The Sun: Structure, Core and Solar Flares
The Sun is the star at the centre of our solar system and the source of almost all the energy on the Earth. This topic explains what the Sun is made of, its internal and atmospheric layers, how it produces energy by nuclear fusion, and the kinds of activity seen on its surface - sunspots, solar flares, coronal mass ejections, the solar wind and auroras. The numbers here are important: the core temperature, the surface temperature, the distance of one astronomical unit and the speed at which sunlight reaches us.
The Sun and its composition
The Sun is a star, a giant ball of hot plasma at the centre of our solar system, and it is classified as a G-type main-sequence star, also called a yellow dwarf. The Sun is mainly composed of hydrogen (about 74 per cent) and helium (about 24 per cent), with only small amounts of other elements. The Sun contains about 99.86 per cent of the total mass of the entire solar system, and its average distance from the Earth is about 149.6 million kilometres, which is called one astronomical unit (AU).
Layers of the Sun
The interior of the Sun has three layers - the core, the radiative zone and the convective zone. The core is the centre where energy is produced, with a temperature of about 15 million degrees Celsius. The atmosphere of the Sun also has three layers - the photosphere, the chromosphere and the corona. The photosphere is the visible surface of the Sun, with a temperature of about 5,500 degrees Celsius, from which sunlight is emitted. The corona is the outermost layer and is surprisingly much hotter than the surface, with temperatures of about 1 to 3 million degrees Celsius; it is visible to the naked eye only during a total solar eclipse.
How the Sun produces energy
The Sun produces its energy through nuclear fusion, in which hydrogen atoms fuse into helium in the core and release enormous amounts of energy. The specific series of reactions in the Sun's core is called the proton-proton chain, which converts hydrogen into helium. Sunlight takes about 8 minutes and 20 seconds to reach the Earth. The Sun is currently in the main sequence stage, about halfway through its total life of about 10 billion years, and it is about 4.6 billion years old.
Solar activity: sunspots, flares and CMEs
Sunspots are darker and cooler areas on the Sun's surface caused by strong magnetic activity. A solar flare is a sudden, intense burst of radiation from the Sun's surface caused by the release of magnetic energy. A coronal mass ejection (CME) is a massive burst of plasma and magnetic field ejected from the Sun's corona into space. The solar wind is a continuous stream of charged particles (plasma) flowing outward from the corona. When these charged solar particles interact with the Earth's magnetic field and atmosphere near the poles, they produce auroras, the northern and southern lights.
The solar cycle and the Sun's future
The solar cycle is the roughly 11-year cycle of sunspot and solar activity. The solar constant is the amount of solar energy received per unit area at the top of the Earth's atmosphere, about 1,361 watts per square metre. The Sun rotates at different speeds at different latitudes - about 25 days at the equator and about 35 days at the poles. In about 5 billion years the Sun will expand into a red giant and finally end as a white dwarf surrounded by a planetary nebula. To study the Sun, India has launched the Aditya-L1 mission, and NASA has sent the Parker Solar Probe.
Quick revision
For quick revision, remember the Sun as a G-type main-sequence star made mostly of hydrogen and helium, containing about 99.86 per cent of the solar system's mass. Its interior layers are the core, radiative zone and convective zone, and its atmosphere layers are the photosphere, chromosphere and corona; the core is about 15 million degrees Celsius and the surface about 5,500 degrees Celsius, while the corona is hotter still. Energy comes from nuclear fusion through the proton-proton chain, sunlight takes about 8 minutes 20 seconds to reach the Earth, and one astronomical unit is about 149.6 million kilometres. Finally fix the activity terms - sunspots, solar flares, CMEs, the solar wind and auroras - along with the 11-year solar cycle and the solar constant of about 1,361 watts per square metre.
How to prepare this topic
- Separate the interior layers (core, radiative zone, convective zone) from the atmosphere layers (photosphere, chromosphere, corona) in a simple diagram.
- Learn the key temperatures: core about 15 million degrees Celsius, photosphere about 5,500 degrees Celsius and the corona 1 to 3 million degrees Celsius.
- Remember the energy facts: nuclear fusion, the proton-proton chain and sunlight taking about 8 minutes 20 seconds to reach the Earth.
- Keep the activity terms in one list - sunspots, solar flares, coronal mass ejections, solar wind and auroras.
- Note the numbers 11-year solar cycle, 1,361 watts per square metre solar constant, 4.6 billion years age and the Aditya-L1 mission.
Common exam traps
- The corona is much hotter than the photosphere, even though it is farther from the core.
- The Sun's energy comes from nuclear fusion, not from burning or chemical reactions.
- A solar flare is a burst of radiation, while a coronal mass ejection is a burst of plasma and magnetic field.
- Auroras are caused by charged solar particles, not by sunlight itself.
- The Sun is about 4.6 billion years old and will become a red giant and then a white dwarf.
Frequently Asked Questions
What are the layers of the Sun's interior and atmosphere?
The interior of the Sun has three layers - the core, the radiative zone and the convective zone. The atmosphere of the Sun also has three layers - the photosphere, the chromosphere and the corona. The photosphere is the visible surface with a temperature of about 5,500 degrees Celsius, and the corona is the outermost layer with a temperature of about 1 to 3 million degrees Celsius, visible during a total solar eclipse.
How does the Sun produce its energy and how long does sunlight take to reach the Earth?
The Sun produces its energy through nuclear fusion, in which hydrogen atoms fuse into helium in the core and release enormous amounts of energy. The specific series of reactions is called the proton-proton chain. The temperature of the Sun's core is about 15 million degrees Celsius, and sunlight takes about 8 minutes and 20 seconds to reach the Earth. The Sun's average distance from the Earth is about 149.6 million kilometres, called one astronomical unit.
What are sunspots, solar flares and coronal mass ejections?
Sunspots are darker and cooler areas on the Sun's surface caused by strong magnetic activity. A solar flare is a sudden, intense burst of radiation from the Sun's surface caused by the release of magnetic energy. A coronal mass ejection (CME) is a massive burst of plasma and magnetic field ejected from the Sun's corona into space. The solar wind is a continuous stream of charged particles flowing outward from the corona, and auroras are caused when these charged particles interact with the Earth's magnetic field near the poles.
What is the solar cycle and what is the solar constant?
The solar cycle is the roughly 11-year cycle of sunspot and solar activity on the Sun. The solar constant is the amount of solar energy received per unit area at the top of the Earth's atmosphere, about 1,361 watts per square metre. The Sun rotates at different speeds at different latitudes - about 25 days at the equator and about 35 days at the poles - and it is about 4.6 billion years old.
What will happen to the Sun in the future and which missions study it?
The Sun is currently in the main sequence stage, about halfway through its total life of about 10 billion years. In about 5 billion years it will expand into a red giant and then finally end as a white dwarf surrounded by a planetary nebula. India's Aditya-L1 mission and NASA's Parker Solar Probe are the missions that study the Sun, helping us understand its atmosphere, corona and solar activity.