Elements and Isotopes

1. Elements and Isotopes We define an “element” by the number of protons in its nucleus. There can be “isotopes” with different numbers of neutrons. The number of protons and neutrons must be similar.

2. Thermonuclear Fusion In order to get fusion, you must overcome the electric repulsion. You can do this by having high density (lots of particles) and high temperature (particles moving very quickly). But actually, you must also have both a proton and a neutron.

3. The Importance of Neutrons • 1) Neutrons : can't build the elements without them • §the strong nuclear force holds nuclei together even though protons repel each other • §it works like velcro : only unlike particles can stick • Result : the elements have almost equal numbers of protons & neutrons • 2) Neutrons fall apart by themselves after about 10 minutes! • §you can make them by squeezing electrons into protons • at high density • §they are held by the weak nuclear force, which makes them pretty stable in nuclei • §sometimes they decay even in nuclei (beta decay)

4. The Proton-Proton Cycle This is the primary energy source for the Sun and most stars. The slowness of the first reaction makes the Sun last billions of years.

5. Inside the Sun The temperature and pressure in the core are extreme enough for fusion (and the Sun’s gravity keeps them that way). Most energy is produced in the inner 20%. Convection carries the energy in the outer 30%. Most of the mass is in the inner 50% because the density is much higher.

6. The Next Fusion Cycles The most common elements are : H (90%) , He (10%), C,N,O (0.1%) They are made by stars… Hydrogen burning in high mass stars The “CNO” cycle Start here Burning Helium in Red Giants (core temperature: 1 billion degrees)

7. What Happens when Hydrogen runs out? A Red Giant

8. Something beautiful… This looked like round blurs to early astronomers, who called them “planetary nebulae” (inappropriately)

9. Final Throes : Planetary Nebulae

10. Space art…

11. The Lifecycle of the Sun

12. Brown Dwarfs If the mass is too low, the object is a “failed star”, and can never stabilize its luminosity with fusion. Brown dwarfs fuse at least deuterium, but then find their pressure support without heat, and fade slowly away.

13. Astro Quiz Which of the following is the weakest reason to call something a planet instead of a brown dwarf? • The object never has any fusion in it. • The object is not in orbit around anything. • The object is smaller than Jupiter.

14. Planets, Brown Dwarfs, Stars

15. High Mass Stars Massive stars have such extreme core pressures that they can continue the process of getting energy from fusion all the way up to iron.

16. Life of High Mass Star 10 million years

17. Stellar Lifetimes

18. Open Star Clusters Hundreds to thousands of stars (and brown dwarfs). Relatively young (still containing massive stars). Found in galactic plane. May not even stay together if really young (might be dispersing). Light dominated by hot stars.

19. Globular Star Clusters Hundreds of thousands of old stars. Often not in plane of Milky Way. Have fewer heavy elements. Pictures dominated by red giants.

20. HR Diagrams and cluster evolution All stars are born at the same time (and distance). You can get the cluster age from the most massive MS star left.

21. Cluster HR Diagrams Good tests of stellar evolution theory.