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The Life Cycle of Stars Formation, Evolution and End States ASTR 101 4/19/2017 1 Hertzsprung–Russell Diagram O B A F G K M Blue giants 107years 10000 Luminosity (solar) 100 Red super giants 10M☉ 108years Red giants A main sequence 1 1010years Sun 100 R☉ 10 R☉ 1011years 0.2M☉ 1 R☉ 0.01 white dwarfs 0.0001 Spectral class 25000 10000 red dwarfs 8000 6000 4500 surface temperature 3000 0.1 R☉ • Main Sequence: The majority of stars (~90%), including the Sun, are in a diagonal band, going from upper left corner (hot, luminous, massive stars) to the lower right corner (cool, dim, low mass stars). – Blue giant: (upper left) large and hot stars – Red dwarfs: (lower right) small low temperature stars • White Dwarfs: Stars in lower left, those are hot but faint stars so must have smaller surface ⇒ must be very small. • Red Giants: : Stars in upper right, those are colder (3000K) but very bright, so they should have large surface area ⇒ must be very big. 2 Star formation and Evolution Brown dwarf • A star begins its life as a collapsing interstellar gas cloud under its own gravity – As the gas cloud contracts, its center heats up and nuclear reactions begins. • A star shines by the energy from those nuclear reactions going on in its core. – In most stars hydrogen is converted into helium. • Because stars are powered by nuclear reactions which consume star material, they have a finite life span. • The theory of stellar evolution describes how stars form and change during that process. 3 Gas/dust clouds in the Milky way Carina Nebula size 300 ly. A young cluster of stars surrounded by the nebula in which they were formed (NGC 3603 ) Orion nebula size 20 ly. Interstellar gas and dust clouds (called nebulae) which could evolve into stars are abundant in galaxies, concentrated in the galactic disk. – They could span few to hundreds of light years in size and have masses many thousand times the Sun. – They are very cold (10-20K) low-density clouds, 102—106 atoms/cm3. (compare with 2x1019 atoms/cm3 in air, better than best vacuums 4 produced on Earth) Horse head nebula • Eagle nebula (M16), about 6000 ly away 70x55 ly in size ‘pillars of creation’ taken by the Hubble space telescope, a star forming in M16 Interstellar Gas clouds are mostly hydrogen and helium, primordial mix of elements: – about 92% hydrogen atoms and 8% helium atoms by number (or about 75% H and 25% He by mass), matter formed right after the Big bang. – There could be 1-2% heavier elements in gas clouds now. • Heavier elements were produced in stars and ejected into the interstellar space as a star blows off its outer layers during the final stages of its life. 5 • Some disturbance, (like a shock wave from a nearby exploding star…) triggers the initial collapse of the gas cloud. • A small over density in the cloud exert an extra gravitational pull on the surrounding lower density gas. • As the cloud contracts the over density grows. A cloud may have few such over dense regions, which pull surrounding matter and grow in size and density. • Contraction subdivides it into smaller pieces, each piece will contact become a star, thus forming a cluster – and stars are not born in isolation, isolated stars (like our Sun) were born in a cluster and later ejected from it due to gravitational interactions. 6 A young cluster of stars embedded in the nebula in which they formed. Pleiades star cluster 100M years old • • Any initial movements in the gas cloud results in some initial rotation. As the gas cloud contracts its rotational speed increases. – Due to the conservation of angular momentum The same reason when a skater spins slow when her arms and legs are extended and faster when she pulls her arms in Angular momentum = 𝑚𝑎𝑠𝑠 × 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑜 𝑎𝑥𝑖𝑠 A protostar 7 8 few light years 100 AU protostar slowly rotating interstellar cloud As the interstellar cloud collapses under self-gravity the small rotation of the cloud is amplified and flattens out the cloud forming a protostar disk. • The rotating ball collapses into a thin disk with most of the mass concentrated near the center forming a protostar (no nuclear burning yet). • The disk may evolve into a planetary system. 