steller evolution and the life of a star

7/23/2019 Steller Evolution and the Life of a Star

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Steller

evolutionand the life

of a star



What is a Star?tars are hot bodies of glowing gas that start their life

n Nebulae. They vary in size, mass and temperature,

iameters ranging from 450x smaller to over 1000xarger than that of the Sun !asses range from awentieth to over 50 solar masses and surfa"eemperature "an range from #,000 degrees $el"ius tover 50,000 degrees $el"iushe "olour of a star is determined by its temperature, the

ottest stars are blue and the "oolest stars are red Theun has a surfa"e temperature of 5,500 degrees $el"ius,s "olour appears yellowhe energy produ"ed by the star is by nu"lear fusion inhe stars "ore The brightness is measured in magnitude,he brighter the star the lower the magnitude goes down

here are two ways to measuring the brightness of a starpparent magnitude is the brghtness seen from %arth,nd absolute magnitude whi"h is the brightness of a stareen from a standard distan"e of 10 parse"s &#'( lightears)



Small Stars* The +ife of a Star ofabout one Solar !ass

Small stars have a mass upto one and a half times that

f the Sunage 1- Stars are born in a region of high density Nebula, and condenses into a huge

obule of gas and dust and contracts under its own gravity.

his image shows the Orion Nebula or M4 .

age - ! region of condensing matter will begin to heat u" and start to glow

rming Protostars. #f a "rotostar contains enough matter the central tem"erature reaches 1$

illion degrees centigrade.

his image is the outflow %coloured red&and "rotostar.

tage # * t this temperature, nu"lear rea"tions in whi"hydrogen fuses to form helium "an start

http://www.seds.org/messier/large/m42.gif

http://www.seds.org/messier/large/m42.gif



Stage 4 - The star begins to release energy, sto""ing it from contracting

even more and causes it to shine. #t is now a Main Sequence Star.

The nearest main se'uence star to (arth, the Sun

Stage $ - ! star of one solar mass remains in main se'uence for about

1) billion years, until all of the hydrogen has fused to form helium.

Stage * - The helium core now starts to contract further and reactions

begin to occur in a shell around the core.

Stage + - The core is hot enough for the helium to fuse to form carbon.

The outer layers begin to e"and, cool and shine less brightly. The

e"anding star is now called a Red Giant.

The star e"ands to a Red Giant, below

Stage - The helium core runs out, and the outer layers drift of away

from the core as a gaseous shell, this gas that surrounds the core is called

a Planetary Nebula.! lanetary Nebula

%/elow, N0 *$42&.

Stage 3 - The remaining core %thats ) of the original star& is now in

its final stages. The core becomes a 5hite Dwarf the star eventually

cools and dims. 5hen it sto"s shining, the now dead star is called

a Black Dwarf .

http://oposite.stsci.edu/pubinfo/jpeg/NGC6543a.jpg

http://oposite.stsci.edu/pubinfo/jpeg/NGC6543a.jpg



!asses!assive Stars * The +ife of a Star of about10 Solar!assive stars have a mass #x times that of the Sun Some are50x that of the Sun

Stage 1 * !assive stars evolve in a simlar way to a small stars

until it rea"es its main se-uen"e stage &see small stars, stages1*4) The stars shine steadily until the hydrogen has fused toform helium & it ta.es billions of years in a small star, but onlymillions in a massive star)

Stage ' * The massive star then be"omes a RedSupergiant and starts of with a helium "ore surrounded by ashell of "ooling, expanding gas

The massive star is mu"h bigger in its expandingstage& /ed Supergiant,below) Stage # * n the next million years a series of nu"lear

rea"tions o""ur forming dierent elements in shells around theiron "ore

Stage 4 * The "ore "ollapses in less than a se"ond, "ausing an

explosion "alled a Supernova, in whi"h a sho". wave blows ofthe outer layers of the star &The a"tual supernova shinesbrighter than the entire galaxy for a short time) The set of images below shows the star going into a stage"alled Supernova and "ontra"ting to be"ome a neutron star

Stage 5 * Sometimes the "ore survives the explosion f thesurviving "ore is between 15 * # solar masses it "ontra"ts tobe"ome a a tiny, very dense Neutron Star f the "ore is mu"hgreater than # solar masses, the "ore "ontra"ts to be"omea Black Hole.



