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Stellar Evolution and the Hertzsprung -Russell Diagram

Stellar Evolution and the Hertzsprung -Russell Diagram. Based on a presentation by Francis P. Wilkin Department of Physics and Astronomy Union College 7/15/13 at RPI Dudley Observatory Astronomy Institute: Planetary Science and Astronomy for the Next Generation of Science Standards.

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Stellar Evolution and the Hertzsprung -Russell Diagram

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  1. Stellar Evolution and the Hertzsprung-Russell Diagram Based on a presentation by Francis P. Wilkin Department of Physics and Astronomy Union College 7/15/13 at RPI Dudley Observatory Astronomy Institute: Planetary Science and Astronomy for the Next Generation of Science Standards

  2. Outline of Presentation • Stars: Their properties and how they work • H-R diagram: Summarizes properties and displays evolution By plotting Luminosity on vertical axis, temperature on horizontal • Evolution: Why and how it occurs

  3. A star is a massive, self-luminous ball of gas held together by its self-gravity, and that shines, or used to shine, due to energy released by nuclear reactions in its core. Self-luminous: not reflecting light like a planet. Shines because it is HOT (not because of nuclear reactions). Nuclear bombs: not held together by gravity –> not man-made stars! Nuclear fusion reactions: convert light element nuclei to heavier nuclei, also converting matter to energy. Dead stars: white dwarfs, neutron stars (black holes? How to tell?) Not stars: Brown Dwarfs (mass from 13-80 x Jupiter)

  4. How a Star Works in Four Ideas • Energy escapes from the photosphere(surface) as light at the location where the atmosphere becomes transparent • Energy is liberated in the core by nuclear fusion reactions (usually converting Hydrogen to Helium: protons to alpha particles). This fusion requires fuel, plus high temperature and density • Energy is “transported” from the core to the photosphere by a combination of photons (radiative transport) and boiling fluid motion (convective transport) • At all locations in the star, the pressure force from hot gas opposes the weight (due to gravity) of the mass above it “hydrostatic” or “gravitational equilibrium”.

  5. The Hertzsprung-Russell (H-R) Diagram Vertical axis: luminosity (the amount of energy) Horizontal axis: surface temperature, spectral type “Sequences” are types of stars All main sequence stars fuse H->He in core Some combinations of L,T Never occur! Why?

  6. Stellar Evolution: The Life cycle of a Star • Unlike biology, does not refer to subsequent generations, only the change in a given star with time • Why must a star evolve? • Isn’t it in equilibrium? No! It continuously loses energy to space! Core’s chemical composition continuously changing -> must evolve! Also loses mass as a wind: a key driver of evolution at the very end Sun’s life: Formation in an interstellar cloud (mostly H) Protostar (accumulating more mass) Pre-main Sequence star (internal adjustment) Main Sequence star (current phase, longest; H->He in core) Red Giant star (He->C fusion in core) Planetary Nebula phase (sheds atmosphere) White Dwarf (“dead”, cooling)

  7. Stellar Properties: Brightness and Magnitudes Magnitudes: backwards (logarithmic) scale used for brightness. Negative numbers are brighter than positive! Each magnitude corresponds to a factor of about 2.51 A difference of 5 magnitudes corresponds exactly to a factor of 100 in brightness Apparent “visual” magnitudes: How bright a star appears to us on Earth. Venus: m = -4.5, Vega: m = 0.0, Polaris: m = 2.0 Faintest star visible to naked eye: m = 6 Polaris is about 2.5 x 2.5 = 6.3 times fainter than Vega! Absolute magnitude: the brightness an object would have if at the standard distance of 10 parsecs (about 32 LY). This is a star’s true brightness.

  8. Brightness, Distance, and Luminosity I. Looking at the sky, we can’t tell whether a bright star is closer or farther than a faint star Vega (magnitude 0.0) is brighter than Deneb (mag 1.25) Vega: d = 25 light years (LY), luminosity 37 Lsun Deneb: d = 1400 LY, luminosity 54000 Lsun Deneb is more luminous than Vega!

  9. Brightness, Distance, and Luminosity II. • Luminosity: the total amount of energy emitted by a star per unit time • Flux (or brightness): the amount of energy crossing a unit area per unit time (normal incidence) 4 π d2 F = L

  10. Surface Temperature and Color • A star is essentially opaque and its spectrum strongly resembles the theoretical “Blackbody” thermal spectrum, depending only on the temperature. Albireo (double star) Note: a blue star is HOTTER than a red one, despite the frequent use of red for hot, and blue for cold in false-color images! Stellar Spectral type goes from hot to cool in the sequence O,B,A,F,G,K,M “Oh, be a fine girl (guy), kiss me!”

