The Sun

The Sun is the central star and the holder of the Solar System. The Sun’s gravity is what drives this system. It accounts for 99.86% of the mass in the entire Solar System. And most importantly, the Sun is the reason for the emergence of life on Earth. However, when we look at the big picture, the Sun is just one among the billion stars in our Milky Way galaxy.

Since ancient times humans have worshipped this star as a bearer of life and as a god. In the centuries to come, we have realised the Sun’s true Nature and only now do we understand, that the bright disk in the sky is far more complex than we could imagine.

Galactic address

The Sun and the Solar System lies in a galaxy called the Milky Way. The Milky Way is a spiral galaxy with many spiral arms. The Sun lies close to the inner rim of the Milky Way’s Orion Arm, in a cloud called the Local Interstellar Cloud. The Sun is 25,000 to 28,000 light years from the centre of the Milky Way. The ionising radiation of the Sun and other neighbouring stars has heated up gas in its surrounding and has created what is known as the Local Bubble.

So, the next time you have to give your solar system’s address to an extraterrestrial, you know what to say!

The Sun has habitable zone in which the Earth lies. Now, the Sun lies in the galaxy’s habitable zone. If the Sun was located anywhere else, life as we know it wouldn’t have been possible. This is because this zone, keeps the Sun away from the dangerous spiral arms, and is at a sufficient distance from the galactic centre.  Thus it does not receive a high dose of radiation from the black hole that lies at the centre of our galaxy.

The Sun orbits around the galactic centre in an elliptical orbit in about 225–250 million years. This is also called a galactic year. The Sun is thought to be 20–25 galactic years old. However the Sun’s orbit is not uniform due to regular tugging by nearby stars. Moreover, the Sun moves above and below the galactic plane approximately 2.7 times in an orbit.

Characteristics:

Stars are classified into various types according to their age, mass, density and so on. The Sun is a G2 type main sequence star that lies at the centre of the Solar System. It comprises about 99.86% of the mass of the Solar System. With a diameter of 696,342 km and a mass of approximately 2 × 10³⁰ kg, the Sun is 109 times as large as Earth and 333,000 times as massive.

And most importantly, the Sun is hot, really hot. The temperature is not constant throughout the star though. The temperature at the centre, is estimated to be as high as 15.7 million K (15,699,726.85 million°C/28,259,540.33 million°F), while the visible surface or the photosphere reaches a temperature of 5778 K (5504.85°C/9940.73°F). Strangely, the atmosphere or the corona experiences temperatures of about 5 million K (4,999,726.85°C/ 8,999,540.33°F).

The Sun is the brightest object in the sky, with an apparent magnitude of -26.74. The lower the value of the magnitude, the brighter the object. For example, the Moon has an apparent magnitude of -12.6 and Sirius, the brightest star in the night sky has an apparent magnitude of -1.46.

 The Sun is approximately 150,000,000 km away from the Earth. This distance is also called one Astronomical Unit or AU and is used to measure distances in our Solar System. However this distance can change due to variations in Earth’s orbit.

Light takes about 8 minutes to travel from the Sun to Earth. This means that if we see the Sun, it will appear as it was 8 minutes ago. Don’t try to see the Sun though at any cost, if you want your eyesight!! The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth’s climate and weather.

Our Sun is larger and brighter than most of the other stars in the galaxy. Only about 5% of stars in the Milky Way are larger than the Sun. Yet, our Sun is classified as a dwarf star because it is a small star when compared to the giants out there. The Sun is classified into a group Population 1, which contains luminous, hot, and young stars that are typically found in the spiral arms of galaxies.

As a fun fact though the Sun appears yellow in our sky, it is actually white. To understand this we need to understand the spectrum of light. We all might have learnt in high school physics class that white light is composed of 7 constituent colours; red, orange, yellow, green, blue, indigo, violet. This is what you see in a rainbow. Now, the reason it appears to be yellow because our atmosphere absorbs all the colours of sunlight except yellow.

The Sun also has a strong magnetic field which is created by the moving plasma the Sun is made of. It has north and south magnetic poles, and the magnetic field lines like any typical magnet. These lines create some beautiful effects on the Sun’s surface which has now come to be known as sunspots.

Sunspot appearances peak on a 11 year cycle but usually there are very few of them. At the low point, called solar minimum, there are very few or no sunspots. And at the high point of the cycle called the solar maximum, there are the most sunspots and the greatest amount of solar activity.

