DOCSLIB.ORG

The Horizontal Branch As a Probe of Stellar Population History

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Horizontal Branch As a Probe of Stellar Population History The Horizontal Branch as a probe of stellar population history DISSERTATION Presented in Partial Ful¯llment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Grant Newsham ***** The Ohio State University 2007 Dissertation Committee: Approved by Professor Donald M. Terndrup, Adviser Professor Marc H. Pinsonneault Adviser Astronomy Graduate Program Professor David H. Weinberg ABSTRACT Color-magnitude diagrams of resolved stellar populations, especially when combined with metal abundance patterns from spectroscopy, are our principal means of determining the evolutionary history of the Galaxy, its stellar clusters, and dwarf satellites. In turn, they also guide our ideas about stellar structure and evolution. Improvements in the capability of evolutionary models allow better interpretations of the data, and better data lead to improvements in the theory. In this dissertation, I concentrate upon the horizontal branch (HB) of composite stellar populations as an aid to understanding the ages, helium content, and mass loss history of stellar systems. Hitherto, the HB has been studied primarily in single stellar populations, such as globular clusters, by comparing observations to theoretical stellar evolutionary models. The characterization of the HB has enabled constraints to be placed upon the ages, helium content, and red giant mass loss of the globular cluster populations. There is no reason in principle why such methods cannot be applied to composite population stellar systems to disentangle their complex history. ii In this dissertation, I utilize stellar evolutionary modeling and speci¯cally the morphology of the HB to investigate the hypothesized helium-rich stars in the globular cluster ! Centauri, and similar methodology to constrain the history of the metal-poor part of the stellar population in the Galactic bulge using the metallicity distribution of RR Lyrae variables. In response to proposed helium-rich stars as an explanation for the double main sequence observed in ! Centauri, I investigated the consequences of this hypothesis elsewhere on the color-magnitude diagram. I concentrated on the horizontal branch where the e®ects of high helium are expected to show themselves most clearly. In the process, I developed a new procedure for comparing the mass loss su®ered by di®ering stellar populations in a physically motivated manner. Stars that have high helium throughout their interiors are absent from the horizontal branch of ! Centauri in the numbers proposed unless their mass loss history is very di®erent from that of the majority population. It is possible to generate a double main sequence from existing ! Centauri stars via accretion of helium rich material consistent with the latest theoretical yields of AGB ejecta; such polluted stars are consistent with the observed HB morphology of ! Centauri. Polluted models are also consistent with the luminosity the main sequence merge as opposed to models of fully helium-rich stars. Using the (B R)=(B + V + R) statistic, I ¯nd that the high helium blue ¡ main sequence stars are required to be younger than the majority population by an iii amount not supported by observations of the main sequence turno®, whereas the pollution scenario stars have no such conict for ordinary mass losses. I then explored the metallicity distribution of RR Lyraes in the bulge to place constraints on the durantion of the early phases of star formation in the bulge. RR Lyrae variables o®er some real advantages as probes of stellar histories, especially if we know their metallicities because the metallicities that become RR Lyrae stars depend on age, mass loss and helium enrichment. I generated arti¯cal colour magnitude diagrams (CMD) of the Galactic bulge by sampling the known bulge metallicity distribution and assigning an age and mass loss for each model star. From these CMD's I created theoretical RR Lyrae metallicity distributions which I compared against the observed data. I used two scenarios to guide my selection of model parameters. The ¯rst is one in which star formation in the bulge starts at some time, t, and continues as a linear function of metallicity up to the highest metallicities, thus giving us an age spread ¢t. I call this a collapse scenario since it is motivated by monolithic collapse models of galaxy formation in which star formation in the bulge occurred all the way to the highest metallicities. The second is based on the hypothesis that the bulge formed from early accretion of small satellite systems already containing low metallicity stars ([Fe=H] 1:0) and gas. After some delay in time after the dwarfs formed, ¢t, · ¡ they merged to form the bulge accompanied by rapid star formation that