SCIENCE REPORTER, November 20108

M.S.S. MURTHY

FAR afar, the Sun appears tobe a calm, serene andunchanging beacon of lightin the sky. But is it really so?

No, say the astronomers. The flatbright disk hides another Sun—active, ever changing with violenteruptions that blaze forth in alldirections and some reaching theearth too. What then is the real natureof the Sun that we see every day?

Astronomers tell us that the Sunis essentially a huge mass of gas,about 330,000 times heavier thanearth, its volume is 1.3 million timesthat of the earth and radius about700,000 kilometers! It contains

mostly hydrogen, the lightestelement in the universe (74%), helium(25%) the second lightest element,and the remaining one percent filledup by other chemicalelements.

The Sun has no solidsurface, unlike earth ormoon. However, because ofits huge mass it hastremendous gravity, whichkeeps the gas fromescaping. As thegravitational force pulls theouter layers of a gas towardsits center, the pressureinside the Sun increaseswith depth and so does thetemperature. Right at thecenter of the Sun, called thecore, the pressure is morethan 200 billion times the

earth’s atmospheric pressure at thesea level (2.334X1011 bars vs 1.013bars) and the density of gas goes upto 151,300 kg per meter cube—about

The Sun is often described as a turbulent ball of fire although to us on the earth itlooks like a white disk. Astronomers have employed innumerable instruments toview the unseen Sun with details not discernible to the naked eye.

The Sun appears to us just like a white disk

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150 times the density of water! Andthen the temperature of the gasreaches 5 million degree Kelvin.

Under these conditions, thehydrogen and helium atoms arestripped of their electrons and thematerial consists of fast movingelectrons, hydrogen nuclei (protons)and helium nuclei, all together calledplasma. The protons undergonuclear fusion reactions in whichfour of them fuse to form a heliumnucleus, releasing tremendousamount of energy. The same basicreaction produces the explosion in ahydrogen bomb, the most powerfuldestructive weapon ever invented byman.

The energy released in onesecond is more than what mankindhas used since the dawn ofcivilization. Astronomers say thatthis has been going on for the past4.5 billion years and will go on foranother four to five billion years. Thisis the source of all the radiation wereceive from the sun. The solar

radiation is notconfined to onlythe light and heat.It also consists ofradio waves,ultraviolet lightand X-rays too.

All the lightand heat and otherforms of radiationswe receive from the

Sun come from the top of theconvective zone, about 500 km thick,known as photosphere. What we seeas the Sun is this layer, whichappears as a smooth white disk. TheSun appears to us this way becauseof many reasons. One, it is the largedistance between the Sun and us—150 million kilometers. At thisdistance, we cannot discern featuressmaller than 43,500 km across—almost four times the diameter of theearth!

Another reason is that our eyesare sensitive to only a small band ofthe radiant energy, the visibleradiation, emitted by the Sun. Wereour eyes sensitive to any of the otherwavelengths in the solar radiation,the Sun would have appearedentirely different! Astronomers havedeveloped various types ofinstruments to view the unseen Sunwith details not discernible to thenaked eye and also in the otherwavelengths for which our eyes arenot sensitive.

Galileo Led the WayThe earliest such instrument was asimple telescope. Four hundred yearsago, when Galileo Galilee, the Italianastronomer turned his newlyinvented small telescope towards theSun he was amazed. The plainlooking bright disk was, after all, notwithout blemishes. He noticedseveral black spots, which came tobe known as “sunspots”. He alsofound that they were not static, butvery dynamic—growing in size,merging with each other, fading,disappearing to appear again atsome other place on the photosphere.Since then astronomers have notstopped exploring the unseen Sun.

In 1908, American astronomerGeorge Ellery Hale discovered thatthe sunspots are areas of strongmagnetic field (0.1 to 0.4 tesla), morethan 10,000 times stronger than theearth’s magnetic field at the equator(0.00003 tesla). Same year, JohnEvershed, the then director of theKodaikanal Observatory in TamilNadu found that sunspots are placesat which the underlying solar gasesflow in and out with a high velocity.

Modern telescopes have revealeddetailed features of the sunspots.They vary in size from about 1000km to 50,000 km. Occasionally theymay grow to 200,000 km (as big as12 times the earth!), becoming visibleto naked eye. The central part of asunspot appears darker than its rim.Since the gas in the photosphereglows at a temperature of nearly 5800degree Kelvin, a black spot meanscooler gas.

