The document discusses messages contained in starlight. It begins with an introduction to astronomy concepts like the electromagnetic spectrum and how different regions can be observed. It then explains how starlight provides information about a star's chemical composition and physical properties through spectroscopy. The document reviews theories of stellar evolution and how observations of starlight have helped develop our understanding of stars and their lifecycles.

1. l1i{,,ftir
{p
The present essay is written .on the tooic rMessages of
Starlightr. The essay is written, assuminq that the reader
pooses some basic knowledge on Astronomy. l,tith an introduct-
ign to the topic, some basic concepts of Physics are narrated
which will be needed for further progress. The origin of lig-
ht is discussed briefly. The essay than proceeds to qualitat-
ive and quantitative analysis of starliqht. The present theo-
ries about stellar evolution is reviewed, with an emphasis on
hotn, the mes.sages of starlight contribute in the developement
of the abo..re theory. !{lth a word on conclution Erie essay is
ended.
0{,
l,_
,4
Y l)]!

2. IvIESSAGE g 5l aiiLIGHT
'fhe study of the light received from the stars gives us
many valuble information about their chemical comr:osition
and physical nature. The radiabions lvhich the hurnan eye can
percieve, constitFte what is catrled the optical windoyr of
ihe eLec tromagnetic spec brurn. OnIy through this very narrow
window, and with his unaided eye, man was ablr: to cognize
the universe in the earry time, whire in the near pas.t, with
the discovery of telescopc and other op'Liual instrumencs,
his Eided eye courd reach stirr farther rearms of L,tro univer-
se. It is not at all an etaggeration bo say that the entire
knowre<lge gained on Astrophlsics, ]48 severar decades tcefo i
re was solely based on the observations made on the op iic . I
region of the eleetromagnetic spec trum.
After the adveni: of lrhe quanuum theory and the theory . :il;L[f
of relativity, Astrophysics,fgyfrd a new foundation in whichFerdshcd^
it courd be rebuilt. Tne tecrrriorogy too, in 'EEla_tF fifty W,- laV;
years, progressed at an unimaginable rate. And wibh the help
of such mordern technorogy, the other regions of the electro
magnetic spectrum could also be studied, which had consiciera
bry accelarated Lhe understanding of sterlar evorution and
cosmology in general.
Each pact<etJ6f light energy received from a distant
star indeed camies a packet of information. To fully aporec-
iate the message carried by the starlight, we have to look
first about their origin, for which we wilL dwetl in physics
fol some 'uirnc to f amiliarise ourself I s ruith some basic conce-
pts.
Fi:W iflOitDS OF RELEVANCE
Radiations similar to light are emitted when an electron
in an excited atom undergoes transition from a higher onergy
Ievel Err to a l-ovrrer enexgy level iirr The frequenc), of emi
tfed raiiaLion is giuen by,
4 c"
f = (Ef-H2)/h
nk 1s constant.
The
u?o n i",itr:
nt. the
tion of
c trutn.
Light f rom lncandf,sent sol lds andt
the photosphere of otil-'.un gives rise to
Even t,hough radiatj-on of all freguencies
the peak value of emision corresponds 'bo
, urhere tht is the p1a
g1o,,^ring fluids like
con tinuous spec trurl .
a re emi. t1: e d b)"' th em
a particular freque
dtstribution of energy levels in an atom depends
0
atomd6 number which is charact';-:'restic of each elerne'
isolated atom of an element when excited ernits radia
cliaractErestic frequencie s giving rise to a Iinc si)e
i- filrplrye--
.-

