beyond solar system 1

7/31/2019 Beyond Solar System 1

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Beyond Solar SystemStudy of

Stars, Nebulae & Galaxies

Author: Prof. Mujtaba Lokhandwala

Published by: Hon. Secretary,

Jyotirvidya ParisansthaTilak Smarak Mandir

Pune 411 030

Jyotirvidya Parisanstha, Pune 411 030

May 2001

1. Introduction :Anybody who looks up at the sky on a clear night is sure to befascinated by the large number of stars. They have been thesubject of a large number of poems, romances and what not. In

a more serious mood man has long speculated about what thestars, really are. Most ancient civilisations had myths associatedwith stars. Some considered them to be holes in the firmamentfrom which the divine light trickled down to us, someconsidered them to be the souls of their dead ancestors. Sincethe last few centuries the stars have been studied scientifically,especially in the last one and today we no longer have suchromantic ideas.Much of the information about the stars can be obtained bymeasuring their radiation. Earlier the stars were listed accordingto their location in a constellation and were designated by aletter or a number. A similar system still persists for the ease of

identification of the stars. The brighter stars have Greek letters, , etc. attached before the name of the constellationin which they occur, usually in the descending order of their

brightness in the constellation. e.g. Sirius is Canis Majorii.The fainter stars are designated by numbers. By universalconsent, the numbers in the Flamsteed British Catalogue of1725 are adopted for stars which have no Greek letters assignedto them. Flamsteed arranged his stars by numbering them inorder of their Right Ascension. Most modern catalogues acceptthis practice, however, with time small changes of star positionsdo occur, because of the precession of equinoxes.

2. Stellar Magnitude :Hipparchus in the year 134 B.C. classified all the stars,

visible to the human eye under ideal conditions in six groups.Magnitude 1 stars were the brightest and magnitude 6 stars werethe stars just visible to the human eye under ideal conditions.


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This system was however later modified such that themagnitude was related to the log of the luminosity and therelationship is

M = k - 2.5 log Bwhere M is the magnitude, B is the Brightness and k is anarbitrary constant. The apparent magnitude of some of the

brighter objects is given in the table 1 at the end. Thus it can be

seen that stars of magnitude 1 are 2.5 times brighter than starsof magnitude 2 and so on.

3.Absolute Magnitude :The distance of the near stars is measured by parallax

and from this it is found that stars are widely scattered throughspace. The parallax method is useful up to a distance of 100

parsecs, beyond which other methods are used.In order to compare the energy output of a star, a

standard is devised. The star is assumed to be placed at adistance of 10 parsecs and its magnitude at that distance iscalled the Absolute Magnitude. If the energy in the visual rangeof radiation is considered, then it is called the visual magnitude

and if measurements are assumed to be obtained by a receiverequally sensitive to all wavelengths, then its magnitude is calledthe Bolometric Magnitude. The energy output of a star isdefined as its intrinsic luminosity. The diameters of a fewnearby stars have been measured by interferometric techniques.We know the radiation from the sun and also its size.Comparing this information with the other stars we can get a lotof information.

4. Hertzsprung - Russel (H-R) Diagram :The relationship between colour and temperature of the

emitting body is well known and is discussed below. The range

of colours seen, shows that stars have a wide range of surfacetemperatures. Thus, if we know the colour index of a star, itseffective temperature can be calculated.

In 1911 Hertzsprung and in 1913 Russel independentlydiscovered the relationship between the colour indices of starsand their absolute magnitudes, which can be seen in fig.1. It isseen that most of the stars are located on a diagonal band called

as the main sequence. There are two more sets of stars, a redcool set more luminous than the main group. They have asmaller absolute magnitude, than stars in the main sequencewith the same colour index and hence the same temperature.They are therefore more luminous, implying thereby that theyare larger. Hence they are called the Red Giants. On the basis ofa similar reasoning the stars at the bottom left of the diagram

are called white dwarfs. From the data about the stars, we comeacross a well defined relation that luminosity of a star isproportional to the mass 3.1.

