Cities gather us in different ways, we dwell them, walk around its streets and avenues attending our own eagerness. Many cities have rise over us with huge buildings that show the ability of societies to overcome our scale, and so in a display of phenomenal energy, cities keep moving.
Now days, electric light seems to be a primary need for todays society. Cities and their lights shine over the night sky, blinding our eyes of the ordinary exercise of looking to the starry night that practice our ancestors. Those who have been in the Atacama Desert know that once is late night, looking at the stars there is a different experience, from the one we are used to when we look at the stars from our “shining” cities. This thought becomes most significant while we start to think about our everyday experience. Have we lost something in the way?
Our culture as human beings, have always been related to the night sky observation. Today, new planetary systems are discovered weekly, observation tools and instruments are increasingly sophisticated; we can look at the origin of the Universe and make an image of it. We live in a fast and fascinating era in which we can dazzle with images of supernova’s explosions, nebula clouds and faraway galaxies. The understanding of what “surround us” changes radically and Astronomy raises new questions, challenging the limits of our comprehension. Now we can look farther and farther and so wider the horizon becomes.
Facing this prospect, we propose to add different points of view. Contributing to the astronomical reflection from different angles, being the scientific perspective one in many others possible. Because we are interested in poetry, music, art and design, also in casual conversation, laughter, awkwardness and contradictions, in things that move and those who stay quiet, we are interested in large and small questions of humanity.
This first edition of Galatic Magazine is the number cero and has as main subject the Birth, the origin of things and also the origin of this project. This is a starting point of a conversation that we hope will spread in time, and so will convene many voices and many views.
Have you ever wondered why the sky is bright during the day? Have you noticed that the blue sky is not even? Do you know what a rainbow is and why we see it that way? These and other fascinating phenomena are closely related to light, more precisely to the particles that form light: photons. Let’s follow the mapping of some photons sent out from different places in the Universe to the thin atmosphere layer that surrounds our planet, so as to understand their behaviour and some of the phenomena that they cause. But before, let’s try to understand some things about this particle.
A photon is an elementary particle that transports energy. It’s associated with light propagation or any electromagnetic wave. Its main properties are the following:
It’s a massless particle. Its symbol is gamma: γ.
In empty space it moves at a constant distance called speed of light (c = 300 000 km/s). This is a constant in physics; there aren’t any particles that move faster than that.
Besides being a particle, it is an electromagnetic wave; therefore it has a wavelength (usually described with the Greek letter lambda: λ). A wave is an oscillation that spreads through space. For example, sound waves are spread through molecules of air, hence they can’t spread in empty space. Human hearing is sensitive to sounds whose wavelength is between 20 mm y 20 m. Light is another type of wave, called electromagnetic, because it consists of oscillation in the electric and magnetic fields. The human eye is sensitive only to a certain range of photons, which we call visible light, composed by photons of wavelengths from 400 nm to 800 nm (1 nm = 10⁻⁹ m = 0.000 001 mm).
There is a historical debate about the nature of light that dates back from the XVII century. Newton thought of light as a particle, whereas Christiaan Huygens stated that light was a wave. Experiments of light refraction and, later, light interference, ran by Thomas Young, support the idea that light is an electromagnetic wave. Nonetheless in 1905 Albert Einstein was only able to explain the photoelectric effect by stating that light is formed by some particles called photons, or quantum of light, which carry discrete amounts of energy. Einstein showed that the energy (E) of a photon is inversely proportional to its wavelength (λ) with the following formula: E = hc / λ.The proportional constant h is referred to as the Planck constant.
The electromagnetic spectrum is a range that contains all the possible wavelengths of electromagnetic waves. It starts with gamma rays, which have the lowest possible wavelength (the size of an atom’s nucleus), until getting to the size of radio waves which can measure several kilometres.
Our Sun sends millions and millions of photons per second to the Earth. However, the photons that get to the Earth’s surface represent a limited portion of the electromagnetic spectrum. Among those are the photons that compose the visible light we mentioned before, also called visible spectrum; the part of the spectrum that is caught by the human eye. Thus, photons with a wavelength from 400 nm, are seen as blue; the ones from 600 nm are seen as green and from 800 nm. are seen as red. In the end all the photons from the visible spectrum are combined in a solar light beam that seems white to our eyes.
