A Solar System Overview

1. A Solar System Overview • A dynamic solar system; • Stellar astronomy; • Subatomic particles and nuclear fusion; • White dwarfs as end to low mass stars; • High mass stellar evolution; • Cepheid variables; • Supernovae; • Neutron stars and black holes • Motivation • Not just Astro 2, 5, 10 content... • Note the origin of Type Ia supernovae • Note the origin of dense supernova remnants.

2. Classifying the planets • The planets fit into two groups: • Outer Jovian planets; • Inner terrestrial planets. • Size, mass, and density • The Jovian planets are larger and more massive; • Terrestrial planets are more dense.

3. Solar system belts & clouds • Asteroid belt Most orbit in or near the plane of the ecliptic; Most between Mars and Jupiter (2.2 to 3.3 AU from the Sun). • Kuiper belt Comets which lie just outside the orbit of Neptune; The largest is called Eris; Pluto is one of these objects; Projections suggest there is much more mass in the Kuiper belt than is in the asteroid belt. • Öort cloud Aphelia of billions of comets; About 10,000–100,000 AU from the Sun; Icy chunks ejected by from inner solar system by Jovian planets? Cause of periodic, near-sterilizing impacts on the Earth?

4. Recent results • Depends upon the semester

5. Extrasolar planetary systems • Until recently, our technology could only find planets Jupiter-sized or larger, which is exactly what we got! • Many of these super planets are closer to their star than Mercury is to the Sun. How does one form such a “hot Jupiter”? • As of February 2012, Kepler has revealed 2321 planets • 281 are Jupiter-sized or larger (6 REarth < R < 22 REarth); • 1118 are Neptune-sized (2 REarth < R < 6 REarth); • 676 are super-Earth (1.25 REarth < R < 2 REarth); • 246 are approximately Earth-sized (R< 1.25 REarth). • 88% are Neptune sized or smaller; • Overall, the size peaks at 2-3 REarth. • 54 are within the habitable zone of its parent solar system; • 5 habitable zone planets are less than 2 REarth.

6. The fiction of a clockwork solar system • Problems • Mars—being in the thick of the protostellar disk—should be 10× larger than it is. • Uranus & Neptune—at the edges of the prostellar disk—should be much less massive than they are. • Why are inner asteroids rocky (S type), and outer asteroids carbon rich (C type)?

7. The proposed, dynamic solar system • Solutions • The locations of the planetary orbits have shifted hugely over time. • Jupiter formed 3.5 a.u. from the Sun. Saturn, Uranus, and Neptune were very close. • Jupiter jostled Saturn into an unstable orbit; Saturn’s close encounters threw Uranus and Neptune into their larger orbits. • Jupiter then crept to 1.5 a.u., plowing through the asteroids and throwing them to the outer solar system. • In the process, Jupiter gravitationally shepherded a mini-disk near the Sun, which ultimately created the terrestrial planets. • A 3:2 orbit-orbit resonance with Saturn saved Jupiter from spiraling into the Sun. Instead, Jupiter was pulled away from the Sun, back to the outer solar system, returning the (S type) asteroids back to the inner solar system. • Mars migrated outwards, also shepherding the S type asteroids. • Further motion outwards by Jupiter and Saturn threw C type asteroids into the belt.

8. Stellar Astronomy • Composition • Stars are formed from interstellar material (gas and dust). • Gas in our part of our galaxy is 70% H, 28% He, 2% heavier elements. • As a result, stellar compositions reflect this. • Vital stats—NOT to be memorized in detail! • Masses range from about 0.1M-100M • Radii range around 0.01R -1000R • Temperatures range around 3000K-30000K • Luminosities range around 0.001L - 106L

9. Spectral Classes In the 1890s, the group of female Harvard astronomers lead by Edward Pickering developed the stellar classification system. Annie Jump Cannon recognized that this system reflects, for the most part, stellar temperatures. O B A F G K M 9

10. The Hertzsprung-Russell diagram • The Hertzsprung-Russell diagram (ca. 1910) is a plot of luminosity versus temperature (or spectral class) for stars. • The strength of the HR diagram lies in the fact that it shows structure, and is not just a “scatter diagram.” • About 90% of all stars fall into a group running diagonally across the diagram called the main sequence. • Other categories in the HR diagram include white dwarfs, red giants, and supergiants.

11. Inside the Stellar Engines Basic nuclear particles Protons—massive, positively charged; Neutrons—massive, no charge; Electrons—low mass (1/2000 proton), negatively charge; Neutrinos—almost massless (~10-9 electron), no charge. Nuclear fusion Reactions convert loosely atoms into more tightly bound atoms; The change in binding corresponds to a mass change from E=mc2. For main sequence stars, 4×(1H) atoms are converted to a single 4He atom. 11

12. Achieving Fusion Overcoming nucleus-nucleus repulsion Protons repel each other because they are both positively charged; Attracted to each other at about 1 fermi (10-15 m); Speed needed to overcome the “Coulomb barrier” as a temperature is about 120 million K; Stars are only about 20 million K in their interior; How does it happen? Quantum tunneling to the rescue! 12

13. Types of Nuclear Fusion • Proton-Proton Chain Hydrogen to Helium, used in stars less than ~1.5 M; • CNO Cycle Hydrogen to Helium, used in stars more than ~1.5 M; • Neutrino Production These reactions predict the production of vast numbers of neutrinos. 50 trillion neutrinos pass through your body/sec! 100 LY of lead shielding are needed to block neutrinos ~30%.

