thanks to the Polaris star, or rather, using its real name α Ursae Minoris or Polaris; it is part of Ursa Minor and we can identify it thanks to Ursa Major which revolves around it during the course of the year.

Vincent Van Gogh, Starry night. Credits: painting-planet.com

What might be puzzling is that the pole star is not fixed: various stars over time will have the title of pole star. This is due to the precession of the equinoxes of the Earth’s axis, which forms a complete cone over a period of 26,000 years. In 11,000 years’ time it will be the turn of the very bright Vega to indicate North, while in the time of the ancient Sumerians it was the turn of the very faint Thuban in the Dragon Constellation.

This phenomenon is mainly due to the gravitational forces exerted by the Sun and Moon on the Earth, which is not a perfect sphere, but is flattened at the poles. Like a spinning top, the axis of our planet rotates around the perpendicular to the plane of the ecliptic, i.e. the plane on which the orbit that describes the Earth around the Sun lies. A further movement, called nutation, tends to make the rotation axis oscillate during its slow precession movement.

Equnozi precession. Credits: ilsapere.org

If in our hemisphere we can be guided by Polaris, in the Southern one the reference star indicating the celestial South is σ Octantis, much less bright than the boreal polar star. Another interesting curiosity: from its height with respect to the horizon we will be able to obtain with good approximation the latitude of the point from which we observe it; for example from Rome it will be possible to admire the polar star at about 41 degrees, which corresponds exactly to the latitude of Urbe.

Endangered stars: light pollution hides the spectacle of the night sky

In our cities, it is becoming increasingly difficult to enjoy a starry sky, and this is once again due to man. Light pollution is becoming more and more of a problem, not only because it prevents us from seeing the stars, but also because it is one of the energy inefficiencies of our behaviour. Mitigating our impact on the environment

Italy at night, focus on the concentration of lights. Credits: ESA

This is why the Cities at Night initiative was born, to create a night-time mapping of the Earth to locate local energy wastage and urge corrective action. Looking at a clear starry sky, such as the one in August, we can think back to how many phenomena are hidden behind our eyes; the small, more or less brilliant flames we can admire conceal the wonders of the cosmos, characterised by impressive physical phenomena, immense distances and distant worlds.

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