3. -2-
. ncy Cepending unon the temperature of the source, given by,
f = cT/k , where rcr is the veloclty
and I T I is 'tlre. temperature . k,=3.6 ',xlO-5 me tre . d'rgre(1 , is kno
'vvn as the :tiens constant.
Ai_oms can also absorb radia;ion oj. same frequenci_es cor
respcnding to Lheir emission frequencies. The spectral analy
sir of a gas sr-lrrounding a hot source, capabLe o1' givi.nq; rise
to a continuous spectrum shol,,rs; darlc absorption lines in it.
Such a conr-inuous spectrum uriuir dark lines in it is ca1led
as an absorption spectrum ,,',,hich is extensivellr employed in I
the st,udy of starlight.
Coming to the gross intensitl,of a ]ight source, 5.t was
observed quite long i:efore by l(ep1er '"hat tho intensiti. of a
Iight source..'aIIs off with an inverse square of its distance
from the observera This prin,iple is used in the cstirnation
.((?cJ*yof the dis;anc:r of the stars when their luminosiLies are kno
wDr and in c:IcuLating t,heir luminosiiies when i;heir distancetJ.
(U'.r( lle known.
oRIGIN gt ST,IRLIGHT
The vast, amount oi'energy radiated by a star comes from
the nuclear reactions Laking place at the coro of the star.
lvlany stars Iike ..ur owrf ltiiii,T fuses ftfydrog0n to lilelium.
.il-re
core r,:,ac l.ions of more *#Biru stars may involve fusion reac
tions producing heavier elements upto Iron.
A11 the radiation whlch we receive from the stars, optr
cal of otherwise, with an exception of a few, sriginate only
6rom the surface of the star. The energy produced by the nuc
lear reactions at the core is carri.ed to tlre surface by racli
ation ancj convection. The surface of the s.uar acts as a regu
Iator in maintaining the equillibrium be tween the energl pro
duced to the energy radj.ated.
Eve:: though our sun is a ordlnary CLASS G star, its pro
ximity enables us to look a lit;.1e closer at, the i:rofiles un
dergoing inside a star. The temperature of the core is about
14 milrion degrees and sumounding it is the radiation zone
and tho cDnvection zone. The surface of Lhe sun is called the
photospherc. whose temprerlture
ttre radiations which ur" roii"3t
it is ihe chromosphe::c.r.in whic
droirs rapridly rruith increasing h
observed in the solar spec trum
ed Fraunhofer rines. At the rimi'bs of chromosph.:)re vue come
accross an energir dump in which ihey'coronq begins, and then
droos again.

4. Almostlin most or"
thc st;rs r the radiations
wnigh tue rec e i.ve , are erni
tted as in our Sun, only
t,he r.,Efl" being differeni,
but Lirer: are quit+ a few
exc,lptions, as; in tlre case
of intrins ic ',/ariable r t r A
$ijAtYs Ig, ol STAIiL IGI{I
Even a casual look at
the nigh t s ky wouI d re'/e aI
tha t the s tars rrEry c ons id
-3-
Txto6
rx6'
b=x td
5xtt
pqrffiit"ios,*
c{eu,,;,W8tr-*.^ Ccil^rnca
L
7x (o''
erebly j-n their brlghtness f x0
wi bh respec't, ,,o one another.
The measureinent of the amo
unt of right received from / v+ YLn-''r lE +tco-€a''
a star is done by determinlild / '"33$::X':r{f,:t"tinlt
tn"'
ing itzlmagnitude, which has
th:r:'ee dif ferent nut/refated definitions -the ap3arenb, Lhe I
absolute
"nfurto*otric.
i'ie sha1l consi<ler these threu' defin
itions of magnitude briefly.
AIPAI{ENI'MAGNITUDEI Devised around 1850 A.0., it is a logr
ithamic scal.e set up such that two stars differing by five
nragnitudes has a brightn,:ss ratio of 1OO. An arbiEary zero is
cirosen to represent certain bright s bars of the northern con
stellations like a-Lyrae M*g") or a-Aurigae (6apella). ihe
scale has been set such that, fatnter the star, g;c-ier Ls l,/.$
i;s magnitude. If LI and L, are th
from tr,,,o stars, and if i'it and M, a
tively then,
G* tr/t, =
In this scale the brightest star in our sky, Sirius""ffa4
a magnitude of -1.6. The full moon and the sun haf magnitudes
-13.6 and -26.8 respectively. A normal eye can percieve fain
ter objects upto -+6th magnltude of ihis scale. The luminous
flux reci"r"afft& the sun ls IO billlon tlmes as that resie
ved from the frlli star Srius.
It is not posslble to estimate the distance of a star
only with@ a knowledge of their apparent
magnltude, since a star vrlth a given apfrarent magnitude may
be a very luminous star a,: a great distance of,a tess lumino
us star at our proximity.
ABSOLTIIE rIIAGNITUDEi Certain sLars in the sky wtren viewed at
an interval of six months show enough displacement Lo be mea