14. Detecting Neutrinos • Homestake Experiment (1960s) Raymond Davis, 400,000 liters of perchloroethylene. Chlorine + neutrino → Argon; Could not detect enough SNUs! • Super Kamiokande Experiment (now) Cylinder, 41.4m tall, 39 m diameter; Filled with 50 million liters of ultra pure water; Neutrino + H2O produces high velocity electrons; Cherenkov radiation produced! Requires a neutrino that has a mass.

15. Cepheid variables • Giant stars that are running out of hydrogen in their interiors. • As they expand, they leave the main sequence. Stars with a certain range of luminosities and temperatures become variable. They are called Cepheid variables. • Cepheid variable stars have a well established period-luminosity relation. This provides a powerful means for determining cosmic distances. • Note: there are many other types of variable stars, so be careful when studying Cepheids.

16. Energy Crisis When stars run out of hydrogen in their interiors… …their outer layers lift off to form a spectacular planetary nebula. …the furiously hot, compact core remains, and is called a white dwarf. …the white dwarf is destined to cool over time, to become a black dwarf. But that will take a looooooong time. …A typical white dwarf has 1.0 M, and a 12,000 km diameter (90% of Earth’s). A teaspoon of white dwarf material would weigh 2 tons. 16

17. White Dwarf Conditions • In normal conditions, “thermal” pressure is determined by a combination of density and temperature: • P=ρkT • In white dwarfs, pressure is instead maintained by “electron degeneracy pressure”. • In degeneracy pressure, the electron cloud of each atom cannot be squeezed any closer to the electron clouds of its neighbor atoms.

18. The Chandrasekhar Limit • More than half of all star systems are binaries or multiple stars. In many cases, the stars are so close that mass transfer can occur. • If mass from a red giant flows onto a white dwarf, explosive brightness changes of 10000× occur (novae), to 150,000 L. • So much matter can flow onto the white dwarf star that the Chandrasekhar Limit (1.4 M) can be exceeded. • Electron degeneracy is defeated!

19. Type I Supernovae • The collapse raises the core temperature and new kinds of fusion reactions occur (helium and carbon fusion). • The white dwarf BLOWS UP! • L=5×109L, which can be brighter than a whole galaxy! • This is called a Type Ia (or white dwarf) supernova. • Type Ia supernovae can be identified by their spectra. Since they are formed by uniform, very repeatable conditions. Therefore, all Type Ia supernovae should reach the same maximum brightness. • This makes them exceptionally reliable “standard candles.”

20. Massive Stars • Stars more massive than about 8 M avoid the white dwarf star stage, by having such high internal pressures and temperatures that they can smash together helium, carbon, and higher mass atoms. • This gives them an extension to their lives. They develop a complicated onion-skin internal structure. • But eventually, even these fuel sources run out. • But in the meantime, they produce all the elements that we find in the Universe, up to iron.

21. The Iron Crisis The inevitable formation of iron in the stellar core is a bad sign. Low mass atoms High mass atoms High mass atoms Low mass atoms Iron produces no energy, either via fusion or fission. The star’s attempt to fuse iron ends in catastrophe. The nuclei in the stellar core are driven together and neutronize, the core collapses, then rebounds, and riding a wave of neutrinos, the star blows up in a Type II supernovae. Nuclear Fusion Nuclear Fission

22. Neutron stars • The end result of a star that was initially perhaps 6-12 M. Neutron stars have masses between 1.4 and 3 M. • The star is supported by neutron degeneracy pressure, and is in a superfluid state—a perfect conductor of energy. • The diameter of a typical neutron star is only about 20 km. • The outer crust of a neutron star is largely electrons and positively charged atomic nuclei. • The neutron star is essentially a gigantic atomic nucleus, albeit one held together by gravity and not the nuclear force.

23. Neutron stars • A pulsar is a neutron star that pulses optical to radio waves with a period 0.0015-8.5 seconds, though nearly all fall between 0.1 and 2.5 seconds. • The lighthouse model explains pulsar behavior as being due to a spinning neutron star whose radiation beam sweeps by us. • If the neutron star is more than 3 M, even neutron degeneracy pressure will not support it against collapse. What then? • Some theories suggest that quark degeneracy pressure might support the core, hence quark stars or strange stars.

24. Black Holes • A black hole is the end result of a supernova explosion of a star with initial mass greater than about 12 M. • The event horizon is the spherical surface around a black hole from which nothing can escape. • Inside a black hole, an object will be crushed out of existence at a central singularity. • You don’t want to go into a black hole.