Life Cycles of Stars

The Hertzsprung-Russell Diagram

How Stars Form Collapsing gas and dust cloud Protostar - mostly infrared

Main Sequence Stars Brown Dwarf (L, T, Y) Red Dwarf (M) Normal Star (O, B, A, F, G, K)

All Objects Exist Because of a Balance Between Gravity and Some Other Force People, Planets-Interatomic Forces Normal Stars-Radiation White Dwarfs-Electron Repulsion Neutron Stars-Nuclear Forces Quark Stars? Black Holes-No Known Force

Mass, Luminosity, Lifetime Luminosity = Mass3.5 (Solar Units) Lifetime = Mass/Luminosity = 1/Mass2.5 Mass = .1: Lifetime = 316 (3160 b.y.) Mass = .5: Lifetime = 5.7 (57 b.y.) Mass = 1: Lifetime = 1 (10 b.y.) Mass = 10: Lifetime = .003 (3 m.y) Mass = 50: Lifetime = .000057 (570,000 yr)

Mass, Luminosity, Lifetime

Before Stars Form Pre-stellar cores Protostars Pre-main sequence star (PMS) Planet system formation. 

Protostars or Young Stellar Objects (YSO’s) Class 0 (T <70K) Emits in microwave range because of opaque surrounding cloud Class I (T = 70-650K) Emits in infrared. Star still invisible but can detect warm material around it. Class II (T = 650-2880K) T Tauri stars. Massive expulsion of material        Class III(T > 2880K) PMS stars

Early Stars and Planets (Class 0) Early main accretion phase (Class I) Late accretion phase (Class II) PMS stars with protoplanetary disks (Class III) PMS stars with debris disks

Super-Massive Stars Stars beyond a certain limit radiate so much that they expel their outer layers W stars (Wolf-Rayet stars) are doing this: T Tauri on steroids Upper limit about 100 solar masses More massive stars can form by merger but don’t last long

Wolf-Rayet Star

How Stars Die Main Sequence Stars Brighten With Age The More Massive a Star, the Faster it Uses Fuel Giant Phase White Dwarf Supernova Neutron Star - Pulsar Black Hole

Leaving the Main Sequence Helium accumulates in core of star Fusion shuts down Star begins to contract under gravity Core becomes denser and hotter Nuclear fusion resumes around helium core Outer layers puff up enormously but cool down Star becomes redder and larger (Red Giant)

“Live hard, die young, leave a good looking corpse”

Peeling off to the Giant Phase

Later Lives of Giants Inert helium core begins to fuse helium to carbon and oxygen Contraction of core stops Outer envelope contracts and heats up Red Giant becomes Yellow Giant Helium core runs out of fuel Helium fusion shell on outside of core, hydrogen fusion above Star loops between red and yellow on H-R plot

Making the Elements Heavy nuclei: Energy from Fission Light Nuclei: Energy from Fusion Both end at Iron: Most stable nucleus Stars can generate H-Fe through Fusion How do we get beyond Fe? Two processes S-Process (Slow) in Red Giants R-Process (Rapid!) in Supernovae

Beyond Helium He + particle = mass 5: not stable He + He = Mass 8: Not Stable The Mass 5-8 Bottleneck Sometimes three He collide to make C Li, Be, B rare in Universe Destroyed in Stars Created by spallation - knocking pieces off heavier atoms

Iron and Beyond Build from C to Fe by fusing successively heavier atoms Can’t Build Beyond Fe by Adding Protons Repulsion of nuclei = Charge1 x Charge2 He + C = O: Repulsion = 2 x 6 = 12 Fe + p = Co: Repulsion = 26 x 1 = 26 Can Add Neutrons Until Atoms Become Unstable n  p + e (Beta Decay)

The End Fate of Medium-Size Stars Core reaches limits of its ability to sustain fusion Fusion shells sputter and become unstable Star expels outermost layers as Planetary Nebulae Inert core left as white dwarf Dwarf has such tiny surface area it takes billions of years to cool Coolest (oldest?) known: 3900 K

Tiny Stars Red Dwarfs are tiny but have huge sunspots and violent flares They have convection throughout their interiors Interiors uniform in composition Do not accumulate helium in core Can use much more of their hydrogen up Never fuse He to C Lifetimes longer than age of Universe

Exploding Stars Nova Type I Supernova Type II Supernova White dwarf attracts matter from neighboring star Nuclear fusion resumes on surface of star Many novae repeat at decade or longer intervals Type I Supernova White dwarf core resumes fusion Type II Supernova Collapse of massive single star

Shell Structure of Massive Star 4H –> He 3He –> C He + C –> O, Ne Ne + He, C –> O, Mg 2O –> Si 2Si –> Fe

Core Collapse Fe core collapses to neutron star in milliseconds Remaining star material falls in at up to 0.1c Nuclei beyond Plutonium created Star blows off outer layers We see the thermonuclear core of the star Much of the light is from radioactive nickel

Historical Supernovae 185 - Chinese 1006 - Chinese, one European record 1054 - Chinese, European, Anasazi? 1572 - Tycho’s Star 1604 - Kepler’s Star 1885 – Andromeda Galaxy 1987 - Small Magellanic Cloud (170,000 l.y.)

Remains of SN 1054 (Crab Nebula)

Life (Briefly!) Near a Supernova Sun’s Energy Output = 90 billion megatons/second Let’s relate that to human scales. What would that be at one kilometer distance? 90 x 1015 tons/(150 x 106km)2 = 4 tons Picture a truckload of explosives a km away giving off a one-second burst of heat and light to rival the Sun

Now Assume the Sun Goes Supernova Brightens by 10 billion times 1010 = 25 magnitudes Our 4 tons of explosive becomes 40,000 megatons Equivalent to entire Earth’s nuclear arsenal going off one km away - every second This energy output would last for days

Neutron Stars and Pulsars Mass of sun but diameter of a few km Rotate at high speed Sun 1,400,000 km –> 10 km Rotation speeds up 140,000 x 28 days –> 17 seconds Pulsars: infalling matter emits jets of radiation Millisecond pulsars: probably “spun up” by accretion, or merger of neutron stars

How a Pulsar Works

Black Holes Singularity: gravity but no size Event horizon (Schwarzschild radius): no information can escape Detectable from infalling matter, which emits X-rays Quantum (atom-sized) black holes may exist Cores of galaxies have supermassive black holes

Black Hole