The intense magnetic fieldworks as a choke to inhibit theoutward flow of hot gas from below,creating cooler (about 3500 degreeKelvin) areas on the photosphere.The number and location of thesunspots vary on an 11-year cycle,known as the sunspot cycle. At thepeak of the sunspot cycle there canbe as many as 100 sunspots. In fact,many other activities of the Sun varyin step with the sunspot cycle.

Sunspots become visible with anordinary telescope. If we zoom witha more powerful telescope with betterresolution, the picture further

Galileo Galilee’s simpletelescope

The flow ofcharged particles,known as thesolar wind, movesat supersonicspeeds in theinterplanetaryspace and createspowerfulmagneticshockwaves.

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changes. We can see texturedgranular patterns on thephotosphere. Kenneth R. Lang in hisbook Sun, Earth and Sky describesthese granules as “supersonicbubbles in an immense boilingcauldron”.

They are columns of hot gas,about 1500 km across, jetting out

from the inner layers to thephotosphere. They areclosely packed with a brightcenter containing the hotgas surrounded by cooldark material, in a wayopposite to the sunspotstructure. The risingcolumns burst open,liberating their energy andthe cool material falls back

to repeat the cycle.Like sunspots they also

exhibit a cosmic dance—changingin shape, merging with one another,disappearing to appear again elsewhere on the photosphere. At anytime, one can see at least a milliongranules across the visible disk, eachlasting about 15 minutes.

As the Sun is 110 times as wide as theEarth most of the sunspots we can seewith a small telescope are bigger thanour planet

Solar AtmosphereSeen from the earth the photosphereappears to be the boundary of theSun. In reality, the Sun has noboundary. Above the photosphereare two more zones called thechromosphere—about 10,000 kmthick and the corona. Coronaextends to millions of kilometers andsome times even reaches the earth’sorbit. Astronomers call these zonessolar atmosphere.

Being so far away from the centerof the Sun, the gravitational forcefalls rapidly in these zones. Hence,the density of the gas decreases tovery low levels. In the photosphere,it is about a billion times less (10-4kg/m3) than the density at the core, inthe chromosphere it is less byanother ten thousand times (10-8kg/m3) and in the corona the density isreduced by another ten thousandtimes!

One would have expected thatthe temperature would also fall asone moved away from the solarcenter. Surprisingly, it is not so. Thetemperature rapidly increasesacross these zones—from about5800 degree Kelvin at thephotosphere to about 10,000 degreeKelvin at the top of thechromospheres and to more than amillion degree in the corona. Hence,the gas in these zones is in a highlyionized state.

Though these gases emitradiations characteristic of therespective temperatures, they are soweak because of the low density, thatthey are not visible to the naked eyeon a normal day against the brightphotosphere. However, they becomevisible for a short duration duringtotal solar eclipse when the mooncompletely covers the photosphere.The chromosphere appears as areddish ring around the darkenedphotosphere and the corona as anirregular whitish halo.

It is in these zones that some ofthe most spectacular events occurdue to the interaction of Sun’smagnetic field with the hot ionizedgases leading to far-reachingconsequences on earth. Hence,astronomers have developed special

The internal structure of the sun

Internal structure

Inner coreRadiative zone

Convection zone

Subsurface flows

Photosphere

Chromosphere

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instruments to study these zoneseven on a normal day.

One such instrument is afiltergram—a type of telescope thatallows only a specific coloured light.A filtergram of chromosphere usingred light emitted by ionizedhydrogen reveals sharp flame-likestructures known as spicules. Eachspicule, 10 to 1000 km across,extends up to 12,000 km into thechromosphere and then collapses. Itlasts for five to 15 minutes.

As the gases in the photospherechurn, they produce shock wavesthat heat up the surrounding gasand push it as a jet through thechromosphere. Seen above the edgeof the visible solar disk these spiculesblend together to appear like burninggrass in a meadow!

The Violent SunSpicules are not the only feature ofthe chromosphere. A filtergram takenduring the maximum sunspotactivity reveals features that are morespectacular. Huge bright columns, inthe form of arches, rise up from thephotosphere, reach tens ofthousands of kilometers into the

chromosphere, and fall back into thephotosphere. These are known asprominences. They are ionizeddense gas columns contained inpowerful magnetic fields.

