Astronomy 162:
Introduction to Stellar, Galactic, & Extragalactic Astronomy
Lecture 14: Energy Transport in Stars
Key Ideas:
- Energy is transported to the surface of stars by:
- Radiation (photons)
- Convection (gas motions)
- Conduction
All the energy in stars is generated in the core, where it is
hot and dense enough for nuclear fusion to take place. The
energy must move from the core to the surface.
Thermal Equilibrium
Heat always flows from hotter regions into cooler regions.
In a star, heat must flow:
- from the hot core,
- out through the cooler envelope,
- to the surface where it is radiated away as light.
Energy Transport
There are 3 ways to transport energy in general:
- Radiation: Energy is carried by photons
- Convection: Energy carried by bulk motions of gas
- Conduction: Energy carried by particle motions
Radiation
Energy is carried by photons.
- Photons leave the core
- Hit an atom or electron within ~1cm and get scattered.
- Slowly stagger to the surface ("random walk")
Takes ~1 Million years to reach the surface.
Convection
Energy carried from hotter regions below to cooler regions above
by bulk buoyant motions of the gas.
Everyday examples of convection are boiling water and hot air "rising" off
of a candle flame or a radiator.
Conduction
Heat is passed from atom-to-atom in a dense material from hot
to cool regions.
Example: Hold the a spoon by the handle and put its bowl into a
candle flame. Over time, heat will get conducted from the bowl up the
handle and you will feel it heat up. If you hang on too long, it will
become almost as hot as the candle flame and you will burn your fingers.
Energy Transport in Stars
Normal Stars:
- A mix of Radiation & Convection transports energy from
the core to the surface.
- Conduction is inefficient as the density is too low.
White Dwarfs:
- Ultra-dense stars
- Conduction dominates energy transport.
Summary:
Energy is transported to the surface by:
- Radiation & Convection in normal stars
- Conduction in white dwarf stars
With Hydrostatic Equilibrium and Energy Generation
mechanisms, these determine the detailed structure
of a star.