Transcript The Lives of Stars From Birth Through Middle Age (Chapter 9)

Life Cycle of a Typical Star
Objectives
• How stars are created
• How they live
• What happens to them at the end of their lives
How Stars Are Created
• Cloud
of hydrogen is compressed to the right density
• Force of gravity is more powerful than the tendency for it to
disperse
• Cloud then irresistibly collapses
• Resultant heating of its interior should eventually
ignite the process of thermonuclear fusion that
powers stars
Eagle Nebula - which astronomers think is the remnants of an exploded star
A Typical Star
Corona
A corona is a type of plasma “atmosphere” of the Sun or other celestial
body, extending millions of kilometers into space, most easily seen
during a total solar eclipse, but also observable in a coronagraph.
The Greek root of the word corona means crown.
Chromosphere
The chromosphere (literally, "color sphere") is a thin layer of the Sun's atmosphere just above
the photosphere, roughly 2,000 kilometers deep.
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The
term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photosmeaning "light" and
σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived to emit light.
It extends into a star's surface until the gas becomes opaque, equivalent to an optical depth of approximately
2/3. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is
transparent to photons of certain wavelengths.
Equilibrium: Life Goal of a Star
This 5-step process works like this:
1. Nuclear fusion. Gravity = gas pressure (equilibrium)
2. Out of fuel.
3. Fusion stops, temperature drops.
4. Core contracts (gravity pulling atoms in).
5. Increased temperature (more atoms, more collisions)
and density in the core reinitiates nuclear fusion,
equilibrium is achieved, and the cycle begins again at
step 1.
Proton-Proton Fusion
Two atoms of hydrogen are combined to create helium-4 and energy in several steps:
1. Two protons combine to form a deuterium atom (hydrogen atom with one neutron and one
proton), a positron (similar to electron, but with a positive charge) and a neutrino.
2. A proton and a deuterium atom combine to form a helium-3 atom (two protons with one
neutron) and a gamma ray.
3. Two helium-3 atoms combine to form a helium-4 atom (two protons and two neutrons) and two
protons.
These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes
from the following reactions:
1. A helium-3 atom and a helium-4 atom combine to form a beryllium-7 (four protons and three
neutrons) and a gamma ray.
2. A beryllium-7 atom captures an electron to become lithium-7 atom (three protons and four
neutrons) and a neutrino.
3. The lithium-7 combines with a proton to form two helium-4 atoms.
The helium-4 atoms are less massive than the two hydrogen atoms that started the process, so
the difference in mass is converted to energy as described by Einstein's theory of relativity
(E=mc²). The energy is emitted in various forms of light: ultraviolet light, X-rays, visible light,
infrared, microwaves and radio waves.
Note: information taken from “How the Sun Works” by by Julia Layton and Craig Freudenrich, Ph.D.
Proton-Proton Fusion
Herzsprung-Russell (HR) Diagram
Death of a Star
• A low mass star becomes a white dwarf
• Medium-mass stars become neutron stars
• The largest mass stars may become black
holes
White Dwarf
Artist’s Conception of a Black Hole
Summary
• Stars form from mostly hydrogen gas +
gravity
• 2 forces in equilibrium: gravity (inward) +
nuclear pressure (outward)
• Once proton fusion stops, star begins to
die
• Size determines death fate of the star