While some prominences last forhours, others may persist for months.When the temperature of the gas inthe prominence reaches five milliondegree Kelvin it violently erupts intowhat is called a ‘solar flare’ releasinglarge amounts of charged particlesand radiation.

Because the average temperatureof coronal gas is more than onemillion degree Kelvin, it emits mostof its energy in the form of X-rays. Inthis respect, the corona outshines thephotosphere. Hence, a coronal X-rayimage can be obtained at any time ofthe day, by using an X-ray telescope.

However, since the earth’satmosphere absorbs X-rays it is notpossible to image the corona fromearth, but from a telescope in space.An X-ray image recorded by theJapanese space telescope Yohkoh on1 February 1992 revealed structuresin the form of huge loops in a state ofconstant agitation. Astronomers callthese “coronal loops”. These are

extremely strong closed magneticloops containing gas at atemperature much higher than thesurrounding corona itself.Occasionally these loops open upejecting a huge amount of solarmaterial, the hot ionized gas. Thisprocess is called the “coronal massejection”.

All these eruptions in the solaratmosphere are intimately connectedwith the complex magnetic fieldassociated with the sunspots. Whilemany small eruptions occureveryday, violent ones synchronizewith the maximum of sunspotactivity. During these violenteruptions millions of tonnes of solarmaterial consisting mainly ofelectrons, protons andelectromagnetic radiation arereleased.

Astronomers estimate that thetotal energy released in sucheruptions can be equivalent to theenergy released in the explosions ofmillions and millions of megatons ofhydrogen bomb! If these are directedtowards the earth they can haveprofound effect on earth and itsatmosphere.

The wild nature of the Sun captured by the Solar andHeliospheric Observatory (SOHO) spacecraft

A raging solar storm

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Fifty to three hundred kilometersabove the earth’s surface is a layer ofearth’s atmosphere called“ionosphere”, which contains freeelectrons and ions. This layer is usedfor long-range radio broadcastingand telecommunication. The intenseX- and UV-radiation emanating fromthese eruptions increase theionization in this layer and disturbthe radio and telecommunications.

That is not all. The flow ofcharged particles, known as the solar

wind, moves at supersonic speeds inthe interplanetary space and createspowerful magnetic shockwaves. Ifthe earth is in the path of thesewaves, they interact with the earth’smagnetic field, producing what areknown as “geomagnetic storms”.These can have disastrous effects onearth.

One such severe event occurredon 13 March 1989. A huge coronalmass ejection produced a severegeomagnetic storm, which plungedthe Canadian province of Quebecinto darkness within seconds. Thegeomagnetic storm induced electriccurrents in power lines, whichcreated voltage surges blowing upcircuit breakers, melting transformerwindings and causing massiveelectrical failure.

For these reasons, many nationshave established space weatherstations like the NASA’s Solar &Heliographic Observatory (SOHO)

launched in 1995 and the twin SolarTerrestrial Relations Observatory(STEREO) launched in 2006. NASArecently launched another solarprobe called the Solar DynamicObservatory, which is capable ofmonitoring the Sun from muchcloser distances. In August this year,it detected a solar eruption directedtowards the earth.

India has optical telescopes atKodaikanal and Udaipur and radiotelescope at Pune dedicated to solarstudies. The Indian Space ResearchOrganization is planning to launcha satellite called “Aditya” to studythe solar activity. While the regularweather stations monitor the visibleSun and report on the temperature,cloud formation etc in the earth’senvironment, these space weatherstations continually monitorthe unseen sun for its manydisturbing effects on earth and itsenvironment.

To top it all, astronomers predictthat when the Sun has used upall its hydrogen and helium in itscore, it will blow up in size in to whatthey call a ‘red giant’. It will be sohuge as to engulf all the planetsin the solar system, including theearth.

The Sun that is nurturing life onearth, finally consumes it. But,astronomers assure that it may nothappen in our lifetime or even in thelifetime of our great, great,great,…grand children. Only afteranother, may be, four billion years.Until then let us enjoy the Sun andits blessings.

Dr M.S.S. Murthy retired as Head, RadiologicalPhysics Division, Bhabha Atomic Research Centre(BARC), Mumbai. Address: B-104, Terrace GardenApartments, 2nd Main Road, BSK IIIrd Stage,Bangalore- 560805

The Sun has no solid surface, unlikeearth or moon. However, becauseof its huge mass it has tremendousgravity, which keeps the gas fromescaping.

Coronal massejection

The KodaikanalObservatory

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