created the higher metallicity stars seen in the bulge ([Fe=H] > 1:0). I chose the dividing ¡ iv line metallicity of [Fe=H] = 1:0 based upon the ¤CDM models of galaxy formation ¡ by satellite mergers. I call this a merger scenario. I found that for plausible giant branch mass loss the RR Lyrae stars need to be very old in both the collapse and merger scenarios. I also found that the age spread of the stars from the most metal-poor to the more metal-rich stars must be less than 1.5 Gyr. Bulge stars of solar metallicity or above would never appear as RR Lyrae variables without extreme mass loss on the giant branch. I also ¯nd that the stars of metallicities comprising the RR Lyrae metallicity distribution cannot be coeval either. Age spreads (collapse scenario), or time delay (merger scenario), of > 0:35 Gyr are implied in either of the two scenarios I investigated. The merger scenario is more restrictive on the start of star formation age and requires somewhat greater mass loss and a larger mass loss dispersion. In both scenarios the e®ect of helium enrichment is to reduce the initial age for the most metal poor stars by about 1 Gyr and to require a larger age spread for the star formation in the collapse scenario or time delay between the two bursts of star formation in the merger scenario. I also showed that the initial assumption that mass loss is independent of metallicity is satisfactory in the regimes of interest. A simple Reimers mass loss prescription is essentially independent of metallicity within our model parameter ranges. Mass losses for the more metal rich stars for a given age would actually be less than those for the lower metallicity stars, by a few hundredths of a solar mass, making them even less likely to appear as RR Lyrae variables. v Dedicated to those who care. vi ACKNOWLEDGMENTS I want to take this opportunity to thank many people without whom this last 5+ years would not have been possible. First I would like to thank my adviser Donald Terndrup whose knowledge, innate scienti¯c common sense (and why is it called common sense when it is seemingly so rare?) and even keel was much appreciated. To sum things up the entire experience of working with him was both interesting and fun. What more could one ask? I also have much appreciated the many conversations on all aspects of stellar physics with Marc Pinsonneault, whose knowledge of stars and physics intuition have always impressed me. I have walked into his o±ce more than a thousand times over the last few years, and the resulting knowledge learned has been a huge bonus in my studies. Another person I want to single out for thanks is Gerry Newsom. His positive demeanor and all around dedication in the teaching environment is truly an inspiration and a model to follow. Plus, how can you go wrong with a name so similar to mine! vii In my time in the Ohio State Astronomy Department I have interacted with numerous others whose help and discussions with have been most welcome. In no particular order and for varied and sundry reasons I shall mention the following: Zheng Zheng - a great o±ce mate, Nik Andronov - for truly strange movie tastes, James Pizagno - for pizza conversations, Julio Chaname - a man who should own a Donato's pizza franchise, Stefan Frank - despite German football, Matthias Dietrich - even more German football, Pavel Denissenkov - for discussions on stars, Rick Pogge - for knowing more than should be possible about Linux systems, David Will & Michael Savage - for their help with untold number of computer problems, David Weinberg - for astrophysical knowledge and obvious dedication and Pat Osmer for maintaining a department with a sure hand of professionalism and expertise. I would also like to thank all the people I was a teaching assistant for (some also mentioned above): Brad Peterson, Barbara Ryden, Smita Mathur, David Ennis, Kris Sellgren and Chris Kochanek. I enjoyed it all. I also am indebted to one inanimate part of the department, the Condor computer batch-job system. Without this I could not have attempted a large part of this dissertation. When you need 18 months plus of CPU time, as I did, then I should take time to acknowledge the University of Wisconsin team responsible for this tool. Thank you. viii Finally, on the personal side of life I want to thank my girlfriend Sue Wolfrum. This has been a di±cult last few years and her patience and understanding have meant the world to me despite the fact I may have been poor at showing how much this mattered to me. Believe me - it was appreciated, every single day. ix VITA June 9, 1965 . Born { Colne, Lancashire, England 1986 . B.Sc.(Honours) Physics, The Royal College of Science, Imperial College, University of London 1986 . Associateship of the Royal College of Science 2001 { 2007 . Graduate Teaching and Research Associate, The Ohio State University PUBLICATIONS Research Publications 1.
