2 Group by mass: Low-mass stars: Less than 2 MSun Intermediate-mass stars: 2-8 MSunHigh-mass stars: Greater than 8 MSun
3 Low-Mass Stars So… what exactly do low mass stars do? They get on the Main Sequence and convert H to He.Then they blow up!
4 Low-Mass Star Evolution They spend most of their life on the Main SequenceThey then evolve into giants, then white dwarfs
5 The Main Sequence Reminder: During a star’s life on the Main Sequence, it doesn’t move very far on the HR diagram.
6 Low Mass: Main Sequence Steady burning of hydrogenNuclear reactions balance gravitySelf-regulating processReactions occur via proton-proton chainSomewhat inefficient compared to other reactions
7 Out of Hydrogen Eventually, the core runs out of hydrogen Still plenty in outer layers, but fusion not happening thereGravity causes the core to collapse on itself
8 Sub-Giant Phase The outer layers actually expand! Collapsing core releases so much heat that outer layers expand and coolThis is the Sub-Giant Phase
9 Red Giant Phase Core continues to shrink Surrounding hydrogen shrinks along with itA shell of hydrogen surrounding the core ignites (begins nuclear fusion)
10 Red Giant Phase Very low mass stars can stop here Leave behind inert helium coresHelium white dwarfs
11 Red Giant PhaseThe burning H shell causes its luminosity to increase into the Red Giant phaseStar expands, almost maintains temperatureLuminosity goes way up
12 H-Shell BurningThe burning H-shell “rains helium” onto the inert helium coreHelium nucleus: 2 + charges, harder to fuse than hydrogenCore continues to contract. Reaches 200,000,000 oF
13 Helium Flash Helium core eventually ignites! This is the end of the Red Giant Phase3 helium nuclei fuse into 1 carbon nucleus
14 Helium Burning He core and H shell expand, cool Outer star layers contract a lot, heatColor moves from red to yellow (down and left)
15 Low-Mass Laboratory: Globular Clusters Globular clusters are some of the oldest objects in the UniversePopulation of blue stars?Population of red stars?Its stars formed at the same time
16 Low-Mass Laboratory: Globular Clusters Its high mass stars have all diedWe see lots of low mass stars in all stages of evolution, sorted by time!
17 Population of Low Mass Stars This observationally-obtained HR diagram agrees with everything said thus far
18 Unrelated Mystery Globular clusters are old; blue stars gone There is no star formationWhy are there blue stars?
19 On Its Way OutHelium gets fused into carbon for 100 million years (Sun-sized star)Core runs out of helium, collapses1,200,000,000 oF required to fuse carbonLow mass stars can’t do it.
20 On Its Way Out A helium shell ignites, star expands A burning hydrogen shell surrounds the helium shellNow double-shell burning giantSwells to larger than red giant phase
21 Death of Low Mass Stars Helium reactions never stabilize They happen in bursts every few thousand yearsCalled thermal pulsesStar is swollen, doesn’t hold onto outer layersCombined with thermal pulses, everything but the core gets blasted out into space
22 Death of Low Mass Stars Results in a planetary nebula Has nothing to do with a planet!
23 Death of Low Mass Stars Exposed stellar core is leftover White Dwarf A sphere of carbon the size of Earth
28 The Fate of EarthThe gradual brightening of the Sun will evaporate the oceans within 3-4 billion yearsWill turn Earth into VenusPrior to helium flash, Earth will be at 2000 oFTitan might feel like Earth does now
29 The Fate of Earth The Sun will shrink after the helium flash 100 million years of reliefWhen it turns into a double-shell burning star:Outer layers will swell to Earth’s orbitWill eject outer layersEarth will be “charred and dark”, if it’s even still around