Pickering set up a system of classification of stars called as theHarvard classification. Here letters designate the class ofspectra and each class is subdivided into 10 parts. The stars areclassified according to the descending order of theirtemperatures and their spectra are designated by the letters of

the mnemonic:(referring to the story of the Red Riding Hood ).Wolf : Oh ! Be A Fine Girl, Kiss Me Right Now,Smack !5. Measuring Distances of Stars

5.1 Visual or Trigonometric Parallax :

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When the earth revolves around the sun, in six months time ittakes up a position that is diametrically opposite, on its orbit. Inthe heaven when we see the nearby stars, they seem to take updifferent positions with respect to the more remote stars in thesetwo positions. This is known as visual or trigonometric parallaxKnowing the baseline i.e. the distance between the two

positions of the earth we can estimate the distance of the star.

This method is valid for the nearest hundred or so, stars.5.2 Spectroscopic Parallax :

At further distances, it is difficult to measure the visualparallax. However the stars spectrum is clearly seen. From thestellar spectrum and classification we can know its luminosity.From this information and its apparent brightness we canestimate its Distance.

5.3 Units of Measuring Distance in SpaceThe conventional units of measuring distances, used by

us on the earth are too small for measuring distances in space.Astronomers use larger units, as they are convenient. The

smallest among them is the Astronomical Unit. It is the meandistance of the earth from the sun and its value is approximately15 Crore kilometres.

The next unit is the light year. This is the distancewhich light travels in a mean solar year. The speed of light isapproximately 3 lakh kilometres per second. Multiplying thisfigure by 60 X 60 X 24 X 365 gives the distance that is the lightyear in kilometres. In this unit the nearest star, ProximaCentauri is 4.18 light years away from us.

Another unit, widely used, is the parsec. It is the shortform of parallax second. With the astronomical unit as the

baseline, if the parallax of an object is one second of an arc,

then the distance of the object is said to be one parsec. It isapproximately equal to 3.26 light years. Larger units are theKiloparsec or Kpc and the Megaparsec or Mpc.

6. Structure of the Stars :The nature of the physical structure of the sun and all

other stars has fascinated mankind for long. With Newtons

discovery of the spectrum and Kirchoffs discoveries about thespectra, we have learnt a great deal about them.

6.1 Temperature :The black body law of radiation was formulated by

Planck. One of the important properties of radiation was theWeins law which stated that the wavelength at which the starsradiated maximum energy is related to its temperature. Thus, by

knowing the colour index of the star we can estimate thetemperature of the atmosphere of stars as they closelycorrespond to ideal black bodies.

6.2 Chemical Composition : Kirchoff showed that :a) A hot and dense gas gives a continuous bright spectrum.

b) A hot tenuous gas gives a spectrum of bright lines withno continuous background called the emission spectrum.

c) A continuous bright spectrum passing through a cooltenuous gas gives a spectrum with dark lines on acontinuous bright background in the same position as b)above. These are called absorption spectra.

Bohr using quantum mechanics showed that theemission and the absorption spectra are due to the changes inthe energy levels of the electrons, for various states of differentelements. Thus, the study of the spectra from various stellarsources can tell us a lot about their composition.

6.3 Composition of Stellar Atmospheres :When the spectral analysis of the atmospheres of

normal stars in the solar neighbourhood is carried out theelements in the stellar atmosphere are found in abundancementioned below :

Element Approximate % by mass

Hydrogen - H 70Helium - He 27C, N, O, Ne 2

Na, Mg, Al, Si, Ca, Fe 1Other elements 1

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In certain high temperature stars, hydrogen is found inlesser abundance and in globular cluster stars in general a lowerabundance of heavier elements is noted. Newer stars generallyhave an abundance of heavier elements and are call Population Istars, whereas older stars formed early in the life of the galaxyhave a lower abundance of heavier elements and are calledPopulation II stars.

6.4 Stellar Interiors :For many stars the mass, radius and luminosity are

known. We can safely assume that the sun is a representativestar. From geological study we know that complicated lifeforms have existed on the earth for around 10 9 years, thus wecan safely deduce that the sun has been giving an almostconstant amount of energy. So it must be in a state of quasiequilibrium while it gives an enormous amount of energy. Wecan therefore safely assume that most stars are also in quasiequilibrium.