These spots are regions which are cooler than the rest of the surface of the Sun. They are caused when two magnetic field lines intersect each other below the surface of the Sun. This prevents plasma from below, to reach the surface causing cooler black regions on the surface. Yet, these ‘cool’ regions are no safe zone. Temperatures here reach up to 3800 K (3526°C/6380°F)

But the interesting part is, when these intersecting magnetic lines snap, they release all that plasma that was trapped below in an event called a Coronal Mass Ejection or CMEs. Sometimes these flares are so huge, that they can knock of Earth based satellites and create total blackouts in cities. This has happened in the past. However, very rarely a tremendously power solar flare has the potential to wipe out all life on Earth, but it has not happened so far.

The Sun, like other planet rotates on its axis. Unlike the planets though, the Sun is made of plasma, not rock. Thus different parts of the Sun have different rotation rates. Near the equator, it rotates about once every 25.4 days; whereas near the poles, it takes up to 36 days to complete a single rotation. Studies have also shown that the core has a rotation rate that is faster than the outer layers of the Sun.

Composition and Structure:

The Sun, like every other star of its kind is mostly composed of 74.9% hydrogen and 23.8% helium. All other elements heavier than helium account for less than 2% of the Sun’s mass with oxygen consisting of roughly 1% of the Sun’s mass, carbon (0.3%), neon (0.2%), and iron (0.2%).

The Sun is not a single ball of plasma but is organised into multiple layers, which includes a core, a radiative zone, a convective zone, a photosphere, and an atmosphere from the interior to the exterior.

Core
With a temperature of 15.7 million K, the core is the hottest region of the Sun. It is here where the fusion reactions from hydrogen to helium take place which support the star. Nearly 99% of the thermal energy produced by the Sun occurs within this region. As we move out from the core, the fusion processes begin to cease.  The rest of the Sun is heated by this energy, which travels outwards to the Sun’s surface or photosphere before escaping into space as sunlight.

Radiative Zone
Further out from the core lies the radiative zone which extends from 173, 925 km to about 486,500 km from the core. The temperature drops with increasing distance from the core. At the interior edge of the radiative zone temperatures reach up to 7 million K but drop to 2 million K at the outer edge. The density also drops a hundredfold from 20 g/cm³ to only 0.2 g/cm³. The energy produced in the fusion processes at the core travels through this region via thermal radiation, hence the name.

Tachocline
Between the radiative zone and the convective zone, lies a transition layer known as the tachocline. The radiative zone has a uniform rotation while the convective zone has a non uniform or sluggish rotation. Scientists think that at the tachocline, these different rotation rates meet and  create a magnetic dynamo. This is what is responsible for generating the Sun’s magnetic field.

Convective Zone
The convective zone, lies above the tachocline which is nearly 200,000 km below the surface and extends all the way up to the surface. The temperature and density in this zone is lower than the radiative zone and the core. The extreme variation in temperature between the bottom and top of the convective zone allows thermal convection to develop. In other words material heated below expands and rises, which then cools and contracts once it reaches the photosphere. Thus causing it to sink again and the cycle continues.

This means that energy from the core rises and falls for thousands of years in this zone before it is finally emitted from the surface. The rising and falling of material causes the surface of the Sun to appear very granulated. Thus, the sunlight you receive every morning was produced at the core thousands of years ago!

Photosphere
The visible surface of the Sun,  is also known as the photosphere. It is from the photosphere, that visible sunlight and solar energy is emitted. The photosphere is hundreds of kilometers thick and is slightly less opaque than the air on Earth. At the photosphere, temperature and density are the lowest anywhere on the Sun, although still very hot. Temperatures reach up to 5,700 K, with a density of 0.2 g/m³ which is about 1/6000th the density of air at sea level.

Atmosphere
Yes, believe it or not the Sun has an atmosphere. the atmosphere is further divided into  three distinct layers: the chromosphere, the transitional region, and the corona moving from the interior to the exterior.

Chromosphere
The chromosphere is roughly 2,000 kilometers deep and has a very low density of about 10^-8 times that of Earth’s atmosphere. As a fun fact, the halo of the Sun we see during a total solar eclipse is actually the chromosphere. Otherwise, the brightness of the photosphere, makes the chromosphere invisible. The temperature in the chromosphere varies from 4500 K to 20,000 K. This is surprising as it means that the Sun’s atmosphere is hotter than the surface

Transitional region
Above the chromosphere lies a thin transition region 200 km thick. Here, temperatures rise rapidly from 20,000 K in the lower layer to close to 1,000,000 K at the corona. These absurd rises in temperature as we go up the solar atmosphere can be explained as follows. The ionisation of helium atoms in the region prevents the cooling of the atmosphere, thus heating it up.

