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Stellar Evolution and Death Lecture 11: Interstellar Matter and Stellar Evolution HR Star Birth • Times spent as protostars: • M-class stars may remain protostars for hundreds of millions of years. • G stars (like the Sun) spend about 30 million years in the protostar phase. • Massive O- and B-type stars may spend only 100,000 years as protostars before joining the main sequence. • Evolutionary track is the path on the H-R diagram taken by the star (and its precursor cocoon and Pre Main Evol - Sun protostar) as its luminosity and color change. © Sierra College Astronomy Department PreMain All stars Lecture 11: Interstellar Matter and Stellar Evolution Star Birth • Upper limit of Star’s Mass: Astronomers calculate that a star with a mass greater than 100 solar masses will emit radiation so intense that it will prevent more material from falling into the star, thereby limiting the star’s size. • Lower limit of Star’s Mass: Protostars with masses of less than 0.08 solar masses do not have enough internal pressure to ignite hydrogen fusion. – What about those stars whose masses are between this and Jupiter? © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Star conditions Stellar Maturity Stellar Nuclear Fusion • Stars of low mass like the Sun (<1.5 M) use the proton-proton chain to generate energy. • Stars of mass greater than 1.5 M have higher core temperatures that allow the CNO cycle to fuse of hydrogen into helium (4H He). – The CNO cycle is more efficient at the higher core temperatures of these stars • This series of reactions involves hydrogen with carbon, nitrogen, and oxygen as catalysts. • Hydrostatic equilibrium (pressure balances gravity) maintains fusion at a uniform rate. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Stellar Maturity Towards Star Death • Until their lives end on the main sequence, the main difference between the evolution of stars of various masses is the amount of time they spend as protostars and main sequence stars. • Stars can be grouped by mass as low-mass or high-mass depending on their eventual end state. • STAR’S LIFETIME ON MAIN SEQUENCE DEPENDS THE STAR’S INITIAL MASS © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Very-Low-Mass Stars Very-Low-Mass Stars • In stars with a mass of less than about 0.4 solar masses, convection occurs throughout most or all of the volume of the star. • Hydrogen from throughout the star is cycled through the core, and the entire star runs low on hydrogen at the same time. • A very-low-mass star will take 20+ billion years to completely burn its hydrogen. © Sierra College Astronomy Department Very Low Mass Conv Lecture 11: Interstellar Matter and Stellar Evolution Very-Low-Mass Stars • Ultimately, very-low-mass stars will (should?) become white dwarfs through gravitational shrinkage. • The hypothetical lifetime of a very-low-mass star is more than the assumed age of the universe. • Consequently, white dwarfs currently observed must have originated in a different manner. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars Flowchart Low-Mass Stars • Low-mass stars include stars with masses To Red Giant between 0.4 and 6 solar masses (includes Sun). • The core shrinks as hydrogen is depleted. • Heat from contraction of the core then heats a shell surrounding the core to temperatures that permit fusion of hydrogen to begin. • These two sources of energy (gravitational HRdia and nuclear) cause the outer portions of the star to expand and cool. A13.10 © Sierra College Astronomy Department expand Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars • Consequently, the star moves to the right on the H-R diagram and upward (due to increasing luminosity) becoming a red giant. • A red giant can have a lower surface temperature (less radiation per square meter) but a higher luminosity because its diameter will expand 200 times or more. 14-14C • As a red giant evolves and hydrogen burning takes place in outer layers of the star, the helium “ashes” are dumped back onto a degenerate core, raising the temperature of the core. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars Electron Degeneracy • The core of a red giant will not continue to Electron degeneracy contract indefinitely because of electron degeneracy. • Electron degeneracy is a quantum state of a gas in which its electrons are packed as densely as nature permits. • The temperature of such a high-density gas is not dependent on the pressure as it is in a “normal” gas. