What Powers the Sun? Nuclear Fusion: An event where the nuclei of two atoms join together. Need high temperatures. Why? To overcome electric repulsion. Energy is produced. (A small amount of mass = a lot of energy)‏ Einstein's conservation of mass and energy, E = mc 2. Where does fusion occur?

Sun converts 600 million tons of Hydrogen into Helium every second. Takes billions of years to fuse all H to 4 He in Sun's core. Rate of fusion sets lifetime of stars. Why doesn't the Sun (or any other star) blow itself apart or collapse?

Hydrostatic Equilibrium

Energy Loss Mechanisms What is the part of the Sun that we can see called and how is energy being liberated from this region? What is the only heat loss mechanism that does not play an important role in the Sun?

Above the photosphere, there is the chromosphere, transition zone, and... The Corona Solar wind => Evaporation of the Sun! Also saw radiation and convection => only unimportant mechanism is conduction!

Sunspots Roughly Earth-sized Last ~2 months Usually in pairs What are sunspots and what causes them?

Sun’s Magnetic Field Rotating sun generates a magnetic field Differential rotation => magnetic field distortion Loops extending beyond photosphere can form

Sunspots They are darker because they are cooler (4500 K vs K). Related to loops in the Sun's magnetic field. Radiation from hot gas flowing along magnetic field loop of Sun.

Apparent Brightness What two things does the apparent (or perceived) brightness of an object depend on? How can this relationship be used to determine distances? Apparent Brightness α Luminosity/Distance^2

Creating the Heavy Elements How are the lives and deaths of stars related to the creation (and distribution) of the heavy elements? What is the heaviest element that can be created in the core of a star?

Stellar Deaths and the Creation of Heavier Elements A star will fuse heavier and heavier elements until: 1) It can no longer achieve the core temperature needed to fuse heavier elements (low mass stars)‏ or 2) Iron is created in the core (highest mass stars)‏ What is left behind when a low mass star dies? Red Supergiant

Stellar Lifetimes Is the lifetime of a high mass star shorter or longer than that of a lower mass star?

Evolution of Stars > 8 M Sun Higher mass stars evolve more rapidly (=> shorter lifetimes). Heaviest element made is iron. Products of outer layers become fuel for inner layers Eventual state of > 8 M Sun star

Novae What conditions are required for a nova to occur?

Stellar Explosions Novae Accreting white dwarf in a binary system How is this process related to a carbon-detonation supernova? What is the Chandrasekhar limit?

A Carbon-Detonation Supernova Despite novae, mass continues to build up on white dwarf (WD). If mass grows to 1.4 M Sun (the "Chandrasekhar limit"), gravity overwhelms the Pauli exclusion pressure supporting the WD. This starts carbon fusion everywhere at once. Tremendous energy makes star explode. No core remnant.

Death of a Very High-Mass Star M > 8 M Sun Iron core at T ~ K radiation photodisintegrates iron nuclei into protons and neutrons. Absorbs enormous amount of energy => core collapses in < 1 sec. Result is a Core-collapse Supernova What is left behind?

Testing our Theories Why are star clusters useful for stellar evolution studies? 1) All stars in a cluster formed at about same time (so all have the same age)‏ 2) All stars are at about the same distance 3) All stars have same chemical composition The only variable property among stars in a cluster is mass!

1. White Dwarf If initial star mass < 8 M Sun or so. (Low Mass)‏ 2. Neutron Star If initial mass > 8 M Sun and < 25 M Sun. (Intermediate Mass)‏ 3. Black Hole If initial mass > 25 M Sun. (High Mass)‏ Final States of a Star

Neutron Stars Conservation of Angular Momentum => Fast Rotation rate: few to many times per second. Huge Magnetic field: x Earth's! What type of object can these conditions produce? A neutron star over the Sandias?

The Lighthouse Model of a Pulsar

Black Hole Geometry What is the “surface” of a black hole called? What physical property determines it's size?

Event horizon: imaginary sphere around object with radius equal to Schwarzschild radius (determined by mass). “Surface” of black hole. Event horizon Schwarzschild Radius

According to Einstein's General Relativity, all masses curve space. How does this change our understanding of the gravitational force?

Black Holes What are some of the strange phenomena we might encounter if we fell into a black hole?

Effects around Black Holes Near event horizon: 1) Enormous tidal forces. 2) Bending of light. 2) Gravitational redshift. 3) Time dilation.

The Equivalence Principle What two phenomenon did Einstein show produce effects that are indistinguishable from one another?

Einstein's Principle of Equivalence According to Einstein, the effects of gravity and acceleration are indistinguishable from one another! The laws of physics in a gravitational field and in a uniformly accelerating frame of reference are identical.