Stellar Evolution - University Of Houston-Clear Lake

Stellar EvolutionSun-like StarsMassive StarsEvidence from Star ClustersBinary Stars

The Life of Main-Sequence Stars Stars graduallyexhaust theirhydrogen fuel. In the process ofaging, theygradually becomebrighter and a littlecooler. They evolve fromzero-age mainsequence (ZAMS)moving up andslightly to the righton the H-R diagram.

Evolution of a Sun-like StarEven while on the mainsequence, the compositionof a star’s core is changing.Hydrogen decreases whilehelium increases.

Evolution of a Sun-like Star As the fuel in the core is used up, the core contracts When the fuel is used up the core begins to collapsebecause the source of thermal pressure is no longer there. Hydrogen begins to fuse outside the core in a shell, whichis called hydrogen shell burning.

Evolution off the Main Sequence:Expansion into a Red Giant Hydrogen in the core iscompletely converted intoHe: “Hydrogen burning” (i.e.fusion of H into He) ceases inthe core. H burning continues in ashell around the core. He core H-burning shellproduces heat that increasespressure causing the massabove the shell to expand Expansion and cooling ofthe outer layers of the star Red Giant

Expansion onto the Giant Branch Expansion andsurface coolingduring the phase of aninactive He core and aH-burning shellSun will expand beyond Earth’s orbit!

Degenerate Matter Matter in the He core has noenergy source left.Electron energy Not enough thermalpressure to resist and tobalance gravity eventhough the core heats upfrom gravitational energy. In stars 2.5 mʘ, matterassumes a new state, calleddegenerate matter Pressure in the degeneratecore comes from electronsthat cannot be packedarbitrarily close together(Pauli exclusion principle)and they have low energies.

Evolution of a Sun-like StarStages of a star leaving the Main Sequence:

Evolution of a Sun-like Star The Sun moves off themain sequence on the H–R diagram to the red giantstage (8 to 9) As the core continues toshrink, the outer layers ofthe star expand and cool. It is now a red giant,extending out as far asthe orbit of Mercury. Despite its coolertemperature, itsluminosity increasesenormously due to itslarge size.

Evolution of a Sun-like StarHelium fusionOnce the core temperature has risen to 108 K,the helium in the core starts to fuse, throughthe triple-alpha process: 4He 8Be energy8Be 4He 12C energy4HeThe 8Be nucleus is highly unstable, and willdecay in about 10–12 s unless an alpha particlefuses with it first. This is why hightemperatures and densities are necessary.

Energy Production Nuclearfusion canproduceenergy up totheproduction ofiron; For elementsheavier thaniron, energyis gained bynuclearfission.Binding energy is a result ofthe strong nuclear force. Ithas a very short range. It isthe strongest of the 4 knownforces: electromagnetic,weak, strong, gravitational

Evolution of a Sun-like StarThe Helium Flash: The pressure within the heliumcore is almost totally due to“electron degeneracy” – twoelectrons cannot be in the samequantum state, so the corecannot contract beyond acertain point. This pressure is almostindependent of temperature –when the helium starts fusing,the pressure cannot adjust. Helium begins to fuse extremelyrapidly; within hours theenormous energy output is over. The star once again reachesequilibrium with steady heliumfusion (Stage 10).

Evolution of a Sun-like Star After the helium flash, the radius decreases, but the starremains a giant on the horizontal branch. As the helium in the core fuses to carbon, the corebecomes hotter and hotter, and the helium burns fasterand faster. When the helium isexhausted, the star isnow similar to itscondition just as it leftthe main sequence,except now there aretwo shells: ahydrogen-burningshell and a heliumburning shell.

Evolution of a Sun-like Star The star expands inradius for the secondtime (10 to 11). A 1 mʘ star is aboutto enter its last stage.

Evolution of Stars MoreMassive than the SunIt can be seen fromthis H–R diagramthat stars moremassive than theSun follow verydifferent paths whenleaving the mainsequence

Evolution of Stars MoreMassive than the Sun High-mass stars, like all stars, leave the mainsequence when there is no more hydrogenfuel in their cores. The first few events are similar to those inlower-mass stars.1. A hydrogen shell and a collapsing core.2. Followed by a core burning helium tocarbon, surrounded by helium- andhydrogen-burning shells.

Evolution of Stars MoreMassive than the Sun Stars with masses morethan 2.5 mʘ do notexperience a helium flashbecause the core doesnot become degenerate.Helium burning startsgradually. A 4 mʘ star makes nosharp moves on the H–Rdiagram – it movessmoothly back and forth.

