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 = mc2.

Where does fusion occur?

Sun converts 600 million tons of Hydrogen into Helium every second. Takes billions of years to fuse all H to 4He 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 FieldRotating sun generates a magnetic fieldDifferential rotation => magnetic field distortion

Loops extending beyond photosphere can form

SunspotsThey are darker because they are cooler (4500 K vs. 5800 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 MSun

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 MSun

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 MSun

(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 MSun

Iron core at T ~ 1010 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 distance3) 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: 1012 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.