6.5 Source of Stellar Energy :

Even though the details of the method of energyproduction in stars is not known, we know that the centraltemperature of main sequence stars varies only by a factor of 2,hence the source of energy has a generation range very sensitiveto temperature change.The energy is available at source attemperatures around 107 K.

In the sun at least, the energy must have been producedfor a minimum period of 5 X 109 years.

Thus it is clear that chemical reactions are inadequateby a wide margin and are therefore ruled out. Also the amountof energy released by gravitational collapse can be ruled out.

The only source of energy known at present that is

adequate is nuclear reaction, where matter is converted intoenergy, by the famous equation by Einstein,

E = mC2

This energy is got by fusion and conversion ofhydrogen into heavier elements. As stars are mostly made ofhydrogen and helium, the major amount of energy is released by

conversion of hydrogen into helium. The two processesacceptable as feasible are :Proton - Proton reaction, made up of the following reactions :

p + p D + b +nD + p 3He3 He + 3 He 4He + p + p

Carbon - Nitrogen Cycle, made up of the following four steps :12C + p 13C + b+ + n13C + p 14N14N + p 15N +b+ + n15N + p 12C +

In both the processes 4 atoms of hydrogen fuse toform, one atom of helium. However there is a slight discrepancyin the mass, which is released as energy. Thus every kg. ofhydrogen converted to helium would release 6.6 X 1014 Joules ofenergy. It can be shown that for sun and stars of lesser mass onthe main sequence, the proton - proton reaction is predominant,whereas for heavier stars on the main sequence the C-N cycle

dominates.

7. Special Stars :

7.1 Binary Stars :Stars which are observed in near proximity on the

celestial sphere are called as optical doubles. Close observationreveals that some of these are in fact gravitationally connectedand orbit around each other. These pairs are relatively close toeach other in space so as to interact gravitationally. They arecalled visual binaries.

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If these two stars are relatively near each other, thenvisually, they cannot be distinguished, however if they orbit in a

plane close to the line of sight of the observer, they wouldeclipse each other and their magnitude would vary. Such pairsare called eclipsing binaries. Algol is a very good example ofthis.

Another situation can arise, when the stars are close

together, however their spectrum shows duplicity, one spectrumshowing red shift and the other showing blue shift, which isperiodically interchanged. Such pairs are called spectroscopicbinaries.

Binaries are important as they help us, study the massesof the stars involved.

7.2 Variable stars :The light curves of some stars exhibiting regular

fluctuations in brightness cannot be explained by the eclipsingbinary model and in some cases, the period is also irregular.Brightness of such stars varies due to variation in the starsluminosity and so they are said to be intrinsic variable. Some of

these stars provide us with much information or are interestingor spectacular, as we will now see.

7.3 Cepheid Variables:They are named after a prominent variable called d-

Cepheus. Cepheids have provided a lot of help in thedevelopment of our knowledge of the Universe. Their mainimportance being providing us with a method of measuringdistances beyond the limits of trigonometric parallax. Thevariation in brightness is regular and changes are smooth.Typically the magnitude variation is of order 1 and the periodvaries from 1 to 50 days, most of them having periods around 5days.

Their spectra show doppler shifts that vary. At itsmaximum the star shows blue shift and at its minimum showsred shift. One very interesting property of Cepheids is that theyshow a close relationship between their period of variation andthe luminosity. In general brighter the star, greater is i ts period.

This relationship called as the period - luminosity lawof Cepheids, is one of the most widely used and accuratemethods of obtaining the distances of star clusters and galaxies,most of whom contain Cepheids. Cepheids are so luminous thatthey enable us to measure distances up to 2 X 106 parsecs.

7.3 Explosive variables :This group contain many type of stars, showing

luminosity changes in a relatively short period of time, which isnot periodic. The change may range from a fraction of amagnitude up to 16 magnitudes i.e. 2.516 times. When such alarge change in luminosity occurs it is considered catastrophicand the star does not return to its original state.

