Illustrated Astronomy

Front matter

JUAN CARLOS BEAMIN

ILLUSTRATED ASTRONOMY THE SUN THE EARTH THE MOON ECLIPSES

In honor of my father, Juan Carlos Beamin Anguita, who during the night, taught me to get amazed by stars, satellites, and the idea of creating new constellations. Without those contemplating hours, this book could not have been possible.

How did we get to this date?

HOW DID WE GET TO THIS DATE? A very brief history of the Universe

Our Universe was born about 13.7 billion years ago. During the first stages, there was no matter, neither atoms, and in a certain way, all the energy, gravitational and electrical forces were all together. Then, in a matter of seconds, the very first particles formed.

It is almost unlikely to believe that, at that very moment, all the matter of the Universe was made up of hydrogen (75 %), helium (25 %), and a pinch of lithium (less than 0.001 %), and a temperature of hundreds of billions of Celsius degrees.

From that moment, and for the next 380,000 years, the Universe only kept growing and getting colder, without forming any new elements.

DID YOU KNOW THAT…

the antimatter is very alike to the ordinary matter? They have the same mass in general, but they differ in their electric charge signs. For instance, both an electron and a positron (or antielectron) have opposite electrical charges. When the matter and the antimatter bind together, they annihilate each other, generating a vast amount of energy.

· At first, the formed matter and light seemed trapped. The latter collided with the first one, being incapable of travel freely as it does now. The early cosmos was so dense and hot that electrons could not remain attached to the nuclei of the atoms. However, as the Universe continued to expand and the temperature cooled down (3,000 °C approximately), the electrons started to remain next to their nuclei, preventing the light from scattering. This stage is known as Recombination or Surface of the Last Scattering since the atoms merge and, from that very moment (380,000 years after the Big Bang), light can travel freely in the Universe.

The light we detect and see today thanks to the microwave telescopes, it has traveled 13.7 billion years, and it has “cooling down” with the Universe, reaching a temperature of only 2.7 K (-271 °C).

Electrons are not bound to the atomic nuclei, and photons collide continuously with electrons and ions.

The Universe cools down. Electrons start to bind to the nuclei, and photons lose energy

The Universe is neutral, and photons don’t have enough energy to interact with atoms. Photons can finally travel freely

When light could finally escape, there were neither planets nor galaxies, and we had to wait other 200 million years before the first stars formed. During that time, the Universe was completely dark, just like a thick, darkened nocturne fog.

It is more than likely that the first formed stars were much hotter, larger, and brighter than our Sun. They needed fuel to shine, so they used both hydrogen and helium to do it. When they merged, they were creating new elements more complex, such as carbon, nitrogen, oxygen, and iron, at the same time when the Universe was releasing energy and light. These stars had very short lives, and when they died, they massively blew up, which is known as supernovae. Thus, they expelled to the cosmos the new materials they had formed, allowing the new generations of stars had access to tiny quantities of them.

Little by little, the dark Universe started to light up. After two hundred million years, the first galaxies showed up, and from then on, a new transformation began: the large amount of light that stars produce makes electrons split up inside hydrogen atoms, so the Universe is ionized again (as in the beginning), and that’s how it is up to date.

Going forward in time, the Universe continues to expand; new galaxies and stars are formed, some of them explode as supernovae and inject some other materials into the Universe; galaxies merge with each other, grow, and trigger the formation of new stars. That’s how the Milky Way, our Galaxy, arouse, which due to its shape is classified as a spiral type.

Even though it is impossible to define an exact age for our Galaxy, we can say that it is almost as old as the Universe itself. Some of the star groups orbiting the Milky Way (known as globular clusters) are nearly 12.5 billion years old.

About 4.5 billion years ago, a great cloud of gas and dust, located at one of the spiral arms of the Milky Way, started to shrink and got divided into smaller and denser clouds. In so doing, hundreds or even thousands of stars of different sizes were formed. Among them is the Sun, that since then is orbiting around the center of the Galaxy, apart from other hotter stars and many other tinnier and colder ones.