Load more

Recommended publications
  • The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server The Variable Stars and Blue Horizontal Branch of the Metal-Rich Globular Cluster NGC 6441 Andrew C. Layden1;2 Physics & Astronomy Dept., Bowling Green State Univ., Bowling Green, OH 43403, U.S.A. Laura A. Ritter Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090, U.S.A. Douglas L. Welch1,TracyM.A.Webb1 Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada ABSTRACT We present time-series VI photometry of the metal-rich ([Fe=H]= 0:53) globular − cluster NGC 6441. Our color-magnitude diagram shows that the extended blue horizontal branch seen in Hubble Space Telescope data exists in the outermost reaches of the cluster. About 17% of the horizontal branch stars lie blueward and brightward of the red clump. The red clump itself slopes nearly parallel to the reddening vector. A component of this slope is due to differential reddening, but part is intrinsic. The blue horizontal branch stars are more centrally concentrated than the red clump stars, suggesting mass segregation and a possible binary origin for the blue horizontal branch stars. We have discovered 50 new variable stars near NGC 6441, among ∼ them eight or more RR Lyrae stars which are highly-probable cluster members. Comprehensive period searches over the range 0.2–1.0 days yielded unusually long periods (0.5–0.9 days) for the fundamental pulsators compared with field RR Lyrae of the same metallicity. Three similar long-period RR Lyrae are known in other metal-rich globulars.
    [Show full text]
  • (GK 1; Pr 1.1, 1.2; Po 1) What Is a Star?
    1) Observed properties of stars. (GK 1; Pr 1.1, 1.2; Po 1) What is a star? Why study stars? The sun Age of the sun Nearby stars and distribution in the galaxy Populations of stars Clusters of stars Distances and characterization of stars Luminosity and flux Magnitudes Parallax Standard candles Cepheid variables SN Ia 2) The HR diagram and stellar masses (GK 2; Pr 1.4; Po 1) Colors of stars B-V Blackbody emission HR Diagram Interpretation of HR diagram stellar radii kinds of stars red giants white dwarfs planetary nebulae AGB stars Cepheids Horizontal branch evolutionary sequence turn off mass and ages Masses from binaries Circular orbits solution General solution Spectroscopic binaries Eclipsing spectroscopic binaries Empirical mass luminosity relation 3) Spectroscopy and abundances (GK 1; Pr 2) Stellar spectra OBFGKM Atomic physics H atom others Spectral types Temperature and spectra Boltzmann equation for levels Saha equation for ionization ionization stages Rotation Stellar Abundances More about ionization stages, e.g. Ca and H Meteorite abundances Standard solar set Abundances in other stars and metallicity 4) Hydrostatic balance, Virial theorem, and time scales (GK 3,4; Pr 1.3, 2; Po 2,8) Assumptions - most of the time Fully ionized gas except very near surface where partially ionized Spherical symmetry Broken by e.g., convection, rotation, magnetic fields, explosion, instabilities, etc Makes equations a lot easier Limits on rotation and magnetic fields Homogeneous composition at birth Isolation (drop this later in course) Thermal
    [Show full text]
  • Hydrogen-Deficient Stars
    Hydrogen-Deficient Stars ASP Conference Series, Vol. 391, c 2008 K. Werner and T. Rauch, eds. Hydrogen-Deficient Stars: An Introduction C. Simon Jeffery Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland, UK Abstract. We describe the discovery, classification and statistics of hydrogen- deficient stars in the Galaxy and beyond. The stars are divided into (i) massive / young star evolution, (ii) low-mass supergiants, (iii) hot subdwarfs, (iv) cen- tral stars of planetary nebulae, and (v) white dwarfs. We introduce some of the challenges presented by these stars for understanding stellar structure and evolution. 1. Beginning Our science begins with a young woman from Dundee in Scotland. The brilliant Williamina Fleming had found herself in the employment of Pickering at the Harvard Observatory where she noted that “the spectrum of υ Sgr is remarkable since the hydrogen lines are very faint and of the same intensity as the additional dark lines” (Fleming 1891). Other stars, well-known at the time, later turned out also to have an unusual hydrogen signature; the spectacular light variations of R CrB had been known for a century (Pigott 1797), while Wolf & Rayet (1867) had discovered their emission-line stars some forty years prior. It was fifteen years after Fleming’s discovery that Ludendorff (1906) observed Hγ to be completely absent from the spectrum of R CrB, while arguments about the hydrogen content of Wolf-Rayet stars continued late into the 20th century. Although these early observations pointed to something unusual in the spec- tra of a variety of stars, there was reluctance to accept (or even suggest) that hydrogen might be deficient.