7.3.1 Flare Stars :Stars that show smaller changes in magnitude remind us

of the flares in the sun and hence are called flare stars.7.3.2 Novae :

Whenever a star increases its luminosity by a largemeasure, typically by 12 magnitudes in a short time, it is calleda nova, meaning a new star. However it is an existing star that

suddenly brightens. After reaching its maximum brightness, itgradually fades, taking several months to reach near its preoutburst luminosity.

Spectroscopic study reveals that shells of material areejected in the space by these stars when they increase inluminosity. Not much is known about why this happens.7.3.3 Supernovae :

Very rarely some stars become unstable and show anextremely large outburst giving much larger increases in

brightness than novae ( by as much as 16 magnitudes ). Afterthe outburst they reduce their brightness gradually over a periodof months. Supernovae are very rare and three have been

recorded in our galaxy in human history in 1054, 1572 and 1604A.D. The latest supernova was seen in 1987 in the largeMagellenic cloud. Supernovae reach from -14 to -16 absolutemagnitudes.

There are other types of stars. Most are comparativelyrare and less dramatic.

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7.3.4 Wolf Rayet Stars :These are extremely hot stars with surface temperatures

ranging from 60,000 K to 100,000 K. They are said to haveextended atmospheres having streaming velocities of the orderof 1000 kms/sec.

7.3.5 Quasars :Many radio sources are associated with galaxies,

however for some radio sources for whom accurate co-ordinatesare available, no galaxy can be seen, but a faint star like objectis seen. Hence the name quasi stellar object was given to them,which is shortened to quasars. When the radio source 3C273was studied, its magnitude (12.8) was sufficiently bright to givea spectrum with a strong red shift indicating a recessing velocityof 37% of that of light. Its spectrum showed a large UVcomponent indicating that it was not radiating as a black body.

Now, some quasars have been shown to have an even greaterrecessing velocities, the maximum being recorded as havingvelocities more than 93 % of the velocity of light.

Quasars fluctuate in optical brightness and this

suggests that quasars are small, perhaps the size of the solarsystem. Their red shift indicates their distance and their

brightness at even these large distances indicate very highluminosity, something comparable to the combined luminosityof a regular galaxy or even brighter. There is only speculationabout their nature and relation to the rest of the Universe. Whatthey are, is not known for sure.

7.3.6 Pulsars:These are small radio sources having a pulsating radio

output with periods ranging from a few hundredths of a secondto a few seconds. Since their discovery in 1968 these sourceshave been called pulsating stars or pulsars. Optically only acouple of pulsars have been identified. Pulsars have a very

precise periodicity.It is now commonly accepted that a pulsar is the

collapsed remnant of a dying star. As the collapse progressesthe pressure becomes enormous and the protons and electronsare forced to coalesce into neutrons. Thus a pulsar may be the

same thing as a neutron star of some 25 kms. diameter having amass typically twice that of the sun.

7.3.7 Black Holes :Stars can have two possible ends, depending on what is

the mass of the star as compared to the Chandrashekhar limitwhich is 1.4 times the mass of the sun. Below this limit the starcollapses to a white dwarf, with a high density. The other

possibility is the explosive supernova event, leaving behind aneutron star or a pulsar behind.

However if the mass of the star is much more than afew solar masses, after the nuclear energy is exhausted, theinternal pressures are insufficient to maintain an equilibriumand a complete gravitational collapse would occur. Such a bodywould be called a black hole, because its surface gravity would

be so great that the escape velocity at its surface would be thespeed of light and no. radiation would ever escape from it.

From the theory of relativity we can deduce that a blackhole of five solar masses would have a diameter of around 7.4kms and a mean density of around 5.9 X 1018 kg/m3. Speculation

in astronomical circles expects a large number of black holes inour galaxy. However methods of searching black holes are

being researched. No unambiguous black hole has been detectedas yet. However it is expected that matter falling in a black holewould radiate short wavelength X - rays. One such X - rayobject Cygnus X - 1, identified as part of the spectroscopic

binary HDE 226868 seems a good possibility.