The Sun is part of the more than 200 billion stars that make up the Milky Way. It orbits around 25,000 light-yearsaway from the galactic center, and it travels at a speed of 200km/s. Completing a single orbit (cosmic year), it takes 250 million Earth years.

When the Sun was forming from its cloud, shrinking to be the size it is today, a minor part of this cloud created a disk around the Sun. Such disk originated all the planets in the solar system, from Mercury to Neptune. Every single one of them took remains of the material existing from the Big Bang, or that was once inside a star, and after thousands of millions of years, they condensed into a planet. Also, comets, asteroids, and the rest of the objects that we can find in the Solar System were formed, such as dwarf planets, where Pluto is the most known. Out of the eight planets that the Solar System has, in the third one closest to the Sun, life developed. In the beginning, 3 billion years ago, there were only bacteria, but this was increasing in time and diversifying. And today, as human beings, we can do research and understand how did we get here and amaze at everything that exists.

The Sun

I

THE SUN

THE BRIGHTNESS OF STARS

The Sun is the nearest star to us, and its luminosity makes it special from the rest. We can see its intense brightness since it is “very close” to us, merely to 150 million kilometers away, whereas the next closest star is no less than 39,762,576,000,000 kilometers away (39 trillion kilometers). Two thousand times farther!

Most of the stars in the Universe are, in fact, smaller and brighter than our Sun. Nevertheless, it is surprising that most of the ones we can see at night are actually, larger, hotter, and brighter. It seems contradictory, but let’s imagine that we can see a light bulb or a streetlight shining from a distance. A match, however, requires to be relatively closer to see if it is lit.

Let’s review a real example. The nearest star is Proxima, located at the Constellation Centaurus. It is part of a triple-star system, where the main one is Alpha Centauri A, following by Alpha Centauri B and then Alpha Centauri C or Proxima. Of those three stars, the first two are visible to the naked eye, even though they seem to be only one because they are very close, and we need a telescope to watch them separately. Alpha Centauri A is similar to the Sun in terms of size, age, temperature, and brightness, being one of the most luminous stars in the southern hemisphere sky. Although it is slightly closer, Proxima has just under one-eighth of the Sun’s mass, and the surface temperature at almost 2800 °C (in contrast to the Sun that is over 5400 °C), which means that its brightness is less than the hundredth part of the Sun’s shine. That is why we cannot see it without help, even though it is very near.

Up here it would be valid to ask: Why all stars are different? Every single one of them owes all their features properties, such as color, brightness, and size to its mass and to their age. Although indeed, the chemical composition does matter, it is a secondary factor. For instance, the Sun has a mass of 1,988,000,000,000,000,000,000,000,000,000 kg (1,988x1030 kg), the so-called solar mass (M ), and we designate a number to the stars in comparison with that value. Then, Proxima has a mass of 0.123 solar masses.

A solar mass is equal to 1.99 x 1030 kg; a solar radius is equivalent to 700,000 km, and the Sun brightness is 3.8 x 1026 W. The Earth could consume between 1,500 and 40,000 Exajoules from the radiation the Sun emits, whereas the whole humanity consumption of one year rise to 600 Exajoules.

In the following table, you can see some examples of stars and their masses, sizes (radius), and brightness or luminosity.

Star | Mass | Radius | Distance [light years] | |Luminosity or brightness

Alpha Centauri A | 1.1 M | 1.22 R | 4.3 | 1.52 L

Alpha Centauri B | 0.907 M | 0.86 R | 4.3 | 0.5 L

Proxima (Alpha Cen. C) | 0.122 M | 0.15 R | 4.24 | 0.0017 L

Sirius | 2.06 M | 1.7 R | 8.6 | 25.4 L

Aldebaran | 1.16 M | 44 R | 65.3 | 518 L

Canopus | 8 M | 71 R | 310 | 10,700 L

Antares | 12 M | 680 R | 550 | 97,700 L

Rigel | 23 M | 79 R | 860 | 120,000 L

In astronomy, this symbol represents the Sun

DID YOU KNOW THAT…

…if the light could travel at infinite speed or if the Universe were endless, the night sky would be crystal clear since we would see stars in every direction? However, as the Universe has a starting point, and the speed of light is finite, our sky looks dark.