9 Protostar stage Protostar HL-Tauri www.eso.org/public/videos/eso1436a • Proto stars in the Orion nebula During contraction, Collisions between infalling gas particles dissipate energy and heat up the protostar and it starts to glow. – A protostar with a one solar mass can reach a surface temperature of a few thousand Kelvins. – and 100 times brighter than the Sun due to its large size. – (no nuclear reactions yet, it is emitting energy from gravitational collapse) 10 + + Electrical repulsion between two positively charged hydrogen nuclei keep them apart at lower temperatures. + + At temperature above 10 MK, H nuclei have enough thermal energy (speed) to overcome the electrical repulsion and come sufficiently close to each other to undergo nuclear interactions. • The protostar continues to collapse under gravity, further heating it up in the process. • It becomes hottest near the center (core) where much of the mass is collected. • When the core reaches 10 million K, hydrogen nuclei (protons) are moving fast enough to overcome the electrical repulsion between them and combine together. • Nuclear fusion of hydrogen begins. The protostar becomes a star. Hydrogen Nuclear Fusion e+ + • + ++ ν + n 2H 1 There is no stable nucleus with just two protons, so when two protons combine, they rearrange to form a deuterium nucleus 2H1 (the hydrogen isotope which has a proton and a neutron) 1H 1 or in simple terms: + 1H1 → 2H1 + e+ + neutrino 1H 1 + 1H1 → 2H1 + energy 12 The Proton-Proton Chain • 1H • Deuterium thus produced combine with a hydrogen nuclei to from a helium 3 isotope 3He, (3He2 = two protons+ a neutron) and releasing some energy 2H1 + 1H1 → 3He + energy 2 • 1 + 1H1 → 2H1 + energy + 3He2 → 4He2 + 1H1 + 1H1 + energy Overall, net result is four hydrogen nuclei combine to form a Helium nucleus releasing some energy. 1H • E Two 3He2 nuclei combine to form helium 4 (4He2 = 2 protons+2 neutrons) 3He • 109 years 4He 2 E 1 second 2 1 + 1H1 + 1H1 + 1H1 E E 106 years → 4He2 + energy nucleus is slightly lighter than four 1H 1 nuclei mass of 4 hydrogen nuclei = 6.693x10-27 kg, mass of a Helium (4He) nucleus = 6.645x10-27 kg Difference in mass = 0.048x10-27 kg, 0.7%, E ●proton, ●neutron , E energy this lost mass is converted to energy according to E=mc2 13 Main Sequence Stars pressure gravity H→He +energy • Once nuclear fusion sets in, energy production is maintained by hydrogen fusion, further gravitational contraction stops and the protostar becomes a star. • The star has reached a hydrogen burning stable state of a ‘main sequence’ star and will remain there as long as it has hydrogen to burn. • A hydrogen burning main sequence star is in a stable equilibrium state, its temperature (and pressure) is maintained by the energy produced by hydrogen fusion. • A star spends most of its life as a main sequence star ⇒ they are the most common type of stars in the galaxy(90%). 14 Life time of a star pressure gravity larger gravitational force balanced by a larger pressure ⇒ higher core temperature massive star • Mass of a star determines its evolutionally path and rate of nuclear burning. – A massive a star has a higher gravitational pull towards the center – To stay balance it has to have a larger internal pressure ,which requires a higher internal temperature. – At a higher temperature star burns hydrogen faster, and releases more energy. ⇒ hydrogen consumption and brightness of a star goes up with the mass of star • Luminosity (energy output) of a star is approximately proportional to the third power of its mass 𝐿 ∝ 𝑀3 15 . Massive stars are short lived! A massive star has more hydrogen, but burns its hydrogen fast, so has a shorter life time – The Sun will stay ~10 billion as a main sequence star. – a 100 solar mass star has a luminosity of 106L☉ (million times the sun) and will stay there less than a million years. – a star with tenth of a solar mass (0.1 M☉) has a luminosity of 10-3L☉ (one thousandth of the Sun) and a lifetime of 100 billion years. • Massive stars are brighter, but has a shorter life. 16 Brown Dwarfs Brown dwarf Gliese 229B, about 20-50 times the mass of Jupiter • If the mass of a star is lower than 0.08 Solar mass (80 times the mass of Jupiter), its core won’t get hot enough to begin hydrogen fusion. • Initially they glow in infrared due to the gravitational energy, and gradually cools off and become brown dwarfs. • Brown dwarfs are difficult to observe directly, as they are very dim. • On the other end, if the protostar is more than about 100 solar masses, nuclear reactions happens so fast, the star blows apart. • Mass of a hydrogen burning main sequence star is between 0.08M☉ and 100M☉ 17 Red Giant stage • As the star ages it gradually runs out of hydrogen, and helium accumulates at the center. • Helium (4He) cannot undergo nuclear fusion until the core temperature is