8. Astronomy using radiation other than visible light8.1 Radio Astronomy :

It started in 1932 when Jansky was investigating whythere was noise in short wave radio receivers. He foundcrackling sound due to thunderstorms and a continuous hissingnoise which he traced to extra terrestrial sources. Studying it hetraced the maximum noise to the centre of the Milky Way inSagittarius. His report was followed by Reber an amateurastronomer who mapped the strength of the radio emission in

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form of a contour map. He noted that the contours closelyfollowed the belt of the Milky Way. He also detected a fewemission peaks. Later on astronomers took up radio astronomyas a serious field of study and confirmed all that Reber hadfound and much more. Much of the general galactic radioemission comes from the regions of hot ionised hydrogen.

The major discovery of radio astronomy has been thelocation of neutral hydrogen in the galaxy. It was predicted byVan de Hulst that Hydrogen in its ground state would emit radiowaves if the spin of the proton and electron which was parallelwere to reverse their directions. The predicted value was afrequency of 1427 MHz or 21 cms. Wavelength. That thisexisted was proved just 5 years later in 1949.

Other radio spectrum lines have been detected and withthem the presence of the hydroxyl radical, ammonia,formaldehyde and formic acid have been found within thegalactic material.

The peaks were studied and called radio stars, but theywere not all identified with visual objects. The source in Tauruswas identified as the Crab Nebula. By radio astronomy methodsthe remnants of other supernovae were searched and two oldsupernovae have been recorded as radio sources.

Sources outside the galaxy were studied and a famoussource Cygnus A was thought to be two colliding galaxies,however later it was accepted as a remarkable single galaxyseen edge on. Some galaxies are much brighter in the radiowave range than the optical range and these enable us to reachthe very edge of the observable Universe.

Other methods used for the study of the heavens havebeen in the Infra Red and the X-ray range of the radiation. Forthis purpose, satellites have to be used as our atmosphere isopaque to almost all X-ray and most of the IR radiation.

9. Life of a Star :

9.1 Early Stages :It is now accepted that stars are formed from interstellar

gas clouds. Parts of the cloud will condense under their own

gravity to form protostars having radii of approximately 1parsec. As the part shrinks its central temperature rises and for astar like the sun, this stage lasts a time of the order of 107 i.e. acrore years, at the end of which the central temperature ismeasured in millions of degrees.

9.2 Evolution after arrival on the Main Sequence :Due to the gravitational condensation and the high

temperature in the star, hydrogen is transformed to helium,producing a core enriched in helium. The nuclear process thendies out in the core and is taken up by an expanding envelope,till some 20% of the hydrogen is consumed, after which the starstarts expanding to become a giant. This, however is a small

phase in the life of the star. It spends most of its life time on themain sequence fusing hydrogen to helium.

9.3 After the Giant Stage of stellar evolution, otherthermonuclear reactions may take place. The expansion of thestar caused by the rise in temperature of the core, which in turnis caused by the contraction of the core. The temperature in the

core rises upto10

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K, when the onset of further reactioninvolving higher elements takes place. The first step is thebuilding up of beryllium.4He + 4He 8Be the second step uses 8Be to make carbon.4He + 8Be 12C9.4 The Final Stage in stellar evolution is not clearlyunderstood and the progress depends upon the mass of the star.Elements more massive than carbon are made as the coretemperature rises further.

In some stars where gravity is the dominant force, thedensity reaches very high values. The matter then enters a statecalled as quantum mechanically degenerate. Densities of the

order of 10

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times of water are expected. These stars thenbecome dwarfs.Other stars may go through the hydrogen burning phase

to the red giant phase, to an unstable phase where they explodeto become novae or supernovae, while some other stars becomemembers of different classes of variable stars.

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10. Stellar Clusters :

In our galaxy their are two types of clusters viz. Theopen or galactic clusters and the globular clusters. It can bereasonably assumed that stars in a particular cluster have beenformed approximately at the same time, however the individualclusters may have formed at different times. They thus enable usto study stars of different masses at the same age.

The brightest stars in galactic clusters are bluesupergiants and are population I stars, while the brightest starsin the globular clusters are red giants and population II stars.

11. Nebulae :

They are hazy patches of hazy light which remain at afixed position relative to the star background.

To avoid confusion with comets, Messier a Frenchobserver first published a list of 103 such objects in 1784. Theseobjects are designated by M followed by a number.