5. -4-
.u*uO accurately. The angle of
parallax(d) thus obtained is
converted in to y"dian me - sure.
( r f"dian = 206 ,265 s u-c onds o f
arc ) . Using Earth-Sun distance
as a circular segment of arc,
wj"iih t e s'Lar at the cAnLre,
subtending thc angle of paralL
ax a t the star, v/e could calcu
laLe 't,he radius of tire circLetfr
wLrich is 'l;ire dis-bance(d) of the
s tar f rom ear i,Lr* 11' the Ear th-
Sun distancr is taken as an I
unit of distance, ihen tire invel{ d{
rs e of i,he paralla x angl e e xpre
t'(€thod of rriansur a L ion
ssed##trfadians would directly give th€ ciistance of the star
in driE* unit.'fhe mean Earth-Sun distane is called as an
&tronomical [Iui.t (4.U. ) =gA+. =
1@,1I. = ]49.6 million km.
Anotl-rer uni't $for maasuring distances involved in ss;r
onomy, and of tln u5s6 in i:he method of triangulation is carll
ed as the rparsect.d{p"r"""n is the distance at which one
A.[r. v,rould ap,)ear as lt'jO of arc. Roughly t F"rou. * ?,.26 Li
ght ye a:: s .
The absolute magniLude of the st,ar is defined as ;he
q11 r-rrrt of liglrt recieved from the s L,ai (its a'>parent mafln:tu
de) at a standard distance 01' Len Darsec/from it. Hence this
is a true measure of Ehe stellar lumino",fti"". Once bhe abso
Lute and ap;carent magnitudes are known, applying the invcrse
sqirare law, for the loss of light 'uhe distance of the star
could i:e estj.mated. By knowing their apparent magnitude ,end
5l€*"rr, the s*asls dis'tance by the method of triangulation, its abso
Iute magnitude could be arrived at, using the same Iaw.
BOtOlsEII(IC I,HGNITUDE: ilolometric magnitude is defined as ihe
total drnount of energy recieved from the star in all Lhe ran
ges of detectable wavelengths, including the non-oprical leu
r.Ls too. This emphasizes t
by the eye, bui:dgre. essent
theore rt"ri i*pBlt?h.
".
STARLIGHT SPECTROSCOPY: On resolvi#*[: st;rrlLght into a
spectrum rrsing a diffraqtion gratt$! andphotographing it ,
many valuble informatiortbly'can be hdd of them. Depenciing uron
their spectrum ;he s-tars are classified 1n1o ffi"major-cao
egor5-es stellar classes, designated by the alphabets O,
G, K and iv{, along with four minor classes t'ilrRrN anO(.
B, A, €7
nce