Stellar

evolution



Stellar evolution is the pro"ess by whi"h

star "hanges during its lifetime 2epending on the mass ofhe star, this lifetime ranges from a few million years for the

most massive to trillions of years for the least massive, whi"h isonsiderably longer than the age of the universe The tablehows the lifetimes of stars as a fun"tion of their masses lltars are born from "ollapsing "louds of gas and dust, oftenalled nebulae or mole"ular "louds 3ver the "ourse of millionsf years, these protostats settle down into a state of-uilibrium, be"oming what is .nown as a main*se-uen"e star

u"lear fusion powers a star for most of its life nitially thenergy is generated by the fusion of hydrogen atoms at theore of the main*se-uen"e star +ater, as the preponderan"e oftoms at the "ore be"omes helium, stars li.e the Sun begin touse hydrogen along a spheri"al shell surrounding the "ore Thisro"ess "auses the star to gradually grow in size, passinghrough the subgiant stage until it rea"hes the red giant phase

tars with at least half the mass of the Sun "an also begin toenerate energy through the fusion of helium at their "ore,hereas more*massive stars "an fuse heavier elements along aeries of "on"entri" shells 3n"e a star li.e the Sun hasxhausted its nu"lear fuel, its "ore "ollapses into a dense whiewarf and the outer layers are expelled as a planetary nebulatars with around ten or more times the mass of the Sun "an

xplode in a supernova as their inert iron "ores "ollapse into anxtremely dense neutron star orbla". hole lthoughhe universe is not old enough for any of the smallest redwarfs to have rea"hed the end of their lives, stellar modelsuggest they will slowly be"ome brighter and hotter beforeunning out of hydrogen fuel and be"oming low*mass whitewarfs



Protostartellar evolution starts with the gravitational "ollapse of a giant

mole"ular "loud Typi"al giant mole"ular "louds are roughly 100ght*years &561014 .m) a"ross and "ontain up to000,000 solar masses &1'610#7 .g) s it "ollapses, a giant

mole"ular "loud brea.s into smaller and smaller pie"es n ea"hf these fragments, the "ollapsing gas releases gravitationalotential energy as heat s its temperature and pressure"rease, a fragment "ondenses into a rotating sphere of

uperhot gas .nown as a protostarprotostar "ontinues to grow by a""retion of gas and dust from

he mole"ular "loud, be"oming a pre*main*se-uen"e star as it

ea"hes its 8nal mass 9urther development is determined by itsmass &!ass is "ompared to the mass of the Sun:

0 !; &'0610#0 .g) means 1 solar mass)

rotostars are en"ompassed in dust, and are thus more readilysible at infrared wavelengths 3bservations from the =ide*8eldfraredurvey %xplorer &=S%) have been espe"ially important for

nveiling numerous >ala"ti" protostars and their parent starustersrown dwarfs and sub-stellar objectsrotostars with masses less than roughly 00? !; &1(610' .g)

ever rea"h temperatures high enough for nu"lear fusion ofydrogen to begin These are .nown as brown dwarfshe nternational stronomi"al @nion de8nes brown dwarfs as

tars massive enough to fuse deuterium at some point in theirves &1# Aupiter masses &m A), '5 6 10'? .g, or 001'5 M;)

bBe"ts smaller than 1# M A are "lassi8ed as sub*brown dwarfs

but if they orbit around another stellar obBe"t they are "lassi8eds planets)C Doth types, deuterium*burning and not, shine dimlynd die away slowly, "ooling gradually over hundreds of millionsf years

https://en.wikipedia.org/wiki/Jupiter_mass

https://en.wikipedia.org/wiki/Jupiter_mass



Hydrogen fusionor a more*massive protostar, the "ore temperature willventually rea"h 10 million .elvin, initiating the protonEprotonhain rea"tion and allowing hydrogen to fuse, 8rst to deuteriumnd then to helium n stars of slightly over 1 !; &'0610#0 .g),

he "arbonEnitrogenEoxygen fusion rea"tion &$3 "y"le)ontributes a large portion of the energy generation The onsetf nu"lear fusion leads relatively -ui".ly to a hydrostati"-uilibrium in whi"h energy released by the "ore exerts aadiation pressureF balan"ing the weight of the starGs matter,reventing further gravitational "ollapse The star thus evolvesapidly to a stable state, beginning the main*se-uen"e phase of

s evolutionnew star will sit at a spe"i8" point on the main se-uen"e ofhe HertzsprungE/ussell diagram, with the main*e-uen"e spe"tral type depending upon the mass of the starmall, relatively "old, low*mass red dwarfs fuse hydrogen slowlynd will remain on the main se-uen"e for hundreds of billions ofears or longer, whereas massive, hot 3*type stars will leave the

main se-uen"e after Bust a few million years mid*sized yellowwarf star, li.e the Sun, will remain on the main se-uen"e forbout 10 billion years The Sun is thought to be in the middle ofs main se-uen"e lifespan