Evolution of Stars MoreMassive than the Sun A star of more than 8 mʘcan fuse elements farbeyond carbon in its core,leading to a very differentfate. Its path across the H–Rdiagram is essentially astraight line – it stays atjust about the sameluminosity as it cools off. Eventually the star dies ina violent explosion calleda supernova.

Fusion into Heavier ElementsFusion intoheavier elementsthan C and ORequires very hightemperatures andoccurs only in verymassive stars(more than 8 Mʘ).

Mass Loss from Giant Stars All stars lose mass by some form ofstellar wind. The most massive stars havethe strongest winds; O- and B-type starscan lose a tenth of their total mass thisway in only a million years. These stellar winds hollow out cavities inthe interstellar medium surrounding giantstars.

Mass Loss from Giant StarsThe sequence below, of actual Hubble images,shows a very unstable red giant star as itemits a burst of light, illuminating the dustaround it

Evidence for Stellar Evolution:Star ClustersStars in a star cluster all have approximately the same age!More massive stars evolve more quickly than less massiveones.If you put all the stars of a star cluster on a H-R diagram, themost massive stars (upper left) will be missing!

Star ClustersTwo types of star clusters:1. Open clusters young clusters of recentlyformed stars within the disk of the GalaxyOpen clusterM 52Globular Cluster M 192. Globular clusters old, centrallyconcentrated star clusters; mostly in ahalo around the galaxy and near thegalactic center

Globular ClustersGlobular ClusterM 80 Dense clusters of 50,000 – a million stars Old ( 11 billion years), lower-main-sequence stars 200 globular clusters in our galaxy

Observing Stellar Evolutionin Star Clusters The following series of H–Rdiagrams shows how stars ofthe same age, but differentmasses, appear as the clusteras a whole ages. After 107 years, some of themost massive stars havealready left the mainsequence, while many of theleast massive have not evenreached it yet. Note that the lowest massbodies are still proto-stars.

Observing Stellar Evolutionin Star Clusters After 108 years, a distinctmain-sequence turnoffbegins to develop. Yet,most of the highest-massstars are still on the mainsequence. After 109 years, the mainsequence turnoff is muchclearer.

Observing Stellar Evolutionin Star Clusters After 1010 years, anumber of features areevident: The subgiant, redgiant, asymptoticgiant, and horizontalbranches are allclearly populated. White dwarfs, solarmass stars in theirlast phases, alsoappear.

Observing Stellar Evolutionin Star ClustersThis double cluster, h and cPersei, must be quite young – itsH–R diagram is that of a newborncluster. Its age cannot be morethan about 107 years.

Observing Stellar Evolutionin Star Clusters The Hyades cluster is also rather young Its main-sequence turnoff indicates an age of about6 x 108 years.

Observing Stellar Evolutionin Star ClustersThis globular cluster, 47 Tucanae, is about 1–1.2 x 1010years old, much older than the previous examples.

The Evolution ofClose Binary-Star Systems If stars in a binary-star system are relativelywidely separated, their evolution proceeds as itwould if they were not companions. If they are close enough for their Roche lobes tobe in contact, it is possible for material totransfer from one star to another, leading tounusual evolutionary paths. These are calledclose binary systems.

Mass Transfer in Close Binary StarsIn a binary system, each star controls a finite region ofspace, bounded by the Roche lobes (or Roche surfaces).Lagrangian points: points ofstability, where matter canremain without being pulledtoward one of the stars.Matter can flow over from one star to anotherthrough the inner Lagrange point L1.

The Evolution of Close Binary-Star Systems There are different types of close binary-star systems,depending on the evolutionary state of the stars. In a detached binary, neither star fills its own Roche lobe.This term is also used for stars whose Roche lobes do nottouch.

The Evolution of Close Binary-Star Systems In a semidetached binary, one star fills its Rochelobe and can transfer mass to the other star. Thiswill alter the evolution of both stars compared toisolated stars.

The Evolution of Close Binary-Star Systems In a contact binary, much of the mass is sharedbetween the two stars and their volumes overlap.

The Evolution of Close Binary-Star Systems As the stars evolve, the type of binary system canevolve as well. This is the Algol system.a) It is thought to have begun as a detached binary.

The Evolution of Close Binary-Star SystemsAlgol systemb) As the blue-giant starenters its red-giantphase, it expands tothe point where masstransfer occurs.c) Eventually enoughmass is accretedonto the smaller starthat it becomes ablue giant, leavingthe other star as a redsubgiant.

Star Clusters Two types of star clusters: 1. Open clusters young clusters of recently formed stars within the disk of the Galaxy 2. Globular clusters old, centrally concentrated star clusters; mostly in a halo around the galaxy and near the galactic center Globular Cluster M 19 Open cluster M 52