    [Show full text]
  • The Deaths of Stars
    The Deaths of Stars 1 Guiding Questions 1. What kinds of nuclear reactions occur within a star like the Sun as it ages? 2. Where did the carbon atoms in our bodies come from? 3. What is a planetary nebula, and what does it have to do with planets? 4. What is a white dwarf star? 5. Why do high-mass stars go through more evolutionary stages than low-mass stars? 6. What happens within a high-mass star to turn it into a supernova? 7. Why was SN 1987A an unusual supernova? 8. What was learned by detecting neutrinos from SN 1987A? 9. How can a white dwarf star give rise to a type of supernova? 10.What remains after a supernova explosion? 2 Pathways of Stellar Evolution GOOD TO KNOW 3 Low-mass stars go through two distinct red-giant stages • A low-mass star becomes – a red giant when shell hydrogen fusion begins – a horizontal-branch star when core helium fusion begins – an asymptotic giant branch (AGB) star when the helium in the core is exhausted and shell helium fusion begins 4 5 6 7 Bringing the products of nuclear fusion to a giant star’s surface • As a low-mass star ages, convection occurs over a larger portion of its volume • This takes heavy elements formed in the star’s interior and distributes them throughout the star 8 9 Low-mass stars die by gently ejecting their outer layers, creating planetary nebulae • Helium shell flashes in an old, low-mass star produce thermal pulses during which more than half the star’s mass may be ejected into space • This exposes the hot carbon-oxygen core of the star • Ultraviolet radiation from the exposed
    [Show full text]
  • White Dwarfs in Globular Clusters 3 Received Additional Support from the Theoretical Investigations of [94, 95, 96]
    White Dwarfs in Globular Clusters S. Moehler1 and G. Bono1,2,3 1 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany, [email protected] 2 Dept. of Physics, Univ. of Rome Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy, [email protected] 3 INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy We review empirical and theoretical findings concerning white dwarfs in Galactic globular clusters. Since their detection is a critical issue we describe in detail the various efforts to find white dwarfs in globular clusters. We then outline the advantages of using cluster white dwarfs to investigate the forma- tion and evolution of white dwarfs and concentrate on evolutionary channels that appear to be unique to globular clusters. We also discuss the usefulness of globular cluster white dwarfs to provide independent information on the distances and ages of globular clusters, information that is very important far beyond the immediate field of white dwarf research. Finally, we mention pos- sible future avenues concerning globular cluster white dwarfs, like the study of strange quark matter or plasma neutrinos. 1 Introduction During the last few years white dwarfs have been the topic of several thorough review papers focused on rather different aspects. The interested reader is referred to [85] and to [86] for a comprehensive discussion concerning the use of white dwarfs as stellar tracers of Galactic stellar populations and the physics of cool white dwarfs. The advanced evolutionary phases and their impact on the dynamical evolution of open and globular clusters have been reviewed by [116], while [4] provide a comprehensive discussion of the use of arXiv:0806.4456v3 [astro-ph] 30 Jun 2011 white dwarfs to constrain stellar and cosmological parameters together with a detailed analysis of the physical mechanisms driving their evolutionary and pulsation properties.