!ature starsventually the "ore exhausts its supply of hydrogen and the staregins to evolve o of the main se-uen"e =ithout the outwardressue generated by the fusion of hydrogen to "ountera"t theor"e of gravity the "ore "ontra"ts until either ele"tronegenera"y pressure be"omes suI"ient to oppose gravity orhe "ore be"omes hot enough &around 100 !J) for heliumusion to begin =hi"h of these happens 8rst depends upon thetarGs mass



ow-mass stars

hat happens after a low*mass star "eases to produ"e energyrough fusion has not been dire"tly observedK the universe isound 1#? billion years old, whi"h is less time &by several ordersmagnitude, in some "ases) than it ta.es for fusion to "ease in"h stars"ent astrophysi"al models suggest that red dwarfs of

1 M; may stay on the main se-uen"e for some six to twelve

lion years, gradually in"reasing inth temperature and luminosity, and ta.e several hundred billioore to "ollapse, slowly, into a white dwarf L Su"h stars will not"ome red giants as they are fully "onve"tive and will notvelop a degenerate helium "ore with a shell burning hydrogen

stead, hydrogen fusion will pro"eed until almost the whole star lium

ernal stru"tures of main*se-uen"e stars, "onve"tion zones withrowed "y"les and radiative zones with red Mashes To the leftow-mass red dwarf , in the "enter a mid-sized yellow

warf and at the right a massive blue*white main*se-uen"e starghtly more massive stars do expand into red giants, but theirlium "ores are not massive enough to rea"h the temperatures

-uired for helium fusion so they never rea"h the tip of the redant bran"h =hen hydrogen shell burning 8nishes, these starsove dire"tly o the red giant bran"h li.e a post >D star, but atwer luminosity, to be"ome a white dwarf star of about5 M

; will be able to rea"h temperatures high enough to fuse

lium, and these Fmid*sizedF stars go on to further stages ofolution beyond the red giant bran"hC1'L

https://en.wikipedia.org/wiki/Stellar_evolution






Mid-sized stars

e $atGs %ye ebula, a planetary nebula formed by the death ofstar with about the same mass as the Sun

Stars of roughly 05E10 M; be"ome red giants, whi"h are large

n*main*se-uen"e stars of stellar "lassi8"ation J or ! /ed gian

e along the right edge of the HertzsprungE/ussell diagram due teir red "olor and large luminosity %xamples in"lude ldebaran

the "onstellation Taurus and r"turus in the "onstellationof DoNtes /ed giants all have inert "ores with hydrogen*burningells: "on"entri" layers atop the "ore that are still fusing hydroge

into heliumd*sized stars are red giants during two dierent phases of their

st*main*se-uen"e evolution: red*giant*bran"h stars, whose ineres are made of helium, and asymptoti"*giant*bran"h stars,

hose inert "ores are made of "arbon symptoti"*giant*bran"hars have helium*burning shells inside the hydrogen*burningells, whereas red*giant*bran"h stars have hydrogen*burningells only n either "ase, the a""elerated fusion in the hydrogen*ntaining layer immediately over the "ore "auses the star to

pand This lifts the outer layers away from the "ore, redu"ing thavitational pull on them, and they expand faster than the energodu"tion in"reases This "auses the outer layers of the star tool, whi"h "auses the star to be"ome redder than it was on theain se-uen"e



Massive stars

e $rab ebula, the shattered remnants of a star whi"h exploded as a supernova, thet of whi"h rea"hed %arth in 1054 2

massive stars, the "ore is already large enough at the onset of the hydrogen burningll that helium ignition will o""ur before ele"tron degenera"y pressure has a "han"e ome prevalent Thus, when these stars expand and "ool, they do not brighten as"h as lower*mass starsK however, they were mu"h brighter than lower*mass stars to

gin with, and are thus still brighter than the red giants formed from less*massive staese stars are unli.ely to survive as red supergianst instead they will destroymselves as type supernovasremely massive stars &more than approximately 40 M

;), whi"h are very luminous an

s have very rapid stellar winds, lose mass so rapidly due to radiation pressure thaty tend to strip o their own envelopes before they "an expand to be"ome redergiants, and thus retain extremely high surfa"e temperatures &and blue*white "olo

m their main*se-uen"e time onwards The largest stars of the "urrent generation areout 100*150 M