extremely high, of the order of 100 million K. • So core cools down and the pressure decreases.The inert He core contracts, and gets hotter. • Hydrogen nuclear burning continues in a shell around the core. • Higher temperatures around the core increase the hydrogen fusion rate and the energy production. • This increased outflow of energy expands the outer layers of the star. The star extends over 100 times becoming a red giant star. – Expanding outer layers cools down, lower temperature of the outer layers makes it appear red. – Despite its cooler temperature, its luminosity increases enormously due to the large size. – When the Sun becomes a red giant in about 7 billion years its will extend beyond the orbit of Mercury. H burning shell inert He core cool extended envelope ++ ++ A helium nucleus has two protons, a larger electrical charge. So mutual repulsion is greater, a higher energy (temperature) is needed to bring them closer to undergo nuclear reactions. 18 Helium Fusion • Meanwhile, the He core continue to contract until it reaches a temperature of about 100 million K. • Once the core temperature has risen to 100M K, helium in the core starts to fuse through the triple-alpha process: • 4He He burning shell inert C core + 4He + 4He → 12C + energy (4He nucleus is often called an alpha particle, hence the name) • This reaction produces carbon in the core of the star. • Now there is hydrogen burning outer shell and Helium burning inner shell and an inert carbon core. • Higher energy production results in a further expansion of the outer layers. • Low escape velocity of the expanded outer layers of the giant star causes the outer layers to escape the star. • The ejected envelope expands into interstellar space, forming a planetary nebula. • Light from the central star makes the planetary nebula shine. – H burning shell Early astronomers thoughts they were newly forming planetary systems, so they called them planetary nebulae, actually they are the opposite, dying stars. cool extended envelope 19 Degenerate Pressure • • • The star now has two parts: • • • A small, extremely dense carbon core An envelope about the size of the solar system As the dead core of the star cools, the nebula continues to expand and dissipate into the surroundings. If the star has a mass less than 8 solar masses the core cannot get hot enough for carbon fusion (due to higher nuclear repulsion). Without pressure generated by nuclear reactions, the core contracts up to the point where electrons resist further packing. – Like electrons have no more room to move around, all available energy states elections can occupy are filled, which creates an ‘electron degenerate pressure’. – No further contraction of the core is possible. – This state of a star core, which had contacted to about the size of Earth and held by the electron degenerate pressure is called a white dwarf. – Densities of 108 to 1011 kg/m3 e- e- + e+ + eee + e+ + + + + + + eeee+ + + + ee- + + e e+ + + eeee+ e- 20 White Dwarfs Comparison of a White Dwarf Star and the Earth. Sirius and its white dwarf companion Serius B • Initially a white dwarf is very hot, and it cools down slowly (in billion year time scale). White dwarf surface temperatures extend from over 150,000 K to barely under 4,000 K. • Densities of ~ 109 kg/m3 (a teaspoon of white dwarf matter would weight over a ton). • Due to their small size, white dwarfs have low luminosities even when they are in the initial hot stage. (down to 10-4 solar luminosities). • The Sun will become a red giant, then lose about 40% of its mass and finally become a white dwarf of 0.6 solar masses. • Maximum mass of a white dwarf is about 1.4 solar masses ( called the Chandrasekhar limit). Beyond that the electron degenerate pressure is not sufficient to counter gravity, and the core further collapses. • A white dwarf is the end product for low and intermediate mass stars (0.08 21 to 8 solar masses). Evolution of massive stars (more than 8 solar masses ) • Core temperature of massive stars reach higher temperatures required for the fusion of heavier elements, before the electron degenerate pressure sets in. • Heavier elements are formed in their cores – – – 12C + 4He → 16O + photon (for 500M K) 16O + 4He → 20Ne + photon (for 500M K) 16O + 16O → 32S + photon (for 1 billion K) • and so on until iron is produced • Reactions to heavier elements go faster and faster : He burning last less than a million years, creating oxygen; Neon, Magnesium takes only few years, production of sulfur takes few days. • Fusion reactions