Nonetheless, under certain circumstances it is possible to decompose the solar light according to the photons’ wavelength. This is produced, for example, when we see a rainbow, which happens when a beam of solar light, which contains millions of photons with different wavelengths of the visible spectrum, crosses a drop of water. This phenomenon is called refraction and happens when the photons divert from their original path when they cross a drop of water. The deviation angle depends on the wavelength: blue photons are more diverted than the red. So, all colours are separated and we can see the rainbow.
dispersion of photons in the atmosphere
If visible light contains photons of all colours; why do we see the sky as blue? And why is this colour not uniform in the sky? In order to understand this, let’s see what happens with photons coming from the Sun when getting into our atmosphere.
When the photon arrives from the Sun to the Earth it can be affected by a phenomenon called dispersion, which is produced when a photon meets a particle from the atmosphere – as a molecule of diatomic Oxygen (O₂) or diatomic Nitrogen (N₂) – this generates a change in the original trajectory of the photon. The dispersion efficiency depends mainly on the wavelength: photons with less wavelength, like blue, are more likely to be dispersed by the molecules of our atmosphere than photons with more wavelength, like red. This explains why the sky is bright in any direction, not only towards the Sun.
Without dispersion, photons would get to our eyes directly form the Sun (with altering its trajectory), with which the sky would look dark as if it was night. This is what happens on the moon for example, since our natural satellite does not have an atmosphere that spreads the photons from the Sun. Astronomers would be charmed if this happened on Earth, since they could study the stars during the day.
So, the sky is blue because blue photons are more spread out throughout the atmosphere, nonetheless the blue colour is not uniform. If you look at the zenith on a sunny day, you will notice that this blue is darker than the blue on the horizon, which looks whiter or bleached out. The brightness in the sky depends on the quantity of dispersion that happens in the line of vision, therefore, the number of molecules that spread light. The bigger the number of molecules in a given direction, the brighter that zone in the sky is.
The density of the atmospheric layer also depends on the direction of the line of vision, when we look at the zenith the atmospheric layer is thin. On the other hand, when the line of vision gets closer to the horizon, the atmospheric layer becomes 38 times thicker, thus there is more dispersal of photons. The atmospheric layer becomes so thick towards the horizon that there are high possibilities that photons will be spread out more than once in their propagation in the atmosphere, independently of their wavelength. That’s how different wavelength photons mix, in other words colours, which makes the sky look neutral or white. For the same reason, the sky looks darker if we look from the top of a mountain (or from a plane) than at sea level, because the layer of the atmosphere is thinner at these heights.
photon absorption in the atmosphere
Molecules of our atmosphere can absorb some photons. This way the energy of the photons is transferred to the molecules, which start to vibrate and later spin. The absorption is very unlikely for long wavelength photons between the 400 to the 800 nm (visible light), but it’s very frequent for ultraviolet (uv) light. In fact, the atmosphere is opaque to this kind of wavelength and this “blindness” protects life in Earth from this kind of radiation that is highly energetic and very harmful for living cells.
The Earth is also opaque to most infrared light due to the presence of water vapour and carbon dioxide in the atmosphere molecules. As a result, the thermic emission of the Earth, in infrared, is trapped inside the atmosphere; it can’t escape from our planet and heats up the surface: this is the greenhouse effect. The greenhouse effect is much stronger on Venus and much weaker on Mars, which explains why Venus is an “oven”, whereas Mars is much colder than the Earth. uv and infrared light absorption is also the reason why we have to go to outer space to study the light of the stars in this wavelength range.
birth of a photon in the sun
Now, let’s go back in time to understand how, when and where photons that get to our atmosphere are created; the ones that come from the Sun, the closest star to the Earth. The Sun is composed by various layers, the centre is called nucleus – the inside part – of the star. In the nucleus the temperature and pressure conditions are extreme (millions of degrees) and it is here where nuclear fusion occurs.