Read more about “Olbers’ paradox.” on this link: https://en.wikipedia.org/wiki/Olbers%27_paradox

WHERE DO STARS GET THEIR ENERGY FROM?

To turn on a light, a telephone or a computer, to move a car, to warm ourselves in winter and refresh in summer, to walk, or even read these lines, we need energy. So, where does this energy come from? There are different sources. We can burn some fuel to warm up or move, or take advantage of the wind or the falling water on a waterfall to turn it into kinetic energy useful for other purposes. We can also take the solar light and turn it into chemical energy, keep it within batteries, and then use them to run our electronic devices. Even plants can transform solar light, air, water, and minerals into sugars to generate energy through other chemical processes, but how is the energy that comes from the Sun produced?

The Sun is mainly made up of hydrogen and helium, the most lightweight and common elements in the whole Universe, besides small quantities of chemical components such as carbon, nitrogen, oxygen, iron, sodium, calcium, and magnesium, among others.

In contrast to the Earth conditions, the Sun possess an enormous gravity due to its gigantic mass, which makes its core particularly hot. Also, it is very dense. There is an immense pressure and, under these extreme conditions, a phenomenon known as nuclear fusion occurs. Nuclear fusion is the process in which small and lightweight atoms, such as hydrogen, come together to create larger and heavier ones such as helium. In fact, it is needed four hydrogen atoms to fuse into one helium, but the process is much more complicated than just put atoms together.

HYDROGEN FUSION

What does this have to do with generating energy? If we put on a balance four hydrogen atoms and one helium atom, we see that the mass of the hydrogen atoms is slightly larger than the helium in 0.7 % or so. This means that during the fusion process of hydrogen atoms, part of the mass is lost!

DID YOU KNOW THAT…

...the core of the Sun can reach 15 million Celsius degrees, and its pressure is 200 billion times higher than the atmospheric pressure on Earth? Only under these conditions, the fusion of lightweight elements can occur, transforming them into heavier ones.

If you had heard that “the mass can neither be created nor destroyed” then, what happened with the mass loss on the fusion process? In this case, the energy is the one which transforms, and the mass is only a type of energy, such as light and heat. The famous formulae E=mc2, which reads “energy = mass times the speed of light squared,” precisely stands for this: if the mass of something is multiplied by the speed of light squared, it is obtained its rest energy.

If we want to or could transform 1 kg stone into energy, we would have to do it this way: E= mc2 Where c = 300 000 000 [m · s-1] (the speed of light in vacuum) m = 1 kg E= 1 x (300 000 000)2 [Kg · m2 · s-2] E= 9 x 1016 [Joules]

This energy is tremendous, similar to the total energy released by an earthquake magnitude 9.1 or half of the energy of the most powerful nuclear weapon ever detonated in human history: The Tsar Bomba.

The Sun has a luminosity of 3.8x10 26 J/s. If we divide this value by the speed of light squared, we obtain the mass that the Sun turns into energy at every second. It is more than four million tons of matter. Fortunately, as our star has so much mass, we can say that it can withstand those levels of energy production for at least 4.5 billion years more.

If it has lived that amount of years already, we might say that it is in the middle of its age as it is estimated that it will live about 9 or 10 billion years.

DID YOU KNOW THAT…

...the energy that the Sun produces takes at least 100,000 years to come out of it and reaches the Earth in only eight minutes?

DID YOU KNOW THAT…

at every second, the Sun produces more energy than all humanity has spent in its history?

THE SUN STRUCTURE, OR WHAT IS IT BETWEEN THE SUN’S CORE AND WHAT WE SEE?

The star has six main layers:

THE CORE

Where its energy is produced. It is the innermost part and is up to 20 % or 25 % of its total radius, which means if the Sun has a radius of 700,000 km, the core from the center goes between 150 and 200,000 km. In this place, a significant quantity of helium builds up at every moment.