undergo like an onion skin structure with more heavy elements concentrated toward the center of the star where temperature is highest. • Iron has the most stable nuclear structure, it does not release energy when undergo nuclear fusion, instead absorb energy. So once iron is formed fusion chain stops. • This process is called Steller Nucleosynthesis most elements other 22 than hydrogen and helium are produced by this process in stars. Steller Nucleosynthesis energy is consumed Relative abundance of elements in the solar system elements with even atomic numbers are more abundant then odd atomic numbers, a possible consequence of alpha capture process alpha capture process (After 56Ni energy is consumed in alpha capture and the process stops, 56Ni decays to iron isotope 56Fe ) 23 Supernova explosions gravity • e- e- + e+ + + eee e+ e+ + + + + + + eeeee+ + e+ + ee- + + ee+ + + ee- gravity A star initially more massive than 8 solar masses will have a core heavier than the Chandrasekhar limit (1.4M☉ ) after the red giant stage. – electron degenerate pressure can not overcome the crushing force of gravity. It is too big to be a white dwarf! – Gravitational crush will overcome the electron degenerate pressure, further squeezing the core. – Under the enormous crushing force of gravity protons and electrons in the core fuse together producing neutrons in a fraction of a second. proton + electron → neutron + neutrino ( p+e- → n + ν ) releasing a large number of neutrinos in the process 24 Supernova explosions the core of the star is compressed into neutrons infalling material bounce off the hard neutron core expelling the stellar material into space, forming the supernova. • At such high densities neutrinos interact with the matter and push the star envelope outward at a high velocity (~5% the speed of light). • In addition infalling stellar material bounces back off the core creating a shock wave that propagates out through the star, expelling the stellar material into space • As a result the whole star blows apart in an explosion called supernova. 25 Supernova explosions Supernova 1987 in the large Magellanic cloud a supernova in the Pinwheel Galaxy • A supernova momentarily outshines the Sun by 10 billion times, as bright as an entire galaxy • During supernova explosion heavier elements (nickel, silver, lead….) are produced and ejected into the interstellar space, along with other elements produced in the star over its life time. • Supernovas are rare : – once in a century in a galaxy. – The last one in the Milky way was in 1604 (observed by Kepler). – There was one in the Large Magellanic Cloud in 1987. 26 Crab nebula 1000 years (size 5.5 light years) Remnant of Tycho’s 1572 Supernova Veil nebula 6000 years (size 50 light years) Images of few supernova remnants • Interstellar material enriched with heavy elements from a supernova becomes the raw material for a new generation of stars and the cycle of stellar birth and death begins again. • The Sun is a second-generation star. Each of the heavy elements in the solar system (including planets, people…) had been produced in the core of a star that blew away in a supernova explosion billions of years ago. 27 Neutron stars • The core of the star is now mostly composed of closely packed neutrons. – Without anywhere to move around, they create neutron degenerate pressure. – Neutron degenerate pressure stops further collapse of the core and forms a neutron star. • • It is like a one giant nucleus made of neutrons If the core is less than 3 solar masses, gravity can be balanced by the neutron degenerate pressure. 28 Neutron stars earth white dwarf neutron star An artist's rendering of a neutron star • Mass of a neutron star is between 1.4-3 solar masses, typical radius ~ 10 to 20 km! • It is the densest form of matter in the observable universe: 1017 kg/m3 – a teaspoon of neutron star matter would be 100 million tons Due to conservation of angular momentum, it is spinning very fast, up to thousands times a second – angular momentum of the star now carried by a small object, hundreds of thousand times smaller in size • 29 Pulsars • • A Neutron star has an extremely strong magnetic field. Material falling to a neutron star are accelerated and taken to the poles by the magnetic field. – This results in strong beams of electromagnetic radiation emitted at the magnetic poles. • • • • If the rotation axis is not the same as the magnetic axis, the two beams will sweep out circular paths. If the Earth is in one of those paths, the beams of radiation from the jets