6. ,n"/,pec rrum ;futffi.;- rrorr
each class has subdivisj-ons adoptir,y
09 is followed by BO on to 89 and AO
one c I ;r s s i:o ano ther ,
a sc[Le of O t,o 9. Thus
and so ofl .
o- B-- A
I
W
- F- G,r K-
i'de would briefly narrate the characterestics of the stars
belonging to each stellar class.
CLASS O: These are hrlue-whitds&ars and with € excep'[ion
t.r4.
gf@qff-nayet stars) nas a very high surface temperature a*otr
e.rrob[r6!t52,ooo Kervin. fs in all spectt"I cl"""esfifydrogen lines
are found in tiieir spectrum. ALso their spectrum shows pror:r
inent absorpt.i-on ]ines clue to doubly ionisedltfelium, and dou
bly and even tripJ.y ioniseA ditrog"n and Oxygen. Any me Lals
present would show chemseLves only at ultra-voileL region at
such a hiOh$temperatures. But our atmosphere shields the
Ultra-voilet region and hence those lines even if present ca
nnot be observed, frrl-A )f b'up *-/i"l ee{}:,.''
CL.qSIi Bl 'I'hese
are lvhite stars having a surface tempe:ature
of 25rOOO Kel'rin. Apart from |fuaroqen Lines, absorption linee
due to unionised fietium and singly ionisedfliirogcn and @xyg| '- o
en are found in their spectra.h'lany s i:ars of [][s class are
found in ;he constellation of Orion. The star f-Orionis (ltni
Ia), the cen'Lral si.ar of ttre belt of Orion j.s an excellent /
example of th6s class.
CLASS A: i'hese 3re also white sLars but lvith a lesser surfa
ce temperat,r:xe of l.$fi IOTOO0 K. 'l'hese stars show ini-ense :lal-
rner ,Iines of $ydroOen in thelr spectrum. Slrius(c-Canis tvtajo
ris) and Vega(c-Lyrae) are the besu examales, which are /fi/
of the 'bype AO.
CLASS G:( t'hese s.ars with a surface temperature of 6000 K,
are yellow in col-oul. Our sun(G2f ifdf// belongs to th&s ste
llar class. The t{ and K lines of Calcium are inten5le , and
the.Hydrogen lines still wealcer in tlieir sDectrum. Thelr @.
ft.ef,6,*rrt"a*t#,iA ire cr,arac i'erised uvfin" absorbpti on filildi tines due
to neutrat
am,
particularly lron.
CLASS K: 1'he orange-red s tars having surf ace temperatures
upto 4OO0 K, belong tdr th&s class. Hydrogen llnes alre very
weak but- we find characterestics bands due to lhe pfesence
of trydrocarbons in their spectrum. They also show presence
of Titanium Oxide. The Arturee(c-Booties) and Al l{asI(y-Sa
gitarii) are the best examples, both of ihe type KO.
CLASS M: These are red stars havlng a surface temperature of
s
I
jVt
i
RAN

7. -6-
CLASS G: These stars with a surface temperature of 6000 K,
are yellow in colour. Our Sun(G2) belongs to thts stellar
class. The H and K lines of Calcium are intenser and the
& Hydrogen lines still weaker in their spectrum. Their spe-
ctrum is characterised by fine absorbption lines due to
neutral metals, particularly lron.
CLASo Kl The orange-red stars having surface temperatures
upto 4OOO K, belong to this class. Hydrogen lines are very
weak but we find characterestLcs bands due to the presence
of lrydrocarbons in their spectrum. They also show presonce
of Titanium Oxide. 'Ihe Artures(c-Booties) and AI Nasl(y-Sa-
qitarii) a e the best examples, both of the type KO.
CLASS M: These are red stars having a surface temperature
of 3OOO K. These tnclude the majority of supergaints and
Iong period variables. The fluted bands of Titanlum Oxide
is prominent in their.spoc',iurr-the reddness of these stars
are due to the absorption of the blue end of the snectrum
by this compound. The best example of this class is the star
Antares(type MI) of the constellation of Scorpio.
'Ttte- CSAS.$ .R'AllD

8. i'-ra;,x-
:* *4 **, u* F ltc"otrt
ff"u (,t * €r'"d -t lt* -'8fe"(*"u"t
^6 u,dL - 'a+*- i.u"-,* 4*dwLY=trL
"l
N, ,Lfa- ,'LiD, ( t:y1." ."i r)
+ "/C". fi **t-*, n
I S W;:lz-t- F**,^cno(,
tgr^ci 7hcu,ncl. [W*- )f1s,r4 ok yut a- h!-J ot i-
Lz*s< yl ,4)5+, @.---a+r.e' *e- "i;#u,L. *"*,*+
,*t*"L1.-^,"r lvcfue @ 'niu
'-tta@c
6fiff rN sTri<;rRr+( r-ir(es '' W'{t&<" I
L,a.wh lK
fi*.^{"YW tlco^^,* Gzr-:nr v o ,)'a,,,u,n ,h;^
Lei /d-s.,"c(