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars • When the degenerate core temperature reaches 100 million K, helium nuclei begin to combine 3-a through the triple alpha process forming carbon. • The initial fusion of helium proceeds in a runaway process called the helium flash, expanding the core, returning the core to a non-degenerate state, HR and shrinking the star to a yellow giant. • Following the helium flash, the center of the star forms three layers - an inner degenerate carbon core, a layer of helium that fuses to carbon in a conventional manner, and an outer hydrogenRed fusing shell. Giant © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars Yellow Giants and Pulsating Stars • Many yellow giants (whether an aging highmass or low-mass star) swell and shrink rhythmically: they pulsate. • These pulsating yellow giants are located in the instability strip of the H-R diagram. • High-mass pulsating giants are Cepheid variables (periods of about 1-70 days). • Low-mass pulsating giants are RR Lyrae variables (periods of about 12 hours). © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars • The cause for the pulsation is a special situation where the yellow giant’s atmosphere can trap some of its radiated energy. • This heats the atmosphere which then expands to a point that the star’s trapped radiation can escape. • This causes the atmosphere to cool, shrink to its original size, and start the process all over again. • The regular pulsation process of variable stars has led to the period-luminosity relation: higher average luminosity leads to longer periods. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars Post Helium Flash • After the helium flash, a yellow giant then expands again into a red giant. • Stars more massive than 2 solar masses do not experience a helium flash, but will simply expand through the yellow giant stage to its one and only red giant stage. © Sierra College Astronomy Department Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars Mass Loss In Stars • The solar wind carries away about 10–14 of the Sun’s mass each year. Over the course of 10 billion years, the Sun will lose only 0.01% of its mass this way. • In red giant stars, it is thought that core instabilities and pulsations are responsible for the large mass loss. • A typical red giant loses 10–7 solar masses a year and hence can last at most 10 million years. © Sierra College Astronomy Department PN drawing Lecture 11: Interstellar Matter and Stellar Evolution Low-Mass Stars M57 Planetary Nebulae • A Planetary nebula is a spherical shell of gasM57IR that is expelled by a red giant near the end of its life. NGC6543 • The material in the shell glows because UV radiation from the central hot star causes it to fluoresce. Others • Pulsations and/or stellar winds are thought to cause planetary nebulae. © Sierra College Astronomy Department Planetary HR-PN Lecture 11b: Stellar Remnants White Dwarfs • White dwarfs are the cores of red giants that remain after the outer parts of the original stars have blown away. • Electron degeneracy supports the star against gravity. Nuclear fusion no longer degen occurs. • White dwarfs have observed surface temperatures between 4,000 K and 85,000 K. Their masses range from perhaps 0.02 solar masses up to 1.4 solar masses. © Sierra College Astronomy Department size Lecture 11b: Stellar Remnants White Dwarfs • When a gas gets extremely dense the electrons have trouble moving to different energy levels • When a gas gets into this state, it is called a degenerate gas – The gas resists further compression, since degen only a certain number of electrons can be at a given energy level – Also, pressure of a degenerate gas does not depend on the temperature (unlike an normal, ideal gas). © Sierra College Astronomy Department Lecture 11b: Stellar Remnants White Dwarfs • A typical white dwarf has 0.8 solar mass, a 10,000 km diameter (3/4 of Earth’s), and a teaspoon of white dwarf material would weigh 2 tons. • Astronomers estimate that 10% of all stars are white dwarfs. • A black dwarf is the theorized “final” state of a star with a main sequence mass less than about 8 solar masses, in which all of its energy sources have been depleted so that it emits no radiation. © Sierra College Astronomy Department size Lecture 11b: Stellar Remnants White Dwarfs • A binary system of a white dwarf and a newly formed red giant will result in the formation of an accretion disk around the white dwarf. • Hydrogen build-up on the white dwarf can ignite an explosive fusion reaction blowing off a gas shell that causes the white