sweep around us as the pulsar rotates (just as the light beam from a lighthouse does.) So from earth we see regular flashes of radiation from the neutron star, at the frequency equal to its rotation rate. Neutron stars for which we see such radiation pulses are called "pulsars"30 • The first pulsar was discovered in 1967. It emitted extraordinarily regular radio pulses (every 1.33733 seconds), nothing like it had ever been seen before. • After some initial confusion, it was realized that it was a neutron star spinning very rapidly (no large object could spin so fast). – It was discovered by Jocelyn Bell, a gradate student at the time. Her advisor (Anthony Hewish) shared the 1974 Nobel Prize for the discovery. (a good article about the discovery by Jocelyn Bell at : http://www.bigear.org/vol1no1/burnell.htm) • Typical period of pulsars very from milliseconds to few seconds. 31 Crab nebula and the Pulsar Crab nebula in visible light, debris ejected from a supernova explosion in 1054 • • • X ray image of the crab nebula, by Chandra X-Ray observatory Slow motion video of the crab pulsar Chinese astronomers had reported of an extremely bright star appeared in 1054, which was bright enough to be visible during the day. Today at that location we see a nebula, with gases in the cloud expanding outward at about 1,500 km/s. In 1967 a pulsar was discovered in it. – period 33 ms (flashes 30 times per second), slowing down by 38 ns/day – about 20 km in diameter 32 Black Holes Nothing can escape from a black hole • If the core has more than 3 solar masses, even the neutron degenerate pressure cannot stop the gravitational collapse. – Gravity wins, and the core collapses to a highly dense state, a space-time singularity called a black hole. • Gravitational force of a black hole is so high that even light cannot escape from it. So it would look “black”. • Its escape velocity is larger than the speed of light (fastest speed an object can have) 33 Event Horizon light circles the black hole at the the event horizon Escape velocity = 2GM c2 Event horizon Escape velocity = speed of light. r 2𝐺𝑀 𝑟 Black hole (M is the mass within a sphere of radius r) • If an object is squeezed to an extremely small size, at some point escape velocity from it exceeds the speed of light. • It is called event horizon, when an object is squeezed beyond the size of its event horizon, it will become a black hole. – Nothing come closer than the event horizon can escape the black hole. • For the Earth, that happens when it is compressed beyond 1 cm. • For the Sun 3km, for a star core of 3 solar masses it is 9 km. • Only gravitational force can crush matter to such extreme densities. – Object has to be massive enough to create the needed gravitational crush – which eventually overcome all mechanisms preventing gravitational collapse. Thermal pressure, electron degeneracy, neutron degeneracy 34 Observing Black holes An Illustration of Cygnus X-1 A composite image (visible, Xray, IR) of the active galaxy M82 • No light is coming out of a black hole as their gravitational fields will cause light to bend around them. • • But when matter fall into a black hole, before they enter the event horizon due to gravitational acceleration they emit radiation, in the form of strong X-rays. Few such X-ray sources have been found and are likely black hole candidates. • Cygnus X-1 a strong X resource is the first suspected black hole candidate. – It is a binary system, visible partner is about 25 solar masses. – The system’s total mass is about 35 solar masses, so the X-ray source must be about 10 solar masses, too large for a neutron star. – Hot gas appears to be flowing from the visible star to an unseen companion, with X-ray emission that flickers in hundredths of a second. • There are strong evidence that the centers of most galaxies contain supermassive black holes—about 1-1000 million solar masses. 35 The Supermassive black hole at the center of Milky way Galaxy visible infrared radio center of Milky way in different wavelength light • Galactic center is a very strong radio source (called Sagittarius A) • Infrared studies of the region done over last two decades have discovered stars orbiting around a very massive object at the location of Sagittarius A, sometimes reaching 4% speed of light. • Very likely a supermassive black hole of 4.1 million solar masses. 