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What Powers the Sun?
What Powers the Sun?
Nuclear Fusion:
An event where nuclei of two atoms join together.
Need high temperatures. Why?
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
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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. 5800 K).
Related to loops in the Sun's magnetic field.
radiation from hot gas
flowing along magnetic
field loop of Sun.
Apparent Brightness
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What two things does the apparent (or perceived)
brightness of an object depend on?
How can this relationship be used to determine
distances?
Creating the Heavy Elements
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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
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Is the lifetime of a high mass star shorter or
longer than that of a lower mass star? Why?
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
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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
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Why are star clusters useful for stellar evolution
studies?
Testing our Theories
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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!
Final States of a Star
1. White Dwarf
If initial star mass < 8 MSun or so. (Low Mass)
2. Neutron Star
If initial mass > 8 MSun and < 25 MSun . (Intermediate Mass)
3. Black Hole
If initial mass > 25 MSun . (High Mass)
Neutron Stars
Conservation of Angular
Momentum => 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
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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
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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
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What two phenomenon did Einstein show
produce effects that are indistinguishable from
one another?
Einstein's Principle of Equivalence
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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.