Hubble Photos Credit: NASA, The Hubble Heritage Team (STScI/AURA) Heading for death. q q q q q q Leaving the main sequence End of the Sunlike star The helium core The Red-Giant Branch Helium Fusion Helium Flash The carbon core Death of a low mass star Death of more massive stars Observing stellar evolution Evolution of binary-stars q Past Pre main sequence: Beginning of its life on main-sequence: Sun had 1/3 luminosity it has now. q Present q Future End of its life on main-sequence: Sun will have twice the luminosity it has now. Post main sequence: Why and when do stars evolve off of the Main Sequence? What structural changes occur inside the star? v they have convective and radiative layers v they have uniform composition zones are determined by zones are NOT determined uniform amounts of hydrogen and helium As nuclear fusion (hydrogen burning) proceeds, the composition of the star s interior changes. Amount of He in the core Amount of H in the core 1
very slight changes in stars luminosity and surface temperature. energy still being generated at about the same rate. 10 Byr star runs out of hydrogen at the core nuclear fusion stops the core begins to contract 2
Core Contraction: Helium core shrinks/contracts energy generated from contraction similar mechanism to pre-main sequence core temperature rises not enough temp for starting of He burning but surrounding hydrogen becomes hotter hydrogen shell burning begins inert helium core shell of hydrogen burns with faster rate Core contraction & or helium core Hydrogen shell burning: H in the shell burns with ever increasing rate luminosity increases more energy generated in shell burning surface temperature drops slightly temp now is = 4000K (was 6000K) non-burning outer H layers increase in radius very high radiation pressure gravity is not able to stop them core is shrinking and heating up, outer layers are expanding and cooling star moves to red giant branch The Red-giant branch: radius of star now 100 time larger density = 10-3 kg/m 3 (in the outer most layers) core is very compact size = 1/1000 the size of the entire star density = 10 8 kg/m 3 (1000 times more dense than MS core) very luminous many 100s solar luminosity relatively slow evolution 100 million years in this stage 3
Core Contraction Continues: amount of inert helium increase shell burning adds more helium ash no energy generated in core gravity contracts core density and temperature increase in core due to its contraction and shell burning density = 10 8 kg/m 3 temperature = 100 million Helium Fusion: temperature > 100 million K 10 times hotter than main sequence core helium fuses into carbon another name of 4 He nuclei is alpha particles next nuclear fusion inside the core begins called triple alpha process (three helium nuclei (or alpha particles) combine to form a nucleus of carbon.) Two energy sources: core and the surrounding shell 4
Electron Degeneracy: before helium burning starts, the core density is very high (density = 10 8 kg/m 3 ) at this density electrons become degenerate --- they can be thought of tiny hard spheres that once brought into contact, present stiff resistance to being compressed any further. this degeneracy pressure is independent of temp this pressure opposes the gravity burning becomes unstable with explosive consequences The Helium Flash: for a few hours, the helium burning is very rapid this ferocious burning is called helium flash in a few hours, the core is no longer degenerate normal radiation pressure once again dominates core expands, density drops (equilibrium is restored) helium burning slows to more normal rate stable core, temp > 10 8 K temp increases and luminosity decreases on HR diagram star jumps from stage 9 to 10 The Horizontal Branch: in core He is burning, in surrounding shell H no degenerate core "Helium main sequence" 50 million year stage of evolution 20 30% of original mass of star escapes due to strong stellar winds. Ferocious helium burning for few hours at the top of red giant branch. After this He starts burning in the core to form carbon with the normal rate. helium core burning, hydrogen shell burning 5