The light produced at the core interacts with charged particles, such as electrons. In every interaction, the light changes its travel direction.

RADIATIVE ZONE

The radiative zone is the second layer, where the energy produced in the core travels to almost 70 % of the Sun size, which means, between 200,000 and 500,000 km through the physics process of radiation. In this region, the temperature decreases dramatically, from 7 million degrees to 2 million degrees when it hits the transition zone called Tachocline, to then move to the third layer.

It is believed that in this transition layer is where the Sun rotation changes from a rigid rotation, such an “iron” sphere, to rotate differentially. Like when we shake a ball with liquid in, and we observe the ball spin faster than the liquid inside. Apparently, this would produce the magnetic field of the Sun.

The interactions occur so often that the light energy takes thousands of years to escape from the Sun.

CONVECTIVE ZONE

The convective layer ranges from 500,000 km to almost its visible surface. Here, energy does not travel by radiation, but by convection. The cooling-down process happens differently from the previous layer because the Sun’s material moves as heat “bubbles” (convective cells) —which comes from the Tachocline and expand, making them “float” or moving upwards — carrying this innermost part heat to the outermost ones. Through the movement of this material (plasma state), the temperature cools down from about 2,000,000 °C in the Tachocline to 6000 °C in the photosphere, which is the following layer.

DID YOU KNOW THAT…

...the movement of material in the solar surface produces the so-called Granulation and Supergranulation?

PHOTOSPHERE

The photosphere is the visible layer of the Sun. When we talk about the temperature of a star, we generally refer to the photosphere’s temperature, and the light of the mentioned layer is also useful to do chemical analysis of the stars. In the case of the Sun, the photosphere’s temperature goes from 4,200 °C to 5,700 °C.

On this layer, we can see different phenomena such as sunspots, which are regions of reduced temperatures caused by local magnetic fields that inhibit the escape of hot material from the innermost layers. These spots appear and then disappear as it is related to a solar cycle of eleven years in which the Sun changes its global magnetic field, which means that the north pole turns into the south pole, and vice-versa. Like so, if in one year we see many sunspots, after five or six years we will see just a few. A small telescope with a suitable solar filter can help to observe these sunspots.

Solar telescopes are specially designed to observe the solar surface with higher contrast. We can obtain high-resolution images of solar flares, sunspots, spicules, and granulation with a particular filter called H-alpha filter (Hα).

Sun images during periods of higher and lower activity in the photosphere.

Another visible feature of the photosphere is granulation, which owes its name to the “granulate” or “granules” appearance that hot plasma takes when it rises from the convective zone and gets colder when it reaches the photosphere. This plasma has a hot core, but its edges are colder, which creates this granulated shape on the surface. The granules have an average size of 1,500 km and dissipate after 10 or 20 minutes, approximately. Granulation is a constant motion, and it can only be seen observing the Sun through a telescope with special filters.

The Sun’s diameter is 1.4 million km.

The granulation of the Sun can be seen as small imperfections or temperature differences on the surface. To have a general idea of the convective cells’ size, on average, they are slightly smaller than Brazil. In other words, in every “grain”, you can place Argentina, Colombia, Perú, Bolivia, and Chile.

Finally, another feature of this layer is the darkening towards the edges, also known as limb darkening, where the Sun’s center appears brighter that the edges.

Why does this happen? When we look at the center of the solar circle, we can see more in-depth, and the light comes straight to us, crossing fewer layers of its atmosphere. On the contrary, if we look at the edges and we want to observe the same depth as if we look at the center, the light would have to go through more layers of the solar atmosphere. So, by observing the edges of the Sun, we can see colder, outer layers, that’s why it looks slightly darker.

Something similar can be observed in the Earth’s atmosphere. The stars in the sky look clearer and static when we look up, whereas if we look to the horizon, they look blurred and blinking because the stars above cross fewer atmospheric layers to approach us.

Nearby the center of the star, a part of the light we observe is in the depths of it, where the temperature is higher, which means more light.