; be"ause the outer layers would be expelled by the extreme radiatio

hough lower*mass stars normally do not burn o their outer layers so rapidly, they "wise avoid be"oming red giants or red supergiants if they are in binary systems "los

ough so that the "ompanion star strips o the envelope as it expands, or if they rotaidly enough so that "onve"tion extends all the way from the "ore to the surfa"e,ulting in the absen"e of a separate "ore and envelope due to thorough mixingC

e "ore grows hotter and denser as it gains material from fusion of hydrogen at thee of the envelope n all massive stars, ele"tron degenera"y pressure is insuI"ient t "ollapse by itself, so as ea"h maBor element is "onsumed in the "enter, progressiveavier elements ignite, temporarily halting "ollapse f the "ore of the star is not too

ssive &less than approximately 14 M;, ta.ing into a""ount mass loss that has o""urthis time), it may then form a white dwarf &possibly surrounded by a planetary nebudes"ribed above for less*massive stars, with the dieren"e that the white dwarf ismposed "hieMy of oxygen,neon, and magnesium

e onion*li.e layers of a massive, evolved star Bust before "ore "ollapse &ot to s"aleove a "ertain mass &estimated at approximately '5 M

; and whose starGs progenitor

s around 10 M;), the "ore will rea"h the temperature &approximately 11 giga.elvins

whi"h neon partially brea.s down to form oxygen and helium, the latter of whi"h

mediately fuses with some of the remaining neon to form magnesiumK then oxygenes to form sulfur, sili"on, and smaller amounts of other elements 9inally, the

mperature gets high enough that any nu"leus "an be partially bro.en down, mostmmonly releasing an alpha parti"le &helium nu"leus) whi"h immediately fuses withother nu"leus, so that several nu"lei are ee"tively rearranged into a smaller numbeheavier nu"lei, with net release of energy be"ause the addition of fragments to nu"leeeds the energy re-uired to brea. them o the parent nu"leitar with a "ore mass too great to form a white dwarf but insuI"ient to a"hievetained "onversion of neon to oxygen and magnesium, will undergo "ore "ollapse &du

ele"tron "apture before a"hieving fusion of the heavier elements Doth heating andling "aused by ele"tron "apture onto minor "onstituent elements &su"h as aluminumd sodium) prior to "ollapse may have a signi8"ant impa"t on total energy generationhin the star shortly before "ollapseCL This may produ"e a noti"eable ee"t on the





Neutron

starsn article: Neutron star

ble*li.e sho". wave still expanding from a supernova explosion 15,000 years agoen a stellar "ore "ollapses, the pressure "auses ele"tron "apture, thus "onverting the greatority of the protons into neutrons The ele"tromagneti" for"es .eeping separate nu"lei apgone &proportionally, if nu"lei were the size of dust mites, atoms would be as large as footb

diums), and most of the "ore of the star be"omes a dense ball of "ontiguous neutrons &in

me ways li.e a giant atomi" nu"leus), with a thin overlying layer of degenerate matter &"hie unless matter of dierent "omposition is added later) The neutrons resist further

mpression by the <auli %x"lusion <rin"iple, in a way analogous to ele"tron degenera"yssure, but strongerse stars, .nown as neutron stars, are extremely smallOon the order of radius 10 .m, no

ger than the size of a large "ityOand are phenomenally dense Their period of rotationrtens dramati"ally as the stars shrin. &due to "onservation of angular momentumK observe

ational periods of neutron stars range from about 15 millise"onds &over (00 revolutions peond) to several se"ondsC=hen these rapidly rotating starsG magneti" poles are aligned with%arth, we dete"t a pulse of radiation ea"h revolution Su"h neutron stars are "alled pulsars

were the 8rst neutron stars to be dis"overed Though ele"tromagneti" radiation dete"tedm pulsars is most often in the form of radio waves, pulsars have also been dete"ted at visiby, and gamma ray wavelengthsC

Black olesn article: Black hole

e mass of the stellar remnant is high enough, the neutron degenera"y pressure will beuI"ient to prevent "ollapse below the S"hwarzs"hild radius The stellar remnant thusomes a bla". hole The mass at whi"h this o""urs is not .nown with "ertainty, but is "urrenmated at between ' and # M

;

". holes are predi"ted by the theory of general relativity ""ording to "lassi"al generaltivity, no matter or information "an Mow from the interior of a bla". hole to an outsideerver, although -uantum allow deviations from this stri"t rule The existen"e of bla". holehe universe is well supported, both theoreti"ally and by astronomi"al observationause the "ore*"ollapse supernova me"hanism itself is imperfe"tly understood, it is still notwn whether it is possible for a star to "ollapse dire"tly to a bla". hole without produ"ing a

ble supernova, or whether some supernovae initially form unstable neutron stars whi"h theapse into bla". holesK the exa"t relation between the initial mass of the star and the 8nal

https://en.wikipedia.org/wiki/Pulsar

https://en.wikipedia.org/wiki/Pulsar

steller evolution and the life of a star

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