  11. Stellar Sizes: The Stellar Zoohttp://www.youtube.com/watch?v=HEheh1BH34Q

  12. Stellar Size related to Temperature and Luminosity • An opaque thermal source (blackbody) emits amount of energy σT4 for every square meter of surface. • A sphere has surface area 4 π R2. (σ is the Stefan-Boltzmann constant) • Thus, the star’s luminosity L = 4 π R2 σ T4 Solve for R, the radius of a star: R = (L/4 π σ)1/2/T2

  13. H-R Diagram (Size Matters)

  14. Key Stellar Properties Sun: Mass=2x1033 kg (about 1000 Jupiters!), Luminosity=4x1026 W, Radius=7x108 m (about 100 Earths!), Teff=5800 K, Tcore=1.6x107 K Masses: 0.08 – 150 Msun Luminosity 10-4-106Lsun Radius: 10-2 – 103Rsun Temperature 3000K – 50,000 K (exceptions) Huge range of size, luminosity Smaller ranges in temperature, mass

  15. Star Formation A,B) Stars form by gravitational collapse in interstellar hydrogen clouds C) Rotation causes a flattening, and a disk forms around the protostar, which is still accumulating mass and invisible at optical wavelengths (Not in H-R diagram) D) Planets form in the disk, nuclear reactions occur in the pre-main sequence (T Tauri) phase

  16. Main Sequence Stars • 90% of all stars • Defining characteristic: H-> He fusion in core • Luminosity roughly proportional to M3 • Long-lasting: star consumes inner 10% of hydrogen. (Compare luminosity to fuel supply to find lifetime) tMS proportional to 1/M2 General trends: • More massive stars are larger, hotter, more luminous • Roughly diagonal from upper left (luminous, hot) to lower right (faint, red)

  17. Post-Main Sequence Evolution • Core runs out of fuel and must contract; core temp rises • H->He continues in a shell surrounding the core • Atmosphere expands and cools • Details depend sensitively on total mass: consider “low mass” stars like the sun, and “high mass” stars over 8 Msun. • “Very low mass” stars end their lives after the main sequence with no further nuclear fuels. • Post-M-S evolution may last 1/10 as long as M-S did • (Sun: 1 billion vs. 10 billion)

  18. Low-Mass Star Death

  19. Planetary Nebula Ejection Not an explosion but a strong wind 30-70% of mass may be ejected Lasts 10-30 thousand years Central star becomes white dwarf

  20. White Dwarfs are Forever! • New type of pressure opposes gravity: Quantum-mechanical degeneracy pressure. • Electrons repel each other when highly compressed (not due to charge) • Requires no energy source – keeps working as the star cools • Star maintains constant radius (about same as Earth) • Example: Sirius B • Maximum mass 1.4 Msun or collapse! Incredible density! 0.6 Msun in one Earth volume! Nearly 106 g/cm3 !

  21. Death of Massive Stars • Concentric shells of successively more massive fuel nuclei • The ashes of each reaction are the fuel for the next inner shell. • Each reaction is less efficient than the previous: they buy very little time • Iron accumulates because energy cannot be gained by making heavier elements (it has the most stable nucleus) • The iron core is supported by degeneracy pressure, just like a white dwarf • When the core of Iron reaches 1.4 Msun, it collapses, triggering a supernova!

  22. Massive Star Supernova (“Type II”) • Core mass exceeds 1.4 Msun and collapses • Electrons collide with protons to make neutrons (removing the pressure source!) • Collapse of center until reaching nuclear density, size about 10 km • Outer parts fall onto “neutron core” and bounce • Neutrinos provide additional push • Convection and non-sphericity may be critically important • Envelope flies out as supernova remnant • Core collapses into either a black hole or neutron star • Ejected matter provides the heavy elements the Earth, and our bodies, are made of

  23. Pulsars: Rapidly-Spinning, Magnetized Neutron Stars • Mass roughly 1.35-2 Msun (limits uncertain) • Size about 12 km radius • Gravity is opposed by neutron degeneracy pressure (requires no energy source, lasts forever) • Lighthouse model: beams along magnetic axis

  24. Binary Stars: Mass Transfer Mass transfer slows the evolution of the losing star, and speeds the evolution of the gainer. It can push a white dwarf over the maximum mass limit, causing a type Ia supernova explosion! Mass striking a surrounding disk can glow in x-rays! Many possibilities

  25. Conclusions • Stellar evolution is primarily determined by the initial stellar mass (higher mass, shorter life) • Nuclear fusion reactions provide the energy source for the stars • Low mass stars end up being white dwarfs • High mass stars explode as supernovae, leaving either a neutron star or a black hole • Many details of star formation, and supernova explosions, are still unknown • All the elements beyond the first five (H,He,Li,Be,B) were formed in stars : we are stardust

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