In the last article, we got familiar with the most magnificent member of our solar system family -The Sun. This article is in continuation to our previous article “Structure of Sun – I”. In this article of the Basics of Astrophysics series, I am going to shed more light on the structure of this giant beast. Today, I am going to talk about some amazing structures that have been observed on the solar surface.

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These are the most striking phenomena that occur in the sun’s photosphere. Sunspots are the regions of strong magnetic fields and lower temperatures. A sunspot may have a temperature of about 3800 K which is about 2000 K cooler than the surrounding photospheric temperature. This is the reason that the sunspots appear to be darker than the bright photospheric background.

Structure of Sun - II 1
Sunspots in H-Alpha (Image: TJ Connolly

First Observation of Sunspots

In 1610, G. Galileo observed the sunspots for the first time using his newly discovered telescope. In the late 18th and early 19th century, observers believed that the sunspots were “holes”. They believed that the sunspots were like a window through which the “cooler interiors” of this star could be seen. Now, we all know that this is just a wild act of thinking. However, in earlier times, these ideas were easy to believe.

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A closer view of sunspot in the visible spectrum 
Image courtesy: the TRACE observatory.
A closer view of sunspot in the visible spectrum
Image courtesy: the TRACE observatory.

A sunspot first appears as a small pore of diameter around 1,000 km. Its size and shape gradually change as a sunspot grows. A fully developed sunspot comprises two well-defined regions. These are – the umbra (inner darker region) and the penumbra (surrounding lighter zone).

Also watch: How will the Sun die?

The sunspots are the regions of concentrated magnetic flux. They usually appear in pairs having opposite magnetic polarities. It is noteworthy that the number of sunspots varies according to the 11.2-year long solar cycle.

The solar cycle is the period of solar activity. The magnetic polarities of sun alter during this period. During the peak of this cycle, the sun is stormy. It generates sunspots and solar flares that can fling plasma toward the Earth. Solar storms can disable satellites used for weather forecasts and GPS navigation and affect the radio, navigation and computer systems of airplanes flying over the poles. 

Solar Prominence

The photographs of the sun during a total solar eclipse exhibit red flames emanating from the chromosphere to the corona. The red-glowing, looped material in a solar prominence is plasma. These looped flames are another important structure on the sun- the Solar Prominences. A solar prominence is a large, bright feature extending outward from the Sun’s surface. A prominence forms over timescales of about a day. Stable prominences may persist in the corona for several months. These prominences may loop hundreds of thousands of miles into space covering distances much greater than even the Earth’s diameter.

Related In Series:
Chapter 11: The Saha’s Equation
Chapter 12: Structure of Sun – I

The internal dynamo of the sun generates a magnetic field. The plasma in solar prominences loop due to the twisting of magnetic field lines. This hot gas comprises of electrically charged hydrogen and helium. An erupting prominence occurs when such a twisted magnetic field structure becomes unstable. Then it bursts outwards while releasing the plasma.

Solar Wind

We all know that sun emits electromagnetic radiation of all wavelengths. As already mentioned during the peak of the solar cycle, the sun also emits a continuous stream of plasma particles having very high velocities towards the Earth. Electrons and protons are primary components of the solar wind. The properties of this wind are continuously changing. The wind also differs based on which region of sun it emerges from. The velocity of the solar wind is higher over coronal holes. It speeds up to 500 miles (800 kilometers) per second in these holes.

This video shows a conceptual animation showing Earth’s polar cusps and the cusp aurora.

Our Earth continuously receives these streams of energetic particles in the solar wind. Earth’s magnetic field prevents solar wind from reaching the Earth’s surface. It deflects these energetic charged particles towards the poles of the Earth. These charged particles are the cause of beautiful polar lighting called the Auroras. A bow shock is formed in the region where Earth’s magnetic field first encounters the solar wind.

Author’s Message

We can understand the fundamental laws of Physics by observing nature.
It is very interesting that observing nature may help to develop technologies that can be used for the benefit of mankind. The study of solar prominences is one such example. The observation of loop-like structures indicated the presence of strong magnetic fields on the Sun. Nuclear physicists investigate these magnetic field lines that form closed loops to confine the plasma in a fusion reactor on Earth. This will lead to the beginning of a new era of sustainable energy called Nuclear energy.

The cover image of this article is the work of astrophotographer Samara Nagle from the U.S. You can find her work on this website.

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[…] kilometers per hour (over 60mph). They also reach a temperature nearly as high as that on the surface of the Sun! The bubbles stun prey, and the slap creates a bioluminescence glow. However, the glow is way too […]

Abhay Shukla
Abhay Shukla

Thank You.


Wow! Great use of words, you made picturing it, much easier.

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