Introduction
Sun is the bright yellow ball that you see in the sky during the day. Man, since time immemorial has always considered the Sun as a great source of light and heat, and energy. The source of the Sun’s enormous energy was a great mystery to the ancient people.
Many of the great ancient civilizations such as ancient Hindus, Egyptians, Romans, Aztecs, and Incas considered the Sun, the God. Astronomers from ancient times have tried to learn more about the Sun. The advent of the Renaissance and Scientific Revolution made possible the scientific study of the Sun and resulted in obtaining enormous knowledge about the Sun.
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What are the Basic Facts about the Sun?
The Sun is the primary source of energy on Earth. The Sun is a hot ball of plasma giving out an enormous amount of energy due to the process of nuclear fusion taking place at its core. The Sun converts 600 mil tons of hydrogen into helium every second.
The energy from the core takes 10,000 to 170,000 years to reach the surface. When this energy reaches the surface of the Earth it is about 1000W/m². To give you an extent of how big the Sun is its radius is 109 times larger than Earth’s, a mass 333 thousand bigger than Earth’s, and Volume 1.3 million times larger than Earth’s.
The Sun is made up of gaseous plasma and doesn’t have a fixed rotation speed at all latitudes. The solar Equator rotates once in 24.47 days whereas at the poles it is 38 days, the average being 28 days. It has a density of only about a quarter of the Earth’s.
Its surface gravity is 28 times that of Earth. To escape from the surface of the Sun into space, you need to be moving at 617.7 km/s, that’s 55 times the escape velocity on the Earth’s surface.
The Sun has an average distance of about 150 mil kilometers from Earth. The sunlight takes 8 minutes and 19 seconds to reach the Earth when it is closest to the Earth. It is 4.6 billion years old and is in the middle of its lifetime. It is expected to be in this stage of life for the next 10 billion years.
About 73% of the Sun is hydrogen, the remaining being mostly helium and a minute percentage of other elements. Its surface temperature is nearly 6000 kelvin. Based on the spectral classification, the Sun is a G-type star. The Sun is the brightest object in the sky, with its apparent magnitude being −26.74, to compare full moon has an apparent magnitude of -12.90.
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How did the Sun form?
ESA/NASA/JPL-Caltech/Source
Planetary Scientists have determined that the Sun formed 4.6 billion years ago, due to the collapse of part of a giant molecular cloud that was made of mainly hydrogen and helium. The ancient meteorites have been found to contain stable daughter nuclei of short-lived isotopes that can only be formed due to exploding stars.
This reveals that the formation of the Sun could have been triggered by an exploding nearby Supernova or that the region where the Sun formed might have had a Supernova previously.
Due to the principle of conservation of angular momentum, the primordial cloud rotated and heated up with increasing pressure. Most of the mass became concentrated in the center which later became the Sun as gravity and pressure were too high to trigger nuclear fusion.
The remaining portion of the cloud flattened out into a disk and became the planets and other astronomical objects in the Solar System.
What’s happening in the Sun’s core?
Kelvinsong/CC BY-SA 3.0/Source
The inner 20-25% of the Sun’s radius is composed of its core in which nuclear fusion reaction happens. Most nuclear fusion occurs through a reaction called the p-p chain since it is favorable for stars with less than 1.3 solar masses, however, 0.8% of the energy of the fusion comes from another reaction called the CNO cycle which is actually dominant in stars with more than 1.3 solar masses.
The Sun produces an energy equivalent of 9.192×10¹⁰ megatons of TNT per second. The core has a temperature of about 15.7 million kelvins and a density of 150 g/cc. In its main sequence, the Sun the temperature of the core and the surface is gradually increasing as well as its radius and luminosity.
What is present between the core and the surface of the Sun?
From 20-25% to 70% of the radius of the Sun there is a Radiative Zone in which the fusion energy from the core transfers towards the surface through radiation. The temperature in this region varies from 7 million to 2 million kelvins.
From 70% of the Solar Radius to almost near the surface, there exists a Convective Zone in which the primary method of energy transfer is Convection. The density of the Convective Zone is less and hence the primary energy transfer occurs by convection.
There is a boundary region called Tachocline between the Convective and Radiative Zones. The Tachocline is predicted to have a large shear due to Radiative Zone having a fixed rotation rate and Convective Zone having a differential rotation rate.