dwarf to brighten by 10 mags in a few days - the Accretion brightening is called a nova. or • The explosion does not disrupt the binary system. Infalling H ignition can recur with periods ranging from months to thousands of board years. © Sierra College Astronomy Department Lecture 11b: Stellar Remnants White Dwarfs • If you add heat to this degenerate gas it does not expand (unlike gas in this room) • As you add more mass to the white dwarf, it gets smaller! Chand • Theory predicts that white dwarfs radius will go to zero when the mass of the white dwarf becomes 1.4 Solar masses . – This is the Chandrasekhar limit – Most isolated stars lose enough mass to avoid this Lecture 11b: Stellar Remnants Type I Supernovae • If accretion brings the white dwarf mass above the Chandrasekhar limit, electron degeneracy can no longer support the star, and it collapses. SN • The collapse raises the core temperature and A14.5 runaway carbon-fusion begins, which ultimately leads to the star’s explosion. • Such an exploding white dwarf is called a Type I supernova. • While a nova may reach an absolute magnitude of –8 (about 150,000 Suns), a Type I supernova attains an absolute magnitude of –19 (5 billion Suns). © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Massive Stars Massive (High-Mass) Stars • Massive stars (> 6 solar masses) will expand beyond the red giant stage to become Red supergiants. supergiant • Typical supergiants have luminosities a million times that of the Sun and absolute magnitudes of –10. • The greater core temperatures and pressures, produce heavier elements, such as neon, silicon, and iron. • The creation of these elements is known as nucleosynthesis. © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Massive Stars • Fusion of heavier and heavier elements continues until the creation of iron (Fe) occurs • Iron is the most tightly bound atom – To fuse iron requires energy, so this is not a natural thing to do – Without fusion the core will collapse (again gravity will win) Lecture 11b: Stellar Remnants Type II Supernovae • Type II supernovae begin with the conversion of silicon to iron. The fusing of silicon to iron in a supergiant star will take only a few days. • Because the iron fusion reaction absorbs more energy than it releases, the core shrinks, heats up, but produces no new more massive elements. • At the Chandrasekhar limit, the core collapses violently. Protons and electrons combine and form neutrons. After reaching its minimum size, the core rebounds, colliding violently with SN infalling material. A14.5 © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Type II Supernovae • This collision between the infalling material and the rebounding core produces two effects: • 1. Enough energy is produced to fuse iron into heavier elements. • 2. Shock waves are sent outward that throw off the outer layers of the supergiant. These shock waves may be further heated by neutrinos escaping the collapsed core. How? © Sierra College Astronomy Department Demo Lecture 11b: Stellar Remnants Supernovae in General SN A14.5 Supernovae Property Type I Type II Spectrum No hydrogen lines Prominent hydrogen lines Peak absolute magnitude Light curve –19 –17 Sharp peak Broader peak Expansion rate 10,000 km/s 5,000 km/s Mass ejected 0.5 solar masses 5 solar masses © Sierra College Astronomy Department Supernova in M81 April 23, 1992 April 1, 1993 Lecture 11b: Stellar Remnants The Neutron Star • Neutron star is the remainder of a 6-12 solar mass star that has collapsed to the point at which it is supported by neutron degeneracy. • The diameter of a typical neutron star is only 0.2% of the diameter of a white dwarf (about 20 km) and the neutron star is a billion times more dense. • Neutron stars have masses between 1.4 and 3 solar masses. • Temperature = 10,000,000 K © Sierra College Astronomy Department Lecture 11b: Stellar Remnants The Neutron Star Observation - The Discovery of Pulsars • In 1967, Jocelyn Bell discovered an unknown source of rapidly pulsating radio waves. • Subsequent discoveries of similar sources gave rise to the name pulsar. • A pulsar is a celestial object of small angular size that emits pulses of radio waves with a regular period between about 0.03 and 5 seconds. © Sierra College Astronomy Department pulsar Lecture 11b: Stellar Remnants The Neutron Star • Objects that emit pulsing signals with a duration of 0.001 sec cannot have a diameter any greater than 0.001 light-secs, which is a lighthouse few hundred kms. • Such a small size ruled out white dwarfs (Earth-sized objects), leaving