On the contrary, the light we observe near to the edge of a star is produced closer to its surface, where the temperature is colder.

CHROMOSPHERE

Is the next-to-last solar layer and is characterized by its reddish color visible during a total solar eclipse or using a solar telescope specially designed for this kind of observations (a telescope with an alpha -Hα hydrogen filter). On this layer, way less dense than the previous ones, it can be observed phenomena such as spicules, which are plasma jets moving at 20 km/s from the photosphere to outer space. Spicules can happen anywhere in the solar sphere, and last around 15 minutes.

Unlike the rest of the Sun, it is observed that on this layer, as it moves away from the center, the temperature increases from 4,200 °C in the outer part of the photosphere to about 25,000 °C, 2,000 km in the “upper” part.

A definitive explanation of what causes a temperature inversion is still under discussion, although the local magnetic field is likely to undergo some variations, known as magnetic reconnection. That is how the energy of the magnetic field turns into motion and heat, which increases the temperature on this layer.

CORONA

The corona is the outermost layer of the Sun. It extends a few million kilometers into outer space but is not that dense, just like a billion times less than the photosphere. On the other hand, its temperature is way hotter, hitting temperatures between 1 million and 3 million Celsius. This layer is visible during a solar eclipse or using coronagraphs designed to observe it. It is worth mentioning that it is probably one of the most breathtaking and iconic views of a total solar eclipse. The corona does not have a well-defined, symmetrical shape, and it depends on the solar magnetic cycle.

What is the solar magnetic cycle?

The Sun, like the Earth, has two magnetic poles: the north pole and the south pole. Every eleven years, these poles turn upside-down, and the one in the north moves to the south, which is known as the magnetic cycle. This situation generates changes in the emitted Sun’s radiation (less than 0,1 %) as well as in the plasma amount ejected from its surface. During the maximum magnetic activity of the Sun, most sunspots, solar flares, and others are observed in the photosphere.

Studying these changes is essential since they cause effects on space and Earth, mainly on an atmospheric level, producing auroras and, in the case of stronger events, they can even affect satellites orbiting around our planet.

The solar corona behavior is still not known precisely and is one of the biggest open questions on solar physics. Nevertheless, the Parker Solar Probe, launched in 2018 by NASA, may provide more information about its origin.

Doubtlessly, the Sun is the star that has received the most attention over human history, and its significance has been recognized in all cultures. We are going in-depth on this topic later in chapter VI about the ancient worldview of eclipses.

Solar corona during a total eclipse of the Sun. Behind the moon’s shadow, redlined, we can see the solar flares, typical phenomena of the chromosphere.

• • • STARS ARE MADE UP OF ATOMS BUT HOW TO KNOW WHICH ATOMS ARE IN EVERY SINGLE ONE OF THEM?

Astronomers research the material that stars are made up of. To do so, they can only see the light they emit. Fortunately, the light produced at the center of the stars pass through their atmosphere and approach us, acting just like the light we produce on Earth. Also, their atoms behave the same way in the star as in the laboratory, which means that first, we study the kind of light atoms produce on Earth, and the colors they emit, and also how bright is every color.

For instance, if the gas of an element is heated, the brightness of its color is always the same. So, thanks to the laws of quantum physics, we can precisely calculate how much energy that brightness means. Atoms can emit or absorb light only on those specific colors.

Astronomers, on their behalf, scatter the light coming from the stars (leaving it as a rainbow), and they observe which colors are NOT there. Later, using spectroscopy, which is a technique used in every natural science, they compare the colors missing with the ones that atoms emit.

The light of the stars interacts with electrons within atoms. Photons with the exact energy are absorbed and drive electrons to higher energy levels. If the electron drops to a less energetic level, it emits a photon but not necessarily in the first direction.

Analyzing the light of the stars, we observe fewer photons with specific energy, which provides information about atoms in their atmosphere.

Review questions

1 · Why does the Sun shine?

2 · What makes the stars different from one another?

3 · How is the Sun inside and how do we know that?

4 · How is energy transferred from the Sun to Earth?