The Carbon Core: eventually, the star runs out of helium in the core the He fuel doesn t last long --- no more than a few 10s of millions of years after the initial flash. a new inner carbon core forms no more He at the very center of the star. core again starts contracting but near the edges of core He still keeps burning, called helium-burning shell. gravitational energy coming out from core contraction, causes H and He-burning rates in the shells to increase. non-burning outer envelop keeps expanding star passes through asymptoic-giant branch and becomes red supergiant. Star is becoming red supergiant and 4 layers are there in the star now. Carbon Core Contraction Continues: carbon core continues to contract but the temperature never reaches more than 600 millions K needed for carbon burning. temperature = 300 millions K though some oxygen may form there via reaction due to the the compression between carbon and helium-burning shell. carbon core becomes degenerate density reaches to a point beyond which electron once again in the core cannot be compressed further. density = 10 10 kg/m 3 6
Expansion and Cooling of outer envelop also Continues: meanwhile the non-burning outer envelop continues to expand and cool reaches the maximum radius about 300 times solar radius now big enough to even engulf the planet Mars. Helium Shell Flashes : electron degeneracy causes enormous pressure on the he-burning shell Helium burning in the shell becomes unstable helium fusion rate depends strongly on temperature. Helium shell flashes begin the star begins to pulsate these pulses also destabilize the outer envelop Stellar Envelope Separates : rapid process: takes ~10 5 years extra energy pushes stellar envelope away from the core hydrogen rich material escapes nuclear burning ends in and around core no pressure from the envelope planetary nebula forms Core and Envelope go their separate ways. 7
Planetary Nebula : Expanding envelope forms a ring nebula around the contracting C-O core. in reality it is not a ring, but a three dimensional shell not a planet, but part of a dead star size ~ roughly more than our solar system becomes thin and dim in < 100,000 years gradually disperse into interstellar space NGC 6369 Credit: JPL, StScI, NASA Hourglass Nebula What Happens to the Core: becomes visible as the envelop recedes takes several 10s of thousands of years to appear by that time shrinks to the size of the Earth mass ~ half the mass of the present Sun. no nuclear reactions not enough surface pressure no hydrogen gas shines only by stored heat becomes a white dwarf about 9% of a stars' lifetime is in the white dwarf stage cools very slowly by emitting radiation small size slows cooling spends 1 billion years cooling 8
White Dwarf: very high density density = 10 10 kg/m 3 1 tablespoon = 1 ton composition mostly carbon some oxygen from more massive stars have size roughly of the earth but mass of the sun. very common about 9% of a stars are white dwarf held up against gravity by Electron Degeneracy Pressure. size doesn t shrinks with time though heat leaks away cools and becomes a black dwarf a cold, dense, burned-out ember in space take ~ 10 trillions years to cool off Galaxy is not old enough for there to be any Black Dwarfs yet. Stars in a cluster have approximately: the same distance formed at the same time same composition They are best lab to test the stellar evolutionary models because the stars are only differ in mass. The HR diagram of a cluster can be used to determine the cluster's age. (massive stars evolve more rapidly than low mass stars) 9
Main sequence turnoff Main sequence turnoff 10
Open Cluster (h and chi Persei) Age: 10 million yrs Globular Cluster (47 Tucanae) Age: 12-14 billion yrs White dwarf cools off in trillions of years and becomes black dwarf but what if it is a part of binary system? A pair of stars bound together by gravity. Orbit each other about their center of mass. Between 20% and 80% of all stars are binaries v In wide binaries, which typically have orbital periods of tens of years or more, the separation between the two stars is larger than the largest radius that either star will ever reach and hence the two stars will go through their entire life cycles independently. v In close binaries (or interacting binaries), which have shorter orbital periods, and the two stars are closer then the gravitational pull between them. There is then the possibility that one star may influence the other and completely new evolutionary paths are opened up for the two stars. 11
The teardrop-shaped region (or volume) around a star in a binary system, inside which material is bound to that star is called Roche lobe, after the French mathematician Edouard Roche. each star is within its Roche lobe and is relatively undistorted. massive star during its evolution expands so much that mass starts transferring onto other star. this happens when both stars overflow their Roche lobes. But..what is the fate of a white dwarf if it is a part of binary system? 12