Illustrated Astronomy/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.[1]
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 images of this exercise were taken from https://sohowww.nascom.nasa.gov/classroom/docs/Spotexerweb.pdf where you can find the complete exercise.