5 · Which layer of the Sun is visible during an eclipse?

DID YOU KNOW THAT…

...the Sun contains 99,9 % of the Solar System’s mass?

In other words, if we gather all the planets and their moons, and asteroids, and comets, they become just a thousandth part of the Solar mass.

DID YOU KNOW THAT…

...the Solar System travels to 220 km/s around the Milky Way’s center and, besides its high speed, it takes 220 million years to orbit once around the Galaxy? This period is called a galactic year or cosmic year.

• • • SUMMARY SELF-TEST

In the next link or QR code you can find Sun pictures taken in different days and the date they were shot.

And by using them:

1 · Calculate the rotation speed of the Sun.

2 · About what speed (in km/h), something on the Sun surface would move (at its Equator)?

3 · Compare that speed to the one we experience on the Earth’s surface.

· Have in mind that the radius of the Sun is 700,000 km.

If you don’t know how to start, we give you the following clues:

CLUE 1: You can calculate the motion of every spot using the grid in the QR code downloaded. To do so, follow the motion of every spot and calculate how many degrees does the spot move per day.

CLUE 2: If you have already calculated the angular distance per day, you can calculate how much it will take to complete the 360° of the solar sphere.

CLUE 3: Everything can be simplified with a higher margin of error. You can get the highest values of the day in which a spot appears and how long it takes to disappear. As it traveled halfway through the Sun (the part we can see), the total travel time has to be twice.

The Earth

II

THE EARTH

The Earth is the third planet in the Solar System counting from the Sun. It is the largest planet among the rocky ones (Mercury, Venus, and Mars), but smaller than gas giants (Jupiter, Saturn, Uranus, and Neptune) and it is located about 150 million kilometers from the Sun. This enormous distance means that the light coming from the Sun doesn’t reach the Earth immediately, but takes around eight minutes, even though light is the fastest thing that exists! Since it travels to almost 300,000 kilometers per second.

If, for any reason, the Sun disappeared or stopped shinning, we would take eight minutes to know it, and the Earth would keep orbiting along that time to continue then traveling in a straight line into space at 30 km/s since the Sun would exert no force upon it.

On the one hand, this enormous distance is something great because if the Earth were closer, the amount of light and energy that it would receive would be much higher. Therefore, the temperature would increase, putting the development of life in danger. On the other hand, if we were further, it would be the opposite: the lack of energy and low temperatures would make the living existence on our planet difficult.

Another interesting feature is that Earth’s orbit is almost circular (is indeed an ellipse), and that makes our radiation levels neither lower nor higher. An eccentric orbit would cause us to get very close to the Sun in a short time, but in that period, life would experience significant changes: it would be fire everywhere and a lot of UV radiation which would kill bacteria, causing damage to the living creatures here on the Earth surface. The rest of the time, we would be far from the Sun, which would bring a brief ice age every year, and by the freeze of everything and the lack of both energy and light, it would be significantly hard to survive.

The term “habitable zone” or “circumstellar habitable zone” was created to describe a place as the one the Earth has in space, which means, in a stable, almost circular orbit at the exact distance of its star. This term refers to the space region around a star, where the temperature of an object or planet is between 0 °C and 100 °C, and water in liquid state is possible under normal pressure conditions.

Venus and Mars are in the inner and outer bounds of the habitable zone. However, it is needed much more than the right temperature to the existence of life. Next, we are discussing four essential conditions for habitability on Earth.

PRESENCE OF LIQUID WATER

More than 70 % of the terrestrial surface is covered in water. In fact, the Earth is the only planet in the Solar System that has the three phases of water on its surface (solid, liquid, and gas).

Some of the properties that make water an essential element for life development on Earth are: it is an excellent solvent, it provides suitable conditions to many chemical reactions, and it allows the transfer of substances between cells and their environment. Also, the heat capacity of water -that is the amount of energy required to increase one Celsius degree- is very high, which means that in comparison with other elements such as rocks or metal, water takes more time to heat, remaining the surface temperature of the planet stable with slight variations between day and night.