9. Ylueo z)L-{ -d']- 4
,,fl,
,lf oro*( K L*y
C,*-&. r.r-(rLC ,,LG /--
€-v-,t(u,liL-,
Srf',a.r*t ,,vt,tssis
' a l'vt*-4)* ,h
"!t.#^ I;. C_Olt-c,Jafi_
Coyu c..r(,,*
. ai_
t /6 ylro,)1
Vr;stLl Lii[ @
-.--
'--.-'-----:- h @i .&' v*le,:cq
6rgaran-'Ettotit
-:*-r i:'- .*rr- -

10. -7-
Hubble found that the ratio of the velocity of reccestion to
the distanc e, in the case of the galaxios, is approximatly
a constant. This method is employed in estimating the dista-
nce, when aII the other cl.rssical methods fail.
ST;LLAII r',|{S,SES: The determination of the mass of a star be-
con'res quite easy if iL is a fia nember of a binary or trinary
system. The knoruledge. of the period of revolut:Lon, and the
maxium angular separation is enough to calculate Ur,:, mass o:fl
the cornponent.s. Ln cases where the star is noi in conjugation
tuith an anot$er star, its inass could be estimated by ex:rao-
olating a grpph, pl_otted with stell_ar luminosities aqainst
sterrar mass of iho stars for which both the above pa::ameters
are obtainable by other methocis.
The de termi-nation of the s tellar cla s s
belongs not only _qives information about its
rnpcrature and luminosi'L,rr but also about its
Iuminosity of a s.i;ar varies as the _secgjtl:l- or
to which a star
mnsx sur "ace te-
mass, sinc': tire
third polver of
its mass tL s lul2 or l,{3]. fb.r-,'tt<
STET.IIN EVOLUIION
Upto now lve were conserned r,"lith wha'u where tirc messages
of sbarligh+u dnd hoyl these messages r^rere classif ied. ir'e vuj-I1
now consj.cler hor,v L,hese rTre-
sriages ilre in-terpreted in
the expLanation of s'Le11ar
evolut,ion.
andI-l-R D IAG.".AJ,{ : I-le r 'uzs prung
Fussel, inciependently in
1912 and 19f i, trieC to
obLain 'tho gross characte-
risLics of the st,ars by,:1
otirrg abso.Iute magnitude
agai-ns; their st,ellar clas
ses. Thu.y found that most
of 'rhe s-i.ars licd around
the diagonal fronr the trp
Lef t 'Eo the hot rorn righ t.
They call<,'d these stars as
the main sequence s i:ars. I
!,t'as a significant si:cp in
explatning'[hr: evolution o
the s i:ars. The o1d L,lieorie
conjecturr:d that a et,ar be
gins its life as a blue-ilh
f:Btc TRrlL 'fyp{z
B,lrtA
r^lgirb-

11. -8-
star at the top left of the H-H diagram and mo.red aLong the
diagonal during its course of evoluLion. But due [o the mul-
titude of data avaiLable novr the interpretation of the H-R
diaqram is done in a dif l'rent way.
According to the present theorj.es, the mechanism which
initiai;es stellar formatioril, from a huge cloud gives birth
,lo sdverai hundred siars. The factor which is resoonsibLc
forLhe contraction o'f the cloud initialy, is the shock vravc
from a nearblr supernovae explotion and the llresence of tracc.s
of dLrs'E parlicles which the makes the cloud opaquo t6 the
radiations. Ilence aL the er:ge oi' Lrre c.r.uriii -ii! r'e sultant
radia'bion pressure acts towards thc c:.:ntre. This malces .:he
cloud to contract, tiIl the gravi. 11, Lalkes over t,hL' conLrac-
i;j.on. Thr: tcrmperatures begins 'Lo rise in the localised whir-
cools of matter caused by the assmytrical magnetic forces.
corrsidering any one such spot, the rising temperature bends
to oppose further contraction. At this stage the temperature
at the sufface of the condensation comes to around 4OOO oC,
at tuhich the atoms their are neither too efficient in radiati-
ng the energy owayr nor they are too efficient in deflecting
the energy back to the core. This eases the internal pressure
anci causes the star to contract furbher.fhe temperai:ures rea-
ches a pointtwhen fusion reacLion begins . This prol-osta::
undergoes large pertrubations, expahding and contracting and
librating excess oii matter. The evidence for this stage is
furnished by T-Tauri variablos. Depending on ihe mass of ;he
post T-Tauri- product the star settles itself in somc place
on the diagonal of the H-R diagram, to become a main sequence
stax. The star spends virtualy, most of iis life at lhe same
position, and deviated from it only at lts a*! caiaclysarnic
ciis integration.
The time in whlch the star remains in the main sequence
is called as stellar longltlxitlevity, and is proportiona'I to
the ratio of i'r,s mass to lts luminosity. Sinte luminosity va-
ries as the third or fourth power of the mass of tfie starr the
stable life of the star ls lnvensl'),' proportional to tha sdco-
nd or third power of their mass. Hence a more massive star W
will consume mole protons (Hydrogen). It is generaly observed
tir.rt a star leaves the main Sequence wnen iu percent of ibS
intial mass of Hydrogen is converted to HeIium'
For the stars whose mass is less than twiCe the mass of
the Sun, Hydrogen is convt:rted into Heliurn by the plo i'on-pro-
ton nuclear reaction, since their core temperatures are le'ss
than 2O mitlion degrees. For more masslve stars the conversion
of i{ydrogen to Heliun is done at a much faster rate by the ea
Carbon.Nitrogen fusion cycle.After its lO percAfit of initial
*^ LI t i