Later in 1888, Dreyer compiled a New General

Catalogue of Nebulae and Cluster of Stars which listed 7840objects. These are designated as NGC followed by a Number.Thus the famous nebulosity in the constellation

Andromeda was called M 31 and later NGC 224.These nebulosities were later found to consist of

various object like the open or galactic clusters, globularclusters and objects now classified as galaxies. If the galaxiesare removed from consideration then the remaining objects arenow called as Nebulae.

Matter in the Universe that has not condensed into starscan be seen in one of the two different ways; either byemitting/reflecting light or by absorbing light generatedelsewhere.

11.1 Emission Nebulae :These nebulae absorb the radiation from the stars

nearby. This is done by the material inside being ionised andthen emitting the radiation in accordance with the energy levelsassociated with the elements present in them. The Orion nebula

is the best example of an emission nebula. It is energised by theO type stars in the side and then emits radiation having linesoriginating from hydrogen, helium, oxygen and other elements.

11.2 Reflection Nebulae :They simply reflect the light of the stars in their

vicinity. Their spectroscopic band shows the continuousradiation associated with the stars around them. The bestexample of this type is the Pleiades cluster in Taurus.

11.3 Dark Nebulae :They are dark clouds of gas which absorb a lot of light

in them. The stars behind them, if any are seen, seem dimmerthan they would otherwise. A good example of the dark nebulais the Horsehead Nebula in Orion.

11.4 Planetary Nebulae :They have nothing to do with the planets or the solar

system as such. Their name is a historical anomaly. They are

usually a central star surrounded by a hazy shell, like a ringaround the star. Their size is typically 20,000 A.U.

11.5 Supernova Remnants :They are the end result of the explosion of a supernova

which spews material in space when it explodes. A goodexample is the Crab Nebula, which was observed as asupernova by the Chinese in 1054 A.D.

12. Our Galaxy : The Milky Way

With the naked eye we see a large no. of stars

around us. A question arises in our mind whether the starsextend uniformly throughout space or is there a limit.

Earlier there was no answer to this question, but now inthe 20th century we know that most of the stars we can see are ina nearby locality called the Galaxy or the Milky way. It is socalled because, it appears as a faint band of light circling thesky.

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Sir William Herschel studied the distribution of stars inthe sky and published his results in 1784. He found that theMilky Way had a finite boundary and nothing could be saidabout the faint patches of light seen around the sky. He foundthat the maximum density of stars is along a band with itscentre around Sagittarius and its minimum near Auriga.

The study of distribution of stars shows that thedistribution is like showed in fig.2. Globular clusters are foundspread uniformly in a sphere around the galaxy. Thus the studyof their distribution enabled us to get a rough idea about the

place of the solar system. It is situated much off the centre ofthe galaxy. The diameter of the galaxy is estimated to be about30,000 parsecs, whereas the sun is about 8,500 parsecs from thecentre.

Fig. 2 : Distribution of Globular Clusters.

Our galaxy is very similar to the Andromeda galaxy i.e.M31 or NGC 224 and rotates around its centre. It has different

angular velocities for various parts. The centre having a greaterangular velocity than the arms, so they show spiralling. The

period of revolution of the sun around the centre isapproximately 24 crore years. Since the Sun is 5 X 109 yearsold, it has completed 20 or more revolutions around the galaxy.

The mass of the galaxy is 1011 times that of the sun. Thedisk of the galaxy has a distinct bulge in the centre and is calledthe galactic nucleus and is spheroidal. Its equatorial plane beingthe same as the disk.

13. Extra Galactic Universe and Cosmology

In early astronomy the nebula was a wideencompassing concept. Lord Rosse with his 72 inch telescopefirst detected the spiral structure of the nebulae that lookedelliptical. Later they were found to have stellar systems andhence it was reasonable to assume that they were extragalacticobjects. That this was so was confirmed in the 20th century whenCepheids were used as the measuring yardsticks.

The nearest such object are the Magellanic Clouds andthe local system of galaxies is considered to have 17 such largeand small galaxies. At present the Universe is estimated to have1011 such galaxies.