Fusion is a process in which two Hydrogen atoms unite to form a Helium atom. A sub product of this reaction is a photon, which is released when both Hydrogen atoms fuse. This new-born photon is not visible yet – such as the one that gets to the Earth – since it is a gamma-ray photon, very energetic and with a shorter wavelength than a visible photon. Once produced, the photon will escape the nucleus crossing the radiative and convective zones, then the photosphere, until it is finally able to reach the Earth
However, crossing the radiative and convective zone is not an easy task, given that this region is highly dense and full of atoms, which means that the photon will bounce in all direction and will start to lose energy. In the end, it will need between 10 000 to 200 000 years to get to the photosphere, despite its incredible speed of 300 000 km/s. During this trip it will lose a lot of energy, which will be used to warm up solar matter. Finally, the photon will emerge from the photosphere with a wavelength mainly within the range of visible or uv light. Then, in eight minutes and 20 seconds it will reach the Earth, which is located 150 million km from the Sun.
detecting photons from an exoplanet
What happens now if photons do not come from our Sun, but from another star? The main difference is related to the travel time through space. For example, for the star closest to the Sun, Proxima Centauri, photons need more than 4 years to get to the Earth. Whereas for distant stars it can take thousands of years.
One of the great challenges of astronomy in the last decade is the detection of photons from exoplanets, that is to say, a planet orbiting a star different from the Sun. What do we do to identify these photons and what their trajectory is? These distant photons can have two different origins:
Photons emitted by the star to which the exoplanet orbits and that bounce off it (by dispersion in the atmosphere, as we saw above).
Photons emitted by the exoplanet.
In the first case, photons abandon the star’s photosphere getting to the exoplanet in a couple of minutes. They bounce off the atmosphere or off the planet’s surface and – luckily – directly towards the Earth; this trip can take from forty to hundreds of years, depending on the distance between both planets. To detect these kinds of photons is very difficult due to the fact that among all the photons emitted by the star, only a few reach the exoplanet, and of these only a few bounce towards the Earth. So, we have a bunch of photons coming from the star and only a miniscule fraction coming from the exoplanet. This fraction can be so small such as 10⁻¹⁰ for a planet of the size of the Earth. This is a contrast problem between a star and an exoplanet.
However, there is an easier way to detect photons from an exoplanet. Instead of trying to identify photons emitted by the star and then bounce off the exoplanet in our direction; we can look for photons emitted directly by the exoplanet. Despite the fact an exoplanet does not give off visible light, it does give off infrared light, which our eyes cannot see, but our telescopes can! In this case the photons we watch are produced directly on the surface of the planet, hence the contrast with the infrared light emitted by the star becomes lower than in the first case.
absorption in an interstellar medium
Let’s go further in space and look for photons coming from very distant stars, located in the centre of our galaxy. These photons need around 27 000 years to travel, from the distant star’s photosphere to get to the Earth. This means we see that star just as it was 27 000 years ago; that’s why we say that looking back in space, is looking back in time, in the past.
Let’s see now what happens when during this long journey, photons meet enormous gas clouds, formed mainly of hydrogen. Our galaxy still contains many of these clouds, which are the places where new stars are born. The wonderful Orion Nebula is an example of these clouds.
If photons that try to go through the gassy cloud have wavelengths of the visible spectrum, the changes would be too big and the majority would be absorbed by the cloud. The energy of these photons would be transferred to the cloud, and as such its temperature rises; these photons end their trip here. This explains why nebulae are similar to veil and hide their visible light form the stars behind them.
Now, if photons are infrared, that is, they have a bigger wavelength, it will be harder for them to be absorbed and cross the nebula. Therefore, looking at the infrared with telescopes specially designed for this purpose allows us to see through the dust and interstellar gas, towards the stars that are beyond the nebulae.
Photons do not have a quiet life; there are many processes that can affect their propagation in space, until being seen by the human eye or telescopes. Understanding these processes to describe in detail the environment with which they interact during their propagation is one of the objectives of astronomers. As we will never send satellites to take measurements in situ, the photon is without a doubt the only source of information for astronomers to study stars, exoplanets, a distant nebula or the centre of the Milky Way. For optimizing the understanding of these particular environments, astronomers need to identify more and more photons. It is for this reason that bigger and bigger telescopes are built in countries with clear dark skies, such as Chile.