    [Show full text]
  • (NASA/Chandra X-Ray Image) Type Ia Supernova Remnant – Thermonuclear Explosion of a White Dwarf
    Stellar Evolution Card Set Description and Links 1. Tycho’s SNR (NASA/Chandra X-ray image) Type Ia supernova remnant – thermonuclear explosion of a white dwarf http://chandra.harvard.edu/photo/2011/tycho2/ 2. Protostar formation (NASA/JPL/Caltech/Spitzer/R. Hurt illustration) A young star/protostar forming within a cloud of gas and dust http://www.spitzer.caltech.edu/images/1852-ssc2007-14d-Planet-Forming-Disk- Around-a-Baby-Star 3. The Crab Nebula (NASA/Chandra X-ray/Hubble optical/Spitzer IR composite image) A type II supernova remnant with a millisecond pulsar stellar core http://chandra.harvard.edu/photo/2009/crab/ 4. Cygnus X-1 (NASA/Chandra/M Weiss illustration) A stellar mass black hole in an X-ray binary system with a main sequence companion star http://chandra.harvard.edu/photo/2011/cygx1/ 5. White dwarf with red giant companion star (ESO/M. Kornmesser illustration/video) A white dwarf accreting material from a red giant companion could result in a Type Ia supernova http://www.eso.org/public/videos/eso0943b/ 6. Eight Burst Nebula (NASA/Hubble optical image) A planetary nebula with a white dwarf and companion star binary system in its center http://apod.nasa.gov/apod/ap150607.html 7. The Carina Nebula star-formation complex (NASA/Hubble optical image) A massive and active star formation region with newly forming protostars and stars http://www.spacetelescope.org/images/heic0707b/ 8. NGC 6826 (Chandra X-ray/Hubble optical composite image) A planetary nebula with a white dwarf stellar core in its center http://chandra.harvard.edu/photo/2012/pne/ 9.
    [Show full text]
  • Announcements
    Announcements • Next Session – Stellar evolution • Low-mass stars • Binaries • High-mass stars – Supernovae – Synthesis of the elements • Note: Thursday Nov 11 is a campus holiday Red Giant 8 100Ro 10 years L 10 3Ro, 10 years Temperature Red Giant Hydrogen fusion shell Contracting helium core Electron Degeneracy • Pauli Exclusion Principle says that you can only have two electrons per unit 6-D phase- space volume in a gas. DxDyDzDpxDpyDpz † Red Giants • RG Helium core is support against gravity by electron degeneracy • Electron-degenerate gases do not expand with increasing temperature (no thermostat) • As the Temperature gets to 100 x 106K the “triple-alpha” process (Helium fusion to Carbon) can happen. Helium fusion/flash Helium fusion requires two steps: He4 + He4 -> Be8 Be8 + He4 -> C12 The Berylium falls apart in 10-6 seconds so you need not only high enough T to overcome the electric forces, you also need very high density. Helium Flash • The Temp and Density get high enough for the triple-alpha reaction as a star approaches the tip of the RGB. • Because the core is supported by electron degeneracy (with no temperature dependence) when the triple-alpha starts, there is no corresponding expansion of the core. So the temperature skyrockets and the fusion rate grows tremendously in the `helium flash’. Helium Flash • The big increase in the core temperature adds momentum phase space and within a couple of hours of the onset of the helium flash, the electrons gas is no longer degenerate and the core settles down into `normal’ helium fusion. • There is little outward sign of the helium flash, but the rearrangment of the core stops the trip up the RGB and the star settles onto the horizontal branch.