13.1 Classification of Galaxies :Galaxies are divided into three classes viz.a)Elliptical b) Spiral c) Irregular

13.1.1 Elliptical Galaxies :Most elliptical galaxies are shaped like a convex lens.

They are designated by E followed by a number from 0-7 whichdescribes the degree of flattening. The higher the number thegreater is the degree of flattening. These galaxies have mostly

population II stars and have little gas and dust present. Theirsize varies from 25,000 to 2000 parsecs. They form a notable

percentage of the galaxies.

13.1.2 Spiral Galaxies :They form between 50 to 70 % of the total populationof the universe. Our own galaxy i.e. the Milky Way belongs tothis type. Another good example of the spiral galaxy is theGalaxy in Andromeda. ( M 31 or NGC 224 )

The disk contains dust and gas. The nucleus usually haspopulation II stars and those in the arm are usually of the

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population I type. They contain all the features of a galaxy withwhich we are familiar viz. Star fields, globular clusters, regionsof gas and dust, various types of stars etc.

These galaxies are further classified as normal orbarred. The barred galaxies have a bar like extension to thenucleus from which the arms spring.

13.2 Hubbles Classification of Galaxies :Hubble classified the galaxies on the basis of anarrangement that suggested evolutionary trends.

13.3 Distances of Galaxies :The earliest distance measurement were done by

studying the presence of Delta Cepheid stars in the galaxies.The nearest 20 or so galaxies are measured by this method. Alittle further away the galaxies are measured by the brightnessof blue supergiants and novae. Still further the size of themembers of a cluster are measured.

13.4 Spectra of Galaxies :In 1920 a major discovery of the Red Shift in the

spectra of the galaxies was observed. The dimmer the galaxy i.e.greater its apparent magnitude, the greater the red shift wasobserved to be. This red shift was interpreted to be due to theDoppler Effect, which meant that the galaxies were movingaway from us at an increasing rate as this red shift was found to

be proportional to the distance of the galaxy. The relationshipcan thus be stated as

V = Hd

where H = Hubbles constant = 55 km per sec per Mpc,

V = velocity of retreat of the galaxy& d = distance of the galaxy from us.

13.5 Cosmological Principle :

States that there is nothing special or outstanding aboutour point of view or location inside the Universe. The humanspecies has learnt this lesson the hard way. All old cosmologies,especially the religion based ones gave primacy to humans inthe scheme of things. The humans were the raison detre of theexistence of the Universe. Gradually humans have been shiftedaway from the centre of the known Universe, first from thecentre of the Solar system, then towards the periphery of thegalaxy and so on.

13.5 Structure of the Universe :

With the red shift of the galaxies as seen throughout theobservable Universe the conclusion was obvious, that theUniverse was expanding. The next obvious conclusion was thatsome time in the past the Universe must have been muchsmaller and that even earlier must have a beginning sometime inthe past.

However not all scientists were willing to accept theconclusions. According to them, though the Universe wasexpanding it was effectively the same through billions of yearsof the life of the Universe and they derisively called theconclusions of the other hypotheses as the Big bang as againstwhat they described their own model of the Universe as theSteady State Theory.

The Big Bang model predicted that the Universe wouldbe very hot at the beginning of the Universe and even now weshould be able to see the remnants of the initial radiation. Thiswas later on, discovered as a uniformly distributed backgroundradiation equivalent to approximately 3 K temperature. This

evidence clinched the issue in favour of the Big Bang Theory,which is now universally accepted as the model for the

beginning of the Universe.

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Table 1Magnitudes of some bright objects

OBJECT MAGNITUDE

Sun - 26.5

Full moon - 12.5Venus - 4Jupiter, Mars - 2Sirius - 1.45Canopus - 0.73

a Centauri - 0.1Arcturus - 0.06Vega 0.04Capella 0.08Rigel 0.11Procyon 0.35Betelgeuse 0.8

Achernar 0.48b Centauri 0.60Altair 0.77

a Crucis 0.9Aldebaran 0.85Spica 0.96Antares 1.0Pollux 1.15Fomalhaut 1.16Deneb 1.25

bCrucis 1.26

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beyond solar system 1

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