The Tachocline is said to be responsible for the magnetic field of the Sun, however magnetic fields about the strength of the Sun have been found in stars with no radiative core suggesting the Convective Zone may be responsible for the magnetic field.
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What is present on the Sun’s surface and its atmosphere?
The Sun is made up of gaseous plasma and doesn’t have a definite surface like the Earth. What we are able to see in the Sun are the Photosphere and its atmosphere. The Photosphere is the most interior region visible from the Earth since the solar atmosphere is transparent.
The Photosphere has a density of about 37% of the Earth at sea level. The solar atmosphere is composed of four regions; the chromosphere, the transition region, the corona, and the heliosphere.
The lowest temperature of about 4100 kelvins in the Sun occurs at a temperature minimum region extending to about 500 km above the photosphere.
Carbon monoxide and water molecules have been discovered in this cool region. The chromosphere, transition region, and Corona have much higher temperatures than the surface of the Sun.
This is thought to be due to Alfvén waves, which are a type of magnetohydrodynamic waves in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.
The chromosphere is the region above the temperature minimum region which is about 3000 -5000 kilometers deep. The density of the chromosphere is about 2×10-⁴ kg/m³ at the inner boundary to about 1.6×10-¹¹ at the outer boundary.
The temperature varies from 6000 kelvins at the inner boundary to 35,000 kelvins at the outer boundary with a minimum of 3800 kelvins somewhere in the middle.
The word Chromosphere comes from the Greek word chroma, meaning color since the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses. Above the Chromosphere there is a region called a Solar Transition Region which is only about 200 kilometers thick.
The temperature rapidly varies from 20,000 kelvins at the inner boundary to about 1 million kelvins at the outer boundary.
The rapid increase in temperature is accounted for due to the full ionization of helium. Above the Solar Transition Region, there is Corona. The lower boundary of Corona has a density of about 10-¹⁶/m³.
The average temperature of Corona ranges from 1 million to 2 million kelvins, although in the hotter regions it can be up to 20 million kelvins.
The reason for the much higher temperatures of the Corona compared to the surface is unknown but it is thought to be due to induction by the Sun’s magnetic field and the action of magnetohydrodynamic waves.
A stream of charged particles is released from the Corona called the Solar Wind. The Solar Wind plasma is made up of electrons, protons, and alpha particles with kinetic energy ranging from 0.5 to 10 keV.
The solar atmosphere is extended towards the Heliosphere which is the cavity formed by the Sun in the surrounding interstellar medium which is continuously inflated by the solar wind.
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What will happen to the Sun after its main sequence life ends?
The Sun will empty its supply of hydrogen in the core about 5 billion years from now. The Sun, having mass less than needed for a Supernova explosion to occur, will have its gravitational potential energy released.
The Sun will expand first into a subgiant and then into a red giant, and its luminosity will increase. There will be unused hydrogen remaining in the shell just outside the core, which will start to fuse due to heating due to gravitational contraction.
This will cause the luminosity of the Sun to increase to 1000 times its current luminosity. The Sun will expand so much that it will swallow Mercury and Venus and cook the Earth so badly. The Sun will remain in this red giant stage for a billion years and will lose 1/3 of its mass.
The core, now containing only degenerate helium ignites undergoes a triple-alpha process converting about 6% of the core into carbon. The Sun’s size then reduces to about 10 times its present size and its luminosity decreases to 50 times the present luminosity.
After the helium in the core is exhausted, the Sun will again expand as it did after hydrogen was exhausted in the core, but the expansion is more rapid. Then the Sun becomes a highly unstable body with its mass rapidly decreasing and thermal pulses happening once in about 100,000 years.
The thermal pulses cause the Sun to become larger and larger and more luminous with its radius approaching the current radius of Earth’s orbit around the Sun. The Earth will move away from the Sun due to Sun’s mass decreasing, but the Earth will again come closer to the Sun due to tidal forces.
The Earth is then swallowed by the Sun. At the end of the red giant phase, after four thermal pulses, the Sun will only have about 1/2 of its mass with the remaining outer envelope going into the planetary nebula.
Then the temperature of the Sun will increase causing ionization of the mass and conversion into planetary nebula while its luminosity remains the same. The remaining Sun’s core will have about 54% of the present Sun’s mass and will turn into a white dwarf.
The planetary nebula will disperse in only 10,000 years. The temperature of the Sun in its white dwarf stage will be about 100,000 kelvins. The Sun will remain in this stage for trillions of years until it converts into a black dwarf.
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