Stellar Evolution: Low Mass Stars

1. Stellar Evolution: Low Mass Stars AST 112

2. Group by mass: • Low-mass stars: Less than 2 MSun • Intermediate-mass stars: 2-8 MSun • High-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 Sequence • They 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 hydrogen • Nuclear reactions balance gravity • Self-regulating process • Reactions occur via proton-proton chain • Somewhat 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 there • Gravity 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 cool • This is the Sub-Giant Phase

9. Red Giant Phase • Core continues to shrink • Surrounding hydrogen shrinks along with it • A 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 cores • Helium white dwarfs

11. Red Giant Phase • The burning H shell causes its luminosity to increase into the Red Giant phase • Star expands, almost maintains temperature • Luminosity goes way up

12. H-Shell Burning • The burning H-shell “rains helium” onto the inert helium core • Helium nucleus: 2 + charges, harder to fuse than hydrogen • Core continues to contract. Reaches 200,000,000 oF

13. Helium Flash • Helium core eventually ignites! • This is the end of the Red Giant Phase • 3 helium nuclei fuse into 1 carbon nucleus

14. Helium Burning • He core and H shell expand, cool • Outer star layers contract a lot, heat • Color moves from red to yellow (down and left)

15. Low-Mass Laboratory: Globular Clusters • Globular clusters are some of the oldest objects in the Universe • Population 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 died • We 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 formation • Why are there blue stars?

19. On Its Way Out • Helium gets fused into carbon for 100 million years (Sun-sized star) • Core runs out of helium, collapses • 1,200,000,000 oF required to fuse carbon • Low mass stars can’t do it.

20. On Its Way Out • A helium shell ignites, star expands • A burning hydrogen shell surrounds the helium shell • Now double-shell burning giant • Swells to larger than red giant phase

21. Death of Low Mass Stars • Helium reactions never stabilize • They happen in bursts every few thousand years • Called thermal pulses • Star is swollen, doesn’t hold onto outer layers • Combined 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

24. Death of Low Mass Stars

25. Death of Low Mass Stars

26. Death of Low Mass Stars This explains everything up to and includingcarbon. That’s six elements of the periodic table.Where do the other 100+ elements come from?

27. Death of Low Mass Stars

28. The Fate of Earth • The gradual brightening of the Sun will evaporate the oceans within 3-4 billion years • Will turn Earth into Venus • Prior to helium flash, Earth will be at 2000 oF • Titan might feel like Earth does now

29. The Fate of Earth • The Sun will shrink after the helium flash • 100 million years of relief • When it turns into a double-shell burning star: • Outer layers will swell to Earth’s orbit • Will eject outer layers • Earth will be “charred and dark”, if it’s even still around