    [Show full text]
  • Evolution of Stars
    Evolution of Stars (Part I: Solar-type stars) 1 Death of the Sun Parts I and II 2 Learning goals: Be able to …. ! summarize the future of the Sun on a rough timescale; ! apply the basics of the conservation of energy and the battle between gravity and outward pressure to what “drives” a star to evolve at each major stage of evolution; ! explain what is meant by main sequence, subgiant, red giant branch, electron degeneracy, helium flash, horizontal branch, asymptotic giant branch, planetary nebula, white dwarf. 3 Be able to summarize the future of the Sun in a rough timescale. Apply the basics of the conservation of energy and the battle between gravely and pressure to what “drives” a star to evolve at each major stage of evolution. Explain what is meant by red giant branch, electron degeneracy, helium flash, horizontal branch, planetary nebula, white dwarf 4 5 http://chandra.harvard.edu/xray_sources/browndwarf_fg.html Brown Dwarf "In between a star and a planet "Jupiter < BD mass < 0.08 Sun "Radiates in infrared due to low temperature. "Mass too low to start fusion in core. "Slowly cools over trillions of years. 6 7 Core producing energy--Fusing H to He 8 A review of what we know the Sun is doing as a main-sequence star 9 Number of particles in core is decreasing. NET RESULT IN: 6 H OUT: 1 He + 2 H FUSION RATE MUST INCREASE TO OFFSET DECREASE IN PARTICLE PRESSURE! Sun is slowly becoming more and more luminous. 10 Explain how the Sun maintains a constant balance in its interior (solar thermostat) Part of the pressure depends on the number of particles Pressure decreases, core shrinks slightly, pressure evens out, fusion rate must increase to offset the smaller pressure provided by the number of particles present.
    [Show full text]
  • Accretion Regimes in the X-Ray Pulsar 4U 1901+03 P
    A&A 594, A45 (2016) Astronomy DOI: 10.1051/0004-6361/201629200 & c ESO 2016 Astrophysics Accretion regimes in the X-ray pulsar 4U 1901+03 P. Reig1; 2 and F. Milonaki2 1 IESL, Foundation for Reseach and Technology-Hellas, 71110 Heraklion, Greece e-mail: [email protected] 2 Physics Department, University of Crete, 71003 Heraklion, Greece Received 28 June 2016 / Accepted 17 August 2016 ABSTRACT Context. The source 4U 1901+03 is a high-mass X-ray pulsar than went into outburst in 2003. Observation performed with the Rossi X-ray Timing Explorer showed spectral and timing variability, including the detection of flares, quasi-periodic oscillations, complex changes in the pulse profiles, and pulse phase dependent spectral variability. Aims. We re-analysed the data covering the 2003 X-ray outburst and focused on several aspects of the variability that have not been discussed so far. These are the 10 keV feature and the X-ray spectral states and their association with accretion regimes, including the transit to the propeller state at the end of the outburst. Methods. We extracted light curves and spectra using data from the Rossi X-ray Timing Explorer. Low time resolution light curves were used to create hardness-intensity diagrams and study daily changes in flux. High time resolution light curves were used to create pulse profiles. An average spectrum per observation allowed us to investigate the evolution of the spectral parameters with time. Results. We find that 4U 1901+03 went through three accretion regimes over the course of the X-ray outburst.
    [Show full text]
  • Stellar Evolution: Evolution Off the Main Sequence
    Evolution of a Low-Mass Star Stellar Evolution: (< 8 M , focus on 1 M case) Evolution off the Main Sequence sun sun - All H converted to He in core. - Core too cool for He burning. Contracts. Main Sequence Lifetimes Heats up. Most massive (O and B stars): millions of years - H burns in shell around core: "H-shell burning phase". Stars like the Sun (G stars): billions of years - Tremendous energy produced. Star must Low mass stars (K and M stars): a trillion years! expand. While on Main Sequence, stellar core has H -> He fusion, by p-p - Star now a "Red Giant". Diameter ~ 1 AU! chain in stars like Sun or less massive. In more massive stars, 9 Red Giant “CNO cycle” becomes more important. - Phase lasts ~ 10 years for 1 MSun star. - Example: Arcturus Red Giant Star on H-R Diagram Eventually: Core Helium Fusion - Core shrinks and heats up to 108 K, helium can now burn into carbon. "Triple-alpha process" 4He + 4He -> 8Be + energy 8Be + 4He -> 12C + energy - First occurs in a runaway process: "the helium flash". Energy from fusion goes into re-expanding and cooling the core. Takes only a few seconds! This slows fusion, so star gets dimmer again. - Then stable He -> C burning. Still have H -> He shell burning surrounding it. - Now star on "Horizontal Branch" of H-R diagram. Lasts ~108 years for 1 MSun star. More massive less massive Helium Runs out in Core Horizontal branch star structure -All He -> C. Not hot enough -for C fusion. - Core shrinks and heats up.