12. -9-
mass of Hydrogen is converted to l{elium, successivelv heav6,er
eLements wiri buird as the core t,emperature rises furilrer. At
this siage fusion reactions of different heavier element will
take place at different shells, and the nuclear react,ons mo-
rres further towards the suriace. Hennl at rtris sra:ju i;he stai
moves io a gaint stage. The long period variabLes are drro to
'uiie starsof t):is stage, vuhich delebratly tries to i.ralance
lits enrgy' productionto its j theenergy producrd to the ene:gy
radiat,ed. The radius of the si;ar j.ncreases many times and
so its luminosity( Iuminosity o fr2T4) too, but ii;s surface
tenrperature dloDs and the star mo',res to ihe to,r right o:f ilrt:
I-l-It diagram.
When iire core ternperatur,.:s reaches very high value gas
cegeneration se-[s in, and the star could no more withtand
prevent ii:s collapse due i;o gravity. stars ress than tiran tvso
solar lnasses shrinks and becomes a white drawf . stars 1L,ss
th;ln 15 solar mas$Lls and greater than two soLar mass(+s sheds
of mos i. of its excesc of mass rnaa catacrrsamic improsion,
urhich is of ten called a Supernovae, and ..he core of the star
becomes a Neutron star. lThen the mass of the atar excer:ds 15
solar masses it could not shed of its excess of matLer by
Ssupernovae explosion, and ends up in a corrapsor orc the bra-
clc ho1c. Ihx Chandrasekhar gave a limit (I.4 solar masses)
for gravity to make the pcotons -Eo combine with bhe electrons
into nutronsr. An lmaginary sphere cai.led the event horizon
is sltuated ataund the black hole which is cllled the slngula
rily is constructed to explain Lhe various phenomena. The
daia available on the black hole is very limi Lde and it would
be safe to remain at resonable distance foomthose theoritical
conjecture for this present smalI essay.
A WORD OF CONCLUSION: The messages wtrich we receive foonr star
light are indeed enormous, and the different sorts of explan-
ations provicied fur thErr on in:.erpreting thos" diversed mess-
agc's are gr:ite numerous. To bring even the outline of ttre
subject withln such EFdft*m)imposed timitationa had been
quite dlfflcult. It is posltive that many potnts mayba left.
The awe which we experience on the expos'lure to the knowledge
about the univeree is indeed unparalled by any other experen-
ces which we have had. After achvelng the space telescone the
hori.zons of the visual universe would expand much further and
id sure to brl,ng nore knowledge about cosmology. With such an
expectation in our mfinds we should at the same time be aware
of the contributions of the ameture to the subject. Its quite
sure that wha L appears as mysterLes today will definitly be
the corner stone of the unwritten science of the future.