36 Star formation and Evolution Life Cycle of a Star nebula Brown dwarf < 0.08M☉ < 8M☉ protostar > 8M☉ Core < 3M☉ 37 Hertzsprung–Russell Diagram O B A F G K M Blue giants 107years 10000 Luminosity (solar) 100 Red super giants 10M☉ 108years Red giants A main sequence 1 1010years Sun 100 R☉ 10 R☉ 1011years 0.2M☉ 1 R☉ 0.01 white dwarfs 0.0001 Spectral class 25000 10000 red dwarfs 8000 6000 4500 surface temperature 3000 0.1 R☉ • Main Sequence: The majority of stars (~90%), including the Sun, are in a diagonal band, going from upper left corner (hot, luminous, massive stars) to the lower right corner (cool, dim, low mass stars). – Blue giant: (upper left) large and hot stars – Red dwarfs: (lower right) small low temperature stars • White Dwarfs: Stars in lower left, those are hot but faint stars so must have smaller surface ⇒ must be very small. • Red Giants: : Stars in upper right, those are colder (3000K) but very bright, so they should have large surface area ⇒ must be very big. 38 Evolutionary path of a star like Sun in the HR diagram 10000L☉ 1000 R☉ planetary nebula forms 500 million years helium burning red giant stage 100L☉ Proto star contraction 50 million years main sequence 1L☉ 10 billion years helium core 10 R☉ contracting Sun 1 R☉ 0.01L☉ white dwarf 0.1 R☉ 0.0001L☉ 10000K 6000K 3000K 39 Evolution of Stars in a Cluster When an interstellar gas cloud collapsed under gravity, a large number of stars, a cluster, with stars of different masses are formed. Massive stars evolve and go through their life cycles faster than less massive stars. The open cluster Hyades: Its H-R diagram shows a high man sequence cut off, and few white dwarfs. Probably it is 600 million years old. Globular cluster 47 Tucana: Main sequence turnoff has reached sun like stars, well developed red giant and white dwarf branches. So it has to be more than 10 billion years old. 40 Cepheid Variable Stars • Later in their evolution, at the end of the red giant phase most stars undergo unstable oscillations. – The star becomes a variable star, its brightness fluctuates periodically. • There are many types of variable stars, and many reason why they change their luminosity periodically. • One type of variable stars, called Cepheid Variables shows a direct relationship between their luminosity and the period of variation. • In 1912 Henrietta Leavitt working at the Harvard College Observatory was looking for variable stars in the Small Magellanic Cloud. • She noticed that one type of variable stars, Cepheids (named after delta Cepheus, first star of that type) had a longer period when they are brighter. 41 apparent brightness log(period) A Cepheid in the Andromeda galaxy. • All stars in the Small Magellanic cloud are at about the same distance, • Thus if its period is known, its luminosity can be estimated. • Cepheids are bright supergiant stars (~1000 times brighter than the Sun), so they can be identified even they are outside the Milky way, in other galaxies. • In 1924 Edwin Hubble showed that Andromeda galaxy was an object outside the Miky way by identifying few Cepheid variables in the Andromeda nebula (as it was called then) and estimating its distance. ⇒ the brighter ones have longer periods suggested that period and luminosity were related. 42 • • • • • • • • • • • • • • • • • • • • • • • • • Review Questions What are the interstellar gas clouds made of? Where does the interstellar dust come from? What cause an interstellar gas cloud to collapse and form a star? What is the source of energy of a protostar? Why does proto stars smaller than 0.08 solar masses never become stars? Why don’t we see stars heavier than 100 solar masses? What is a main sequence star? What is a planetary nebula? Why are most stars we see are main sequence stars? What keeps a main sequence star from collapsing under gravity? Why does a massive star have a shorter life span while less massive stars lives longer? Why isn’t hydrogen fusion occur at temperatures below 10 million K. Why does Helium fusion need a higher temperature than for hydrogen fusion? What keeps a white dwarf star collapsing under gravity? What is a neutron star? How is it different from a white dwarf? Why does a neutron star spins so fast? What is the difference between a neutron star and a pulsar? What is the main factor which determine the course of a star’s evolution? Why isn’t elements heavier than iron produced by nuclear fusion in stars? How are the elements heavier than iron produced? What would be the end state of a star like the Sun? What is the end state of a massive star? What are the evidence that black holes exist? How is it possible to estimate the age of a star cluster from its HR diagram. 43 Why do Cepheid variable stars important in astronomy?