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

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

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 x Jupiter)

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”.

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?

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)

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.

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 L sun Deneb: d = 1400 LY,luminosity L sun Deneb is more luminous than Vega!

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 d 2 F = L

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!”

Stellar Sizes: The Stellar Zoo

Stellar Size related to Temperature and Luminosity An opaque thermal source (blackbody) emits amount of energy σT 4 for every square meter of surface. A sphere has surface area 4 π R 2. (σ is the Stefan-Boltzmann constant) Thus, the star’s luminosity L = 4 π R 2 σ T 4 Solve for R, the radius of a star: R = (L/4 π σ) 1/2 /T 2

H-R Diagram (Size Matters)

Key Stellar Properties Sun: Mass=2x10 33 kg (about 1000 Jupiters!), Luminosity=4x10 26 W, Radius=7x10 8 m (about 100 Earths!), T eff =5800 K, T core =1.6x10 7 K Masses: 0.08 – 150 M sun Luminosity L sun Radius: – 10 3 R sun Temperature 3000K – 50,000 K (exceptions) Huge range of size, luminosity Smaller ranges in temperature, mass

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

Main Sequence Stars 90% of all stars Defining characteristic: H-> He fusion in core Luminosity roughly proportional to M 3 Long-lasting: star consumes inner 10% of hydrogen. (Compare luminosity to fuel supply to find lifetime) t MS proportional to 1/M 2 General trends: More massive stars are larger, hotter, more luminous Roughly diagonal from upper left (luminous, hot) to lower right (faint, red)

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)

Low-Mass Star Death

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

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 10 6 g/cm 3 !

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!

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

Pulsars: Rapidly-Spinning, Magnetized Neutron Stars Mass roughly 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

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

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