The transition layer is not well-defined, and is in constant, chaotic motion. The transition region is not easily visible from Earth’s surface, even during a solar eclipse. However it is visible when viewed in ultraviolet light.

Corona
The topmost layer of the solar atmosphere is the corona. In some regions of the corona the average temperature is about 1 – 2 million K. However in the hottest regions the temperatures soar up to 8 and 20 million K. This is believed to be due to the Sun’s magnetic field which causes various particles to accelerate at high speeds, creating a lot of friction and heat in the process.

Evolution:

The Sun formed around 4.5 billion years ago from the collapse  of a giant molecular cloud we now call the solar nebula. The rest of the solar system also formed from the same part of the nebula that gave birth to the Sun. It is thought that this nebula may also have given birth to other stars that maybe still wandering out there. They maybe our Sun’s long-lost siblings. The birth of our Sun went something like this.

A fragment of the solar nebula collapsed in on itself, due to gravity. Soon after the collapsed cloud began to rotate and heat up due to increasing pressure. The centre of this disk began accreting mass and rose rapidly in temperature, whereas the rest flattened out into a disk where rocks pulled on each other to form planetesimals.

Gravity and pressure at the centre of the cloud generated a lot of heat. Gradually the immense heat triggering nuclear fusion of hydrogen atoms to helium atoms.  This was marked by a grand explosion which vaporised the remaining dust, thus stopping further planet formation. The Sun was born.

Now, some 4.5 billion years after that event the Sun is a main sequence star, or in other words a middle-aged star. The Sun sustains itself by the balance of two forces. Gravity which crushes it inward and the force of thermal energy through nuclear fusion pushing it outward.

Currently, more than four million tonnes of mass is converted into energy within the core every second, producing light and heat which supports all life on Earth. At this rate, the Sun has converted 200 times the mass of our Earth into energy which is only about 0.03% of its total mass.

As the Sun fuses more and more hydrogen atoms to helium it becomes hotter because the helium atoms in its core occupy less volume than all the hydrogen that’s been fused. Therefore the core is shrinking, allowing the outer layers of the Sun to move closer to the centre. Thus the outer layers experience a stronger gravitational force as they are closer to the centre of the Sun. The outer layers also exert a strong inward force on the core, which in turn is making the core denser.

Once the hydrogen in the core is exhausted the Sun will begin to expand and become a red giant. It is hypothesized that it will grow large to eat up Mercury, Venus, and maybe even Earth. Don’t worry though, as it won’t happen for another 5.4 billion years.

When the Sun turns into a red giant, it is said to have reached the Red Giant Branch (RGB) phase. However, the story of our star is far from being over. Now, the core is full of  helium. With no fusion process happening, our Sun will begin to collapse under its own gravity.

This will increase the temperatures to such a point that helium will begin to fuse to carbon. Fusion starts again and the collapse is halted. This is known as a helium flash. During this event approximately 6% of the core and 40% of the Sun’s mass will be converted into carbon within a matter of minutes.

The Sun is now 10 times its current size and 50 times its luminosity and has dropped in temperature. Now, the fusion of helium to carbon will sustain the star for the next 100 million years. After which it will reach a phase called the Asymptotic Giant Branch (AGB) phase.

Over the course of the next 20 million years, the Sun will expand much faster than before and then become unstable. Soon it will begin losing mass as it sheds layer by layer of its mass. This will occur periodically every 100,000 years or so. Each time the Sun will become larger and brighter before giving off a layer.

At this point, scientists are not sure whether Earth will be engulfed or not, but it will certainly be inhospitable for life as we know it. Planets in the Outer Solar System are likely to change dramatically, as more energy is absorbed from the Sun, causing their gases in the atmosphere to vaporise. Moreover the ices on the moons of these planets will melt leading to oceans.

After 500,000 years or so, the Sun will eject its final layers. All the layers it had shed until now has formed a beautiful planetary nebula.

In the end a naked core is all that remains of what was a majestic star called the Sun. The temperature of the core will be over 100,000 K. This remnant is called a white dwarf. The white dwarf will roam the void for trillions of years before cooling down to form a black dwarf.

However, the planetary nebula may collapse again and give birth to new stars that may host new planetary systems. One of these planets may host life-like Earth. Some even may host intelligent life.

Exploration:

The Sun won’t be here for long, hence we must do our best to explore it when we can. As of 2018 we have sent many missions to the Sun, the first of which were NASA’s Pioneer 5, 6, 7, 8 and 9 probes, which were launched in the years 1959 to 1968. However, these probes didn’t ‘go’ to the Sun (you may have guessed why) but orbited at a distance similar to that of Earth, and made detailed measurements of the solar wind and magnetic field.