the hypothesized neutron star as the explanation for pulsars. • The lighthouse model is a theory that explains pulsar behavior as being due to a spinning neutron star whose radiation beam we see as it sweeps by. 15-12 © Sierra College Astronomy Department Lecture 11b: Stellar Remnants The Neutron Star • The beam is created by charged electrons spiraling in the magnetic poles of the neutron star leading to the emission of synchrotron radiation (nonthermal radiation). • The high spin rate of a neutron star is obtained from the original star’s spin as a result of angular momentum conservation. Ang • Neutron stars may undergo “glitches” in their Mom rotation rates. • Neutron stars may be members of x-ray binaries (caused by infall of material from other star). © Sierra College Astronomy Department Lecture 11b: Stellar Remnants The Neutron Star • More than 1000 pulsars have been discovered, most with periods between 0.1 and 4 seconds. • The Crab pulsar spins so rapidly because it formed so recently. Over time it will lose rotational energy, slow down, and emit less energy. • The Crab pulsar is slowing down because of the “drag” of the electrons propelled out into the nebula surrounding the pulsar. © Sierra College Astronomy Department Crab Nebula Supernova remnant– the Crab Nebula Pulsar in the middle Lecture 11b: Stellar Remnants Very-Massive Stars Very-Massive Stars • Very-massive stars differ primarily from massive stars in what happens to them when their core is compressed to a density greater than electron degeneracy can support. • In a massive star, the resulting supernova leaves a neutron star. In a supermassive star, the core collapses into a black hole. • Neutron degeneracy cannot support a neutron star whose mass is greater than about 3 solar masses. © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Black Holes • The Schwarzschild radius is the radius of a spherical region in space within which no light can escape: RS = 3M (RS in km; M in solar masses) • The size of the Schwarzschild radius depends on the mass within the sphere. • A black hole is a spherical volume of space with a radius given by the Schwarzschild formula above and with an escape velocity that exceeds the speed of light. © Sierra College Astronomy Department board Lecture 11b: Stellar Remnants Warp space Photon deflects Black Holes • The event horizon is the spherical surface of radius RS around a black hole from which nothing can escape. • Inside a black hole, an object will eventually be subjected to extreme tidal forces pulled apart. • The final destination of an object (or so it is thought) in a black hole is to be crushed out of existence at a central singularity. • Spinning and charged black holes are more complex. © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Black Holes Detecting Black Holes • If a black hole has a close binary companion, material may be pulled from the companion to form an accretion disk around the black hole. • The accretion disk radiates x-rays and gamma rays as the gas is heated to very high temperatures as it approaches the event horizon. • From their x-ray emissions, Cygnus X-1 and AO620-00 are two very good black hole candidates. binary © Sierra College Astronomy Department Lecture 11b: Stellar Remnants Black Holes Black Holes Forever? • Jacob Bekenstein discovered that the black holes can be assigned a temperature. • Soon thereafter in 1974, Stephen Hawking showed that this temperature meant that a black hole emits thermal radiation through a quantum/gravity energy exchange. • Hawking radiation means black holes radiate away, although a one-solar-mass black hole will take 1067 years to do so. © Sierra College Astronomy Department Deep Sky Objects Star Quiz practice #1 Review – the fates of stars What happens after all the H is used up in the core? • Very Low-mass stars (0.4 Msun or less): Star cease fusing material in the core after all the Hydrogen is used up • Low-mass stars: Hydrogen shell burning, eventually leads to Helium flash in core, planetary nebula phase, leaving a carbon-oxygen white dwarf (if in a close binary, accretion may create nova or type I supernova) • Medium-Low-mass stars: same as Low-mass stars except Helium burning is steady • High-mass stars: keep burning heavier atoms in core and in shell, until iron is left in core – Core collapse results in supernovae (Type II) explosion – Neutron star forms in the core Fates flowchart • Very High-mass stars: same as High-mass stars except that a black hole forms in core The End © Sierra College Astronomy Department