Life of a Low-Mass Star AST 101 Introduction to Astronomy: Stars & Galaxies Planetary Nebulae White dwarfs REVIEW END STATE: PLANETARY NEBULA + WHITE DWARF WHAS IS A WHITE DWARF? Exposed core of a low-mass star that has died Mostly made of Carbon and Oxygen No fusion to maintain heat and pressure to balance gravity pull Electron degeneracy pressure balances inward crush of its own gravity Very high density and hence gravity Maximum mass=1.4 Msun (Chandrasekar limit)
Funky properties of white dwarf material! Size Of A White Dwarf Size of Earth 1 Kg chocolate cake! 2 Kg chocolate cake! 0.4 Msun white dwarf! 0.8 Msun white dwarf! Clicker Question Hubble Space Telescope spies 12-13 billion year old white dwarfs Formed less than 1 billion years after the creation of the universe HST and H. Richer (University of British Columbia) Which is correct order for some stages of life in a low-mass star? A. protostar, main-sequence star, red giant, planetary nebula, white dwarf B. protostar, main-sequence star, red giant, supernova, neutron star C. main-sequence star, white dwarf, red giant, planetary nebula, protostar D. protostar, main-sequence star, planetary nebula, red giant E. protostar, red giant, main-sequence star, planetary nebula, white dwarf
Clicker Question Which is correct order for some stages of life in a low-mass star? A. protostar, main-sequence star, red giant, planetary nebula, white dwarf B. protostar, main-sequence star, red giant, supernova, neutron star C. main-sequence star, white dwarf, red giant, planetary nebula, protostar D. protostar, main-sequence star, planetary nebula, red giant E. protostar, red giant, main-sequence star, planetary nebula, white dwarf Sirius A & B Main Sequence & White Dwarf Chandra image, X-ray light Time scales for Evolution of Sun-like Star H core burning Main Sequence 1010 yr 10 billion years Inactive He core, H shell burning Red Giant 108 yr 100 million years He core burning (unstable), Helium Flash Hours He core burning (stable), Horizontal Branch 107 yr 10 million years C core, He + H shells burning Bright Red Giant 104 yr 10 thousand years Envelope ejected Planetary Nebula 105 yr 100 thousand years Cooling C/O core White Dwarf Cold C/O core Black Dwarf Clicker Question The Big Bang produced only hydrogen and helium. Suppose the universe contained only low mass stars. Would elements heavier than Carbon and Oxygen exist? A. Yes B. No Hubble image, visible light
Clicker Question The Big Bang produced only hydrogen and helium. Suppose the universe contained only low mass stars. Would elements heavier than Carbon and Oxygen exist? A. Yes B. No Lives of Intermediate/High-Mass Stars Low mass: < 2 times the Sun Intermediate mass: 2-8 times the Sun High mass: > 8 times the Sun General Principles Are the Same: Battle Between Pressure and Gravity Main sequence lifetimes are much shorter Early stages after main sequence Similar to a low mass star, but happen much faster No helium flash Intermediate-Mass Stars May burn up to carbon but do not have enough mass to get temperatures high enough to go any higher up the periodic table Degeneracy pressure stops the core from collapsing and heating enough: particles are squashed together as much as possible End their lives with planetary nebulae, white dwarfs, similarly to low-mass stars.
High-Mass Stars (M >8 M SUN ) Sequence of expansion/contraction repeats as higher and higher elements begin to fuse Each heavier element requires higher core temperatures to fuse Core structure keeps on building successive shell - Like an onion Most elements are formed via Helium Capture A helium (2 protons) nucleus is absorbed, energy is released The elements are created going up the periodic table in steps of 2 Lighter elements on the outside, heavier ones on the inside Other Reactions Carbon (6), Oxygen (8), Neon (10) Magnesium (12).
WE ARE ALL STAR STUFF! - Carl Sagan We are all star-stuff All heavy elements are created and dispersed through the galaxy by stars Without high mass stars, no heavy elements Our atoms were once parts of stars that died more than 4.6 billion years ago, whose remains were swept up into the solar system when the Sun formed HIGH mass stars keep creating elements up the period table UNTIL. IRON (Fe, 26 protons ) Iron does not release energy through fusion or fission Remember: All energy created by the loss of mass from the fusion or the fission (E=mc 2 ) There Is No Way Iron Can Produce Any Energy to Push Back Against the Crush of Gravity in the Star s Core The star is DOOMED!!!
Clicker Question Clicker Question What is the heaviest element that can be created through fusion? A. Carbon B. Silicon C. Iron D. Uranium What is the heaviest element that can be created through fusion? A. Carbon B. Silicon C. Iron D. Uranium No significant changes in luminosity Star travels back and forth on the HR diagram In the most massive stars, changes happen so quickly that the outer layers do not have time to respond Massive red giant or supergiant: Fierce hot winds and pulsed ejecta Outer layers subject to strong winds Hubble
Wildest of all! Question: why do we see the glowing gas surrounding the star to grow in time? ETA CARINAE Supermassive star (150 M SUN ) late in life, giant outburst 160 yr ago Note: the star emitted a pulse of radiation some time ago. Violent bipolar ejecta + disk at equator Red Giant with intense brightening `Light Echo from pulse Star V838 Monocerotis HST-ACS The core of a high mass star accumulates iron as the layers above it fuse Without any outward pressure, the core once again starts to contract. Electron degeneracy pressure supports the core for awhile until the mass of iron gets too heavy (how heavy?)
When mass is too large (>1.4M sun ), core collapses and iron atoms get compressed into pure neutrons protons + electrons! neutrons + neutrinos This takes less than 0.01 seconds Electron degeneracy pressure - GONE! Core collapses completely Eventually neutron degeneracy pressure stops the collapse abruptly Infalling atmosphere impacts on the core. Time for a demo Big and small balls Demo What do you think will happen? A. The little ball will bounce up together with the others B. The little ball will bounce higher than the others, but no higher than when the little ball is dropped alone C. The little ball will bounce much higher than the other balls Supernova! The lightweight atmosphere impacts on the heavy core and is bounced off in a huge explosion Plus huge energy release from neutrinos! The star s former surface zooms outward with a velocity of 10,000 km/s!