Next up was the Helios 1 and 2 probes, sent in the 1970s. This time, we went closer; as close as Mercury’s orbit. The mission was a U.S.-German collaboration that studied and gained data on the solar wind and the solar corona.

During this time (1973) NASA launched the Skylab Space Station. Although its purpose was not to solely study the Sun, it made some important discoveries using the Apollo Telescope Mount. These included the discovery of coronal mass ejections and coronal holes.

In 1980, NASA launched the Solar Maximum Mission. Unfortunately, an electrical failure caused the probe to go into standby. It was subsequently repaired by the Space Shuttle Challenger in 1984. The mission acquired thousands of images of the solar corona and observed gamma rays, x-rays and UV radiation released by solar flares before returning to Earth in June of 1989.

In 1991, Japan sent the Yohkoh satellite from the Japan Aerospace and Exploration Agency (JAXA). Its mission was to observe the solar flares at X-ray wavelengths. It was a success in the sense that it observed an entire solar cycle of sunspots. However in 2001, an annular eclipse halted its observations of the Sun.

However one of the most important solar mission was launched in 1995, by the joint efforts of ESA-NASA. Solar and Heliospheric Observatory (SOHO) was a mission designed to observe the Sun constantly in many wavelengths of light. But it didn’t go too close to the Sun and was situated at a special point called a Lagrange point between the Earth and the Sun. A Lagrange point is a special point in space where the gravity between two objects neatly cancel out and we can place a probe without spending much energy.

Originally intended to end in 1997 the SOHO mission was extended to 2012. Moreover a follow-on mission was launched in 2010, called the Solar Dynamics Observatory(SDO).

In 1990, scientists tried something new. All these years they had observed the Sun at its equator. Now, they tried to observe its polar regions. Thus, NASA and ESA sent the Ulysses probe whose mission was to observe the solar wind and magnetic field strength near the Sun’s poles. It found that, near the poles the solar wind moved slower than expected at about 750 km/s. It also found large magnetic waves emerging from these regions that had not been observed before.

In 2006, NASA launched the Solar TErrestrial RElations Observatory (STEREO) mission. Its mission was to conduct stereoscopic imaging of the Sun and solar phenomena. It consisted of two identical spacecraft which were launched away from Earth and gradually made to go behind the planet which enabled it to perform its mission better.

As for now, that is all that we have sent to the Sun. But our curious Nature will send more probes in the coming years. For example, NASA is launching the Solar Probe Plus this year. The mission is designed to take measurements of the particles and energy released by the corona.

Moreover, the Indian Space Research Organisation (ISRO) plans to send a mission called Aditya. Aditya, which means ‘Sun’ in the Sanskrit language is a 400 kg satellite that is slated for launch in 2019–20. Its main mission will be to study the working of the corona.

Next year, in 2019, the ESA will launch the Solar Orbiter mission which will study the mechanism that is responsible for the formation of the heliosphere at the edges of the Solar System. The mission will fly as close as 0.28 AU to perform its measurements.

And finally, in an unspecified future NASA plans to send the Solar Sentinels mission. Their objective is to study the sun during solar maximum, research energetic particles, coronal mass ejections and interplanetary shocks in the inner heliosphere. This mission will be an important step to predict solar weather so as to protect astronauts in future missions.

The mission will involve a total of six spacecraft.  Four of which, will be stationed inside the orbits of Venus and Mercury. The remaining two will be stationed behind the Sun and in the orbit of Earth respectively.

Conclusion

The Sun is a star and is the primary source of energy for all life on Earth. Without it no life form would be able to support chemical and metabolic reactions for its sustenance on this planet. Moreover the Sun gives out heat and light which illuminates your good day.

No doubt that the Sun releases harmful rays, solar winds and flares that would kill us all if not for the Earth’s atmosphere and magnetic field. However, the solar winds carry charged particles that slam into the interstellar medium at the edge of the Solar System. This  forms a protective magnetic field that prevents dangerous cosmic rays from entering the Solar System. This is called the heliopause and without it, the Solar System would be bombarded by cosmic rays.

No doubt that the Sun isn’t here forever, but an expanding Sun also means an expanding habitable zone. Maybe the giant Sun may support life on Mars or Jupiter’s moons. Maybe humans will colonise other planets and stay long after the Sun is dead.

As of today, the Sun is the primary and the most important member of the Solar System without which there would be no solar system, no life on Earth and no humans to look above and wonder about the place we call the Universe.