    [Show full text]
  • Today's Outline
    Today's outline Review High and low mass Low-mass Stars Summary of evolution Main sequence Hydrogen Exhaustion Review: Clusters, Birth of Stars Giant phase Helium flash Horizontal Branch Evolution of low mass stars Helium Exhaustion Planetary Nebula I Low and high mass stars Summary again I Interior of a giant star I Phases of burning I White dwarf formation Review High and low mass Low-mass Stars Summary of evolution Main sequence Evolution of low mass stars Hydrogen Exhaustion Giant phase I Low and high mass stars Helium flash Horizontal Branch Helium Exhaustion I Interior of a giant star Planetary Nebula Summary again I Phases of burning I White dwarf formation Reference stars Low Mass: Review I Sun - low-mass "dwarf" High and low mass Low-mass Stars I Vega - low-mass "dwarf" Summary of evolution Main sequence I Sirius - low-mass "dwarf" Hydrogen Exhaustion Giant phase Helium flash I Arcturus - low-mass giant Horizontal Branch Helium Exhaustion I Sirius B - white dwarf (very small) Planetary Nebula Summary again High Mass: I Rigil - high-mass (blue) giant I Betelgeuse - high-mass (red) giant High and low mass stars Black hole Review "high−mass" High and low mass Low-mass Stars (hydrogen burning) Neutron Star Summary of evolution Main Sequence Main sequence 8 Hydrogen Exhaustion Giants Giant phase Protostar Helium flash Horizontal Branch "low−mass" Helium Exhaustion Planetary Nebula 2 White dwarf Summary again Birth Mass Time Stars generally classified by their end-of-life I Low mass { form white dwarf stars, no supernova I High mass { form
    [Show full text]
  • 37 — the End of Low-Mass Stars [Revision : 1.2]
    37 — The End of Low-Mass Stars [Revision : 1.2] • Burning to a degenerate core – Stars with masses M & 9 M do not burn as far as iron (although some may burn further than helium) – Recall: after helium flash, star settles onto horizontal branch (HB), with (convective) core helium burning and shell hydrogen burning – As helium burns to carbon and also oxygen, molecular weight increases, core contracts – Contraction accompanied by envelope expansion; star moves redward in HRD, toward Hayashi line – Eventually, core runs out of helium, burning stops; contraction continues until electron degeneracy pressure halts it – Helium burning resumes in shell • The Asymptotic Giant Branch – When star reaches Hayashi line, turns upwards with luminosity increase; this asymptotic giant branch is similar to red giant branch, but star now has He-burning shell, and is more luminous – Initially, star is on lower part of AGB — the early AGB (E-AGB) – On E-AGB, helium shell source is dominant source of energy; hydrogen shell is almost dormant – Because on Hayashi line, deep convection zone – When convection zone dips through hydrogen shell, mixes helium and nitrogen (produced by incomplete CNO-cycle burning) up to surface — second dredge-up (first dredge up occured on RGB) – As star moves to upper AGB, hydrogen shell reignites and becomes dormant – Helium burning shell become unstable, switching on and off periodically — thermal pulses. This part of AGB known as thermal pulse AGB (TP-AGB) – When helium shell switched off, helium accumulates from above due to outer
    [Show full text]